1533 MT PI Cover_Layout 1 9/3/15 10:16 AM Page 1

Programmed Instruction Series: Magnetic Particle Testing

PROGRAMMED INSTRUCTION SERIES

Magnetic Particle
Testing
Written for ASNT by
Michael A. Kowatch

Catalog No.: 1533 The American Society for

ISBN: 978-1-57117-363-8 Nondestructive Testing, Inc.
 PROGRAMMED INSTRUCTION SERIES

Magnetic Particle
Testing
Written for ASNT by
Michael A. Kowatch
Product Evaluation Systems, Inc.
 Copyright © 2015 by The American Society for Nondestructive Testing, Inc.

The American Society for Nondestructive Testing, Inc. (ASNT) is not responsible for the authenticity or
accuracy of information herein. Published opinions and statements do not necessarily reflect the opinion of
ASNT. Products or services that are advertised or mentioned do not carry the endorsement or
recommendation of ASNT.

No part of this publication may be reproduced or transmitted in any form, by means electronic or
mechanical including photocopying, recording or otherwise, without the expressed prior written
permission of ASNT.

IRRSP, NDT Handbook, The NDT Technician, and www.asnt.org are trademarks of ASNT. ACCP, ASNT, Level III
Study Guide, Materials Evaluation, Nondestructive Testing Handbook, Research in Nondestructive Evaluation and
RNDE are registered trademarks of ASNT.

first printing 08/15

Errata if available for this printing may be obtained from ASNT’s web site, www.asnt.org.

ISBN: 978-1-57117-363-8

Printed in the United States of America

Published by: The American Society for Nondestructive Testing, Inc.

1711 Arlingate Lane
Columbus, OH 43228-0518
www.asnt.org

Edited by: Bob Conklin, Educational Materials Editor

Assisted by: Cynthia M. Leeman, Educational Materials Supervisor

Tim Jones, Senior Manager of Publications

ASNT Mission Statement:

ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing.

ii Programmed Instruction Series

About the Author
Michael A. Kowatch began his career in nondestructive testing (NDT)
nearly 30 years ago while enlisted in the U.S. Navy. He currently holds an
ASNT/NAS 410 Level III in MT and PT, and was previously a Level III for UT,
as well as a Level II for RT. He is also a certified welding inspector (CWI).
He spent 11 years in the Navy, eight as an NDT specialist. Much of his early
NDT experience was gained performing examinations of various
repairs aboard Navy submarines and surface ships. After his time in the
Navy, he attended Old Dominion University in Norfolk, VA. For the past
10 years Kowatch has worked for Product Evaluation Systems, Inc. (PES),
an independent laboratory, serving as their QC Manager and NDT Level III.
Since joining PES, he has been involved with a wide variety of industries
that utilize NDT, including foundries, casters, power plants, aerospace, oil
and gas, and others. He is also a corporate Level III for several
corporations.

Magnetic Particle Testing iii

 Contributors
The American Society for Nondestructive Testing, Inc., is grateful for the
volunteer contributions, technical expertise, knowledge, and dedication
of the following individuals who helped make this work possible.

Technical Reviewers

Eric Henry – Applied Technical Services, Inc.

George Hopman – NDE Solutions Inc.
Douglas G. Kraus – Huddleston Technical Services, Inc.
Luis Payano – The Port Authority of New York and New Jersey
Robert F. Plumstead
Scott N. Smotherman – Huddleston Technical Services, Inc.
Mark Stowers – Crossroads Institute

Publications Review Committee

Joseph L. Mackin, Chair – Reel Group

Martin T. Anderson – Global Diving & Salvage, Inc.
Shant Kenderian – The Aerospace Corporation

iv Programmed Instruction Series

Foreword
The Programmed Instruction Series developed by the American Society
for Nondestructive Testing (ASNT) is intended for the independent study
of five major methods of nondestructive testing:

• Electromagnetic testing
• Liquid penetrant testing
• Magnetic particle testing
• Radiographic testing
• Ultrasonic testing

The aim of this educational series is to provide a review of the body of

knowledge for Levels I and II presented in ANSI/ASNT CP-105: ASNT
Standard Topical Outlines for Qualification of Nondestructive Testing
Personnel (2011). Readers are given the following opportunities within
each volume to evaluate their progress:

• Chapter questions – a variety of on-the-spot comprehension

checks woven into the body of each chapter following the
presentation of major ideas and concepts.
• Chapter reviews – quizzes at the end of each chapter to test
retention and application of core material.
• Volume self-test – comprehensive test at the end of the
volume. Multiple-choice questions with four unique answers
reflect the format of ASNT exams.

Need for Nondestructive Testing

Nondestructive testing (NDT), with its ability to detect discontinuities,

measure dimensions, and assess material characteristics, is a primary tool in
evaluating component performance and predicting remaining service life.

NDT is particularly effective at locating performance-degrading conditions

such as cracking, distributions of voids, inclusions, and unbonded or
poorly welded conditions, as well as identifying assemblies that contain
improper or misassembled subcomponents. Essentially, NDT assists in the
detection and characterization of material deficiencies without inhibiting
the part’s usefulness in any way.

Magnetic Particle Testing v

 NDT Methods

Nondestructive testing technology has matured to the extent that some

methods have become standard practice. These methods include:

Electromagnetic testing (ET) – ET comprises three techniques:

alternating current field measurement, eddy current, and remote
field testing. The eddy current technique is one of the most
common. This technique uses an electric circuit or coil to induce
an electric current—referred to as an eddy current—in a metal
test object. When the current flow is impeded by a discontinuity,
instrumentation will detect a significant change in the
electromagnetic field. Industrial applications of ET include the
testing of thin-wall tubing and aircraft maintenance.
Liquid penetrant testing (PT) – The PT method reveals the
appearance of surface discontinuities in solid and nonporous
materials through the use of liquid penetrants that penetrate
cracks and cavities open to the surface through capillary action.
These openings may be very small, otherwise undetectable by the
unaided eye.
Magnetic particle testing (MT) – MT is a method used to locate
surface and near-surface discontinuities in materials capable of
being magnetized. Discontinuities in a magnetized part produce
a magnetic flux leakage field when transverse to the direction of
the original magnetic field. This leakage field is, in turn, detected
by an application of fine ferromagnetic particles, producing an
indication of the discontinuity. Both PT and MT are used in quality
control, machinery maintenance, and the testing of large
components, among other applications.
Radiographic testing (RT) – The effectiveness of RT as an NDT
method is based on images of discontinuities recorded digitally or
on radiation-sensitive film. In industry, RT is principally used to test
the integrity of castings and weldments. Applications also include
the testing of mechanical assemblies for proper placement of
internal components.

vi Programmed Instruction Series

 Ultrasonic testing (UT) – UT is useful for the detection of
surface and subsurface discontinuities. High-frequency sound
waves are beamed into a material and reflections of the sound
are analyzed. This method can be used to locate internal
discontinuities in most metals and alloys in a wide range of
applications, including the testing of pressure vessels, machinery,
and bridges.

All of these methods coexist and may indeed complement each other. In
most cases, the choice of nondestructive testing method is based on the
process that will best locate the discontinuities sought under the given set
of conditions.

NDT Personnel Qualification

Nondestructive testing has historically required personnel with a high

degree of integrity, dedication, and training in order to successfully apply
even the most common forms of inspection. Inspectors, for example, may
be required to interpret test results by reading radiographs, estimating
discontinuity sizes, or analyzing strip-chart recordings. Nondestructive
tests involving technician judgment, such as interpreting indications in
critical welds, require that the technician is certified, as well as qualified, to
do the job.

There are three basic levels of qualification applied to nondestructive

testing personnel and used by companies that follow the
recommendations in the ASNT document Recommended Practice
No. SNT-TC-1A or the standard published as ANSI/ASNT CP-189.

An individual in the process of becoming qualified or certified to Level I is

considered a trainee. A trainee does not independently conduct tests,
interpret, evaluate, or report test results of any nondestructive testing
method. A trainee works under the direct guidance of certified individuals.

Qualification for Level I

Level I personnel are qualified to perform the following tasks:

1. Perform specific calibrations and nondestructive tests in

accordance with specific written instructions.

Magnetic Particle Testing vii

 2. Record test results. Normally, the Level I does not have the
authority to sign off on the acceptance and completion of the
nondestructive test unless specifically trained to do so with clearly
written instructions.
3. Perform nondestructive testing job activities in accordance with
written instructions or direct supervision from Level II or Level III
personnel.

Qualification for Level II

A Level II must be thoroughly familiar with the scope and limitations of

each method for which the individual is certified. Level II personnel are
qualified to perform the following tasks:

1. Set up and calibrate equipment.

2. Interpret and evaluate results with respect to applicable codes,
standards, and specifications.
3. Organize and report the results of nondestructive tests.
4. Exercise assigned responsibility for on-the-job training and
guidance of Level I and trainee personnel.

Qualification for Level III

A Level III is responsible for nondestructive testing operations to which

assigned and for which certified. A Level III must also be generally familiar
with appropriate nondestructive testing methods other than those for
which specifically certified. Level III personnel are qualified to perform the
following tasks:

1. Develop, qualify, and approve procedures; establish and approve

nondestructive testing methods and techniques to be used by
Level I and Level II personnel.
2. Interpret and evaluate test results in terms of applicable codes,
standards, specifications, and procedures.
3. Assist in establishing acceptance criteria based on a practical
background in applicable materials, fabrication, and product
technology.
4. In the methods for which certified, be responsible for the training
and examination of Level I and Level II personnel for certification
in those methods.

viii Programmed Instruction Series

Certification

It is important to understand the difference between two terms that are

often confused within the field of NDT: qualification and certification.
Qualification is a process that should take place before a person can
become certified.

According to Recommended Practice No. SNT-TC-1A, the qualification

process for any NDT method should involve the following:

1. Training in the fundamental principles and applications of the

method.
2. Experience in the application of the method under the guidance
of a certified individual (on-the-job training).
3. Demonstrated ability to pass written and practical (hands-on)
tests that prove a comprehensive understanding of the method
and an ability to perform actual tests using the specific
nondestructive testing method.
4. The ability to pass a vision test for visual acuity and color
perception or shades of gray, as needed for the method.

The actual certification of a person in NDT to a Level I, Level II, or Level III is
written testimony that the individual has been properly qualified. It
should contain the name of the individual being certified, identification of
the method and level of certification, the date, and the name of the
person issuing the certification. Certification is meant to document the
actual qualification of the individual.

Certification of NDT personnel is the responsibility of the employer.

Personnel may be certified when they have completed the initial training,
experience, and examination requirements described in the employer’s
written practice. The length of certification is also stated in the employer’s
written practice. All applicants should have documentation that states
their qualifications according to the requirements of the written practice
before certifications are issued.

Magnetic Particle Testing ix

 Central Certification

The American Society for Nondestructive Testing developed the ASNT

Central Certification Program (ACCP) for the third-party certification of
nondestructive testing personnel in the U.S. This certification program is
intended to meet or exceed the requirements of ISO-9712.

The employer’s written practice may accept such third-party certificates as

proof of qualification, but the employer must still certify its
nondestructive testing personnel to perform nondestructive testing.

x Programmed Instruction Series

Preface
The magnetic particle testing volume of the Programmed Instruction
series provides an overview of the Level I and Level II body of knowledge
contained in ANSI/ASNT CP-105: ASNT Standard Topical Outlines for
Qualification of Nondestructive Testing Personnel (2011).

Chapters are broken down as follows:

Level I: Chapters 1–7

Level II: Chapters 8–14

Level I and II candidates are encouraged to use this programmed

instruction book as a review for magnetic particle testing examination and
certification. Please keep in mind that a Level I or Level II candidate may
be expected to know additional information depending on industry or
employer requirements.

Magnetic Particle Testing xi

 USB Flash Drive for Computerized Study

A computer-based training program is provided on the USB flash drive

that accompanies this volume in the programmed instruction series.
The material presented complements the content of the print volume.
Navigation from the main menu allows selection of chapters for review.
Each chapter is broken into a number of individual lessons. Learners are
not locked into a sequence but may exit a lesson at any time.

Features include:
• Learning objectives and summaries – bulleted points that
clearly preview and sum up the material covered in each lesson.
• Interactive “pop-up” questions – multiple-choice questions
interspersed within each lesson, with the opportunity to review
content for questions missed. Brief explanations of correct
responses are also provided. Learners must answer a question
correctly in order to advance through a given lesson.
• Chapter quizzes – multiple-choice format with immediate
question-by-question grading of correct/incorrect responses.
A final percentage score is displayed at the end of each quiz.
Answers are randomized for repeat quiz taking.
• Volume self-tests – multiple-choice questions with immediate
results as well as a final percentage score. Test data includes
chapters and lessons missed so that learners may review specific
areas for improvement. Answers are randomized for repeated use
of each test.

For more information, please refer to the user instruction file included on
the USB flash drive.

xii Programmed Instruction Series

Table of Contents
About the Author ...........................................................................................iii
Contributors....................................................................................................iv
Foreword ..........................................................................................................v
Preface.............................................................................................................xi
USB Flash Drive for Computerized Study ...................................................xii

Chapter 1: Introduction to Magnetic Particle Testing..................................1

From the Beginning: The History of MT Inspection ..............................................2
The Purpose of MT Inspection ..............................................................................4
Benefits of MT..............................................................................................................4
Limitations of MT .......................................................................................................5
Precautions with MT .................................................................................................6
Magnets 101 ........................................................................................................................6
Types of Magnets .......................................................................................................6
Magnetic Poles............................................................................................................8
Magnetic Flux ..............................................................................................................9
Magnetization Forces.............................................................................................10
Chapter 1 Summary........................................................................................................11
Answers to Chapter 1 Questions ...............................................................................13
Chapter 1 Review ...........................................................................................................14
Chapter 1 Review Key ....................................................................................................16

Chapter 2: Electricity and Magnetism .........................................................17

Alternating Current (AC) Magnetization.................................................................18
Skin Effect...................................................................................................................19
Alternating Current Equipment .........................................................................20
Yokes.....................................................................................................................20
Prods.....................................................................................................................21
Rectified Alternating Current Magnetization........................................................22
Half-Wave Current ...................................................................................................23
Single-Phase Full-Wave Current .........................................................................24
Three-Phase Full-Wave Current ..........................................................................26
Chapter 2 Summary........................................................................................................29
Answers to Chapter 2 Questions ...............................................................................30
Chapter 2 Review.............................................................................................................31
Chapter 2 Review Key ....................................................................................................32

Magnetic Particle Testing xiii

 Chapter 3: Performing Magnetic Particle Testing Inspections..................33
Defining Techniques and Field Direction ...............................................................34
Technique ...................................................................................................................34
Direction ....................................................................................................................34
Right-Hand Rule ...............................................................................................35
MT Techniques..................................................................................................................36
Yoke (Longitudinal).................................................................................................36
Coil (Longitudinal)...................................................................................................37
Prods (Circular)..........................................................................................................38
Head Shot and Central Conductor....................................................................40
Wet Bench Units...............................................................................................41
Chapter 3 Summary........................................................................................................43
Answers to Chapter 3 Questions ...............................................................................44
Chapter 3 Review.............................................................................................................46
Chapter 3 Review Key ....................................................................................................48

Chapter 4: Detecting Discontinuities with MT............................................49

Defining Discontinuities ...............................................................................................50
Types of Discontinuities ................................................................................................52
Inherent ......................................................................................................................53
Processing ..................................................................................................................53
Inservice ......................................................................................................................54
Locating Discontinuities ...............................................................................................55
Magnetic Direction and Strength......................................................................57
Quantitative Quality Indicator ....................................................................57
Pie Gage ..............................................................................................................58
Interpreting Indications ................................................................................................59
Relevant versus Nonrelevant Discontinuities ...............................................59
Magnetic Writing .....................................................................................................60
Mapping and Measuring Indications ...............................................................60
Linear versus Rounded Indications...................................................................61
Subsurface Discontinuities ..................................................................................62
Evaluating Discontinuities............................................................................................63
Chapter 4 Summary........................................................................................................65
Answers to Chapter 4 Questions................................................................................66
Chapter 4 Review ...........................................................................................................68
Chapter 4 Review Key ....................................................................................................70

xiv Programmed Instruction Series

Chapter 5: MT Equipment.............................................................................71
Factors Affecting MT Equipment Selection ...........................................................72
Customer/Inspection Requirements ................................................................72
Power Availability ....................................................................................................72
Location of the Test.................................................................................................73
Type of Discontinuities Expected ......................................................................73
MT Materials ......................................................................................................................73
Dry Particles ..............................................................................................................74
Wet Particles ..............................................................................................................75
Suspension ................................................................................................................75
MT Testing Equipment...................................................................................................76
Yoke ..............................................................................................................................77
Wet Bench Unit.........................................................................................................78
Heads....................................................................................................................80
Coil ........................................................................................................................80
Bath Tank.............................................................................................................80
Amperage Control...........................................................................................81
Accessories.........................................................................................................................82
Pie Gage ......................................................................................................................82
Gauss or Tesla Meter...............................................................................................82
Quantitative Quality Indicator (QQI).................................................................83
Field Indicator ...........................................................................................................83
Ultraviolet Lamps.....................................................................................................84
Calibration..........................................................................................................................86
Quick Break Functionality.............................................................................................87
Chapter 5 Summary........................................................................................................88
Answers to Chapter 5 Questions ...............................................................................90
Chapter 5 Review ...........................................................................................................92
Chapter 5 Review Key ....................................................................................................94

Chapter 6: Demagnetization Principles ......................................................95

Introduction.......................................................................................................................96
Why Demagnetize?.........................................................................................................96
Demagnetization Techniques .....................................................................................98
Coil/Yoke Demagnetization .................................................................................98
Bench Demagnetization .......................................................................................99
Effect of Heat-Treating ...........................................................................................99
Measuring Residual Magnetism ..............................................................................100
Chapter 6 Summary .....................................................................................................103
Answers to Chapter 6 Questions ............................................................................104

Magnetic Particle Testing xv

 Chapter 6 Review ..........................................................................................................105
Chapter 6 Review Key..................................................................................................106

Chapter 7: Inspection Personnel................................................................107

NDT Personnel................................................................................................................108
NDT Specifications Governing Certification........................................................109
Training .............................................................................................................................111
Experience .......................................................................................................................112
Certification Examinations.........................................................................................115
Performance Reviews and Recertification ...........................................................116
Chapter 7 Summary .....................................................................................................119
Answers to Chapter 7 Questions.............................................................................120
Chapter 7 Review ..........................................................................................................122
Chapter 7 Review Key..................................................................................................124

Chapter 8: Review of MT Fundamentals ...................................................125

Understanding Magnetism .......................................................................................126
Magnetic Domains .......................................................................................................127
Magnetic Flux and Poles.............................................................................................128
Magnetic Field Strength and Direction ................................................................131
Hall Effect Meter ...................................................................................................132
Field Indicator ........................................................................................................133
Quantitative Quality Indicator (QQI) ..............................................................134
Flexible Laminated Strip.....................................................................................134
Magnetic Fields..............................................................................................................135
Direct Magnetization ...........................................................................................135
Indirect Magnetization........................................................................................136
Magnetic Flux and Discontinuities .........................................................................137
Electrical Currents and Electromagnets ...............................................................138
Alternating Current ..............................................................................................139
Half-Wave Current.................................................................................................139
Single-Phase Full-Wave Current.......................................................................140
Three-Phase Full-Wave Current........................................................................141
Magnetic Permeability ................................................................................................142
Hysteresis Curves ..................................................................................................142
Choosing the Correct MT Technique .....................................................................145
Chapter 8 Summary .....................................................................................................147
Answers to Chapter 8 Questions.............................................................................149
Chapter 8 Review ..........................................................................................................151
Chapter 8 Review Key..................................................................................................153

xvi Programmed Instruction Series

Chapter 9: Characteristics of Electromagnetic Fields ..............................155
Types of Magnetic Fields............................................................................................156
Circular Magnetic Field ...............................................................................................157
Head Shot/Contact Plates..................................................................................158
Central Conductor ................................................................................................160
Prods ..........................................................................................................................162
Longitudinal Magnetic Field .....................................................................................165
Yoke............................................................................................................................166
Coil..............................................................................................................................167
Wrapped Cables.....................................................................................................168
Calculating Current in Cable Wraps........................................................169
Chapter 9 Summary .....................................................................................................173
Answers to Chapter 9 Questions.............................................................................174
Chapter 9 Review ..........................................................................................................176
Chapter 9 Review Key..................................................................................................178

Chapter 10: Demagnetization Techniques................................................179

Residual Magnetic Fields............................................................................................180
Magnetic Properties.............................................................................................180
Retentivity........................................................................................................180
Coercive Force ................................................................................................180
Current ......................................................................................................................182
Alternating Current ......................................................................................182
Direct Current .................................................................................................182
Deciding to Demagnetize..........................................................................................183
Reasons to Demagnetize....................................................................................183
Reasons Not to Demagnetize...........................................................................184
Measuring Residual Magnetic Fields .....................................................................184
Removing Residual Fields ..........................................................................................185
Longitudinal versus Circular Residual Fields...............................................186
Techniques for Removing a Residual Magnetic Field..............................186
Using a Coil or Yoke......................................................................................186
Step-Down Technique or Downcycling ................................................186
Demagnetizing Procedures.......................................................................................187
AC Demagnetization............................................................................................188
Direct Demagnetization .....................................................................................188
Cable Wrap...............................................................................................................189
Demagnetizing Do’s and Don’ts ..............................................................................189
Current ......................................................................................................................189
Demagnetizing Requirements .........................................................................190

Magnetic Particle Testing xvii

 Demagnetizing Field Strength.........................................................................190
Direction...................................................................................................................190
Verifying the Residual Field...............................................................................190
Summary ..........................................................................................................................191
Chapter 10 Summary...................................................................................................193
Answers to Chapter 10 Questions ..........................................................................194
Chapter 10 Review........................................................................................................195
Chapter 10 Review Key ...............................................................................................196

Chapter 11: Materials and Accessories......................................................197

MT Particle Types and Characteristics ...................................................................198
Dry Particles ............................................................................................................198
Wet Particles............................................................................................................199
Specific Characteristics of Particles ................................................................199
Size......................................................................................................................199
Shape .................................................................................................................201
Color...................................................................................................................202
Permeability ....................................................................................................203
Retentivity........................................................................................................203
MT Inspection Lighting: Visible and Fluorescent ..............................................204
White Light ..............................................................................................................205
Ultraviolet Light .....................................................................................................206
Ultraviolet Radiation Intensity Requirements ...........................................208
Ambient Light Measurements ................................................................209
Ultraviolet Light Measurements .............................................................209
Chapter 11 Summary...................................................................................................211
Answers to Chapter 11 Questions .........................................................................212
Chapter 11 Review........................................................................................................214
Chapter 11 Review Key ...............................................................................................216

Chapter 12: NDT Certification and Test Standards...................................217

NDT Personnel Certification Standards.................................................................218
Certification Requirements................................................................................218
Training .............................................................................................................219
Experience .......................................................................................................219
Written Examinations ..................................................................................220
Practical Examination ..................................................................................220
Eye Examinations ..........................................................................................220
Certification Standards........................................................................................221

xviii Programmed Instruction Series

 NDT Inspection Standards .........................................................................................223
Examples of Inspection Standards .................................................................224
Information Contained in Inspection Standards.......................................224
NDT Acceptance Standards.......................................................................................225
Interpreting Inspection Standards..................................................................226
Variations in Acceptance Standards...............................................................228
Summary ..........................................................................................................................230
Chapter 12 Summary...................................................................................................231
Answers to Chapter 12 Questions ..........................................................................232
Chapter 12 Review........................................................................................................234
Chapter 12 Review Key ...............................................................................................236

Chapter 13: Discontinuities Detectable by MT .........................................237

What Is a Discontinuity? .............................................................................................238
Inherent Discontinuities .............................................................................................239
Shrinkage .................................................................................................................241
Porosity .....................................................................................................................241
Blowholes.................................................................................................................242
Cold Shuts................................................................................................................242
Nonmetallic Inclusions........................................................................................243
Hot Tears...................................................................................................................244
Primary Processing Discontinuities ........................................................................244
Forging, Rolling, and Extruding Discontinuities ........................................245
Forging Cracks and Laps.............................................................................245
Forging Bursts.................................................................................................247
Laminations.....................................................................................................247
Seams.................................................................................................................248
Hydrogen Flakes ............................................................................................248
Welding Discontinuities......................................................................................249
Cold Cracking (Heat-Affected Zone Cracks) ........................................249
Hot Cracking (Crater Cracks) .....................................................................250
Lamellar Tearing.............................................................................................252
Lack of Fusion .................................................................................................252
Lack of Penetration.......................................................................................253
Inclusions .........................................................................................................253
Arc Strikes.........................................................................................................254
Secondary Processing Discontinuities ..................................................................255
Machining Tears.....................................................................................................255
Grinding Cracks......................................................................................................255
Heat-Treating and Quench Cracks ..................................................................255

Magnetic Particle Testing xix

 Inservice Discontinuities ............................................................................................256
Fatigue Cracks ........................................................................................................257
Creep Cracking.......................................................................................................257
Stress-Corrosion Cracking..................................................................................258
Hydrogen Cracking (Embrittlement) .............................................................258
Summary ..........................................................................................................................260
Chapter 13 Summary...................................................................................................261
Answers to Chapter 13 Questions ..........................................................................262
Chapter 13 Review........................................................................................................264
Chapter 13 Review Key ...............................................................................................266

Chapter 14: Process Quality Assurance.....................................................267

Calibration .......................................................................................................................268
Calibration Requirements ..................................................................................269
System Checks................................................................................................................270
System Check Specifications.............................................................................271
Ketos Steel Ring .....................................................................................................272
Using the Ring Standard.............................................................................274
Water Bath Quality........................................................................................................274
Water Bath Safety..................................................................................................274
Water Bath Concentration .................................................................................275
Settling Test.............................................................................................................275
Bath Maintenance ................................................................................................276
Surface Preparation for Wet Fluorescent Method.....................................277
Recording the Examination.......................................................................................277
Examination Report Priorities...........................................................................279
Chapter 14 Summary...................................................................................................280
Answers to Chapter 14 Questions ..........................................................................282
Chapter 14 Review........................................................................................................283
Chapter 14 Review Key ...............................................................................................284

Magnetic Particle Testing Self-Test............................................................285

Self-Test Answer Key...................................................................................296
References ...................................................................................................297
Figure Sources.............................................................................................299
Glossary .......................................................................................................301

xx Programmed Instruction Series

 Chapter 1
Chapterto
Introduction #
Chapter
Magnetic Particle Title
Testing

In this chapter:
Historical overview of magnetic particle testing
Purpose and benefits of magnetic particle testing
Definitions of ferromagnetic, paramagnetic, and diamagnetic
Types of magnets: permanent and electromagnetic
Ring versus horseshoe magnets
Magnetic poles, flux, and forces
Definitions of reluctance, permeability, coercive force, and retentivity

1
 From the Beginning: The History of MT
Inspection
Magnetism has been around since the formation of the Earth. The Greeks
were probably the first to recognize and document its existence. Since
then, magnetism has been used in many ways. For example, explorers,
both on land and sea, have used the Earth’s magnetism for navigation.
As for using magnetism to inspect for discontinuities, a crude form of
magnetic particle testing (MT) was used to inspect rifle and cannon
barrels in the mid to late 19th century.

In the mid to late 1800s, when the Industrial Revolution began, steel was
being used more than it had ever been. Industries such as shipbuilding,
automobile manufacturing, and railroading, just to name a few, were
demanding massive amounts of steel to build their products. At the time,
quality was not as important as it is today. Steel and other product
manufacturers had goals similar to that of contemporary businesses: ship
as much product, quickly, with the highest possible profits. However, the
difference between then and now is the quality of the products being
manufactured. Compared with today, quality was an afterthought.
Companies often only reacted after users discovered a defect. Now, quality
is designed into and is a critical part of the manufacturing process.

There are many factors as to why this is the case, but one of the main
reasons is the lack of technology available in the 19th and early 20th
centuries; technology was limited. The main tool for performing quality
inspections was almost exclusively visual testing (VT). Although visual
inspection was, and is, useful, many discontinuities could not be seen
visually. These discontinuities were missed and would eventually lead to
failures after the product was placed into service.

2 Programmed Instruction Series

Question 1.1

Magnetism has been present for millions of years. Who is credited with
discovering it?
A. 19th-century navigators.
B. Ancient Greeks.
C. Shipbuilders.
D. Rifle-and cannon-barrel inspectors.

 Please turn to the end of the chapter for the answer.

During this period, there were some very crude magnetic particle techniques
being used; however, it was not until after the turn of the 20th century, during
World War I, that magnetic particle inspection began to take shape. This was
when the magnetic particle testing method that most closely resembles what we
use today was first discovered. During World War I, Major William E. Hoke was
experimenting with permanent magnets and metal shavings and noticed that he
could find cracks in metal by placing the magnet next to the part and sprinkling
metal shavings onto its surface. At the time, Hoke did not fully understand
magnetism and why or how it worked, but his research would lead to great
advancements in magnetic particle testing equipment, materials, and inspection
techniques.

The equipment and materials in use today are manufactured to very

precise specifications. Although there are many different industry
specifications governing magnetic particle inspection, the theory used to
perform and evaluate MT inspections is the same. MT remains one of the
most widely used methods of nondestructive testing (NDT) and is used by
all manufacturing segments, including aerospace, shipbuilding, metal
fabrication, railroading, and nuclear propulsion.

Question 1.2

Major Hoke experimented with magnetic particle testing during World

War I using:
A. electromagnets.
B. magnets and metallic shavings.
C. refrigerator magnets.
D. horseshoe magnets.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 3

 The Purpose of MT Inspection
Magnetic particle inspection is used in many industries to detect a wide
variety of discontinuities in metallic products. One of the most common
uses is to inspect welds, but many metallic test objects, including castings,
forgings, and machined parts, can be inspected, as long as they are capable
of being magnetized. The goal for the technician using MT is to find
discontinuities before products are put into service, which saves time and
money, and more importantly can prevent injuries and loss of life.

There are many opportunities for using MT in the manufacturing process

prior to final machining; for instance, after primary processing and before
secondary processing begins. More specifically, MT inspection is
performed after bending operations, at various stages of the welding
process, on final surfaces of castings and forgings, and after final
machining. Inservice discontinuities are also detected with MT.
Essentially, the method is used to identify discontinuities, such as cracks,
on or under the surface of ferromagnetic material; that is, material highly
susceptible to magnetization. The aerospace industry is a major example
of a sector that relies on magnetic particle testing to ensure safety.

Question 1.3

What is the purpose of MT inspection?

A. Finding discontinuities only after final machining.
B. Finding discontinuities only before final machining.
C. Permanently magnetizing parts.
D. Finding discontinuities in ferromagnetic material.

 Please turn to the end of the chapter for the answer.

Benefits of MT
Today, there are many inspection methods used by nondestructive test
personnel, including liquid penetrant, radiographic, ultrasonic, and
electromagnetic testing, but magnetic particle testing remains one of the most
widely used nondestructive testing methods available. With advancements in
NDT technology—guided wave, phased array, laser methods, thermal/
infrared, computed radiography, and others—it might seem that simpler NDT
methods such as magnetic particle and liquid penetrant testing would soon
become obsolete, but that is not the case at all. Both are more popular than
ever before. Why? Here are a few factors to consider:

4 Programmed Instruction Series

 • Cost: Although there are some exceptions, the equipment and
materials used to perform MT inspection are relatively inexpensive
when compared with other NDT methods. For example, ultrasonic
testing equipment can cost thousands of dollars.
• Time: As the old saying goes, “Time is money,” and that is still true
today. In some cases, an MT inspection may take only a few minutes
to complete. Other nondestructive test methods can have lengthy
setup and inspection times.
• Reliability: MT techniques, when used properly, are extremely
reliable. As long as the inspectors are using the correct technique
and are trained adequately, MT inspection is highly capable of
finding surface and near-surface discontinuities.
• Personnel: MT equipment and inspection techniques can be applied
with minimal personnel training and experience. Of all of the NDT
methods, training and experience hour requirements for MT
inspection personnel are among the lowest.
• Simplicity: Although there are several MT inspection techniques, most
of them are relatively simple to perform. There are some exceptions, such
as when using magnetic particle benches. These units are not portable
and take time to set up prior to actually performing tests.

Question 1.4

Which of the following statements is most accurate with regard to MT

testing?
A. It’s one of the more costly NDT test methods.
B. It’s an extremely complicated test method.
C. MT is less costly and simpler than most NDT methods.
D. All MT equipment has special electrical needs, which limit their use.

 Please turn to the end of the chapter for the answer.

Limitations of MT
Limitations of MT include:

• Limited to ferromagnetic materials.

• Surface discontinuities that are parallel to the magnetic field will not
produce indications because they do not interrupt or distort the
magnetic field.

Magnetic Particle Testing 5

 • Discontinuities may go undetected when improper contact exists
between the ferromagnetic particles and the part surface.
• Foreign material not removed prior to testing may cause an invalid
test.
• Parts may need to be disassembled before inspection.
• Objects with large cross sections require a high current to generate
an adequate magnetic field.

Precautions with MT
Precautions and considerations involving MT include:

• Where the general direction of a discontinuity is questionable, it may

be necessary to magnetize in two or more directions.
• Visible or fluorescent particles must produce a contrast color when
evaluated against the surface of the part.
• Demagnetization of parts is usually required. For example, while
performing MT prior to and during welding, demagnetization
may be necessary so as not to interfere with subsequent welding
operations.

Magnets 101
Although the basics of magnetic particle testing are relatively simple, it’s
necessary to have a thorough understanding of the characteristics of
magnets—both permanent and those created by electric current.

Magnetism is the ability of a material to be attracted to a magnetic force.

Not all materials are magnetic or have the ability to be magnetized to a
strength that is adequate for MT inspection. For instance, aluminum,
platinum, and chromium are materials that are only slightly attracted to a
magnetic field and are referred to as paramagnetic. On the other hand,
some materials, such as gold, copper, quartz, and mercury, are weakly
repelled by a magnetic field and are referred to as diamagnetic.
As for magnetic particle inspection, only ferromagnetic materials can be
sufficiently magnetized and used for testing. Common ferromagnetic
materials include iron, nickel, and cobalt.

Types of Magnets
Magnets come in many different shapes and sizes and are used in a wide
variety of applications. There are two main types of magnets: permanent

6 Programmed Instruction Series

and electromagnetic (temporary). Permanent magnets tend to be weaker
than electromagnets and are usually not preferred for MT inspections when
electromagnets are available for use. Also, permanent magnets do not lose
their magnetism unless they are demagnetized. Some examples of fabricated
permanent magnets that most of us are familiar with include horseshoe
magnets, cabinet door closures, and refrigerator door magnets. For example,
refrigerator door magnets or sheet magnets are made by mixing ferric oxide
and plastic together and extruding the mixture into a thin sheet. The sheet is
then magnetized by passing it through strong permanent magnets. Unlike
the magnets used for magnetic particle testing, sheet magnets have
alternating north and south poles. (See Figure 1.1.)

Figure 1.1: Alternating poles of a sheet magnet.

Another type of magnet is a ring magnet. If a horseshoe magnet is bent so

that its poles are close together, the poles still attract magnetic materials.
Iron filings or other magnetic materials cling to the poles and bridge the
gap between them. In the absence of a slot, the magnetic flux lines are
enclosed within the ring, as illustrated in Figure 1.2. No external poles
exist, and ferromagnetic particles dusted over the ring are not attracted to
the ring even though there are magnetic flux lines flowing through it.
Magnetized materials attract external ferromagnetic objects only when
poles exist. A ring magnetized in this manner is said to contain a circular
magnetic field that is wholly within the object.

Temporary magnets are magnets that lose some or all of their magnetism
once the magnetic force is removed. A scrapyard magnet is an example of
a temporary magnet or electromagnet. Once the electric current is shut
off, the magnetic lines of force do not exist in the material and it is no
longer magnetic. Any magnetism remaining in the part is referred to as
residual magnetism.

Magnetic Particle Testing 7

 Magnetic Poles
With the exception of a ring magnet,
all magnets, permanent and temporary,
have north and south poles. In a bar
magnet, the poles are at the ends and
anywhere there is a break or fracture,
(a)
such as a crack or cut in the material.
In Figure 1.3, you can see how the poles
react to each other. Opposite poles are
attracted to each other and like poles
are repelled.

Question 1.5 (b)

Magnetic
particles
Which type of magnet does not have
north and south poles?
A. None; all magnets have poles.
B. Weak magnets.
C. Electromagnets. (c)
D. Ring magnets.
Figure 1.2: Types of magnets: (a) horse-
 Please turn to the end of the shoe; (b) magnet with poles moved
closer together showing magnetic flux
chapter for the answer. in air around poles; (c) ring.

N S S N

Repel

N S N S

Attract

Figure 1.3: Attraction and repulsion of magnetic poles.

8 Programmed Instruction Series

 Ë From the Field: Residual magnetism can be detrimental and
in most cases must be removed following MT inspection. If a
part such as a driveshaft were to be left magnetized, metal
shavings would likely collect on its surface during operation,
which could lead to its failure or the failure of associated
components.

Magnetic Flux
Magnetic lines of force outside of a magnet exit at one pole (conventionally,
labeled the north pole) and enter at the other (south) pole. These lines are all
the same strength and never cross each other. However, the magnetic field is
stronger at the poles because of more flux lines (flux density) per unit area.
Magnetic lines of force are sometimes referred to as magnetic flux. When
passing through nonferromagnetic material, flux lines are forced to spread
out and thus create poles where they are forced out of the material. It is at
these poles where ferromagnetic particles in a magnetic particle test
accumulate, indicating possible discontinuities. Try dipping a bar magnet in
metal shavings; the largest concentration of the shavings tends to accumulate
at the poles.

All magnets, which include any materials that have a magnetic field,
regardless of strength, have magnetic lines of force or magnetic flux.
Although you cannot see these lines with the naked eye, the image in
Figure 1.4 may help you to visualize magnetism by showing the magnetic
lines of force as they exit and enter a magnet. The metal shavings form
magnetic lines on and around the magnet, extending to and from the
north and south poles.

N S

Figure 1.4: Magnetic lines of force.

Magnetic Particle Testing 9

 Question 1.6

Residual magnetism is removed from parts because:

A. it could interfere with the product’s intended use.
B. it improves the part’s cosmetic appearance.
C. it is a mandatory inspection practice.
D. all specifications require demagnetization.

 Please turn to the end of the chapter for the answer.

Magnetization Forces
As mentioned previously, not all materials can be magnetized. However,
the ease with which ferromagnetic materials are magnetized varies. Some
are more easily magnetized than others. The term reluctance is used to
describe a material’s level of resistance to being magnetized. Permeability,
the reciprocal of reluctance, refers to the ease or difficulty with which a
material can be magnetized; in other words, how easily or difficult it is to
induce a magnetic field into a material. The higher the permeability, the
more easily a material may be magnetized. Coercive force is defined as the
force necessary or required to remove residual magnetism from a material,
more specifically, the strength of the reverse magnetic field that must be
applied to a magnetized material to make the magnetic flux return to zero.
Retentivity refers to the ability of a material to maintain a remnant or
residual magnetic field in the absence of a coercive force. Ideally, a test
object subjected to MT would be relatively easy to magnetize and
demagnetize.

Question 1.7

Which of the following characteristics of a test object is best suited for MT

inspection?
A. High coercive force.
B. High retentivity.
C. Low coercive force.
D. Low permeability.

 Please turn to the end of the chapter for the answer.

10 Programmed Instruction Series

Chapter 1 Summary

r Magnetic particle testing (MT) began to resemble its current form

during World War I with the experiments of Major William E. Hoke.
r MT is a widely used method of nondestructive testing (NDT) in all
manufacturing segments.
r The goal of MT is to find surface and subsurface discontinuities in
ferromagnetic material before products are put into service.
r Benefits of magnetic particle testing include cost and time
savings, reliability of results, ease of personnel training, and
simplicity of the method.
r Technicians should be aware of the limitations and precautions
associated with MT.
r Paramagnetic materials are slightly attracted to a magnetic force,
whereas diamagnetic materials are slightly repelled.
r With MT, only ferromagnetic materials can be sufficiently
magnetized and tested.
r There are two types of magnets: permanent and temporary
(electromagnetic).
r In a ring magnet, no external poles exist and the magnetic field
within the magnet is considered to be circular.
r Temporary magnets are magnets that lose some or all of their
magnetism once the magnetic force is removed; any magnetism
remaining in the part is referred to as residual magnetism.
r With the exception of a ring magnet, all magnets, permanent and
electromagnetic, have north and south poles.
r Magnetic lines of force are referred to as magnetic flux.
r In MT, ferromagnetic particles accumulate where there is a
disruption in a magnetized material, indicating possible
discontinuities.
r Permeability, the reciprocal of reluctance, refers to the ease or
difficulty with which a material can be magnetized.
r Coercive force is defined as the force necessary or required to
remove residual magnetism from a material.
r Retentivity refers to the ability of a material to maintain a remnant
or residual magnetic field in the absence of a coercive force.

Magnetic Particle Testing 11

Answers to Chapter 1 Questions
Question 1.1

Answer: B – The Greeks were likely the first to discover magnetism in

iron ore.

Question 1.2

Answer: B – Major Hoke was one of the first to understand and describe
magnetic particle testing using permanent magnets and metal shavings.

Question 1.3

Answer: D – In MT, ferromagnetic particles are used to indicate or outline

discontinuities in magnetized ferromagnetic test objects, which facilitates
identification and interpretation of discontinuities.

Question 1.4

Answer: C – MT is simple to perform and is one of the least expensive NDT

test methods.

Question 1.5

Answer: D – Only ring magnets do not have north and south poles.

Question 1.6

Answer: A – Residual magnetism can attract ferromagnetic materials while

in service. These particles could cause the part to fail prematurely.

Question 1.7

Answer: C – Materials best suited for magnetic particle inspection are

easily magnetized and retain little residual magnetism, thus requiring less
coercive force to remove.

Magnetic Particle Testing 13

 Chapter 1 Review
1. Ancient explorers used magnetism for:
A. finding metal objects.
B. hanging maps.
C. navigation.
D. lifting metal objects.

2. Modern magnetic particle inspection techniques began to be

developed:
A. during the late 1700s.
B. after World War II.
C. at the start of the Cold War.
D. during World War I.

3. MT inspection is not typically used to find discontinuities:

A. such as internal voids in castings.
B. after final machining.
C. on weld surfaces.
D. after bending.

4. Diamagnetic materials are:

A. highly magnetic.
B. slightly magnetic.
C. moderately attracted by magnetism.
D. weakly repelled by magnetism.

5. Which type of magnet has alternating positive and negative poles?

A. Ring magnets.
B. Sheet magnets.
C. Horseshoe magnets.
D. Bar magnets.

14 Programmed Instruction Series

6. Opposite magnetic poles:
A. repel each other.
B. are nonmagnetic.
C. attract each other.
D. do not affect each other.

7. Magnetic lines of force:

A. enter the south and exit the north pole.
B. exit and enter a magnet at both poles.
C. enter the north and exit the south pole.
D. do not exit a magnet.

Magnetic Particle Testing 15

 Chapter 1 Review Key
1. C

2. D

3. A

4. D

5. B

6. C

7. A

16 Programmed Instruction Series

 Chapter 2
Electricity and Magnetism

In this chapter:
Characteristics of alternating current versus direct current
Skin effect and depth of penetration
AC equipment: advantages and disadvantages of yokes and prods
Use of rectified alternating current in magnetic particle testing
Half-wave, single-phase full-wave, and three-phase full-wave current

17
 The principles of magnetic particle testing are simple. First, create a
magnet and, second, look for cracks or other kinds of discontinuities in
the magnet. Yes, it really is that simple—sort of. How do you create a
magnet? In this chapter, we will discuss the techniques for creating
magnetic fields using alternating and rectified alternating current.

Alternating Current (AC) Magnetization

Magnetization using alternating current (AC) has very different
characteristics when compared to direct current (DC). One of the more
important features that must be considered is its limited penetrating
ability. Primarily, AC has less penetrating power than DC. In fact, when
NDT supervisors are choosing which type of current to use for an MT
test, this must be taken into consideration. Typically, AC is most useful for
the detection of near-surface and surface-breaking discontinuities not
more than 0.1 in. (2.54 mm) deep. A surface-breaking discontinuity is
shown in Figure 2.1(a).

If subsurface discontinuities more than 0.1 in. (2.54 mm) deep are to be
detected, direct current (or rectified alternating current) is the best choice.
The leakage field from a subsurface discontinuity is shown in Figure 2.1(b).
In MT, rectified alternating current is used to produce a current as close to
direct as possible.

The difference between DC and AC is that AC changes its polarity either

100 or 120 times per second, completing 50 or 60 cycles per second,
depending on the country, with current flow in each direction. One cycle
is called a hertz, abbreviated Hz; thus, 60 cycles per second is 60 Hz. This
is the standard alternation for AC in the U.S., whereas in many other
countries AC alternates at 50 Hz. Direct current, such as that obtained
from an automotive battery, does not change polarity. However, pure DC
is seldom used in MT, except in emergencies when a battery may be used
to power a handheld magnetizing device.

DC does not change polarity. This constant change in polarity prevents

AC from penetrating appreciably below the surface of the material being
tested. Ultimately, the frequency of the current determines how deep it can
penetrate.

18 Programmed Instruction Series

Skin Effect
For alternating or pulsed
currents, there is a Leakage field Test object
tendency for the current to
flow near the surface rather
than penetrating deeply Flux lines
into the test object. As (a)
the frequency of the
magnetization current Particle buildup at leakage field
increases, the depth of
penetration decreases.
This is called the skin effect, Flux lines
whereby circular magnetic
(b)
fields induced by
alternating current
produce very high surface Figure 2.1: Leakage fields in a longitudinally
magnetized test object: a) surface discontinuity
sensitivities. This is (air gap); (b) subsurface discontinuity.
recommended when testing
for service-induced surface
discontinuities. However, the skin effect of AC is less at lower frequencies,
resulting in deeper penetration of the lines of force. At 25 cycles, the
penetration is demonstrably deeper, and at frequencies of 10 cycles per
second or less, the skin effect is almost nonexistent.

As a rule, skin depth (also called standard depth of penetration) for 60 Hz

alternating current fields in steel is typically about 0.04 in. (1 mm),
depending on the permeability and electrical conductivity of the test
object. The field intensity falls to 37 percent at this depth. At two skin
depths, field intensity falls to 13 percent of its surface value.

Due to AC reversing its polarity, a pulsating or vibrating action is created

on the surface of the material being tested. This is advantageous for
magnetic particle testing because the pulsations increase the mobility of
the ferromagnetic particles as they lie on the material, which helps move
them toward any discontinuities on the surface. This surface effect makes
AC the best choice for finding surface discontinuities. Another advantage
of AC is that it is easily demagnetized. Because the current does not
penetrate deeply into the material, very little residual magnetism remains
after the current is removed.

Magnetic Particle Testing 19

 Question 2.1

In the U.S., alternating current reverses its polarity, completing a full cycle:
A. 20 times per second.
B. 30 times per second.
C. 50 times per second.
D. 60 times per second.

 Please turn to the end of the chapter for the answer.

Ë From the Field: In most cases, the MT technique will be

determined by an engineer or the NDT Level III. They will
determine the best technique to use based on the type of
material, its condition, the location of the inspection site, and
the power source available at the inspection site.

Alternating Current Equipment

There are two types of portable equipment most commonly used with
alternating current: yokes and prods. Let’s start with yokes.

Yokes
The yoke, shown in Figure 2.2, is the most common choice of MT
equipment used. Some yokes also include DC, but their penetrating power
is limited.

• Advantage – Readily available power supply.

• Disadvantage – Cannot penetrate below the surface of the material.

Figure 2.2: AC yoke for magnetic particle testing.

20 Programmed Instruction Series

Yokes are portable, handheld testing devices that are most often used in
the field where there are electrical sources available. Yokes require 110 V
AC, which is very common and easy to find. Although we have been
discussing alternating current being induced into a part, a yoke does not
actually induce alternating current into the material being tested. Yokes
use alternating current to create a magnetic field by passing current
through a coil that surrounds the handle of the yoke. The magnetic field
flows perpendicular to the current flow, as shown in Figure 2.3.

AC coils Current

Yoke

(+) (−)

Magnetic field

Figure 2.3: Current flow in a yoke.

Question 2.2

A yoke induces __________ into the test material.

A. a magnetic field
B. ferromagnetic particles
C. alternating current
D. direct current

 Please turn to the end of the chapter for the answer.

Prods
Prods are a second type of portable equipment used with magnetic
particle testing. Essentially, prods are a specialized form of small contact
points, as shown in Figure 2.4. They are often used to test welds. Prods are
firmly pressed against the surface to be magnetized. As current flows

Magnetic Particle Testing 21

 Magnetic field Current Current

Weld Magnetic field Weld

Figure 2.4: Use of prods.

through that surface, a circular magnetic field is set up around the prods.
Often, wet horizontal bench machines are equipped with prods for
irregular-shaped test objects. Small alternating currents of 1000 to 2000 A
in portable magnetic particle machines are the most common type of prod
testing equipment.

• Advantage – A portable type of magnetizing equipment most

effective when spaced 6 in. to 8 in. (150 mm to 200 mm) apart.
• Disadvantage – The use of prods may be restricted for many
applications due to the possibility of burns at the points of contact
with the test object.

Yokes and prods will be discussed in more detail in Chapter 3, along with
other types of magnetic particle testing equipment, including the use of
coils and head shots with bench units.

Rectified Alternating Current Magnetization

As we discussed in Chapter 1, there are two types of magnets:

1. Permanent magnet – made from minerals found in the Earth.

2. Electromagnet – created using electrical current.

For magnetic particle testing, electromagnets are the most common

type used. Permanent magnets can be used for some applications, but they
are usually not strong enough. In fact, many industry specifications limit
the use of permanent magnets, and some actually prohibit their use.
Permanent magnets are typically only used for special applications.

22 Programmed Instruction Series

 As discussed, electromagnets used in magnetic particle inspection are
created by using alternating current (AC), which periodically reverses
direction or changes polarity, going through a complete cycle either 50 or
60 times a second (50 Hz or 60 Hz), depending on the country.
Alternating current can be passed through an electronic circuit called a
rectifier and rectified, or converted, into a current that flows in only one
direction, approximating direct current. There are three types of rectified
alternating current used to perform MT inspections: half-wave, single-
phase full-wave, and three-phase full-wave current.

Question 2.3

Which of the following is not a type of rectified alternating current?

A. Half-wave.
B. Single-phase full-wave.
C. Current produced by a battery.
D. Three-phase full-wave.

 Please turn to the end of the chapter for the answer.

Half-Wave Current
To create half-wave current (HW), alternating current is passed through a
rectifier where it loses the negative part of its wave. It is often called
pulsating direct current. (See Figure 2.5.) This is because it flows in one
direction but is pulsed only half of the time. HW provides the greatest
sensitivity for detecting discontinuities that lie below the surface,
particularly when using dry powder and continuous magnetization. The
waves from HW cause pulsations that vibrate the ferromagnetic particles.
This vibration causes the particles to move across the surface of the part

– Half-wave
current output
Alternating Half-wave
current input rectifier

Figure 2.5: Half-wave current.

Magnetic Particle Testing 23

 being inspected, which increases the chance that the particles are attracted
to and, thus, indicate discontinuities. This particle mobility, which is very
pronounced when dry magnetic powder is used, contrasts with the
reduced mobility of the powder when pure direct current is used.

• Advantage – Deep-penetrating, sensitive magnetic field.

• Disadvantage – Not as effective as other types of current when used
for demagnetization.

There are several models of MT equipment designed to produce HW. A

portable unit, shown in Figure 2.6, is one example.

Figure 2.6: Portable MT unit.

Question 2.4

Half-wave current is effective for detecting __________ discontinuities.

A. surface
B. subsurface
C. visible
D. dry powder

 Please turn to the end of the chapter for the answer.

Single-Phase Full-Wave Current

One way to describe single-phase full-wave current (FW) is to say that the
negative side of the wave has been turned upright or flipped to the positive

24 Programmed Instruction Series

 +

Current

0 Time

Figure 2.7: Single-phase full-wave current.

side, therefore multiplying the positive waves twofold. All of the current
flows in one direction but goes from zero to full current strength, then
back to zero, over and over, with no negative current. (See Figure 2.7.)
Single-phase FW has slightly better penetrating ability compared with
HW but lacks the same pulsating ability, limiting the mobility of the
ferromagnetic particles.

• Advantages – Penetrating ability; less expensive than three-phase

FW machines.
• Disadvantage – The current requirement is almost twice that of
three-phase FW machines.

The bench unit in Figure 2.8 is similar to the MT unit shown in Figure 2.6,
but this unit has the capability of producing both HW and single-phase FW.

Figure 2.8: MT bench unit.

Magnetic Particle Testing 25

 Question 2.5

Single-phase full-wave current has which of the following characteristics?

A. Less penetrating power than half-wave current.
B. More penetrating power than half-wave current.
C. Low current requirements.
D. Greater particle mobility than half-wave current.

 Please turn to the end of the chapter for the answer.

Three-Phase Full-Wave Current

Three-phase FW adds two positive sides of the current in between the single-
phase FW waveform. It adds a third positive side of the current in between
the two positive sides of the single-
phase FW waveform. This is as +
close to actual DC as it is possible Current
to get with MT using AC instead
of a battery. (See Figure 2.9.)
0 Time
Machines equipped with this
current have very high amperage
capabilities and allow the inspector Figure 2.9: Three-phase full-wave current.
to choose either AC or three-phase
FW to perform an inspection. Although desirable for heavy-duty industrial
applications, it is not the most common type of rectified alternating current
used because of the power source required. Machines equipped with three-
phase FW are usually less expensive than single-phase FW machines.

• Advantage – Capable of producing alternating and three-phase

FW currents.
• Disadvantage – Requires 220 V or 440 V power supply.

Three-phase FW units (see Figure 2.10) are large and require special
electrical connections. They are usually enclosed in a darkened room
or tent for fluorescent MT inspections.

26 Programmed Instruction Series

Figure 2.10: Three-phase full-wave current unit.

Question 2.6

Which type of magnetic particle testing unit allows the inspector to switch
from AC to a current that comes close to DC?
A. Three-phase full-wave unit.
B. Half-wave.
C. Yoke.
D. Prod.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 27

Chapter 2 Summary

r Alternating current (AC) alternates at 50 or 60 cycles per second

(50 Hz or 60 Hz), depending on the country.
r Compared with direct current (DC), AC has limited penetrating
power.
r Direct (or rectified alternating) current is better for detecting
subsurface discontinuities.
r AC is preferred for forming indications of surface and near-surface
discontinuities.
r As the frequency of the magnetization current increases, the
depth of penetration decreases.
r The skin effect occurs when circular magnetic fields induced by AC
produce very high surface sensitivities.
r Because AC reverses its polarity, it creates a pulsating or vibrating
action that increases the mobility of ferromagnetic particles.
r An advantage of AC is that it is easily demagnetized because the
current does not penetrate into the material.
r There are two types of portable equipment most commonly used
with alternating current: yokes and prods.
r Yokes are portable, handheld devices that use AC to create a
magnetic field that flows perpendicular to the current flow.
r Prods are a specialized form of small contact plates often used to
test welds using a circular magnetic field.
r Alternating current can be rectified or converted into a current
that flows in only one direction, approximating direct current.
r There are three types of rectified alternating current used to
perform MT inspections: half-wave (HW), single-phase full-wave
(FW), and three-phase FW current.
r HW provides the greatest sensitivity for detecting discontinuities
that lie below the surface, particularly when using dry powder and
continuous magnetization.
r Single-phase FW has slightly better penetrating ability compared
with HW but lacks the same pulsating ability, limiting the mobility
of the ferromagnetic particles.
r Three-phase FW is as close to actual DC as it is possible to get
with MT.
r High amperage capabilities make three-phase FW desirable for
heavy-duty industrial applications.

Magnetic Particle Testing 29

 Answers to Chapter 2 Questions
Question 2.1
Answer: D – In the U.S., AC completes 60 full cycles of polarity reversal per
second (60 Hz). In many other countries, the alternations occur at 50 Hz.

Question 2.2

Answer: A – Alternating current is used to generate a magnetic field in the

coil surrounding the handle of this type of test equipment. Technically, it is
a magnetic field that is induced into the test object—not alternating
current.

Question 2.3

Answer: C – The current produced by a battery is pure direct current.

Rectified alternating current uses available power supplies to simulate
direct current in magnetic particle testing applications.

Question 2.4

Answer: B – Half-wave current is very effective at finding discontinuities

below the surface of a part.

Question 2.5

Answer: B – Single-phase full-wave current is equivalent to doubling the

positive waves of half-wave current. Although single-phase FW has slightly
better penetrating ability than HW, particle mobility is more limited when
it comes to producing indications of subsurface discontinuities.

Question 2.6

Answer: A – Three-phase full-wave comes closest to DC. In addition, a

three-phase FW unit can provide the inspector with AC to perform a
magnetic particle inspection.

30 Programmed Instruction Series

Chapter 2 Review
1. Which type of magnet is most commonly used in magnetic
particle testing?
A. Bar magnet.
B. Horseshoe magnet.
C. Electromagnet.
D. Permanent magnet.

2. Permanent magnets are used for MT inspection:

A. in most situations.
B. never.
C. for special applications.
D. always.

3. Which of the following is a disadvantage of half-wave current?

A. It is not effective for demagnetization.
B. It requires 440 volts.
C. It is used to locate surface discontinuities only.
D. It has no disadvantages.

4. Which of the following is not a characteristic of AC?

A. It’s effective for detecting surface discontinuities.
B. It constantly changes polarity.
C. A power source is readily available.
D. It leaves a high residual magnetic field.

5. Which current is most similar to DC?

A. HW.
B. FW.
C. Three-phase FW.
D. Nonrectified HW.

6. The most commonly used MT equipment is a:

A. coil.
B. yoke.
C. bench unit.
D. permanent magnet.

Magnetic Particle Testing 31

 Chapter 2 Review Key
1. C

2. C

3. A

4. D

5. C

6. B

32 Programmed Instruction Series

 Chapter 3
Performing Magnetic Particle
Testing Inspections

In this chapter:
Difference between an NDT method and technique
Circular versus longitudinal magnetization
Right-hand rule for determining the direction of a magnetic field
Yoke and coil techniques to induce a longitudinal magnetic field
Prod technique to induce a circular magnetic field
Use of head shots and central conductors in magnetic particle testing

33
 Defining Techniques and Field Direction
When talking about magnetic particle testing, two terms are often used
that all inspectors must know and understand the meanings of: technique
and field direction.

Technique
The term technique is often mistakenly used to describe an NDT method
and vice versa. In NDT, technique refers to a specific type of test within
one of the NDT methods. In some cases, the actual equipment used is the
technique. For example, a yoke is the name of a type of machine used to
perform MT. Yoke is also considered an MT technique. Other techniques
used in MT testing are coil, prods, and central conductor. The word
method should always be used to refer to an inspection discipline—for
example, radiographic, ultrasonic, and liquid penetrant, as well as
magnetic particle testing.

Direction
A magnetic field travels in one of two directions: circular or longitudinal.
Each MT technique creates a magnetic field with a known direction. Field
direction, as it relates to MT, is the direction that the magnetic lines of
force travel when the equipment is energized. For instance, a yoke creates
a longitudinal magnetic field, which means that the magnetic lines of force
travel longitudinally from the north to the south poles of the yoke, as in
Figure 3.1. Conversely, circular magnetic fields travel perpendicular to the
direction of current flow. This concept is often referred to as the right-
hand rule.

Current-carrying wire

Magnetic flux
in legs of yoke

S
N

Crack Magnetic flux in part

indication

Figure 3.1: Magnetic lines of force in a yoke.

34 Programmed Instruction Series

Question 3.1

Which of the following is an example of an NDT technique?

A. Radiographic testing.
B. Magnetic particle yoke testing.
C. Magnetic particle testing.
D. Liquid penetrant testing.

 Please turn to the end of the chapter for the answer.

Right-Hand Rule
The right-hand rule assists inspectors with remembering the direction
of a magnetic field. This rule applies to magnetization techniques where
rectified alternating current is used to induce a magnetic field into the
part. Here’s how it works. When current passes through a conductor,
such as a copper wire or bar, a magnetic field is created that flows
perpendicular to the direction of the current. As shown in Figure 3.2, the
current flows through the bar and travels in the direction of the thumb.
The magnetic field travels perpendicular to the bar, in the same direction
as the fingers holding the bar.

Figure 3.2: Right-hand rule.

Magnetic Particle Testing 35

 Question 3.2

Magnetic fields are either circular or:

A. intermittent.
B. constant.
C. longitudinal.
D. paramagnetic.

 Please turn to the end of the chapter for the answer.

MT Techniques
Several techniques are used to perform MT. The names of these
techniques are closely related to the equipment or components used to
create and apply the magnetic field:
• Yoke.
• Coil.
• Prods.
• Head shot and central conductor.

Yoke (Longitudinal)
A yoke is an extremely portable and easy-to-use piece of MT equipment.
It is an excellent choice for inspecting small castings, welds, steel plate, and
machined parts. Yokes induce a strong magnetic field in a concentrated
area between the legs of the yoke. Unlike direct-current techniques, yokes
do not induce electric current into the material; instead, they create a
magnetic field that travels longitudinally from one leg of the yoke to the
other. The magnetic field is created by passing current through a coil in
the handle. (Refer back to Figure 3.1.) Yokes can be purchased with
articulated or fixed legs. Yokes with articulated legs are more versatile,
allowing the operator to manipulate the legs in order to inspect parts with
complex geometries. Some common characteristics of a yoke include:

• Powered using 115 V or a 12 V battery.

• Can be used with dry powder or a wet bath.
• Used for surface indications only.
• Creates excellent particle motion.
• Establishes a longitudinal magnetic field.
• Extremely portable.

36 Programmed Instruction Series

Question 3.3

What type of magnetic current does a yoke induce in a test part?

A. Longitudinal magnetic field between the legs of the yoke.
B. Circular magnetic field between the legs of the yoke.
C. Circular magnetic field around the legs of the yoke.
D. Longitudinal magnetic field moving away from the legs of the
yoke in opposite directions.

 Please turn to the end of the chapter for the answer.

Coil (Longitudinal)
Just like a yoke, coils produce a longitudinal magnetic field and are
portable; however, unlike a yoke, the size of the part that can be inspected
is limited by the size of the coil. A coil is a round, donut-shaped
electromagnet formed by wrapping cables, usually five or six times. These
cables are hidden inside a hard, durable shell. (See Figure 3.3.) A typical
coil has a 12 in. (305 mm) inside diameter (ID), but larger coils, some big
enough to drive a car through, have been used.

Figure 3.3: Examples of coils.

Magnetic Particle Testing 37

 Coils are very good for demagnetizing parts. Demagnetization is
accomplished by passing the part through the inside of the coil while it is
energized. Characteristics of a coil are similar to those of a yoke and
include:

• Powered using 115 V.

• Can be used with dry powder or a wet bath.
• Used for surface indications only.
• Creates excellent particle motion.
• Establishes a longitudinal magnetic field.
• Uses AC, HW, FW, or DC.

Question 3.4

A coil usually consists of how many cable wraps?

A. 1.
B. 3 or 4.
C. 5 or 6.
D. At least 10.

 Please turn to the end of the chapter for the answer.

Prods (Circular)
The prod technique is very
different from those we have
discussed so far. This technique
induces rectified alternating
electrical current, usually HW (as
discussed in Chapter 2), into the
material instead of a magnetic
field. The electric current passing
through the material creates a
magnetic field that travels
circularly around the prods.
(See Figure 3.4.)

The equipment setup consists

of a power transformer (see Figure 3.4: Prod technique.
Figure 3.5), electrical cables, and

38 Programmed Instruction Series

Figure 3.5: Power transformer for prod Figure 3.6: Demonstration using prods.
technique.

two prods with insulated handles, one with a power switch, as shown in
Figure 3.6. Because the prod technique uses HW, it’s an excellent choice for
detecting discontinuities at the surface and below the surface of the part.
Despite good particle mobility with dry powder, there is some skin effect
when HW is used, caused by the pulsating magnetic fields produced by HW
current. However, the effect on field penetration is small at the usual power
frequency of 50 Hz or 60 Hz. Nevertheless, the inspector must be skilled
with applying ferromagnetic particles and interpreting subsurface
indications that may form.

Question 3.5

The most common type of electrical current used with the prod technique
is:
A. 115 V.
B. alternating current.
C. three-phase direct current.
D. half-wave (HW) current.

 Please turn to the end of the chapter for the answer.

The prod technique does have some drawbacks. First, it requires two
people to operate. Second, it can damage the surface of the material being
inspected because it can cause arc strikes. An arc strike is localized heat
damage to a test object caused by poor coupling between the test object
and the contact prods.

Magnetic Particle Testing 39

 Ë From the Field: No matter how careful the operator is, arc
strikes are difficult to prevent. Care must be taken not to turn
the amperage up too high, and the prod ends must be held
tightly against the surface being inspected in order to minimize
the possibility of creating arc strikes. This technique should only
be used when arc strikes would not permanently damage the
part should they occur.

Characteristics of the prod technique include:

• Uses half-wave current (HW).

• Portable but requires two technicians to operate.
• Good for finding surface and subsurface discontinuities.
• Induces current into the material.
• Produces a circular magnetic field.

Question 3.6

What type of discontinuity is the prod technique best suited for?

A. Surface discontinuities only.
B. Subsurface discontinuities only.
C. Both surface and subsurface discontinuities.
D. Detecting arc strikes in the test material.

 Please turn to the end of the chapter for the answer.

Head Shot and Central Conductor

For inspecting long parts or a large quantity of parts, a horizontal bench
unit is often used to produce a head shot. A typical bench is capable of
generating both longitudinal and circular magnetic fields. For circular
fields, there are two heads, one that is fixed and another that can be
adjusted to the length of the part, as shown in Figure 3.7(a). Some units
have a pneumatic system installed that clamps the heads together for a
stronger hold on the part. This is especially useful for preventing arc
strikes.

As mentioned, an arc strike is a mark left on the surface of a part or

material. It is caused by a short circuit at the point where the part and
head make contact. Each of the heads has a copper pad attached to it.

40 Programmed Instruction Series

The copper pad helps to create good contact between the head and the
part, lessening the chance for arc strikes.

Another technique for magnetizing hollow parts and preventing arc

strikes is to use a copper central conductor, as shown in Figure 3.7(b). One
type of central conductor is a copper rod that is passed through the part
and then clamped between the heads. The current passing through the
central conductor creates the circular magnetic field.

Field
Field
Head

Current Current
(a) (b)

Figure 3.7: Circular magnetization by direct and indirect current induction: (a) head shot
(direct); (b) central conductor (indirect).

Question 3.7

Arc strikes are caused by which of the following techniques?

A. Yoke.
B. Permanent magnet.
C. Coil.
D. Prods and bench unit.

 Please turn to the end of the chapter for the answer.

Wet Bench Units

Standard wet bench units use a fluorescent suspension instead of dry
particles. The ferromagnetic particles are mixed with oil or water and are
constantly circulated through the system by a pump. This keeps the
particles mixed and ensures that the concentration of particles in the
suspension is maintained. To perform an inspection, the part is clamped
between the heads. A suspension is flowed or sprayed over the part and
then the machine is energized. The ferromagnetic particles suspended in
the bath are attracted to any discontinuities. A multidirectional bench unit

Magnetic Particle Testing 41

 Figure 3.8: Small parts application of suspension from the overhead nozzle is timed by
the machine.

capable of both longitudinal and circular magnetization is shown in

Figure 3.8.

Characteristics of a bench unit include:

• Powered using three-phase FW (some units have AC as well).

• Uses wet particle bath.
• Used for surface and subsurface indications.
• Has good penetrating power.
• Establishes both longitudinal and circular fields.
• Not portable.

All of the equipment and techniques described in this chapter can also be
used to demagnetize material having residual magnetization remaining
after the inspection.

42 Programmed Instruction Series

Chapter 3 Summary

r In NDT, a technique is a specific type of test within one of the NDT

methods.
r In MT, the actual equipment used is the technique; for example,
prod or yoke technique.
r A magnetic field travels in one of two directions: circular or
longitudinal.
r The right-hand rule assists inspectors with remembering the
direction of a magnetic field.
r Several techniques are used to perform MT: yoke, coil, prod, head
shot, and central conductor.
r Yokes induce a strong magnetic field that travels longitudinally
from one leg of the yoke to the other.
r A coil is a round, donut-shaped electromagnet formed by
wrapping cables, usually five or six times, inside a hard, durable
shell.
r Characteristics of yokes and coils include: powered using 115 V;
can be used with dry powder or a wet bath; used for surface
indications only; create excellent particle motion; extremely
portable.
r The prod technique induces rectified alternating electrical current,
usually HW, into the material to create a circular magnetic field.
r Prods are an excellent choice for detecting both surface and
subsurface discontinuities.
r One drawback of the prod technique is that it can cause arc
strikes.
r A horizontal bench unit is capable of generating both longitudinal
and circular magnetic fields to inspect long parts using a head
shot.
r A technique for preventing arc strikes is to use a central
conductor, in which a copper rod is passed through the part and
then clamped between the heads to create a circular magnetic
field.
r Standard wet bench units use a fluorescent suspension that is
flowed or sprayed over the part.

Magnetic Particle Testing 43

 Answers to Chapter 3 Questions

Question 3.1

Answer: B – Techniques are specific tests within the method. Yokes, prods,
and coils are techniques used in the magnetic particle testing method.

Question 3.2
Answer: C – A longitudinal field is created by the yoke and coil techniques.

Question 3.3

Answer: A – Electrical cables are wrapped around the handle of a yoke.

Alternating current passing through the cables creates a longitudinal
magnetic field between the legs of the yoke.

Question 3.4

Answer: C – There are usually five or six cable wraps inside of a coil. There
must be enough current to create adequate field strength.

Question 3.5

Answer: D – HW sources are easy to find. HW does have some penetrating

and good surface agitation characteristics, both of which are
advantageous for MT inspection.

44 Programmed Instruction Series

Question 3.6

Answer: C – Because prods use HW, which does agitate the surface of the
test part, prods are an excellent choice for locating surface and subsurface
discontinuities.

Question 3.7

Answer: D – Both prods and horizontal benches can cause arc strikes. Any
time current is directly applied to a part, there is a risk of arc strikes.

Magnetic Particle Testing 45

 Chapter 3 Review
1. Liquid penetrant, magnetic particle, radiographic, and ultrasonic
testing are all examples of NDT:
A. techniques.
B. certification levels.
C. methods.
D. equipment.

2. Circular magnetic fields are produced by which of the following

equipment?
A. Yoke.
B. Permanent magnet.
C. Coil.
D. Head shot.

3. The best central conductors are made from:

A. copper.
B. steel.
C. aluminum.
D. lead.

4. Which type of unit is capable of producing both circular and

horizontal longitudinal magnetic fields?
A. Horizontal bench unit.
B. Coil.
C. Yoke.
D. Prod.

5. The MT technique that is not considered portable is the use of:

A. prods.
B. yoke.
C. coil.
D. head shot.

6. Which technique creates a circular magnetic field?

A. Yoke.
B. Prod.
C. Coil.
D. Cable wraps.

46 Programmed Instruction Series

7. Which of the following is a characteristic of a yoke?
A. Establishes a circular magnetic field.
B. Establishes a longitudinal magnetic field.
C. Induces limited particle motion.
D. Used mainly for detecting subsurface discontinuities.

8. Current passing through cables that are wrapped inside a round,

hard shell describes which MT technique?
A. Head Shot.
B. Prods.
C. Coil.
D. Yoke.

Magnetic Particle Testing 47

 Chapter 3 Review Key
1. C

2. D

3. A

4. A

5. D

6. B

7. B

8. C

48 Programmed Instruction Series

 Chapter 4
Detecting Discontinuities with MT

In this chapter:
Causes and examples of inherent, processing, and
inservice discontinuities
Effect of stress on discontinuities in service
Locating and mapping discontinuities using magnetic particle testing
Measuring magnetic field strength and determining field direction
Relevant and nonrelevant discontinuities including magnetic writing
Interpreting and evaluating discontinuities, such as rounded
or linear, for acceptability

49
 Defining Discontinuities
MT inspection is used to detect discontinuities—referred to as defects if
they cause a part or object to be considered rejectable—in a wide variety
of ferromagnetic materials. Discontinuities come in a wide array of shapes
and sizes, and their cause(s) can vary. There are three main categories of
discontinuities:

• Inherent – caused during initial manufacturing processes such as

metal forming and casting.
• Processing – caused by processes such as forming, bending, rolling,
extruding, forging, finishing, heat-treating, machining, and welding.
• Inservice – occurring during the use of a product or object and may
be caused by overuse or fatigue, parts being used for jobs for which
they are not designed, overloading, erosion, and corrosion.

Discontinuities are defined as an interruption in the physical structure

or configuration of a test object. Some discontinuities are cosmetic and
would have no structural impact on the material, but some almost
certainly would. When design engineers choose a material for fabricating
a part, they must consider all of the possible discontinuities that might be
present in that material. Subsequently, the engineer must determine to
what extent those discontinuities will be permitted, should they be found.

Many factors determine what is acceptable and what is not. How the
material is used in service and the potential impact should it fail are just two
factors that are considered when determining which discontinuities are
unacceptable. A discontinuity found in one part might be unacceptable, but
the same discontinuity found in another part might be acceptable. Why?
While in service, the stresses placed on the material can vary.

Question 4.1

A discontinuity is:
A. always unacceptable.
B. always perpendicular with the rolling direction.
C. a disruption or imperfection in the material.
D. unlikely to be found with MT.

 Please turn to the end of the chapter for the answer.

50 Programmed Instruction Series

Take a look at Figure 4.1. Here, two similar parts are represented: part A
and part B. Each part has a linear discontinuity running parallel to its
length, but one of these discontinuities would have a much greater
potential for causing the material to fail. The direction of the arrows
indicates the direction of the inservice stresses placed upon each part.
Can you tell which of these parts would be more likely to fail?

Part A Part B

Figure 4.1: Parts with linear discontinuities subjected to different directions of stress.

Consider the following: The discontinuity in part A runs parallel with the
direction of the inservice stresses and a smaller portion of its cross section
is affected, which has less of an impact on the overall strength of the part.
However, the opposite is true for part B. In this part, the discontinuity is
oriented transversely to the direction of the inservice stresses, which
means that a larger cross section of the part is affected. As stresses are
applied to the part, the discontinuity weakens the material and eventually
causes it to fail.

In either case, this type of discontinuity could ultimately lead to the part
failing, but a part with stresses oriented transversely to the discontinuity
(crack) is likely to fail sooner due to the location and orientation of the
discontinuity.

Figure 4.2 shows how the material in part B might react as transversely
oriented inservice stresses are placed on the material over a period of time.

Magnetic Particle Testing 51

 Time

Figure 4.2: The effect of transverse stresses on a discontinuity over time .

Question 4.2

Which of the following discontinuity characteristics has the greatest

impact contributing to failure of a test object in service?
A. Variations in stress.
B. Type of discontinuity.
C. End use of the part.
D. Type of material.

 Please turn to the end of the chapter for the answer.

Types of Discontinuities
There are many different types of discontinuities that MT inspection is
used to detect. What causes a discontinuity to occur varies as well, but to
make it easier to understand, discontinuities are divided into three basic
categories spanning the origin and use of the part, as presented earlier:

1. Inherent.
2. Processing.
3. Inservice.

Let’s discuss each of these in more detail.

52 Programmed Instruction Series

Inherent
This type of discontinuity is usually the first to appear in the material.
Inherent discontinuities are created during the initial manufacturing
process, for example, in the pouring of ingots and other castings. As the
molten steel in ingots and castings begins to cool down and the material
solidifies, discontinuities can form. Their causes vary. Examples are
nonmetallic inclusions, solidification cracks, and shrinkage cracks.
Inherent discontinuities can be found above and below the surface of the
material. Typical inherent discontinuities are listed in Table 4.1.

Table 4.1: Inherent discontinuities.

Discontinuity Cause

Two streams of molten metal that do not fuse

Cold shut
together

Entrapped gases in the molten steel as it begins to

Porosity
solidify

Inclusions Contaminants in the molten steel

Question 4.3

Which is not one of the categories of discontinuities?

A. Inservice.
B. Inherent.
C. Processing.
D. Manufacturing.

 Please turn to the end of the chapter for the answer.

Processing
Nearly all raw materials are processed in some way in order to achieve
their final shape, size, and geometry. Each finishing process can produce
its own unique type(s) of discontinuity. For instance, the forging process
can cause laps, bursts, and flakes. Additional examples of processing
discontinuities are listed in Table 4.2.

Magnetic Particle Testing 53

 Table 4.2: Processing discontinuities.

Process Discontinuity

Rolling Stringers

Cold-working Cupping

Welding Lack of fusion

Heat-treating Quench cracks

Machining Tears

Question 4.4

Stringers may occur during which of the following processes?

A. Welding.
B. Rolling.
C. Casting.
D. Forging.

 Please turn to the end of the chapter for the answer.

Inservice
Most materials endure some type of inservice stress once they are installed
and in use. Stresses from tension, compression, and cylindrical loads can
cause discontinuities to occur. In some cases, a combination of both stress
and environmental conditions, for example, cold or elevated temperatures

Table 4.3: Inservice discontinuities.

Discontinuity Cause

Stress crack Tensile stress and corrosion

Fatigue crack Twisting or circular stress

Creep High temperature and stress

54 Programmed Instruction Series

within high-moisture environments,
combine to cause discontinuities, such as
intergranular corrosion cracking.
Although inservice discontinuities
sometimes are caused solely by stresses or
the environment, often they are inherent
or processing discontinuities that have
been subjected to inservice conditions.
See Table 4.3 for examples of inservice
discontinuities. A stress crack is shown in
Figure 4.3. Figure 4.3: Fluorescent magnetic
particle indication of stress crack in
mounting bracket at machined
interface.

Question 4.5

Which of the following contributes to inservice discontinuities?

A. Welding amperage.
B. Heat-treat temperature.
C. Atmosphere or working temperature.
D. Rolling direction.

 Please turn to the end of the chapter for the answer.

Locating Discontinuities
Now that you have an understanding of the types of discontinuities MT
inspection is used to find, it is necessary to know how magnetic fields are
used to find them.

Why do discontinuities appear as indications when performing MT

inspections? Magnetic fields, just like electrical currents, tend to travel
along the path of least resistance. As long as nothing gets in their way, all
is well. However, if there is a discontinuity in the material and it is located
along the path of the magnetic field, the lines of force encountering the
discontinuity will reflow, as shown in Figure 4.4. When this occurs,
magnetic poles are created and a leakage field is formed around the edges
of the discontinuity.

Magnetic Particle Testing 55

 As we learned in previous chapters, leakage fields attract ferromagnetic
materials. Therefore, when ferromagnetic particles are spread over the
surface where the discontinuity is located, as shown in Figure 4.5, the
particles are attracted to the leakage field surrounding the discontinuity
and outline its path from one pole to another as formed by the geometry
of the discontinuity. The magnetic field in this case behaves similarly to
the lines of force established in a horseshoe magnet. Ferromagnetic
particles are attracted to both poles and form an indication.

Figure 4.4: Flux backage field from a discontinuity perpendicular to the magnetic flux.

Magnetic field lines Magnetic particles

Crack

Figure 4.5: Magnetic lines of force attracting ferromagnetic particles.

56 Programmed Instruction Series

Question 4.6

What occurs around a discontinuity when ferromagnetic material is

magnetized?
A. Corrosion.
B. Reduction of field strength.
C. Repulsion of ferromagnetic particles.
D. Attraction of ferromagnetic particles.

 Please turn to the end of the chapter for the answer.

Magnetic Direction and Strength

The strength of the leakage field for each discontinuity is different. There
are two main factors that affect strength of the leakage field:

1. The strength of the magnetic field.

2. The size and shape of the discontinuity.

It’s important that the strength of the magnetic field be adequate so

that a strong leakage field is created if a discontinuity is in the material.
Adequate current must be induced into the part to ensure a strong
leakage field.

Quantitative Quality Indicator

One method for determining the adequacy of field strength is the use of a
small, metal indicator referred to as a quantitative quality indicator (QQI).
(Alternatively, they are referred to as AS5371 shims). The indicator is made
of a silicon iron material and measures about 0.5 in. (13 mm) long by
0.25 in. (6 mm) wide by 0.01 in. (3 mm) thick. A 0.003 in. (0.08 mm)
square slot is cut across the 0.25 in. (6 mm) dimension.

In practice, the indicator is placed in a critical area on the test object with
the slotted side firmly against the test object surface to ensure intimate
contact. (QQIs may be purchased with adhesive backings for this purpose
or superglue may be used.) The testing process is carried out, and if an
indication forms at the slot, it is assumed that there is adequate field
strength to reveal actual discontinuities in the test object.

Magnetic Particle Testing 57

 If the indicator has a length-to-diameter ratio that is different from the
test object, there may be a considerable difference between the
permeability of the indicator and test object. Consequently, the indicator
could develop a greater field strength than the test object with the slot
readily indicated but with insufficient field strength in the test object.

Pie Gage
The most common tool used to measure field direction is called a pie gage.
(See Figure 4.6.) A pie gage has known discontinuities built into it that
allow leakage fields to form when adequate field strength is achieved.

Figure 4.6: Pie gage.

The term pie gage is derived from the division of the indicator plate into
eight sections, just as an apple pie would be sectioned. The area between
each of the slices is nonferromagnetic. Each space causes a leakage field in
this area when an adequate magnetic field is introduced. Indications begin
to form on a pie gage at magnetic field levels as low as 5 G (0.0005 T).

Question 4.7

What is a pie gage equipped to do?

A. Determine the size of a discontinuity.
B. Identify the type of discontinuity.
C. Indicate the direction of a magnetizing force.
D. Determine adequate field strength.

 Please turn to the end of the chapter for the answer.

58 Programmed Instruction Series

Interpreting Indications
During the inspection process, the inspector evaluates any indications that are
formed. Indications form as ferromagnetic particles gather at flux leakage
areas, which are normally relevant discontinuities in the material. However,
indications can also form when there are no relevant discontinuities. This type
of indication is either a nonrelevant indication or a false indication. Nonrelevant
indications may be caused by magnetic writing or part geometry. False
indications can be caused by an improperly followed procedure, mishandling
marks, and other factors not related to actual discontinuities.

Relevant versus Nonrelevant Discontinuities

An indication caused by a true discontinuity is considered to be either
relevant or nonrelevant. To be considered relevant, an indication normally
has to be a certain length. For instance, some specifications require that an
indication be longer than 0.25 in. (6.35 mm) to be considered relevant. The
minimum size may be smaller or larger, depending on the specification. In
most cases, only relevant indications need to be evaluated. A relevant
indication using MT is shown in Figure 4.7.

(a) (b)

Figure 4.7: Relevant magnetic particle indication; (a) visual appearance of part;
(b) magnetic particle indication.

Question 4.8

A relevant indication is caused by:

A. part geometry.
B. too strong of a magnetic field.
C. mishandling marks.
D. a discontinuity in the material being inspected.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 59

 Magnetic Writing
Magnetic writing occurs when magnetized test objects are rubbed together
or against other ferromagnetic test objects, producing local magnetized areas
on surfaces that attract and hold ferromagnetic particles. Magnetic writing is
considered a nonrelevant indication since it is not caused by a processing
error. Rather, at the point of contact between the two parts or components,
the magnetic field is distorted, causing magnetic poles (local polarity) to
form. Whether an indication is caused by magnetic writing or
by a subsurface discontinuity can be determined by demagnetizing and
reprocessing the test object to see if the indication recurs.

Mapping and Measuring Indications

Once all of the relevant indications have been identified, the inspector will
record each one in a report. In some cases, each relevant indication will be
mapped out. When done accurately, mapping out of the indications allows
repair personnel to easily find the indications without the need for the
inspector to be present and provides a permanent record of the
indications.

In Figure 4.7, the diagram to the right of the pipe represents how the
parameters of the weld and indications identified during inspection might
be recorded or mapped following an MT inspection. The area marked
“Weld” represents the actual length of the weld around the pipe.

To calculate the length of the weld in a pipe, multiply the outside

diameter (OD) of the pipe by pi (π) (3.14159). If the pipe in Figure 4.8
were 3 in. (76.2 mm) in diameter, the length of the weld would be
9.425 in. (239.4 mm). If this were given to repair personnel, they could
measure from the reference line (usually marked with a paint marker by
the inspector) to the start of each indication and begin the repairs. Each
inspection company records its inspection results differently, but this is a
typical method used in many industries.

60 Programmed Instruction Series

 REF

0.5 in. (12.7 mm)

1.5 in.
REF
(38 mm) Weld

Weld
5.75 in. (146 mm)

9.425 in. (239 mm)

Figure 4.8: Recording of magnetic particle indications in a pipe weld.

Question 4.9

The circumference of a pipe is calculated using which of the following

formulas?
A. Thickness × diameter × pi (π).
B. Outside diameter (OD) × pi (π).
C. Inside diameter (ID) × pi (π).
D. ID ÷ pi (π).

 Please turn to the end of the chapter for the answer.

Linear versus Rounded Indications

Linear indications are typically caused by cracks, seams, cold shuts,
forging laps, scratches, or die marks. A linear indication, as shown in
Figure 4.9(a), is defined as one that is at least 3× longer than its width.
Machining operations may close or remove portions of surface
discontinuities or, conversely, expose portions of continuous subsurface
discontinuities. This type of linear discontinuity may appear as
intermittent lines, as illustrated in Figure 4.9(b), depending on the
MT technique used.

Magnetic Particle Testing 61

 (a) (b)

(c) (d)

Figure 4.9: Typical relevant indications: (a) linear; (b) intermittent linear; (c) rounded;
(d) weak or diffused.

Rounded indications are usually caused by porosity. By definition, a

rounded indication has a length that is equal to or less than 3× its width,
as shown in Figure 4.9(c). Deep cracks may also appear rounded due to
excessive buildup of ferromagnetic particles.

Weak indications, neither linear nor rounded, are difficult to interpret. If

they cover a large area, as in Figure 4.9(d), they are always suspect. When
they appear, the test object should be retested.

Subsurface Discontinuities
The size, orientation, and composition of subsurface discontinuities
affect evaluation capabilities other than locating the discontinuity. In
general, the magnetic image is very broad and fuzzy. Additionally, changes
in test object thickness and holes from the opposite side may produce
misleading indications. The use of alternative testing technologies, such as
radiography, after demagnetization and retesting may be required to
confirm the magnetic particle indication.

62 Programmed Instruction Series

Evaluating Discontinuities
Once all of the relevant indications have been recorded, the inspector
must determine if they are acceptable or unacceptable. Acceptance criteria
for the material being inspected are usually developed by the design
engineers. The acceptance criteria are used to filter out any discontinuities
that could cause the material to fail during its lifetime.

Question 4.10
Which type of indication is at least 3× longer than it is wide?
A. Nonrelevant indication.
B. Magnetic writing.
C. Linear indication.
D. Rejectable indication.

 Please turn to the end of the chapter for the answer.

Ë From the Field: Not only is it very important to find

discontinuities, it is equally important to disposition them
correctly. Inspectors must be careful not to reject a part just
because a relevant indication is present. Some indications may
be tolerated by the acceptance criteria. Relevant but acceptable
indications are often allowed and need not be removed or
cause the part to be rejected. It’s the inspector’s job to record
and determine which, if any, indications are unacceptable and
report them appropriately.

Magnetic Particle Testing 63

Chapter 4 Summary

r Discontinuities are defined as an interruption in the physical

structure or configuration of a test object.
r There are three main categories of discontinuities: inherent,
processing, and inservice.
r A discontinuity oriented transversely to the direction of inservice
stress may weaken and fail earlier than a discontinuity oriented
parallel to the stress.
r Inherent discontinuities are created during the initial
manufacturing process and include nonmetallic inclusions,
solidification cracks, and shrinkage cracks.
r Each finishing process, such as forging, extruding, or rolling, can
produce its own unique type(s) of discontinuity.
r Inservice stresses and environmental conditions can cause
discontinuities to occur.
r Two main factors affect the strength of a magnetic leakage field:
(1) the strength of the magnetic field and (2) the size and shape of
the discontinuity.
r One method for determining the adequacy of field strength is the
use of a small, metal, adhesive indicator made of silicon iron.
r A pie gage is used to accurately indicate the direction of the
magnetizing force.
r Indications may be categorized as relevant, nonrelevant, and false.
r Magnetic writing occurs when magnetized test objects are rubbed
together or against other ferromagnetic test objects, producing
localized nonrelevant indications.
r Mapping allows repair personnel to easily find the indications and
provides a permanent record.
r A linear indication is defined as one that is at least 3× greater in
length than width; a rounded indication has a length that is equal
to or less than 3× its width.
r Not only is it very important to find discontinuities, it is equally
important to disposition them correctly as acceptable or
unacceptable.

Magnetic Particle Testing 65

 Answers to Chapter 4 Questions
Question 4.1
Answer: C – Discontinuities are defined as disruptions or imperfections
that may be considered undesirable after nondestructive testing.

Question 4.2

Answer: A – The type and size of a discontinuity for a given material play a
role; however, the degree and orientation of stress that the discontinuity is
subjected to have the greater impact.

Question 4.3

Answer: D – The three categories are inherent, processing, and inservice.

The term manufacturing is not technically used to describe a category of
discontinuities.

Question 4.4

Answer: B – When bar stock is rolled, nonmetallic inclusions become

elongated and form stringers.

Question 4.5

Answer: C – Cold or elevated temperatures and high-moisture

environments contribute to the formation of inservice discontinuities.

Question 4.6

Answer: D – A leakage field is created that attracts ferromagnetic particles.

Question 4.7

Answer: C – A pie gage is used to indicate the direction of the

magnetizing force through the indicator.

66 Programmed Instruction Series

Question 4.8

Answer: D – A relevant indication is caused by a discontinuity in the

material and must be evaluated.

Question 4.9

Answer: B – The formula is the outside diameter multiplied by pi (π).

Question 4.10

Answer: C – A linear indication would need to be evaluated to determine if

it is acceptable or rejectable.

Magnetic Particle Testing 67

 Chapter 4 Review
1. Discontinuities are caused by any of the following except:
A. manufacturing processes.
B. inservice overloading.
C. forming processes.
D. magnetic testing procedure using a yoke technique.

2. A cold shut is what type of discontinuity?

A. Inherent.
B. Manufacturing.
C. Processing.
D. Inservice.

3. Magnetic leakage fields are created when:

A. ferromagnetic particles are applied on a part.
B. electricity is induced into a part.
C. a magnetic field encounters a discontinuity.
D. a yoke is energized.

4. Stresses from tension, compression, and cylindrical loads may cause:

A. inherent discontinuities.
B. welding discontinuities.
C. inservice discontinuities.
D. processing discontinuities.

5. The strength of a leakage field is affected by all of the following

except:
A. the size of a discontinuity.
B. magnetic field strength.
C. location of a discontinuity.
D. the type of field strength indicator used.

6. Discontinuities are defined as:

A. any indication that is rejectable.
B. imperfections or abnormalities.
C. manufacturing process marks.
D. arc strikes left on the material’s surface.

68 Programmed Instruction Series

7. Magnetic field strength may be measured with a:
A. quantitative quality indicator.
B. pie gage.
C. steel rule.
D. hydrometer.

8. What is the circumferential length of a weld in a pipe with an outside

diameter of 4.25 in. (108 mm)?
A. 13.4 in. (339 mm).
B. 12.6 in. (319 mm).
C. 14.1 in. (359 mm).
D. 13.6 in. (345 mm).

Magnetic Particle Testing 69

 Chapter 4 Review Key
1. D

2. A

3. C

4. C

5. D

6. B

7. A

8. A

70 Programmed Instruction Series

 Chapter 5
MT Equipment

In this chapter:
Factors affecting the choice of MT equipment
Differences between application of dry and wet ferromagnetic particles
Oil-based and water-based magnetic particle suspensions
Theory, use, and advantages of yokes
Single-phase and three-phase wet bench units
Use of MT accessories to determine field strength and direction
Ultraviolet lamp requirements for fluorescent MT
MT equipment calibration and quick break check

71
 Factors Affecting MT Equipment Selection
There are many choices of equipment, materials, and accessories for
performing an MT inspection. Which combination of these you use will
be determined by some or all of the following factors:

• Customer inspection requirements.

• Power availability.
• Test location.
• Anticipated type of discontinuity.

Customer/Inspection Requirements
Inspection requirements usually come from the customer, design engineer,
or end user of the product being tested. The requirements can be
communicated through standards, specifications, purchase orders, and
drawings. In some cases, the specification may only state, “Perform an
MT Inspection in accordance with …” and then reference an inspection
standard. The technique is left to the Level II or III to decide. Some
industries, such as nuclear power, will typically specify the technique that
must be used, as well.

Question 5.1

Inspection requirements are usually determined by the:

A. NDT Level II.
B. NDT supervisor.
C. customer.
D. NDT Level I.

 Please turn to the end of the chapter for the answer.

Power Availability
Because there is more than one choice of MT equipment that can be used
and the power source for each one is different, the power available at the
worksite may determine the equipment and technique that will be used.
For example, if DC is required, but only 110 V is available, a yoke with
DC capability would be a good choice.

72 Programmed Instruction Series

Location of the Test
Whether the inspection will be performed in a shop or in the field will
make a difference regarding which equipment can be used. The power
supply and accessibility must be considered. Fieldwork usually requires
portable MT equipment. If the part can be brought to the shop, the only
limitations would be the size and shape of the part and the type of
equipment available.

Type of Discontinuities Expected

If the part being inspected is more likely to have subsurface discontinuities,
the engineer will most likely want an inspection performed using equipment
with rectified alternating current capability. If only surface discontinuities
are likely, an AC yoke or coil would be adequate.

Question 5.2

For an inspection at a remote site where subsurface discontinuities are

expected, what type of equipment would most likely be used?
A. Yoke with rectified AC.
B. AC-capable yoke.
C. AC coil.
D. Prod unit with AC.

 Please turn to the end of the chapter for the answer.

MT Materials
The materials used for MT inspection consist of ferromagnetic particles,
which are made for either dry or wet MT testing, and suspension fluid
used for wet MT testing. For MT inspections, the materials used to
perform a test will be greatly influenced by the specification that is
designated for the inspection and what equipment is available to the
inspector.

There are two common types of ferromagnetic particles used for MT

inspections: dry and wet. Both types are made of ferromagnetic metal or
oxide, but their sizes are different, as well as the way they are applied to
the test piece during an inspection.

Magnetic Particle Testing 73

 Dry Particles
Dry particles (see Figure 5.1) are made of small pieces of colored iron or
iron oxide that vary in size. They range in size from 0.002 in. (50 µm) to
0.006 in. (150 µm). These particles are highly permeable and easy to
magnetize but have low retentivity. Low retentivity is crucial in order to
prevent the particles from retaining magnetism. If the particles were to
retain magnetism, they would eventually clump together and become
useless during an inspection.

Dry particles are colored. The color of the particles

used is highly dependent on the background of the
material to be inspected and, to a lesser extent, the
color of particles available. An inspection performed on
material with a light-colored background, such as
unpainted steel, would work best with dark particles.
Conversely, light-colored particles work best on a dark
background. The most common color used for MT
inspections is red. However, there are other color
choices available, including: Figure 5.1:
Container of dry
ferromagnetic
• Gray. particles.
• Yellow.
• Black.

The inspector determines which color will work best for each situation.
Although not all colors are readily available, inspectors may need to
perform an inspection with the best color they can obtain that will
provide the greatest contrast between the particles and the surface of the
test object.

Question 5.3

Which of the following factors may determine the color of dry

ferromagnetic particles to be used?
A. Particle retentivity.
B. Size of the test object.
C. Color of the item being tested.
D. Type of MT equipment used.

 Please turn to the end of the chapter for the answer.

74 Programmed Instruction Series

Wet Particles
Wet ferromagnetic particles are smaller than dry particles. They are
typically around 0.0004 in. (10 µm). Particles much smaller than this
would not work very well, as they would have difficulty settling out of the
suspension and would wash off of the part being inspected. Wet particles
have one of two shapes:

1. Long and narrow.

2. Globular.

Both of these shapes are mixed together in the suspension. They are
available in both visible and fluorescent varieties. Visible particles are
usually dyed brown or black and are made from ferromagnetic oxides.
Fluorescent particles are coated with fluorescent pigments that react when
exposed to ultraviolet light and fluoresce green-yellow. This color is most
sensitive to the human eye.

Question 5.4

The size of dry MT particles is typically:

A. 0.0002 in. to 0.002 in. (5 µm to 50 µm).
B. 0.0004 in. to 0.004 in. (10 µm to 100 µm).
C. 0.002 in. to 0.02 in. (50 µm to 500 µm).
D. 0.002 in. to 0.006 in. (50 µm to 150 µm).

 Please turn to the end of the chapter for the answer.

Suspension
The types of carrier solutions available are either water- or oil-based.
Water-based carriers have certain advantages over oil-based carriers.
Advantages include:

• Less expensive.
• No fumes or fire hazard.
• Minimal personnel hazards.

They also allow indications to form more quickly. As for the downside of
water-based carriers, additives for corrosion prevention, anti-foaming
agents, and water conditioners are usually needed for them to work at

Magnetic Particle Testing 75

 their best. (See Figure 5.2.) Oil-based carriers, as shown in Figure 5.3, are
slightly hazardous, give off fumes, and are more expensive. However, they
offer a much higher level of corrosion resistance and lessen the chance for
hydrogen embrittlement.

Figure 5.2: Water-based carrier. Figure 5.3: Oil-based carrier.

Question 5.5

Which type of particle application requires additives for maximum

effectiveness?
A. Wet particles in oil-based carrier.
B. Wet particles in water-based carrier.
C. Red-colored dry particles.
D. Fluorescent particles in oil-based carrier.

 Please turn to the end of the chapter for the answer.

MT Testing Equipment
Not so long ago, the type of equipment available to inspectors was limited.
That’s not so true today as the number of manufacturers in step with an
ever-changing technology has increased the choices offered. The biggest
influence has been technology. Improved technology has allowed
manufacturers to produce MT equipment that weighs less and is more

76 Programmed Instruction Series

portable than what was available just 10 or 20 years ago. The choices of
available equipment continue to change.

Yoke
The yoke is by far the most common piece of equipment used to perform
MT inspections. Several manufacturers produce yokes, but the basic shape
and function are the same regardless of who makes it. It is basically an
electromagnet that is powered by either alternating or rectified alternating
current. Some are equipped with DC, in addition to AC. The mechanics
behind a yoke are fairly simple. An electrical cable is wrapped around the
handle, as shown in Figure 5.4. When current passes through the cable,
magnetic lines of flux are created and travel down one of the legs, across
the material being tested and then up through the other leg.

Current –
Yoke
+

Weld

Magnetic
lines of force

Test object

Figure 5.4: Diagram of current within a yoke.

Question 5.6

Which of the following is true regarding an MT yoke?

A. It is not used very often.
B. It always uses DC current.
C. It is not very portable.
D. It is the MT technique used most often.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 77

 A yoke has many advantages over other types of MT equipment:

• Portable – A yoke can be taken almost anywhere. As long as there is

a source of 110 V current, an MT inspection is possible.
• Easy to use – Compared to other NDT methods, the yoke is one of
the simplest inspection techniques. Performing an MT inspection
with a yoke continues to be one of the most common inspections.
• Fast – An MT inspection using a yoke can be done quickly and
effectively. Far more surface area can be inspected in a shorter
amount of time compared to liquid penetrant inspection.

Although the yoke is a widely used piece of NDT equipment, it does have
its drawbacks. The AC yoke has no penetrating power to detect subsurface
discontinuities. Only yokes with DC current capability can actually
penetrate below the surface of the material being inspected.

Question 5.7

An AC yoke is used to detect which of the following?

A. Surface and subsurface discontinuities.
B. Surface discontinuities.
C. Subsurface discontinuities.
D. Internal voids.

 Please turn to the end of the chapter for the answer.

Wet Bench Unit

The wet bench unit is probably the second-most used piece of MT
equipment. It is not a portable device but stationary. Most models are
equipped with much higher amperage capabilities than other MT
equipment, a factor that makes it desirable for finding subsurface
discontinuities. In addition, bench units have built-in demagnetization
controls, which make them an all-inclusive test system.

78 Programmed Instruction Series

Question 5.8

Compared to a yoke, a wet bench unit is considered to be which of the

following?
A. Stationary.
B. The most-used technique.
C. Equipped with lower amperages.
D. Portable.

 Please turn to the end of the chapter for the answer.

There are only a handful of manufacturers who make wet bench units.
Although there are several variations and options, wet bench units have
either single- or three-phase electrical control systems. (See Figure 5.5.)
Single-phase units are economical and tend to be smaller than three-phase
units.

Figure 5.5: Wet multidirectional bench unit used for magnetization of aircraft landing
gear.

Although very useful for MT inspections, single-phase wet bench units are
not capable of producing the same amperage levels and, therefore, do not
have the power of a three-phase unit.

Three-phase MT units tend to be larger and require higher electrical

currents, which limit where they can be located. Their magnetization
capabilities, both longitudinal and circular, make these units very useful
for inspecting long test pieces, parts with odd or complicated geometries,
and large quantities of small- and medium-sized parts. Large, heavy parts
are usually inspected using other types of equipment.

Magnetic Particle Testing 79

 Question 5.9

An MT bench unit is especially useful for inspecting:

A. large parts.
B. heavy parts.
C. long parts.
D. brass parts.

 Please turn to the end of the chapter for the answer.

Almost all bench units have similar characteristics: two heads, a coil, bath
tank and circulating pump, and amperage adjustment control.

Heads
The bench unit has two heads used to hold the piece or part being
inspected. One of the heads is fixed while the other can be moved in order
to accommodate parts of various lengths. Some units have a pneumatic
clamping system that allows the operator to tightly clamp the piece
between the two heads. This is important to prevent arcing, which can
damage finished parts, requiring them to be repaired or, in some cases,
scrapped.

Coil
A commonly used coil for bench units is a 12 in. (305 mm) diameter,
five-wrap coil, although coils are available with diameters as large as
24 in. (610 mm). The coil is mounted on a sliding base that allows it to be
placed anywhere along the length of the part being inspected. Portable
coils are also available. These units come in a variety of sizes and can be
either AC or DC or both. A bench unit with both heads and coil is shown
in Figure 5.6.

Bath Tank
Bench units use a fluorescent MT bath. The bath, comprising either a
water- or an oil-based carrier and MT particles, is stored below the top of
the unit in a storage tank. Inside the storage tank is a recirculating pump
that keeps the ferromagnetic particles suspended in the carrier. The pump
also supplies the MT bath to the spray hose used to spray the bath onto
the part.

80 Programmed Instruction Series

Figure 5.6: An engine block positioned for multidirectional magnetization using contact
circular magnetization (head shot) and longitudinal magnetization (coil shot).

Amperage Control
The amperage for each inspection will vary, depending on the size of the
part and other factors. The amperage control mechanism allows the
inspector to change the amperage for each inspection, as necessary.

Depending on which bench unit you’re using, it may have more options
than those listed above. Other options include automatic demagnetization,
an air-clamping device, and both AC and DC voltage choices.

Question 5.10

Compared to single-phase units, three-phase MT bench units have:

A. higher amperage capabilities.
B. two heads.
C. a fluorescent bath tank.
D. a smaller, more portable size.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 81

 Accessories
In addition to the equipment and materials used with MT, several
accessories play an important role in the testing process. Some are used to
measure the strength of the magnetic field, some to verify that the field
direction is correct, and others to measure residual magnetism.

Pie Gage
A pie gage is primarily used to verify
the direction of a magnetic field. (See
Figure 5.7.) It is placed in the inspection
area while the magnetic field is being
applied. Pie gages are available in many
configurations. Any variation used must be
qualified to a particular procedure or
standard. Qualification of the pie gage is the
responsibility of the Level III technician. Figure 5.7: Pie gage on flat surface
of alloy steel using a DC yoke.
The device is essentially a disk of high-
permeability material divided into four or more sections by equally spaced
cuts that simulate discontinuities. In practice, the indicator is held firmly
on a magnetized test object, causing some of the magnetic field to pass
through the disk to indicate the direction of the magnetizing force.

Gauss or Tesla Meter

Gauss or tesla meters measure the strength of a magnetic field during an
inspection. The meter’s probe, often called a hall effect probe, measures the
magnetic field by turning its forces into voltage and then displaying the
result as a gauss or tesla
measurement. In Figure 5.8,
an inspector is moving the
sensor along a part in order
to view the localized
magnetic field strength.
The field strength may vary
due to polarity, part
geometry, and so on. In the
International System of
Units (SI), the tesla has
replaced the gauss as a Figure 5.8: Digital gauss (tesla) meter.

82 Programmed Instruction Series

measurement of magnetic flux density. Magnetic flux density is expressed
in webers per square meter (W · m–2) or tesla (T) to indicate flux per unit
area. One tesla equals 10,000 gauss as in the following expressions:

(Eq. 5.1) 1 T = 104 G = 10 kG

Conversely:

(Eq. 5.2) 1 G = 10–4 T = 0.1 mT

Tesla meters are available that give measurements in SI units.

Quantitative Quality Indicator (QQI)

A quantitative quality indicator (QQI) can be used to verify both field
direction and field strength. (See Figure 5.9.) Because it can be used for
both factors, and due to its relatively low cost, it is the preferred method
for determining the characteristics of
a magnetic field in many industries,
including aerospace and nuclear.
QQIs are thin strips of either 0.05
mm (0.002 in.) or 0.1 mm (0.004 in.)
thick AISI 1005 steel. Specific
patterns—concentric circles or a plus
(+) sign—are scribed in the material.
A QQI can be fitted on most part
geometries using clear tape and may
be reused if carefully removed after
each use.

Field Indicator Figure 5.9: QQI under ultraviolet

radiation.
For practical purposes, a field
indicator, either analog or digital,
may be used after performing demagnetization to determine that the
residual field strength has been reduced to a desired level. The field
indicator, diagrammed in Figure 5.10, is a pocket-sized device that
measures the strength of a field against a set of small, enclosed permanent
magnets that restricts the needle movement on a relative scale.

Magnetic Particle Testing 83

 Field indicator

Plan view
S N S N

Side view
N S

S N

Figure 5.10: Diagram of a typical analog magnetic field indicator

Question 5.11

A pie gage is used to verify:

A. field strength.
B. field direction.
C. gauss or tesla level.
D. residual field strength.

 Please turn to the end of the chapter for the answer.

Ultraviolet Lamps
Ultraviolet lamps are used when performing fluorescent magnetic
particle inspection, as shown in Figure 5.11. Fluorescent lamps come
in a variety of shapes and sizes. Most are flashlights, but there are
also fixed-housing lamps available. Ultraviolet light (black light) is
electromagnetic radiation between 100 to 400 nm (1000 to 4000 Å)
and is invisible to the human eye.

84 Programmed Instruction Series

 (a) (b)

Figure 5.11: Two styles of ultraviolet lamps: (a) high-intensity self-ballasted UV-A kmp;
(b) UV-A inspection flashlight.

Ultraviolet light is measured in units of irradiance, either watts per square

meter or microwatts per square centimeter, where

(Eq. 5.3) 1 W/m2 = 100 μW/cm2

All ultraviolet measuring devices are selective, and their sensitivity
depends upon the wavelength of the radiation being measured. The UV-A
range, referred to as long ultraviolet radiation, between 320 and 400 nm,
is used for fluorescent magnetic particle testing. The UV-A meter must
have a filter system to produce the maximum response at 365 nm ±5 nm
(the wavelength used by magnetic particle pigments to produce
fluorescence).

Question 5.12

Ultraviolet light is between:

A. 0.1 to 4 nm (1 to 40 Å).
B. 1 to 14 nm (10 to 140 Å).
C. 10 to 40 nm (100 to 400 Å).
D. 100 to 400 nm (1000 to 4000 Å).

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 85

 Calibration
Much of the equipment used for magnetic particle testing requires
periodic calibrations to be performed. Calibration involves several
measurements and a comparison to a standard that is more accurate than
what is being checked. Calibration ensures that the equipment is operating
correctly and provides a higher level of confidence that the test results are
accurate. Calibration of MT equipment is usually performed at six- or
12-month intervals.

The following is a list of MT equipment that normally requires calibration:

• Yoke – dead weight test at 10, 30, or 50 lb (4.5, 13.6, or 22.7 kg ).

• Ammeter – gage accuracy. In MT, an ammeter serves to measure the
alternating current, as well as the half-wave and full-wave current,
output.
• Timers – timer accuracy.
• White light or ultraviolet light meter – frequency of output.
• Gauss (tesla) meter – accuracy.

Ë From the Field: The frequency of calibration for MT

equipment is dependent on the procedure and/or specification
that the inspector is working to. Some require six-month
calibration cycles and others require 12-month (annual)
calibrations. Calibration should also be performed following
major repairs, whenever a malfunction is suspected, when
specified by the cognizant engineering organization, or
whenever electrical maintenance that might affect equipment
accuracy is performed. The requirements governing calibration
should be spelled out in the inspection procedure of the
company performing MT testing.

86 Programmed Instruction Series

Quick Break Functionality
When a coil is used to impart a longitudinal field in a bar-shaped object,
special circuitry is required to ensure a sufficient field near the ends of the
test object for the detection of circumferential discontinuities. The
condition or effect required has been called quick break or fast break. On
older magnetic particle testing machines, periodic checking of the break is
critical. On electronically triggered equipment, a malfunctioning firing
module could result in quick-break failure. Thus, newer units incorporate
the function so there is no need to verify its functionality. Battery-
powered units also have this feature built in. On the other hand, yokes and
prods make quick-break magnetization unnecessary because the test
object is part of the magnetic circuit.

Magnetic Particle Testing 87

 Chapter 5 Summary

r Inspection requirements usually come from the customer, design

engineer, or end user of the product being tested.
r The power available at the worksite may determine the
equipment and technique that will be used.
r The location of an inspection (shop or field) helps determine
whether the equipment to be used is portable or stationary.
r Dry ferromagnetic particles ranging in size from 0.002 in. (50 µm)
to 0.006 in. (150 µm) are designed to be highly permeable with
low retentivity.
r Dry particles come in gray, yellow, black, and red to provide
contrast with the background of the material to be inspected.
r Wet ferromagnetic particles are smaller than dry particles and
come in two shapes, long and globular, which are mixed together
in a suspension.
r Visible wet particles are dyed brown, whereas fluorescent particles
fluoresce green-yellow, the color most sensitive to the human eye
under ultraviolet light.
r Two types of carrier solutions are available for wet particles: water-
based and oil-based.
r The yoke is by far the most common piece of equipment used to
perform MT inspections.
r A yoke has several advantages including portability, ease of use,
and ability to conduct a quick and effective test.
r A wet bench is a stationary unit with high amperage capabilities
for the detection of subsurface discontinuities.
r Wet bench units have either single- or three-phase electrical
control systems.
r Wet bench units typically comprise two heads, a coil, bath tank
and circulating pump, and amperage adjustment control.
r Accessories used in MT include a pie gage to determine field
direction and gauss (tesla) meter to measure magnetic field
strength.
r A quantitative quality indicator (QQI) can be used to verify both
field direction and field strength.
r Ultraviolet lamps used in MT are in the UV-A range, between
320 and 400 nm, with 365 nm ±5nm being the wavelength used
to produce fluorescence of particles.

88 Programmed Instruction Series

r The success of an MT inspection depends on calibration of
equipment, including yoke, ammeter, timers, white or ultraviolet
light meters, and gauss (tesla) meter.
r Quick break magnetization is when a coil is used to impart a
longitudinal field in a bar-shaped object for the detection of
circumferential discontinuities.
r Calibration of MT equipment is usually performed at six- or
12-month intervals depending on the inspection procedure.

Magnetic Particle Testing 89

 Answers to Chapter 5 Questions
Question 5.1
Answer: C – The customer, end user, or design engineer typically
determines the requirements of an MT inspection.

Question 5.2

Answer: A – A yoke powered with rectified alternating current would be a

good choice for detecting subsurface discontinuities at a remote location.

Question 5.3

Answer: C –The color of dry ferromagnetic particles used for an MT

inspection is determined by the color of the material being inspected.

Question 5.4

Answer: D – Dry particles are larger than wet particles, which are typically
around 0.0004 in. (10 µm) in comparison.

Question 5.5

Answer: B – Water-based carriers typically require additives for corrosion

prevention, anti-foaming agents, and water conditioners.

Question 5.6

Answer: D – A yoke is the MT technique used most often for inspections.

Question 5.7

Answer: B – Limited in its ability to detect subsurface discontinuities,

alternating current is used for the detection of surface discontinuities.

90 Programmed Instruction Series

Question 5.8

Answer: A – A wet bench unit can produce higher amperages than a yoke,
which is portable and used more often in the field.

Question 5.9

Answer: C – Because it is stationary, a wet bench unit is especially effective

at inspecting long parts.

Question 5.10

Answer: A – Three-phase units usually have higher amperage capabilities.

Question 5.11

Answer: B – A pie gage primarily indicates field direction.

Question 5.12

Answer: D – Although ultraviolet light ranges from 100 to 400 nm (1000 to

4000 Å), the UV-A range, between 320 and 400 nm, is used for fluorescent
magnetic particle testing with the maximum response at 365 nm ±5nm.

Magnetic Particle Testing 91

 Chapter 5 Review
1. MT Inspection requirements are usually specified by the:
A. customer.
B. inspector.
C. NDT Level III.
D. welder.

2. Which of the following is not a factor in determining the type

of MT inspection used?
A. Power available.
B. Location of the test.
C. Time of inspection.
D. Type of discontinuity expected.

3. Dry ferromagnetic particles are made from:

A. brass shavings.
B. Inconel™ shavings.
C. stainless steel shavings.
D. iron oxide.

4. Wet MT particles are typically around:

A. 100 µm.
B. 50 µm.
C. 10 µm.
D. 1000 µm.

5. Compared to oil-based carriers, water-based carriers are:

A. more expensive.
B. less corrosion resistant.
C. not hazardous.
D. slower at forming indications.

6. Which type of MT equipment is basically an electromagnet formed

by a cable wrapped around a handle to induce either alternating
or rectified alternating current?
A. Wet bench unit.
B. Coil.
C. Head shot.
D. Yoke.

92 Programmed Instruction Series

7. The yoke technique is:
A. one of the simplest MT techniques used.
B. the least common MT technique used.
C. no longer used.
D. the most difficult MT technique to use.

8. An MT bench unit is considered:

A. portable.
B. best for welded components.
C. stationary.
D. best for large parts.

9. A pie gage is primarily used to:

A. measure field strength.
B. verify field direction.
C. locate discontinuities.
D. measure field strength and verify field direction.

10. Ultraviolet light is light that ranges from:

A. 100 to 400 nm (1000 to 4000 Å).
B. 1 to 4 nm (10 to 40 Å).
C. 10 to 40 nm (100 to 400 Å).
D. 0.1 to 0.4 nm (1 to 4 Å).

11. Calibration of MT equipment is performed:

A. monthly.
B. usually at six- or 12-month intervals.
C. weekly.
D. quarterly.

Magnetic Particle Testing 93

 Chapter 5 Review Key
1. A

2. C

3. D

4. C

5. B

6. D

7. A

8. C

9. B

10. A

11. B

94 Programmed Instruction Series

 Chapter 6
Demagnetization Principles

In this chapter:
Factors affecting the amount of residual magnetism in a test object
Applications requiring demagnetization
Demagnetizing for welding, machining, rotating parts, and gages
Demagnetization using coils and yokes
Process of demagnetizing using bench units
Effect of heat-treating above the curie point on magnetized parts
Use of a magnetic field indicator to measure residual magnetism
Maximum allowable gauss or tesla measurement

95
 Introduction
As a result of the magnetizing force used to inspect ferromagnetic parts
when they are magnetic particle tested, the material being inspected
usually retains some level of the magnetization force. This is called
residual magnetization. The amount of residual magnetization retained is
different for each type of material; some retain more than others. For
example, materials with low carbon content retain less magnetization, and
materials with higher carbon content retain more. Although it’s not
necessarily that simple, it’s a good rule of thumb.

The other factors that affect how much magnetization is retained include:

• The direction of the magnetic field, either circular or longitudinal.

• The strength of the magnetizing force.
• The type of current used, AC or DC.

Because most types of carbon steel retain some magnetization, it’s

almost always necessary to demagnetize the parts/material following an
MT inspection.

Question 6.1

Magnetism that is retained in metal following an MT examination is called:

A. excess magnetism.
B. demagnetism.
C. residual magnetism.
D. ferromagnetism.

 Please turn to the end of the chapter for the answer.

Why Demagnetize?
There are many reasons why material is demagnetized after an inspection.
It mostly depends on how the material will be used in its final application.
In general, if it is anticipated that residual magnetism will interfere with
subsequent manufacturing processes, it should be removed. Applications
that would require demagnetization include:

96 Programmed Instruction Series

 • Welding – Even the slightest residual magnetic field will play havoc
on the welding process. Magnetism interacts with the welding arc,
causing the arc to be deflected sideways, which disturbs the weld
pool. This disturbance is termed magnetic arc blow. Arc blow can
cause weld discontinuities and, depending on the level of residual
magnetism, slow down the welding process.
• Machining – If a part is to be machined after an MT inspection,
demagnetization is important because the residual magnetic field
may hold metal shavings to its surface that could interfere with the
machining process. The shavings may cause finish imperfections as
the part turns and the shavings collect around the tooling.

Equipment containing the following should also be demagnetized:

• Rotating parts – For parts such as drives and camshafts, residual

magnetism may allow metal particles to adhere to their surface,
which can cause premature wear.
• Gages and equipment – Residual fields, when strong enough, can
affect gages and other equipment that is sensitive to magnetism.

Every industry is different. When demagnetizing parts is required, it will

usually be specified by the customer’s purchase order, part drawing, or
specification used to perform the MT inspection. For most parts/
materials, demagnetization is fairly simple. For highly retentive or odd-
shaped test objects, demagnetizing can be challenging. For these materials,
special techniques may be required.

Question 6.2

Residual magnetism needs to be removed when:

A. it interferes with manufacturing processes after the examination.
B. it exceeds 10 G (0.001 T).
C. it exceeds 5 G (0.0005 T).
D. any residual magnetism exists.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 97

 Demagnetization Techniques
Although several techniques are available to demagnetize parts, the easiest
way is to pass the test object through an alternating current coil or
between the legs of a yoke. Alternatively, you can demagnetize using a
bench unit. Regardless of the method you choose, there are two keys to
removing a residual magnetic field:

1. If the last magnetic field used during an inspection was circular,

it should be followed up with a longitudinal field inspection
magnetization, which is easier to remove.
2. Whenever possible, the demagnetizing field should start out
slightly stronger than the field used during the inspection.
Demagnetizing with a stronger field is easy to do when you’re
using a bench type of machine, but when an MT inspection is
performed using an AC coil or yoke, an equivalent field strength
is usually all that is possible.

Question 6.3

One of the simplest techniques for demagnetizing parts is:

A. demagnetizing using a DC circular field.
B. by placing prods in the examination area and demagnetizing.
C. removing the residual field using a permanent magnet.
D. passing the part through an AC coil or between the legs of a yoke.

 Please turn to the end of the chapter for the answer.

Coil/Yoke Demagnetization
For coil/yoke demagnetization, start by
energizing the coil/yoke. Next, while
holding the part, pass it through the
coil/legs and then slowly move it away
until it is approximately 12 in. to 18 in.
(305 mm to 457 mm) distant. This
process may need to be repeated a few
times to adequately lower the residual
field strength. If the coil is equipped
with a rheostat controller that permits
the operator to change the current level, Figure: 6.1: Coil used for demagnetization.

98 Programmed Instruction Series

the part need only be placed inside the coil. Instead of moving the part
away from the coil, the operator reduces the current level using the
controller. Both of these methods will adequately demagnetize the part. A
coil demagnetizing unit is shown in Figure 6.1.

Bench Demagnetization
Demagnetizing using a bench unit is usually only done when the inspection
was performed on the same unit. Although you can demagnetize using the
coil after a bench inspection, most bench units come equipped with a coil
and automated demagnetizing capabilities. Other than setup, most of the
demagnetizing process is automated.
It’s almost as simple as just pressing
a button. An automated unit for
demagnetization is shown in
Figure 6.2. Since bench units have
the capability of both circular and
longitudinal fields, it’s important to
perform the circular magnetic field
inspection first, followed by the
longitudinal magnetic field inspection.
As stated previously, it’s easier to
remove a longitudinal field than it is Figure 6.2: Production AC coil
to remove a circular field. demagnetizer in MT conveyor unit.

Effect of Heat-Treating
Although not common, if the material inspected will be heat-treated above
its curie point—1200 to 1600 °F (650 to 870 °C) for ferrous alloys—there
would be no reason to demagnetize it. The heat-treating process will
remove all residual magnetism.

Question 6.4

Demagnetization on a bench unit should be done:

A. after longitudinal MT, when circular MT is performed first.
B. after circular MT, when longitudinal MT is performed first.
C. after each MT examination, regardless of direction.
D. never; demagnetization cannot be done on a bench unit.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 99

 Measuring Residual Magnetism
Most NDT specifications and procedures specify when demagnetizing is
required. The specifications or procedures should also provide you with
the maximum gauss (G) or tesla (T) measurement that is allowed.
Normally, the maximum residual field is 3 G (3 × 10–4 or 0.0003 T). If,
after the inspection, you measure the residual field and it’s not above
3 G (0.0003 T)—or the maximum allowed by your specification—you may
not need to demagnetize at all. This is often true when the inspection is
performed using an AC coil or yoke. These techniques do not use a
penetrating field and are less likely to leave as strong of a residual field
as with direct current techniques.

Question 6.5

The maximum residual field that is normally allowed is:

A. 1 G (0.0001 T).
B. 3 G (0.0003 T).
C. 5 G (0.0005 T).
D. 10 G (0.001 T).

 Please turn to the end of the chapter for the answer.

Residual magnetic fields are usually

much lower than what is needed
for the inspection, typically less than
20 G (0.002 T). To measure residual
fields in gauss or tesla, a magnetic
field indicator (magnetometer), either
analog or digital, is used. A common
type of field indicator is pictured in
Figure 6.3.
Figure 6.3: Typical pocket-sized magnetic
These meters are simple to use and field indicator.
are usually very accurate. They
typically do not need any maintenance, except that most specifications
require that they be calibrated periodically.

100 Programmed Instruction Series

 Ë From the Field: When measuring residual fields using a field
indicator, make sure you slide the gage over each side of the
part and in more than one direction, to ensure you're finding
residual fields regardless of their orientation.

Demagnetization is a critical part of the magnetic particle inspection

process. As we discussed earlier in this chapter, residual magnetization can
have a serious negative effect on parts and equipment if not removed
properly. The cost of not removing the residual field can be huge. Imagine
the effect on engines, drivetrains, and other assemblies due to improper
magnetic particle testing procedures.

Magnetic Particle Testing 101

Chapter 6 Summary

r Material being inspected with MT usually retains some level of the

magnetization force called residual magnetization.
r Materials with low carbon content retain less magnetization, and
materials with higher carbon content retain more.
r Factors affecting how much magnetization is retained include the
direction of the magnetic field, strength of the magnetizing force,
and type of current.
r Applications that require demagnetization include welding and
machining; equipment with rotating parts and gages should also
be demagnetized.
r The easiest ways to demagnetize are to pass the test object
through an alternating current coil or between the legs of a yoke,
or by using a bench unit.
r If the last magnetic field used during an inspection was circular, it
should be followed up with a longitudinal field inspection
magnetization.
r Whenever possible, the demagnetizing field should start out
slightly stronger than the field used during the inspection.
r Since bench units have the capability of both circular and
longitudinal fields, it’s important to perform the circular magnetic
field inspection first.
r If the material inspected will be heat-treated above its curie point,
there would be no reason to demagnetize it.
r If, after an inspection, the residual field is less than 3 G (0.0003 T)—
or the maximum allowed by your specification—you may not be
required to demagnetize.
r To measure residual fields, magnetic field indicator or magnetometer
is used.
r Demagnetization is a critical part of the magnetic particle
inspection process.
r Residual magnetization can have a serious negative effect on parts
and equipment if not removed properly.

Magnetic Particle Testing 103

 Answers to Chapter 6 Questions
Question 6.1
Answer: C – Magnetism retained by metal is called residual magnetism or
magnetization.

Question 6.2

Answer: A – When it interferes with subsequent manufacturing processes,

residual magnetism should be removed.

Question 6.3

Answer: D – Most residual fields can be removed by passing parts through

a coil or between yoke legs.

Question 6.4

Answer: A – Demagnetization using a bench unit must be done after

performing longitudinal MT using the coil; demagnetizing after
performing circular MT is usually ineffective.

Question 6.5

Answer: B – Most specifications allow no more than 3 G (0.0003 T)

without demagnetizing.

104 Programmed Instruction Series

Chapter 6 Review
1. Residual magnetism is defined as magnetism:
A. in excess of that required to perform the MT exam.
B. leftover after performing an MT exam.
C. that cannot be removed.
D. that is allowed to remain.

2. When is residual magnetism required to be removed?

A. When greater than 15 G (0.0015 T).
B. When greater than 20 G (0.002 T).
C. When less than 3 G (0.0003 T).
D. When more than 3 G (0.0003 T).

3. Which of the following is not an effective method of demagnetizing?

A. Passing the part through a coil.
B. Passing the part between the legs of a yoke.
C. Using a circular MT field.
D. Using a longitudinal MT field.

4. Residual MT can cause:

A. cracks.
B. nonrelevant indications.
C. hardening of metal.
D. arc blow during welding.

5. Residual magnetization is measured in:

A. gauss or tesla.
B. volts or electronvolts.
C. amps or milliamps.
D. hertz or megahertz.

Magnetic Particle Testing 105

 Chapter 6 Review Key
1. B

2. D

3. C

4. D

5. A

106 Programmed Instruction Series

 Chapter 7
Inspection Personnel

In this chapter:
Responsibilities of Level I, II, and III personnel
Recommended practices and specifications for certification
Training hours recommended for Level I and II certification
Minimum hours required in MT and NDT for certification
Tasks associated with the experience portion of the certification process
Overview of certification examinations: theory, specific, and practical
Importance of an annual performance review to ensure competency
Recertification timeframe and procedure

107
 NDT Personnel
Personnel who perform NDT inspections are arguably the most important
part of the inspection process. Without well-trained, experienced
inspectors, the integrity of an inspection is lessened. To ensure inspection
integrity, NDT inspectors must complete an extensive certification process
prior to being allowed to perform inspections without supervision from a
certified inspector. Aside from the initial period of training when a
technician is considered a Trainee, there are three levels of certified NDT
inspectors: Levels I, II, and III.

Question 7.1

Which of the following is not a certification level for NDT inspectors?

A. I.
B. II.
C. III.
D. Trainee.

 Please turn to the end of the chapter for the answer.

A Level I, when authorized by the employer’s customer, may inspect parts

or components using a specific magnetic particle technique. A Level II
may perform inspections using any of the techniques for which he or she
is certified. The NDT Level III is responsible for providing adequate
training and ensuring that Level I and II candidates receive in-depth
experience that encompasses all aspects of each method and the
techniques in which they are attempting to certify. The NDT Level III is
also responsible for guiding personnel through the certification process by
administering the required examinations. If certified to do so, a Level III
can also inspect parts.

When it comes to obtaining certification to perform magnetic particle

inspections, the certified inspector has to complete a lengthy, arduous
journey. The certification process takes months or years to complete and
requires a commitment from not only the person seeking certification but
also from his or her employer. Many hours are spent in formal training
classes, working directly with certified personnel, and observing and
assisting with inspections.

108 Programmed Instruction Series

Question 7.2

The NDT certification level responsible for personnel training and

examination is a(n):
A. human resource manager.
B. Level II.
C. Level III.
D. outside certifying agency.

 Please turn to the end of the chapter for the answer.

NDT Specifications Governing Certification

The training, experience, and examination requirements used for
certifying as a Level I, II, or III magnetic particle inspector/examiner vary
depending on the industry. Table 7.1 provides examples of NDT personnel
qualification specifications, as well as one recommended practice, and the
industries where they are commonly used. Many of the requirements
found in each of these are similar, but there are also distinct differences
between them as well. A similarity is that both NAS 410: NAS Certification
& Qualification of Nondestructive Test Personnel and ANSI/ASNT CP-189:
ASNT Standard for Qualification and Certification of Nondestructive
Testing Personnel permit an outside agency to supply an employer with
NDT Level III personnel to perform NDT. One of the differences between
CP-189 and NAS 410 is the number of training and experience hours
required prior to certification. CP-189 allows for fewer hours of training
and experience than does NAS 410.

Table 7.1: NDT personnel qualification standards and recommended practices.

Specification Industry

SNT-TC-1A
General NDT (power plant, pipeline, pressure
vessels, etc.)
ANSI/ASNT CP-189

NAS 410 Aerospace

NS 250-1500-1 Navy nuclear

EN 473
European NDT certification
ISO 9712

Magnetic Particle Testing 109

 Other differences include the number of questions for written tests and
whether there is a requirement for an annual review of all inspectors. The
differences in the requirements found in these documents can best
be explained by understanding the industries from which they derive.
SNT-TC-1A is a recommended practice used by a wide variety of
industries. The requirements within this document must work for a far
greater number and type of user than NAS 410. In contrast, NAS 410 is
used almost solely by the aerospace industry. Its requirements are written
for a very specific group of users. In the United States, the majority of
NDT inspectors certify to requirements that follow ASNT’s recommended
practice, SNT-TC-1A.

Question 7.3

The specification having certification requirements for aerospace is:

A. SNT-TC-1A
B. NAS 410
C. NAVSEA 248
D. NS 250-1500-1

 Please turn to the end of the chapter for the answer.

The specifications found in these two documents are similar, but

the major difference is that SNT-TC-1A is a recommended practice,
which means that instead of having specific training, experience, and
examination requirements that cannot be altered, its contents are only
recommendations, which may be adjusted in certain situations.

Here’s an example of how this might work. For a Level II magnetic particle
inspector, the total number of training hours listed in SNT-TC-1A is eight;
however, let’s say ABC Manufacturing Company only performs MT using
a yoke and dry powder, and does not use any of the other MT techniques.
For this specific case, the hours required for Level II certification, as set
forth in SNT-TC-1A, may be reduced to fewer hours—four, for example—
because technicians only perform this one specific type of MT test. For
this type of limited NDT certification, the NDT Level III must specify the
inspection limitations on the certification document.

110 Programmed Instruction Series

The majority of other NDT certification specifications do not allow for
this type of adjustment of the written requirements. Ultimately, the
company’s written practice, an internal procedure that outlines specific
requirements for certifying its NDT personnel, must be met. The
requirements within the written practice must be compatible with
customer requirements.

Ë From the Field: NDT certifications issued by employers do not

travel with you if you should decide to switch jobs and work for
another company that needs NDT certifications. Directly
certifying through the ASNT Central Certification Program
and ASNT NDT Level II Program provides you with a
certification that you can take anywhere, without necessarily
having to retake the examinations for your new employer.

Question 7.4

A company’s written practice defines:

A. the certification requirements for NDT personnel.
B. preparation of the test surface for NDT exams.
C. how to perform MT exams.
D. how to choose candidates to be NDT inspectors.

 Please turn to the end of the chapter for the answer.

Training
One of the first things that candidates must do as they begin the journey
to become certified NDT inspectors is complete comprehensive training.
The training program must cover all of the theoretical, specific, and
practical aspects of the NDT method to which they will be certifying.
The training must also be appropriate for the certification level that the
individual is working toward. Some candidates certify as a Level I prior
to progressing to Level II. Some may only certify to Level I.

The training must also meet the requirements of the written practice.
Additionally, the Level III must review and approve of the training outline
to ensure it adequately encompasses all of the necessary aspects for

Magnetic Particle Testing 111

 magnetic particle testing. Depending on the specific NDT method that the
candidate must certify to, the training requirements will vary. Table 7.2
lists the number of training hours recommended for magnetic particle
testing by SNT-TC-1A (2011) in comparison with other methods.

Table 7.2: Training hours by level per SNT-TC-1A (2011).

NDT Method (Technique) Level I Training (Hours) Level II Training (Hours)

Electromagnetic 40 40

Liquid Penetrant 4 8

Magnetic Flux Leakage 16 12

Magnetic Particle 12 8

Radiographic 40 40

Ultrasonic 40 40

Question 7.5

A technician would require how many hours of training to be certified as a

Level II MT inspector?
A. 12
B. 8
C. 20
D. 24

 Please turn to the end of the chapter for the answer.

Experience
In addition to formal training, candidates for magnetic particle inspection
must also acquire knowledge through hands-on experience. Most Level III
personnel would agree that the experience portion of the certification
process is the most critical part. It’s not just about the number of hours of
experience; it’s really the quality of the experience provided by the

112 Programmed Instruction Series

employer that’s important. The experience should provide opportunities
for the technician or inspector to use all of the techniques that he or she
will be certified to use. At the end of the day, the experience that
candidates are exposed to will, more than anything else, affect the integrity
of their inspections after they achieve certification.

Each level of certification has a requirement for experience hours.

The number of experience hours for a Level I and II inspector, as
recommended by SNT-TC-1A (2011), is listed in Table 7.3 for various
methods including MT.

Table 7.3: Hours of experience recommended by SNT-TC-1A (2011) by level.

NDT Method Minimum Hours in
NDT Level Total Hours in NDT
(Technique) Method

I 210 400
Electromagnetic

II 630 1200

I 70 130
Liquid Penetrant
II 140 270

I 70 130
Magnetic Flux Leakage
II 210 400

I 70 130
Magnetic Particle
II 210 400

I 210 400
Radiographic

II 630 1200

I 210 400
Ultrasonic
II 630 1200

Note: For NDT Level II certification, the experience should consist of time at NDT Level I or equivalent. If
a person is being qualified directly to NDT Level II with no time at NDT Level I, the experience (both
Method and Total NDT) should consist of the sum of the hours for NDT Level I and Level II and the
training should consist of the sum of the hours for NDT Level I and Level II.

Magnetic Particle Testing 113

 The total number of hours of experience required by SNT-TC-1A (2011)
for qualifying to Levels I and II in the magnetic particle testing method, in
succession, is 280, but some specifications require even more.

Question 7.6

The total number of experience hours required for an NDT Level II in MT is:
A. 350.
B. 70.
C. 210.
D. 280.

 Please turn to the end of the chapter for the answer.

The following should be covered during the experience portion of the

certification process:

1. Interpreting and understanding NDT procedures, drawings,

specifications, and inspection standards.
2. Knowledge of applicable NDT procedures.
3. Sample preparation/cleaning of parts.
4. Demonstration of magnetic particle testing procedure.
5. Evaluation of inspection results.
6. Documenting the results.

The combination of training and experience is intended to produce a

knowledgeable and competent NDT inspector. Note that the training of
other NDT personnel is not part of the experience required for NDT
certification.

Question 7.7

Which of the following is not a part of the experience portion of the

certification process for Trainees qualifying to Level I?
A. Evaluation of inspection results.
B. Sample preparation/cleaning.
C. Training other NDT personnel.
D. Documenting the inspection results.

 Please turn to the end of the chapter for the answer.

114 Programmed Instruction Series

Certification Examinations
The last phase of the certification process is the examination of the
candidate. This portion of the overall journey to certification is used to
test the candidate’s understanding and knowledge of the theory,
procedures, and specifications, as well as hands-on application of testing
techniques, for magnetic particle testing. For Levels I and II, three
examinations are required:

• Theory – The theory exam is a closed-book, multiple-choice

examination, which is intended to test candidates’ understanding of
the principles behind magnetic particle testing.
• Specific – A specific examination tests candidates’ knowledge and
understanding of the procedures, codes, specifications, and
acceptance criteria that they will use once they certify. This is an
open-book exam.
• Practical – To test candidates’ skills for performing magnetic particle
testing, a practical examination is administered. Candidates are
usually required to test two practical samples for each technique and
correctly evaluate any indications that they find. The Level III should
also evaluate the candidates’ processing techniques to ensure that
they are competent in all aspects of each technique.

Question 7.8

Of the following choices, which one best describes the practical exam
portion of the NDT certification process?
A. Testing the candidate’s skills in performing and evaluating MT
exams.
B. Testing the candidate’s general knowledge of MT theory.
C. Testing the candidate’s knowledge of NDT specifications.
D. Testing the candidate’s knowledge level of the history of MT test
techniques.
 Please turn to the end of the chapter for the answer.

After successfully completing the entire process—training, experience, and

examinations—candidates are now certified and ready to go out on their
own to perform magnetic particle testing with no supervision. However,
it’s not a bad idea for seasoned Level II inspectors to accompany newly
certified personnel when complicated parts/components are to be

Magnetic Particle Testing 115

 inspected, if for no other reason than to provide support if they are faced
with unfamiliar circumstances.

Performance Reviews and Recertification

All certified NDT inspectors are required to undergo performance reviews
periodically, usually on an annual basis. The review is performed to ensure
continued competency with each of the inspection techniques for which
the inspector is certified. The review is usually performed by the NDT
Level III, who observes the inspector as he or she performs an
examination. The purpose of the review is to ensure that the inspector is
correctly performing the examination in accordance with the procedure
and that his or her technique is sound.

Recertification of NDT personnel is usually required every three or five

years, depending on the specification that the inspector is certified to. The
exact timeframe between recertification examinations is based on the
company’s written practice and/or the specification used by the inspector’s
employer to certify its personnel. Recertification may require retaking all
three exams, or only taking a specific and practical exam. The written
practice specifies exactly what is required.

Some specifications allow an NDT Level III to recertify by acquiring

points through attending seminars, training inspectors, administering
NDT examinations, and taking part in other NDT-related activities, such
as completing a technical review of a publication. Evidence is usually
required.

As long as inspectors recertify within the required timeframe, one other

requirement for maintaining their certification is that they must perform
magnetic particle inspections no less than every six months. In addition,
per CP-189, annual visual acuity exams are required as part of the
recertification process; however, color differentiation examinations are
repeated only once per recertification. Annual performance evaluation by
the supervising Level III may also be required. If an inspector does not
perform an examination for a period exceeding six months, his or her
certification can be revoked. To recertify, they must meet the requirements
of their company’s written practice.

116 Programmed Instruction Series

Qualifying to be an NDT inspector, completing the certification process,
and maintaining an NDT certification is not an easy task, but with the
proper training, experience, and guidance from your Level III, most
technicians are well prepared for the examinations and have little trouble
achieving certification.

Question 7.9

Which of the following is not typically part of the recertification process

for MT technicians?
A. Earning recertification points by training other NDT personnel.
B. Submitting an annual self-evaluation to the supervising Level III.
C. Performing MT inspections at least every six months.
D. Retaking one or more qualifying examinations.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 117

Chapter 7 Summary

r To ensure inspection integrity, NDT inspectors must complete an

extensive certification process.
r Aside from the designation of Trainee, there are three levels of
certified NDT inspectors: Levels I, II, and III.
r The training, experience, and examination requirements used for
certifying as a Level I, II, or III magnetic particle inspector vary
depending on the industry.
r SNT-TC-1A is a recommended practice, which means that its
contents may be adjusted in certain situations.
r SNT-TC-1A is used by a wide variety of industries, whereas NAS 410
is used almost solely in aerospace.
r The company’s written practice outlines specific requirements for
certifying its NDT personnel that must be met.
r Level I or II candidates must complete comprehensive training
involving theoretical, specific, and practical aspects of the NDT
method to which they are certifying.
r Candidates for magnetic particle inspection must also acquire
knowledge through hands-on experience including all of the
techniques that they will be certified to use.
r The total number of hours of experience recommended by
SNT-TC-1A (2011) for qualifying to Levels I and II in the magnetic
particle testing method, in succession, is 280, but some
specifications require even more.
r The last phase of the certification process is the examination of
the candidate.
r For Levels I and II, three examinations are required: theory,
specific, and practical.
r All certified NDT inspectors are required to undergo performance
reviews periodically, usually annually, to ensure continued
competency.
r Recertification of NDT personnel is usually required every three or
five years,
r One requirement for maintaining certification is performance of
magnetic particle inspections no less than every six months.

Magnetic Particle Testing 119

 Answers to Chapter 7 Questions
Question 7.1
Answer: D – There is no NDT certification to a Trainee level. Technicians
typically start out as Trainees before training and studying for Level I
qualifying examinations.

Question 7.2

Answer: C – The NDT Level III trains and tests personnel, although it is
the employer that technically certifies NDT technicians. After training
candidates, the Level III recommends certification to the employer when
the candidates meet all of the requirements of the written practice.

Question 7.3

Answer: B – NAS 410 is the National Aerospace Standard for certifying

NDT personnel.

Question 7.4

Answer: A – The written practice specifies certification requirements

specific to the company writing it.

Question 7.5

Answer: C – The individual would require 12 hours for Level I and 8 hours
for Level II, for a total of 20 hours.

Question 7.6

Answer: D – For Level II, a minimum of 280 hours are required: 70 hours
(Level I) + 210 hours (Level II).

120 Programmed Instruction Series

Question 7.7

Answer: C – Trainees are not experienced enough to train other NDT

personnel.

Question 7.8

Answer: A – The practical is used to test the candidate’s hands-on

examination/evaluation skills.

Question 7.9

Answer: B – An annual performance review may be conducted by the

supervising Level III, but not necessarily a self-evaluation.

Magnetic Particle Testing 121

 Chapter 7 Review
1. The certification process is not intended to:
A. make it difficult to certify candidates.
B. prepare NDT Level II personnel.
C. adequately train NDT personnel.
D. provide adequate experience for MT inspectors.

2. The requirements for certifying NDT personnel are derived from:

A. experienced Level II personnel.
B. the company president.
C. ASNT.
D. the certification specification applicable to the NDT inspection
being performed.

3. Performance reviews are typically required:

A. every six months.
B. annually.
C. every five years.
D. at the time of recertification.

4. Most NDT Level IIIs feel that the most critical part of the certification
process is:
A. the practical exam.
B. the specific exam.
C. training.
D. experience.

5. Level I personnel are usually limited to:

A. certifying other NDT personnel.
B. examination of a specific part.
C. performing NDT audits.
D. supervising NDT personnel.

6. The differences between NDT certification documents is due in

part to:
A. different experts writing them.
B. when they were written.
C. the industry using them.
D. the types of MT being performed.

122 Programmed Instruction Series

7. For an MT Level I certifying to SNT-TC-1A, the amount of experience
recommended in NDT is:
A. 20 hours.
B. 70 hours.
C. 70 workdays.
D. 130 hours.

8. SNT-TC-1A is a recommend practice. This means that:

A. the requirements outlined are recommended and can be modified
to suit the needs of the user.
B. it’s only recommend that you use this document.
C. it still requires that you meet all of its requirements.
D. it’s only recommended for certain industries.

Magnetic Particle Testing 123

 Chapter 7 Review Key
1. A

2. D

3. B

4. D

5. B

6. C

7. D

8. A

124 Programmed Instruction Series

 Chapter 8
Review of MT Fundamentals

In this chapter:
Review of magnetic principles and domain theory
Understanding magnetic flux lines in relation to poles
Uniformity of circular versus longitudinal fields
Determining magnetic field strength and direction
Use of hall effect meter, field indicator, and quantitative quality indicator
Direct versus indirect magnetization of test parts
Magnetic flux leakage as an attractor of ferromagnetic particles
Review of alternating, half-wave, and full-wave currents
Recognizing differences in permeability with hysteresis curves
Factors to consider when choosing an MT technique

125
 Understanding Magnetism
We have all been taught about magnetic poles. Here are three basic facts to
keep in mind about magnets:

• There are north and south poles in a magnet.

• Like poles repel and opposite poles attract each other, as shown in
Figure 8.1.
• Magnetic lines of flux travel from the north pole to the south pole
outside of a magnet.

Lines of magnetic flux

S N S N

Unlike poles - “attract”

S N N S

Like poles - “repel”

Figure 8.1: Magnetic fields between magnets based on poles. (Electronics Tutorials,
www.electronics-tutorials.ws.)

Question 8.1

Alike magnetic poles:

A. attract each other.
B. neither attract nor repel each other.
C. repel each other.
D. both repel and attract each other.

 Please turn to the end of the chapter for the answer.

126 Programmed Instruction Series

Magnetic Domains
As with all metal, ferromagnetic material is made up of atoms and
crystalline grains. Within each crystalline grain are regions called magnetic
domains. (See Figure 8.2.)

(a)

(b)

Figure 8.2: Magnetic domains: (a) unmagnetized; (b) magnetized.

The magnetization or magnetic moments within each domain are

magnetically aligned, but not all of the domains are aligned with one
another. When a magnetic force is applied to ferromagnetic material, the
domains align, which increases the strength of the material’s external
magnetic field and provides the ability to perform a magnetic particle
inspection. As more domains align, the magnetic field strength increases.
Note: Figure 8.2 is for illustrative purposes only. In reality, domain walls
shrink or grow depending on the magnetic force exerted.

This is not necessarily true for all ferromagnetic material. Each type and
grade of material is different with regard to the ease with which the
domains can be aligned. Materials that can be magnetized with ease
generally have domains that can be easily aligned as compared to materials
that cannot. Figure 8.3 shows magnetic domains within the grains of a
ferromagnetic material. The light and dark stripes are the magnetic
domains.

Magnetic Particle Testing 127

 Figure 8.3: Magnetic domains within ferromagnetic material.

Question 8.2

Magnetic domains, when ferromagnetic steel is magnetized:

A. always grow in size.
B. always shrink in size.
C. repel magnetism.
D. align with the other magnetic domains.

 Please turn to the end of the chapter for the answer.

Magnetic Flux and Poles

Magnetism is defined as the ability of any material to be attracted to a
magnetic force. The greater the amount of iron a material contains, the
stronger its attraction will be to a magnet. Thus, austenitic stainless steel,
although it contains iron, would be considered only paramagnetic (slight
magnetic attraction). Magnetic flux lines—also referred to as magnetic lines
of flux or even more simply as lines of flux—are the heart and soul of
magnetism. Without magnetic flux lines there would be no such force as
magnetism. They are found within and around all magnets.

128 Programmed Instruction Series

 S N

Figure 8.4: Lines of flux in a bar magnet.

Both permanent and electromagnets have magnetic flux lines. Although

most NDT experts discuss magnetic flux as something that moves or
travels, there is no scientific evidence proving that it does. The accepted
belief among scientists is that magnetic lines of flux are stationary and do
not travel. Nevertheless, it is helpful to imagine them as traveling so as to
define a direction. Although lines of flux cannot be detected visually, they
can be measured and do in fact exist. The lines of flux in a permanent bar
magnet are illustrated in Figure 8.4. Lines of flux seek the path of least
resistance and are more concentrated and closer to each other as they get
nearer the magnetic poles.

Figure 8.5 illustrates the magnetic lines of flux by showing a bar magnet
with iron particles sprinkled on and around it. The iron particles collect
and form a distinct pattern, which shows how the lines of flux are aligned.
As can be seen, the lines of magnetic flux do not cross each other. The
concentration or number of lines of flux and their distance from one
another are used to estimate the strength of the magnetic field.

Magnetic Particle Testing 129

 N S

Figure 8.5: Iron particles illustrating lines of flux.

Question 8.3

Which statement is true regarding magnetic lines of flux?

A. They are often forced to cross due to part geometry.
B. They are most dense at the poles of a magnet.
C. They seek the path of greatest resistance.
D. They cannot be detected by any known means.

 Please turn to the end of the chapter for the answer.

With magnetic lines of force, more lines that are closer together indicate a
stronger magnetic field. The type of flux pattern on the surface and within
a part depends on the type of current being used. With a longitudinal
magnetic field, the lines of force run parallel to the long axis of the part.
With a circular magnetic field, the lines of force run circumferentially
around the perimeter of the part. If a solid bar is magnetized using a head
shot, the circular magnetic field is maximized at the surface of the part
and weakens as it gets closer to the center of the part. However, the
opposite is true if a tubular part is magnetized with a central conductor. In
this case, the maximum strength of the field will be on the inside surface
of the tube and become progressively weaker as the outside surface is
approached.

130 Programmed Instruction Series

Question 8.4

The magnetic field strength increases when the lines of flux are:
A. farther from the magnet.
B. farther apart from one another.
C. closer together.
D. closer to the magnet.

 Please turn to the end of the chapter for the answer.

Magnetic Field Strength and Direction

Measuring the strength of the magnetic flux applied to or remaining in a
part is done for two reasons:

1. When a magnetic field is applied to a test piece, especially a part

with a complex geometry, the magnetic flux strength cannot be
easily determined. For these situations, measuring the flux is
essential in determining if the correct amperage is being used to
magnetize the part.
2. After completing the inspection, the operator needs to know if
there is a residual field still in the part and how strong it is. If it is
strong enough, the part must be demagnetized.

There are three main types of devices used to measure magnetic field
strength and/or direction:

• Hall effect meter – measures magnetizing field strength.

• Field indicator – may be used at any time during an MT inspection
but is typically used to detect the presence of residual fields.
• Quantitative quality indicator (QQI) – preferred method for
determining field strength in nuclear and aerospace sectors.

Magnetic Particle Testing 131

 Hall Effect Meter
The hall effect meter is used to
measure the magnetizing field
strength during a magnetic particle
inspection. The meter is used together
with a probe to measure the field
strength. To take a reading, the
operator places the probe close to the
current-carrying cable or material and
the meter measures the field strength.
Multiple readings can be taken to
ensure that the magnetic field is Figure 8.6: Hall effect meter and probe.
adequate throughout the inspection
area, especially in parts with a complex
geometry. A typical meter/probe setup is shown in Figure 8.6.

Ë From the Field: It’s better to take several gauss (tesla) readings
prior to an MT examination and know that you have adequate
magnetic field strength throughout the part than to take only
one or two readings and possibly have an area that did not
have a strong field, which could cause you to not find
discontinuities.

Question 8.5

A hall effect meter is used to:

A. measure the strength of a magnetic field.
B. determine the amperage of the magnetizing force current.
C. measure the iron content in steel.
D. measure the depth of a magnetic field.

 Please turn to the end of the chapter for the answer.

132 Programmed Instruction Series

Field Indicator
The most common method for
measuring residual fields, especially
for field inspections, is the field
indicator. This indicator is a small
handheld device that measures
residual fields after the inspection is
performed. To check for a residual
magnetic field, the indicator is
placed on and perpendicular to the
surface of the part. The test arrow
should be pointing toward the
Figure 8.7: Field indicator.
surface. If a uniform longitudinal
field was used for the inspection,
the indicator should be aligned parallel to the direction of the lines of flux.
Figure 8.7 shows a typical field indicator.

Ë From the Field: If you’re unsure of the direction of the flux,

rotate the indicator 180° and look for the maximum reading.
Once you find the maximum reading, turn the indicator
another 180°. You should now get close to the same reading
that was indicated when you began. (Note: The reading will
not be exact because the sensor is under the brass needle
pivot and the distance from the part changes with the 180°
rotation.) The maximum reading indicates where the field is
perpendicular to the part’s surface. Once you have determined
the direction of the field, you can accurately measure the
strength of the residual field and demagnetize the part more
effectively.

Question 8.6

A field indicator measures the:

A. degree of rotation of a magnetic field.
B. strength of a magnetic pole.
C. amount of energy required to magnetize metal.
D. strength of a magnetic field including a residual field.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 133

 Quantitative Quality Indicator (QQI)
With wet horizontal testing equipment, a suitable reference standard with
known discontinuities should be used to ensure adequate field intensity at
the midpoint of the test object. A quantitative quality indicator (QQI)
serves such a purpose. Essentially, a QQI is a thin 0.002 or 0.004 in.
(0.05 or 0.1 mm) strip of AISI 1005 steel nominally 0.75 in.2 (4.8 mm2).
(See Figure 8.8.)

The QQI contains an artificial discontinuity that may be circular or linear

and is defined in terms of percent of total QQI thickness. It is held in
intimate contact with a test object’s surface during active magnetization.
QQIs can be placed in the area of interest to verify that the proper
magnitude and direction of magnetic induction have been obtained for
testing.

Figure 8.8: Quantitative quality indicator placed on curved surface of alloy steel part.

Flexible Laminated Strip

Flexible laminated strips, also known as slotted strips, are pieces of highly
permeable ferromagnetic material with slots of different widths. (See
Figure 8.9.) These strips are typically used to ensure proper field direction
and, to a limited extent, the adequacy of field intensity during magnetic
particle examination in general-purpose and aerospace applications.
They are placed on areas of interest on the test object as it is inspected.

134 Programmed Instruction Series

The strips contain three longitudinal slots in the center steel layer, and
the longitudinal axis of the strip should be placed perpendicular to the
direction of the magnetic field of interest to generate the strongest particle
indications on the strip. However, they do not measure the internal field
intensity of the test object due to multiple air gaps at different orientations.

(a)

(b)

Figure 8.9: Flexible laminated strip: (a) diagram; (b) photograph under ultraviolet radiation.

Magnetic Fields
For magnetic particle inspection, a magnetic field is created in one of
two ways: either with permanent magnets or electromagnets. The most
widely used method of creating magnetic fields for magnetic particle
testing is with electricity pulsed through an electromagnet. Compared
to a permanent magnet, an electromagnet is by far the most common
source of a magnetic field used for MT inspection. Electromagnets create
magnetic fields that travel in one of two directions: circular or
longitudinal. In addition, magnetic fields may be classified as direct or
indirect, depending on how they are produced.

Direct Magnetization
Magnetic fields created using the direct magnetization technique are a
product of passing electricity directly into a part or material that is being
tested using a head shot or prods. Most inspection codes require the
application of particles while the current is on. This is referred to as the

Magnetic Particle Testing 135

 continuous method of particle application. With the residual method, the
residual magnetic field needs to be strong enough or the test object
retentive enough to hold the ferromagnetic particles when the current is
switched off. Test objects with low retentivity must be tested using the
continuous method. In addition, the continuous method offers the greater
sensitivity for detecting discontinuities.

A direct magnetic field can be created using either direct current (DC) or
alternating current (AC). Direct current (or rectified alternating current)
is used when subsurface discontinuities need to be identified. Alternating
current is generally limited to detecting surface-breaking and near-surface
discontinuities. (See Figure 8.10.)

Magnetic field

Electric
current

Figure 8.10: Magnetic field produced by direct magnetization.

Indirect Magnetization
Indirect magnetic fields are much different from direct fields. This type
of magnetic field is created by continually passing current through a
conductor, such as a yoke or coil, which is not in contact with the part or
material being tested. The magnetic field created from the passing current
is then applied to the part, or the part may be placed within a magnetic
field using a central conductor, which magnetizes the part.

136 Programmed Instruction Series

Question 8.7

The process of creating a magnetic field by passing electricty into a part is

referred to as:
A. indirect magnetization.
B. yoke magnetization.
C. direct magnetization.
D. coil magnetization.

 Please turn to the end of the chapter for the answer.

Magnetic Flux and Discontinuities

Now that we have talked about magnetic flux properties and how to
measure magnetic flux field strength and residual flux, let’s discuss how
flux is related to discontinuity indications. When there is a discontinuity
located in a part and a magnetic field is applied, the flux traveling across
or in the part encounters this disruption in the material. This disruption
or discontinuity in the material creates a flux leakage field.

This is the same as saying that north and south poles are created, which is
exactly what is happening. The discontinuity has become a small magnet
capable of attracting small iron particles. As we said earlier, magnetic flux
prefers to travel the path of least resistance. A discontinuity equals
resistance, and resistance causes flux leakage fields to be created at the new
poles. The strength of these leakage fields depends on the type and
strength of the current used. If the correct current is used and the leakage
field is strong enough, the discontinuity will attract the ferromagnetic
particles. (See Figure 8.11.)

As the particles collect at the poles of the discontinuity, they begin to build
a bridge over it, which eventually lowers the resistance for the flux field
and stops the further collection of ferromagnetic particles. It’s kind of a
balancing act of flux strength. Initially there is a strong enough flux field
to attract the particles, but as the particle bridge grows, the flux field at the
surface of the particle accumulation weakens and prevents more particles
from collecting in this area. The amount of particles that can be attracted
to one discontinuity depends on the strength of the leakage field and the
size of the discontinuity.

Magnetic Particle Testing 137

 Magnetic particles Magnetic field lines

N S N S

Crack

Figure 8.11: Ferromagnetic particles attracted by flux leakage field. (Image courtesy of
The National Board of Boiler and Pressure Vessel Inspectors.)

Question 8.8

A magnetic leakage field is defined as:

A. the amount of residual magnetism remaining after the
examination.
B. where the lines of flux exit a discontinuity and form poles, and
where ferromagnetic particles are attracted.
C. where the electrical current exits the material.
D. where the electrical current enters the material.

 Please turn to the end of the chapter for the answer.

Electrical Currents and Electromagnets

Two types of electrical current are used in magnetic particle testing:
alternating current (AC) and rectified alternating current. When magnetic
particle testing was first being developed, direct current was used much
more than alternating current. It was believed that true direct current—for
example, current supplied by a battery—was the best current option for
finding discontinuities.

This concept was challenged over the years, and we now know that
alternating current, or rectified alternating current, works best. As
technology has improved, information regarding electrical currents and
magnetic particle inspection has evolved, which has led to many different
types of test equipment and electrical currents to choose from. Currently,

138 Programmed Instruction Series

almost all equipment used for magnetic particle testing is either
alternating current or rectified alternating current that uses an alternating
current source. Here are the four main types of current used for magnetic
particle inspection and important characteristics regarding each one.

Alternating Current
Characteristics of alternating current, shown in Figure 8.12, include:

• Readily available power source (120 V to 440 V at 60 Hz or 50 Hz).

• Flux density is concentrated at the surface (skin effect).
• Limited penetrating ability for detecting subsurface discontinuities.
• Creates a pulsing effect (good for dry particle mobility).
• Excellent power source for portable equipment.
• Good for demagnetization of parts.
• Residual fields are easily demagnetized.

Voltage One cycle One cycle

VPEAK

ZERO Time

–VPEAK

Figure 8.12: Alternating current.

Half-Wave Current
Characteristics of half-wave current, shown in Figure 8.13, include:

• Uses a single-phase AC source.

• Blocks the reversing current.
• Varying current.
• Good penetrating power.
• Creates a pulsing effect (good for dry particle mobility).
• Good power source for portable equipment.
• Cannot be used for demagnetization.
• Part must be demagnetized using full-wave current.

Magnetic Particle Testing 139

 Input voltage Half-wave rectification
VPEAK

Time (t)
Half-wave rectifer ZERO

–VPEAK

Input Output Output voltage

VPEAK

ZERO
Time (t)

Figure 8.13: Half-wave current.

Single-Phase Full-Wave Current

Characteristics of single-phase full-wave current, shown in Figure 8.14,
include:

• Uses a single-phase AC source.

• Inverts the reversing current.
• Varying current.
• Good penetrating power.
• Does not create a significant pulsing effect.
• Not typically used for portable equipment.
• Requires more power input than the other currents do.
• Excellent for demagnetization of parts.
• Residual fields are easily demagnetized.

Input voltage
Full-wave rectification
VPEAK
Full-wave rectifer
Time (t)
ZERO

–VPEAK
Input

Output Output voltage

VPEAK

ZERO
Time (t)

Figure 8.14: Single-phase full-wave current.

140 Programmed Instruction Series

Three-Phase Full-Wave Current
Characteristics of three-phase full-wave current, shown in Figure 8.15,
include:

• Uses a three-phase AC source.

• Inverts all three reversing current phases.
• Good penetrating power.
• Does not create a significant pulsing effect.
• Not typically used for portable equipment.
• Excellent for demagnetization of parts.
• Residual fields are easily demagnetized.

1 2 3

TIME
Resultant CD waveform

Figure 8.15: Three-phase full-wave current.

Question 8.9

Which of the following power sources is limited in its ability to detect

subsurface discontinuities?
A. Half-wave.
B. Three-phase full-wave.
C. AC.
D. Single-phase full-wave.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 141

 Magnetic Permeability
Permeability refers to the ease with which a specific ferromagnetic
material can be magnetized when an electrical current or magnetic field
is induced into it. Some grades of steel are easier to magnetize than
others. Typically, materials with lower percentages of carbon are much
easier to magnetize and are said to have higher permeability. Those with a
higher percentage of carbon are harder to magnetize and are described as
having lower permeability.

Hysteresis Curves

Differences in magnetic permeability can be illustrated using a hysteresis

curve. (See Figure 8.16.) A hysteresis curve shows several magnetic
characteristics of ferromagnetic material. The main characteristics that a
hysteresis curve shows for a specific material are:

1. The force it takes to achieve the magnetization saturation point.

2. The amount of residual magnetization retained.
3. The force required to remove a residual field.

B Flux density Saturation

Retentivity

Coercivity

–H H
Magnetizing force Magnetizing
in opposite force
direction

Saturation
in opposite Flux density
direction in opposite
–B
direction

Figure 8.16: Hysteresis curve.

A hysteresis curve graphically depicts the relationship between the

magnetizing force (X axis) and the resultant magnetic flux (Y axis)
produced and is generally used to compare materials with high versus low

142 Programmed Instruction Series

permeability. The X axis is the magnetizing field strength of magnetic
force (H). The two points at which the curve intersects the X axis is the
coercive force. The Y axis is the flux density or magnetic flux (B). The two
points at which the curve intersects the Y axis are the residual magnetism.

All ferromagnetic materials have their own unique hysteresis curve.

Figure 8.17 illustrates hysteresis curves for two different materials. The
first, curve (a), represents hardened steel with high carbon content, a
material with low permeability, high reluctance, high retentivity, and high
residual magnetism that requires high coercive force for removal. The
second, curve (b), represents annealed steel with low carbon content, a
material with high permeability, low reluctance, low retentivity, and low
residual magnetism that requires a low coercive force for removal.

B+ B+

Residual
magnetism Residual
magnetism
Flux density

Flux density

H– H+ H– H+

Coercive
force
Coercive
force
(a) B- (b) B-
Magnetizing force Magnetizing force

Figure 8.17: Hysteresis curves for two different materials: (a) high carbon content;
(b) low carbon content.

Where the two curves meet at the end of the dashed line in the positive
quadrant in (b) is the saturation point, the point at which a material
becomes magnetically saturated. In (a), the end of the dashed line in the
negative quadrant is the reverse saturation point.

In addition to showing the coercive force, the hysteresis curve also shows
the ease with which these materials can be demagnetized. For example, in
Figure 8.17, the material in curve (b) would have less residual magnetism
than the material in curve (a), making it easier to demagnetize. Just as
some materials are easier than others to magnetize, the same is true with
regard to their demagnetizing characteristics. For magnetic particle

Magnetic Particle Testing 143

 inspection, the optimal materials are those that can be easily magnetized
and demagnetized. As mentioned earlier, time is money; therefore, the
harder it is to magnetize and/or demagnetize the material being tested, the
longer the inspection will take to complete.

For the technician, understanding permeability can be very helpful when

determining how much amperage to use for an inspection. However, this
concept is applicable only when the magnetic particle technique allows the
technician to control the amount of current that will be used, such as with a
head shot or when using a prod unit. The current of even a portable AC
yoke or coil can be adjusted. Thus, if a technician has a basic understanding
of magnetic permeability and knows the permeability of the material prior
to performing an inspection, he or she can better choose a more appropriate
starting current to use from the current range calculated using the formula
allowed by the inspection specification or procedure in use.

For instance, if a current range between 800 A to 2200 A may be used, and
a highly permeable material is being inspected, the technician may opt for
the lower end of the current range, perhaps 900 A or 1000 A. Conversely,
the technician may choose from the higher end, 1600 A or 1800 A, if the
material has a lower permeability, to achieve a similar magnetic field
strength in either of these materials. Of course, as with all MT inspections,
the field must be verified with the appropriate measuring device.

Question 8.10

Compared to materials with high carbon content, steel with low carbon
content would:
A. be harder to magnetize.
B. be easier to magnetize and demagnetize.
C. have lower permeability.
D. have more residual magnetism.

 Please turn to the end of the chapter for the answer.

144 Programmed Instruction Series

Choosing the Correct MT Technique
To determine which MT technique would work best for a specific part or
assembly, an engineer or technician must consider some important factors,
including:

• The type of material being inspected – Knowing the type of material—

specification, grade, processing (rolled, forged, cast, and/or welded), and
so on—is important because the discontinuities associated with each
process used to make and form steel can be predicted. For instance, if it’s
important to locate internal discontinuities, we know that the MT
technique must be capable of penetrating below the surface, which
would require that we use direct-current techniques, such as a head
shot, prods, or coil. Alternating-current techniques are only effective for
locating surface discontinuities.
• The intended use of a part – Depending on how a part or material
is used, some discontinuities may not be considered detrimental to
its inservice integrity; therefore, certain discontinuities might not
need to be located. For example, if an internal discontinuity is not
considered detrimental, then only those discontinuities on the
surface would need to be found. In this case, an AC yoke or coil
could be used.
• Material processing prior to inspection – Processing, such as
machining, rolling, forging, welding, or bending, can induce very
specific discontinuities. For instance, if only welding discontinuities
are of concern to the engineer or technician, he or she would have to
consider the welding process used, the type of weld joint used, and
other welding-related factors to determine which types of
discontinuities are likely to have been introduced during the welding
process. If lack of fusion is possible, the MT technique would need to
be able to locate discontinuities below the surface.
• Test location – Where the inspection will take place can determine
which technique must be used. If the inspection can only be
performed in the field, it would be impossible to inspect using a
head shot, as a stationary bench unit is not very portable. The
majority of field inspections would be performed using either an
AC yoke or coil, or a rectified-AC prod unit.

Magnetic Particle Testing 145

 • Type of electrical current and equipment available – Because the
equipment for each MT technique has different electrical current
requirements, it’s important to know what types of equipment and
electrical current are available when choosing an MT technique. If
only an AC yoke is available, calling for a prod inspection would not
be practical. For example, prods can be plugged into a portable
power unit supplying AC.

Understanding the fundamentals of magnetism and electromagnets is very

important to ensure that an adequate magnetic particle inspection is
performed using a technique that provides the best chance that the
intended discontinuities, when present, are identified so that appropriate
corrective actions can be taken.

Question 8.11

Which factor would probably be most pertinent when deciding whether

to use a head shot with a bench unit?
A. The location of the test.
B. Type of material to be inspected.
C. Material processing of the part prior to an inspection.
D. The type of discontinuities being sought.

 Please turn to the end of the chapter for the answer.

146 Programmed Instruction Series

Chapter 8 Summary

r With magnets, like poles repel and opposite poles attract;

magnetic lines of flux travel from the north to south pole.
r Ferromagnetic material is made up of crystalline grain regions
called magnetic domains that align when a magnetic force is
applied.
r Both permanent and electromagnets have magnetic flux lines that
are more concentrated and closer to each other near the magnetic
poles.
r Lines of magnetic flux seek the path of least resistance and do not
cross each other.
r In longitudinal fields, the lines of force run parallel to the long axis
of the part; in circular fields, they run circumferentially around the
perimeter of the part.
r Measuring the strength of the magnetic flux in a part is done to
determine the correct amperage for magnetization and whether
demagnetization is needed.
r Devices used to measure magnetic field strength and/or direction
include hall effect meters, field indicators, and quantitative quality
indicators (QQIs).
r Magnetization of parts may be categorized as direct or indirect,
depending on how electricity is used to induce magnetism.
r The disruption caused by a discontinuity in the test material
creates a flux leakage field that will attract ferromagnetic particles
if strong enough.
r Two types of electrical current are used in magnetic particle
testing: alternating current (AC) and rectified alternating current.
r Alternating current is limited in its ability to detect subsurface
discontinuities unless rectified as half-wave or full-wave current.
r Permeability is the ease with which a ferromagnetic material can
be magnetized when an electrical current or magnetic field is
induced into it.
r Materials with lower percentages of carbon are easier to
magnetize (have higher permeability) than those with a higher
percentage of carbon.
r A hysteresis curve shows several magnetic characteristics of
ferromagnetic material including the saturation point at which full
magnetism is reached.

Magnetic Particle Testing 147

 r Hysteresis curves graphically depict the relationship between the
magnetizing force (X axis) and the resultant magnetic flux (Y axis)
produced.
r Several factors are considered when choosing an MT technique,
including the type of material inspected, the test location, and the
type of current available.

148 Programmed Instruction Series

Answers to Chapter 8 Questions
Question 8.1
Answer: C – Like magnetic poles (north-north or south-south) always repel
each other.

Question 8.2

Answer: D – Magnetic domains align whenever ferromagnetic materials

are magnetized.

Question 8.3

Answer: B – In addition, magnetic lines of flux do not cross each other and
seek the path of least resistance.

Question 8.4

Answer: C – When the lines of flux are closer together, the magnetic field
would be stronger.

Question 8.5

Answer: A – Hall effect meters measure the magnetic field strength.

Question 8.6

Answer: D – Residual fields can be measured using a field indicator.

Question 8.7

Answer: C – Direct MT techniques, for example, using a head shot or

prods, pass electricity directly into the part.

Magnetic Particle Testing 149

 Question 8.8

Answer: B – A leakage field is where the lines of flux exit a discontinuity

and where ferromagnetic particles are attracted to the material.

Question 8.9

Answer: C – All three types of rectified AC can locate subsurface

discontinuities, whereas AC is limited to surface and near-surface
discontinuities.

Question 8.10

Answer: B – Materials with lower carbon content are easier to magnetize

and demagnetize and are thus described as having high permeability and
less residual magnetism.

Question 8.11

Answer: A – For example, a bench unit would be unsuitable for a field test.

150 Programmed Instruction Series

Chapter 8 Review
1. Which of the following statements is true about magnetic lines
of flux?
A. They travel from the south pole to the north pole.
B. Lines of flux travel both ways.
C. They travel from the north pole to the south pole.
D. Lines of flux do not travel.

2. Which of the following statements about magnetic domains is true?

A. As more magnetic domains become aligned, the magnetic field
strengthens.
B. As fewer magnetic domains become aligned, the magnetic field
strengthens.
C. As more magnetic domains become aligned, the magnetic field
weakens.
D. Magnetic domains do not affect the strength of a magnetic field.
3. The main benefit to knowing what processes preceded the
MT examination is:
A. there is no benefit in knowing this.
B. so the inspector knows how to clean a part.
C. so the inspector knows where the part goes following the
examination.
D. to help the inspector understand what type of discontinuities to
look out for.
4. Which of the following is true regarding half-wave direct current?
A. It does not create a pulsing effect.
B. It uses a single-phase AC source.
C. It uses three-phased AC source.
D. It is commonly used for demagnetization.

5. Which of the following is true regarding direct, as opposed to indirect,

magnetization of a part?
A. It uses a central conductor.
B. It can be created only with direct current.
C. It passes magnetism directly into a part.
D. It passes electricity directly into a part.

Magnetic Particle Testing 151

 6. A hysteresis curve illustrates:
A. the gauss (tesla) level of a magnetic field for a specific material.
B. how long it will take to perform an MT examination.
C. whether a specific material will be easy or hard to magnetize.
D. if you should use an AC or DC power source.

7. When using a hall effect meter, how many readings should be taken in
the area where the examination is being performed?
A. One.
B. Five.
C. 10.
D. Enough to verify that the entire part will be exposed to an
adequate magnetic field strength.

8. Which of the following statements about AC is true?

A. AC has excellent penetrating power.
B. AC produces a pulsing effect.
C. AC limits MT equipment portability.
D. AC creates strong residual magnetic fields.

9. The permeability of a ferromagnetic material refers to:

A. how easy it will be to magnetize a specific material.
B. how easy it is to demagnetize the material.
C. whether the material will retain high levels of residual magnetism.
D. whether the material will retain low levels of residual magnetism.

10. A bar magnet with a surface-breaking discontinuity would have

how many poles?
A. 1
B. 2
C. 3
D. 4

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Chapter 8 Review Key
1. C

2. A

3. D

4. B

5. D

6. C

7. D

8. B

9. A

10. D

Magnetic Particle Testing 153

 Chapter 9
Characteristics of
Electromagnetic Fields

N S

In this chapter:
Characteristics of circular and longitudinal magnetic fields
Orientation of discontinuities for maximum detectability
Use of a head shot for creating a circular magnetic field
Central conductor technique for inspecting hollow parts
Use of prods for portable weld inspections with a circular field
Amperage calculations for prod spacing
Longitudinal fields created with yokes, coils, and wrapped cables
Relation between number of cable wraps and strength of a magnetic field
Calculating the amount of current needed for a cable-wrapped coil
Definition of fill factor of test objects positioned within a coil

155
 Types of Magnetic Fields
Electromagnetic fields created by a yoke, prod unit, coil, horizontal
bench, or other equipment travel in one of two directions: circular or
longitudinal. This is important to know because you will need to ensure
that the entire surface of the area being inspected, or complete coverage, is
achieved. Complete coverage refers to finding all of the discontinuities,
regardless of their orientation, within the material being inspected.

Question 9.1
Magnetic fields used in magnetic particle testing travel in two directions:
A. longitudinal and horizontal.
B. horizontal and circular.
C. circular and reversing.
D. circular and longitudinal.

 Please turn to the end of the chapter for the answer.

As we learned previously, discontinuities must be oriented perpendicular,

or at no more than 45°, to the direction of the magnetic field. If they are
not, it’s unlikely that you will find them. To ensure complete coverage of
the area being inspected, two magnetic fields must be introduced into the
material, the second one perpendicular to the first. There are two ways to
achieve this:

1. First inspect using a circular field; then inspect using a

longitudinal field.
2. Inspect using only a longitudinal field, but the second inspection is
performed with the part oriented 90° to the first.

For new inspectors, this is one of the most important fundamentals

that must be understood in order to perform a good magnetic particle
inspection. Knowing the direction of the magnetic field being produced
by a given technique is crucial. If an inspector does not fully recognize
the direction of the field being produced during the inspection, the
discontinuities being sought may not be identified at all. This could lead
to some parts being accepted and released for use, which in fact have
discontinuities.

156 Programmed Instruction Series

 Ë From the Field: When magnetizing using both circular and
longitudinal fields, always perform the first examination with
the circular field and then with the longitudinal field. Not only
are circular fields difficult to accurately measure, they are
difficult to remove. The longitudinal field will eliminate the
circular field and make it much easier to demagnetize the part
being examined.

Question 9.2
In order for an inspector to locate discontinuities, they must be oriented:
A. parallel to the magnetic field.
B. 45° to 90° to the magnetic field.
C. only perpendicular to the magnetic field.
D. greater than 90° to the magnetic field.

 Please turn to the end of the chapter for the answer.

Circular Magnetic Field

Circular fields may be created by the direct or indirect technique. Direct
magnetization means that an electrical current is applied directly to the
part being inspected using a head shot or prods, which in turn creates a
circular magnetic field. When a central conductor is used for creating a
circular field, it's considered to be an indirect technique. Circular fields
always travel 90° to the current flow. For instance, if current is passed
through a steel bar, a magnetic field forms on the outside of the bar and
travels around its circumference. (See Figure 9.1.)

A circular field is usually the product of rectified alternating current but

may also be produced using alternating current. When using rectified
alternating current, the circular field is excellent for locating both surface
and subsurface discontinuities. Many factors determine the depth of a
subsurface discontinuity that can be located using a half-wave or full-wave
circular field, such as the type of material, the diameter or width of the test
object, the geometry of the part, and the type and amount of direct
current used to create the magnetic field.

Magnetic Particle Testing 157

 Current

Magnetic field

Figure 9.1: Current inducing a magnetic field around a steel bar.

Circular magnetic fields can be created using any one of the following
three techniques:

1. Head shot/contact plates.

2. Central conductor.
3. Prods.

Question 9.3

When a circular field is created using rectified alternating current, the

magnetic field:
A. can only detect surface discontinuities.
B. can only detect subsurface discontinuities.
C. can detect surface and subsurface discontinuities.
D. is nullified.

 Please turn to the end of the chapter for the answer.

Head Shot/Contact Plates

A head shot refers to a technique for creating circular fields using a
stationary wet bench unit. A head shot induces a circular field by passing
electrical current, typically alternating or rectified alternating current,
through the part being inspected. (See Figure 9.2.) True direct current
would not be practical with a stationary unit and is not typically an option
with MT in general.

158 Programmed Instruction Series

 Magnetic field

Zero field
strength Maximum
field
strength

Figure 9.2: Head shot with wet bench Figure 9.3: Magnetic field strength in cross sec-
unit. tion of solid bar.

When a head shot is used to create circular magnetic fields, the strength of
the magnetic field along the length of the part remains the same; it does
not change or diminish regardless of how long the part is. However, the
strength of the magnetic field through the cross section of a part does
change. As a general rule of thumb, the strength of a circular magnetic
field in a solid round bar is zero at its center and strongest on the outside
surface, as shown in Figure 9.3.

This is only true when inspecting with rectified alternating current. For
AC circular fields, the field strength is concentrated on the surface of the
part, but does not penetrate into the part.

Question 9.4

Circular fields created using a head shot typically do not utilize:

A. rectified alternating current.
B. alternating current.
C. true direct current.
D. half-wave alternating current.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 159

 Central Conductor
To inspect the inside and
outside of hollow parts
adequately, a central conductor (a)
may be used, as shown in
Figure 9.4. This is an especially
good technique for inspection
of the inside of a hollow part,
but it is not always used for this
purpose. Sometimes the inside
of a part is not required to be (b)
inspected, but a central
conductor may still be used if Figure 9.4: Central conductor method used to
there is a concern about arc produce circular magnetization: (a) several ring-
strikes damaging the surface shaped test objects magnetized simultaneously;
(b) closeup of ring with cracks in several loca-
where the clamping pads tions and orientations.
contact the ends of a part.

Question 9.5

A head shot would be used without a central conductor when the part
being examined is:
A. hollow.
B. greater than 6 in. (150 mm) in diameter.
C. solid.
D. a hollow part that has been fine machined.

 Please turn to the end of the chapter for the answer.

The central conductor is placed through the hollow area of the part and
then clamped between the heads of the horizontal bench unit. The central
conductor carries the current and a circular field is created on its outside
surface. Placement of the central conductor depends on a few factors:

• Inside diameter of the part.

• Surface of the part being inspected.
• Wall thickness.

160 Programmed Instruction Series

Depending on where the central conductor is located on the inside of the
hollow part, the field strength will differ on both the inside and outside
surfaces. Figure 9.5 illustrates how the magnetic field strength changes
when a central conductor is used.

Magnetic field

Test
part

Weaker Maximum
field field
strength strength

Central
conductor

Figure 9.5: Magnetic field strength when using a central conductor.

Question 9.6

The direction of a magnetic field that is created using a central conductor is:
A. longitudinal.
B. horizontal.
C. circular.
D. reversing.

 Please turn to the end of the chapter for the answer.

Centering the central conductor works for small-diameter central

conductors and hollow parts. If centered, the whole part is magnetized.
When the central conductor is small compared to the diameter of the
hollow part, it must be moved closer to the inside edge of the part. If
offset, the magnetized area is limited to 4× the diameter of the conductor

Magnetic Particle Testing 161

 on each side, meaning more than one shot to inspect the entire surface of
the part will likely be needed. Figure 9.6 shows how the central conductor
would be placed on the inside of the part.

To ensure that 100% of the

circumference of this part is Effective magnetic field
properly inspected, the part is
1
rotated around the central
conductor until all three zones
Zero
are tested. The number of
times that the part must be
inspected is dependent on the
outside diameter of the central
conductor and the inside 2 3
diameter of the hollow part. The
closer they are in size to one
another, the fewer the number of
inspections that will be needed.
Your procedure should provide a
formula for calculating the Figure 9.6: Central conductor placement.
effective magnetic field when
using a central conductor.

Question 9.7

The size of the central conductor versus the size of the hollow part
determines:
A. the distance of the effective magnetic field on either side of the
central conductor.
B. the direction of the magnetic field.
C. the sensitivity of the examination.
D. where to place the central conductor.

 Please turn to the end of the chapter for the answer.

Prods
A third option for creating a circular magnetic field is the prod technique.
A prod unit is used for weld inspections and for inspecting large areas of
material, such as a plate, with a circular field. A prod unit can be portable

162 Programmed Instruction Series

but is mainly used in shops. Some smaller prod units can be used in the
field if a suitable power source is available. If a circular-field inspection is
required but the size of the part prevents it from being inspected in a
stationary unit, the prod unit is a good alternative. The prod unit has a
control function that allows the technician to adjust the amount of
amperage used. It also has two cables—one negative and the other
positive—each having a handle with a copper prod attached to it.
Figure 9.7 shows a portable prod unit and a set of prods. A mobile
unit is shown in Figure 9.8.

Figure 9.7: Handheld prods for magnetic particle testing of a

casting.

Top storage On/off switch

Current value selector

Circuit breaker

Current output light

Amperage meter

Current selector
switch
Output power
indicator light
110 V output
socket

Ground output lug

110 V output
socket Direct current
output lug
Alternating current
output lug

Figure 9.8: Mobile unit for use with prods.

Magnetic Particle Testing 163

 Question 9.8

Prods are primarily used to inspect:

A. hollow parts when a central conductor is not available.
B. welds using longitudinal magnetization.
C. large test objects in stationary units.
D. welds using circular magnetization.

 Please turn to the end of the chapter for the answer.

The amperage used for each inspection is determined by the spacing of

the two prods and the thickness of the part being inspected. With prods,
the circular magnetization strength is proportional to the amperage used
but varies with prod spacing and the thickness of the section being tested.
It is recommended that a magnetizing current of 90 to 110 A for each 1 in.
(25 mm) of prod spacing should be used for material less than 0.75 in.
(19 mm) thick. Per ASTM E 1444, Standard Practice for Magnetic Particle
Testing, prods can be adjusted from 100 to 125 A/in. on material over
0.75 in. (19 mm) thickness. Based on this range, a prod inspection using
a 6 in. (152 mm) prod spacing would use anywhere from 600 A to 750 A.

The prod spacing is Magnetizing

Switch
also used to calculate current
the effective width of
the inspection zone.
Figure 9.9 shows a
typical setup for a
weld inspection using
prods. Since prod
amperage can be
adjusted and prods Weld Magnetic lines of force
can be spaced less Figure 9.9: Typical prod setup for weld inspection.
than 6 in. (150 mm)
apart, the effective area
of an inspection will vary.

Prod spacing less than 3 in. (76 mm) is usually not practical because the
particles tend to band around the prods, making interpretation difficult.
When the area of examination exceeds a width of one-quarter of the prod

164 Programmed Instruction Series

spacing, measured from a centerline connecting the prod centers, the
magnetic field intensity should be verified at the edges of the area being
examined.

Prolonged energizing cycles may cause undesirable localized overheating.

The main drawback of the prod unit is that where the prods touch the
material, arc strikes can occur. Arc strikes can ruin some parts/materials,
depending on their intended use.

Question 9.9
The amperage used when using the prods is based on:
A. type and grade of material.
B. diameter of prods.
C. length of prod leads.
D. thickness of material and prod spacing.

 Please turn to the end of the chapter for the answer.

Longitudinal Magnetic Field

Longitudinal magnetic fields differ from circular fields in terms of the
direction they travel. Unlike circular fields, longitudinal fields travel along
the long axis of a part from end to end with some loss of magnetic flux.

Longitudinal fields are created using an indirect technique, which means

that a magnetic field—not an electrical current—is induced into the part.
This alleviates the possibility of arc strikes on the inspection surface and
allows for much more mobile equipment to be used to complete the
inspection. Three main types of equipment are used to create longitudinal
fields:

• Yokes.
• Coils.
• Wrapped cables.

Magnetic Particle Testing 165

 Yoke
A yoke is a portable, handheld inspection device that uses either AC or
DC to create a magnetic field; however, the majority of yokes do not have
DC capability, only AC. The longitudinal field is created when current
travels through a solenoid that is located in the handle of the yoke. (See
Figure 9.10.) Note: A solenoid is a cylindrical coil of wire acting as a
magnet when carrying electric current.

Solenoid Magnetic particles

collect at crack

Test object

Magnetic lines
of force

Figure 9.10: Schematic of a yoke.

Regardless of how current travels, there is always a magnetic field created

perpendicular to the direction of the current, even if it is not being used
for magnetic particle inspection purposes. In fact, a typical shop extension
cord creates a slight magnetic field as current travels through it. When
using a yoke, the magnetic field created by the solenoid travels
longitudinally from one leg (north pole) to the other (south pole) of the
yoke, but don’t confuse the prod and yoke.

A prod unit induces current into a part, whereas a yoke induces a

magnetic field. Depending on the type of current used, AC or DC, the
longitudinal field may or may not penetrate below the surface of the part
being inspected. As with an AC magnetic field, only surface inspections
are possible. DC yokes can penetrate slightly below the surface but not
nearly as deep as the fields created by the stationary bench or prod unit.
This is because the amount of current used with those techniques is much
greater than a DC-capable yoke.

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Coil
A coil can comprise either cables looped into a circle within a protective
shell, available in sizes as large as 24 in. (610 mm) in diameter, or the
cables from a prod unit wrapped loosely around the part being inspected.
Let’s take a look at the first type.

The usual way to longitudinally magnetize a test object is by placing the

test object in a rigid coil on a stationary magnetic particle testing unit, as
shown in Figure 9.11. The test object may be laid on the bottom inside of
the coil where the field is strongest, or the test object may be supported in
the coil by the contact heads of the unit. Special supports are provided on
some testing units for long, heavy test objects, permitting rotation of the
objects for testing. Coils are usually mounted on rails, permitting
movement along a long test object for multiple tests (multiple coil shots).

Figure 9.11: Bench-type magnetizing unit with coil.

Magnetic Particle Testing 167

 Question 9.10

What type of MT equipment uses a coil of wire in its handle to induce a

magnetic field into a part?
A. Prods.
B. Yoke.
C. Head shot.
D. Coil.

 Please turn to the end of the chapter for the answer.

Wrapped Cables
Cable wrapping a coil around large or heavy test objects is a common
practice. Flexible, insulated copper cable is used. A cable-wrapped coil is
connected to a magnetic particle mobile machine or portable power pack,
or it can be connected to the contact heads of a stationary test unit. The
type of power source depends on the kind of current and amperages
needed to perform a particular test, both magnetizing and demagnetizing.

For either of these techniques, the part being inspected is placed inside of
the coil during the inspection. The diameter of a wrapped cable coil is
dependent on the size of the part being inspected, which can vary, and the
length of the cables being used for the inspection.

A coil is typically made using five cable wraps, but in some cases there
may be as few as three and as many as seven, depending on the strength of
the field desired. Although the amount of current used plays a role in the
strength of the magnetic field, the total number of wrapped cables is
important too. More wraps equal a stronger magnetic field regardless of
the amount of current used. For instance, a three-wrap coil using 1000 A
would create a weaker magnetic field than a five-wrap coil. The cable
wraps act like the solenoid in the yoke. As current travels through the
cables, a longitudinal magnetic field is created by the coil, which travels
perpendicular to the direction of the current. (See Figure 9.12.)

The coil is an excellent choice for completing the second, opposite-

direction inspection after performing a head shot in a stationary bench
unit. It can also be used to perform both of the inspections required to get

168 Programmed Instruction Series

 Magnetizing current

Crack

Coil
and
crac
k are
para
llel

Coil

Figure 9.12: Flow of magnetic current in a coil.

complete coverage in order to locate discontinuities oriented in any

direction.

Question 9.11

Regardless of the amperage setting, adding more coil wraps:

A. decreases magnetic field strength.
B. increases magnetic field strength.
C. has no effect on magnetic field strength.
D. changes the direction that the magnetic field travels.

 Please turn to the end of the chapter for the answer.

Calculating Current in Cable Wraps

Portable coils typically use AC current, but some are capable of both AC
and DC. For these types of coils, the current usually cannot be adjusted.
The type of current used with a coil that is part of a stationary bench unit
is dependent on the capability of the machine. Some bench units have
both AC and DC, whereas some use only DC. For coils that use direct or
alternating current that can be adjusted, the amount of current needed

Magnetic Particle Testing 169

 must be calculated using predetermined formulas and other variables.
The variables used for calculating the current are:

• diameter of the part versus diameter of the coil,

• length and diameter of the part, and
• number of cable wraps.

In the following example, we will use a part

with a low fill factor, which means that the
Test
diameter of the part is less than 10% of the object
diameter of the coil. Note: In magnetic
particle testing, the fill factor is a quantity
for characterizing how closely the outside Coil
diameter of a test object matches the inside (a)
diameter of the magnetizing coil. A test
object with a low fill factor positioned Test
object
close to the inside of a coil is shown in
Figure 9.13(a). Note how this setup
contrasts with a test object with a high fill
factor positioned in the center of a coil in Coil
Figure 9.13(b). A high fill factor is when (b)
the cross-sectional area of the coil is less
Figure 9.13: Fill factors of test
than twice the cross-sectional area of the objects in coils: (a) low fill factor;
test object (including hollow portions). (b) high fill factor.

Let’s calculate the current for a test object with a low fill factor using the
following data:

L = 15 in. (381 mm)

D = 2 in. (51 mm)

N = 5 (number of cable wraps)

I = current

K = constant (45 000)

NI = the number of ampere turns

170 Programmed Instruction Series

To solve, begin with the following equation and plug in the appropriate
variables:

K
NI =
L/D
45 000
L/D
=

45 000
15 in. (381 mm) / 2 in. (51 mm)
(Eq. 9.1)
=

45 000
7.5
=

= 6000

Now you’re ready to determine the amount of current needed:

6000
I=
N
6000
5
(Eq. 9.2) =

= 1200 A ± 10%

For this part, the amperage range that can be used to perform the
inspection is 1080 A to 1320 A based on an allowance of ±10%. Once the
amperage range is calculated, the inspector must determine the amperage
to start with. Usually, it’s recommended that the inspector begin at the
lower end of the range and check the magnetic field strength with a QQI
or hall effect meter to ensure that the field is strong enough. However, the
field should not be too strong as to produce furring of dry particles or
premature pulling of wet particles out of the suspension. Note: This is a
different condition than magnetic writing.

Magnetic writing may occur when two magnetized parts rub against each
other or against other ferromagnetic test objects. Such rubbing may
produce localized magnetic areas on the surfaces that attract and hold
ferromagnetic particles. This type of indication is considered nonrelevant.

Magnetic Particle Testing 171

 Another consideration is the length of the part. To establish an effective
longitudinal magnetic field in a test part using a coil or cable wrap, the ratio of
the length of the part in the direction of the desired field to its diameter or
thickness must be estimated. Ideally, a length-to-diameter (L/D) ratio of 2:1 is
required. If the L/D ratio is considerably less than 2:1, it may be impossible to
establish a magnetic field strong enough to form an indication. For long parts,
more than one inspection may be required to adequately inspect along their
entire length, depending on the size of the coil and part.

Question 9.12
In MT, a low fill factor means that:
A. the diameter of the part is less than 10% of the diameter of the
coil.
B. the diameter of the central conductor is less than 10% of the
diameter of the coil.
C. the length of the part is less than 10% of the diameter of the coil.
D. the cross-sectional area of the coil is less than 2× the cross-
sectional area of the test object.
 Please turn to the end of the chapter for the answer.

Understanding the various techniques and electrical currents used to

perform magnetic particle inspections is very important. Using the wrong
type of equipment or not understanding the characteristics of a specific
technique can seriously undermine the inspection process.

172 Programmed Instruction Series

Chapter 9 Summary

r Electromagnetic fields travel in one of two directions: circular or

longitudinal.
r Discontinuities must be oriented between 45° and 90° to the
direction of the magnetic field in order to be detected with MT.
r To ensure complete coverage with MT, two magnetic fields must
be introduced into the material, the second one perpendicular to
the first.
r Direct magnetization applies an electrical current to the part,
creating a circular magnetic field.
r Circular fields always travel 90° to the current flow.
r With rectified alternating current, the circular field is excellent for
locating both subsurface and surface discontinuities.
r Circular magnetic fields can be created using head shot/contact
plates, central conductor, or prods.
r A head shot is a technique for creating circular fields using a
stationary wet bench unit.
r A central conductor is used to inspect the inside and outside of
hollow parts.
r A prod unit is used for weld inspections and for inspecting large
areas of material with a circular field.
r With prods, the circular magnetization strength varies with prod
spacing and the thickness of the section being tested.
r Longitudinal fields are created using an indirect technique, by
which a magnetic field—not an electrical current—is induced into
the part.
r Three main types of equipment are used to create longitudinal
fields: yokes, coils, and wrapped cables.
r A yoke creates a longitudinal field by passing current through a
solenoid located in the handle.
r Coils take the form of either cables looped into a large circle
within a protective shell or the cables from a prod unit wrapped
loosely around the part being inspected.
r A coil is typically made using five cable wraps; more wraps equal a
stronger magnetic field regardless of the amount of current used.
r A part with a low fill factor means that the diameter of the part is
less than 10% of the diameter of the coil.

Magnetic Particle Testing 173

 Answers to Chapter 9 Questions
Question 9.1
Answer: D – Magnetic fields only travel in circular and longitudinal
directions.

Question 9.2

Answer: B – Optimally, discontinuities must be oriented 90° to the

magnetic field, but those oriented to 45° can also be located.

Question 9.3

Answer: C – Whereas alternating current is limited in its penetrating

ability, rectified alternating current has deeper penetrating capabilities for
detecting subsurface discontinuities.

Question 9.4

Answer: A – Rectified alternating current is used in MT instead of pure DC.

Question 9.5

Answer: C – A central conductor is used to inspect hollow parts. A head

shot is used when the part is solid or when arc strikes can be tolerated.

Question 9.6

Answer: C – As with a head shot, a circular field is created when using a

central conductor.

Question 9.7

Answer: A – When using a central conductor, the effective area extending

on either side of it is dependent on its diameter.

174 Programmed Instruction Series

Question 9.8

Answer: D – Prods are portable units used to inspect welds and other
objects in the field with circular magnetization.

Question 9.9

Answer: D – Most specifications base prod amperage on prod spacing and

material thickness.

Question 9.10

Answer: B – The magnetic field created by the coil of wire (solenoid)

travels longitudinally from one leg of the yoke to the other. A prod unit
induces current into a part, whereas a yoke induces a magnetic field.

Question 9.11

Answer: B – Adding more coil wraps always increases the magnetic field
strength, even if amperage is not increased.

Question 9.12

Answer: A – The fill factor is a quantity for characterizing how closely the
outside diameter of a test object matches the inside diameter of the
magnetizing coil. The fill factor of a part with a diameter less than 10% of
the coil diameter is considered low.

Magnetic Particle Testing 175

 Chapter 9 Review
1. When using prods, the minimum prod spacing allowed is usually:
A. 3 in. (76 mm).
B. 4 in. (102 mm).
C. 6 in. (152 mm).
D. 8 in. (203 mm).

2. A hall effect meter is used to:

A. measure conductivity.
B. create a magnetic field.
C. calculate the fill factor.
D. measure the strength of a magnetic field for an MT inspection.

3. Which of the following is true regarding magnetic fields created by a

head shot?
A. They lose their strength along the length of the material.
B. Their strength remains the same along the length of the material.
C. They create a longitudinal field.
D. They can only be used for surface examinations.

4. Prods create a __________ magnetic field.

A. longitudinal
B. surface
C. AC
D. circular

5. To establish an effective longitudinal magnetic field using a coil or

wrapped cables, the L/D ratio of the test part should be at least:
A. 1:1
B. 2:1
C. 5:1
D. 10:1

6. Relative to the current flow, a circular magnetic field travels:

A. in the same direction.
B. in the opposite direction.
C. perpendicular to its direction.
D. parallel to its direction.

176 Programmed Instruction Series

7. A coil typically has how many cable wraps?
A. 2
B. 3
C. 5
D. 7

8. A circular magnetic field can be created using all of the following

except:
A. a coil.
B. a head shot.
C. a central conductor.
D. prods.

9. The main problem with prods is:

A. finding an alternating current power source.
B. arc strikes.
C. they can only locate surface discontinuities.
D. the amperage cannot be adjusted.

10. When magnetizing a solid bar using a head shot, the magnetic field
is strongest:
A. in the center of the bar.
B. on the surface of the bar.
C. 1 in. (25 mm) from the surface of the bar.
D. just below the surface of the bar.

Magnetic Particle Testing 177

 Chapter 9 Review Key
1. A

2. D

3. B

4. D

5. B

6. C

7. C

8. A

9. B

10. B

178 Programmed Instruction Series

 Chapter 10
Demagnetization Techniques

In this chapter:
Magnetic properties of retentivity and coercive force
Differences between AC and DC regarding residual magnetism
Reasons to demagnetize based on range of factors
Occasions when demagnetization may not be necessary
Using a magnetic field indicator to measure residual magnetism
Ease of demagnetizing longitudinal versus circular magnetic fields
Demagnetization techniques including step-down or downcycling approach
Demagnetization procedures using AC, direct, and cable wrap methods
Demagnetizing requirements including current, field strength, and direction

179
 Residual Magnetic Fields
Following a magnetic particle inspection, it’s usually necessary to
demagnetize ferromagnetic materials. Although the amount of residual
magnetism (the amount of magnetization remaining in the material) varies
from one ferromagnetic material to another, there is almost always some
remaining. Demagnetization is necessary in order to remove as much of the
residual magnetism as possible. The process of demagnetization removes
most of the residual magnetic field that remains but does not always remove
100% of it. The amount of the residual magnetic field remaining in a
material after inspection is determined by two main factors:

1. The magnetic properties of the material being inspected.

2. The type of current used to create a magnetic field.

Magnetic Properties
The magnetic properties of the material being inspected include retentivity
and coercive force. Hysteresis curves for an entire cycle of magnetization
and demagnetization are presented in Figure 10.1.

Retentivity
Each material has its own level of retentivity (the amount of magnetism
that a given material can hold onto after a magnetizing force has been
removed). The point marked “B” in the hysteresis curve shown in
Figure 10.1 indicates the retentivity for a given material.

Coercive Force
The coercive force of a material determines how easy or difficult it is to
magnetize. Materials with a high coercive force tend to be more difficult
to magnetize than those with a low coercive force. To better explain
coercive force, if the same amperage were used to magnetize materials
with high and low coercive forces, the material with the low coercive force
may have a magnetic field with a gauss (tesla) reading of 45 G (0.0045 T),
but the material with a high coercive force may have a gauss (tesla)
reading of 30 G (0.003 T). The point marked “C” in the hysteresis curves
shown in Figure 10.1 designates the coercive force for a given material.

180 Programmed Instruction Series

 B+ B+
Zero flux density Residual
A A
magnetism
Zero magnetic Saturation
field strength B
point

H- H+ H-
{ H+
0 0

B- B-
(a) (b)

B+ B+
A A
B Saturation
B
point

H- C H+ C
H- H+
0 0

Coercive force D
Reverse
B- magnetization B-
saturation point

(c) (d)

B+ B+
A A
B Saturation
B
point

H- C H+ C
H- H+
0 0 F

E Residual E
D magnetization D
Reverse Reverse
B- B-
magnetization magnetization
point point

(e) (f)

Figure 10.1: Hysteresis data for unmagnetized steel: (a) virgin curve of a hysteresis loop
(occurs only during the initial magnetization of unmagnetized material); (b) hysteresis loop
showing residual magnetism; (c) hysteresis loop showing coercive force; (d) hysteresis loop
showing reverse magnetism; (e) hysteresis loop showing reverse residual magnetism;
(f) complete hysteresis loop.

Magnetic Particle Testing 181

 Question 10.1

Retentivity refers to the:

A. ease of magnetizing ferromagnetic material.
B. depth that magnetic lines of force can penetrate.
C. strength of the magnetic field.
D. ability of ferromagnetic material to retain magnetism.

 Please turn to the end of the chapter for the answer.

Current
The type of current used to create a magnetic field—alternating or
direct—is another factor that affects the residual magnetic field.

Alternating Current
A yoke and coil typically create magnetic fields using alternating current,
which does not usually leave a strong residual magnetic field and is much
easier to remove when compared to a direct-current residual field. The
residual field has the same skin effect as the inspection magnetic field
residing on the surface of the material.

Direct Current
Head shots, cable wraps, and prod inspections using direct current (or
rectified alternating current) have the potential for leaving behind a more
penetrating residual magnetic field than alternating current does. Circular
fields are particularly difficult to remove compared to longitudinal fields.
Direct-current inspection techniques, especially when using higher
amperages with ferromagnetic material that has high retentivity
characteristics, can also be challenging to remove.

Question 10.2

Which type of part would be hardest to demagnetize?

A. Part magnetized with a yoke using alternating current.
B. Part magnetized with a coil using alternating current.
C. Part magnetized with prods using rectified alternating current.
D. Part magnetized with a longitudinal field.

 Please turn to the end of the chapter for the answer.

182 Programmed Instruction Series

Deciding to Demagnetize
Regardless of the inspection technique, electrical current, and the type of
ferromagnetic material being inspected, there will probably be some level of
residual field remaining. Whether you have to demagnetize will usually be
determined by the testing specification governing the inspection. The
procedure used by the inspector to perform an MT inspection and/or the
specification called out by the customer may also provide guidance on
demagnetization requirements. If no guidance is provided, you should
contact your Level III to determine what the demagnification requirement is.

Ë From the Field: In most cases, the inspector can locate the
demagnetizing requirements from the inspection procedure or
specification provided. Demagnetization is required 98%
percent of the time. When unsure, demagnetize.

Reasons to Demagnetize
When engineers specify an MT inspection, they must also determine
whether the part/material will need to be demagnetized. They must
consider several factors in deciding whether or not to demagnetize.
Some of the most common factors are:

• Type of magnetizing current used.

• Material.
• Subsequent manufacturing processes.
• End use of the product/material.

Of all of these, probably the most important consideration would be to

determine what the subsequent manufacturing processes are and the end use
of the part/material. These are critical because residual magnetic fields can:

• Detrimentally affect the machining process. Machining chips can

cling to the material and cause surface imperfections and damage
tooling.
• Interfere with electronic equipment such as gages.
• Cause arc blow in the welding process. This occurs when the
magnetic field causes the weld arc to be deflected away from where
it’s intended to be deposited.

Magnetic Particle Testing 183

 • Cause premature wear on surfaces of parts due to a buildup of metal
particles on the surface.

Reasons Not to Demagnetize

Demagnetization is not always necessary, though. Metals that have low
retentivity, such as low carbon steels, often do not retain a great deal of
magnetism once the magnetizing current is removed and therefore may not
require demagnetization. Secondly, when the part/material will be heat-
treated above 1400 °F (760 °C), no demagnetization is necessary because the
heat-treating process removes all residual magnetism. This temperature is
referred to as the curie point. Also, if you perform a circular field MT
inspection using a head shot and subsequently use a longitudinal field,
demagnetization following the head shot is not required. The longitudinal
field removes any residual circular field that remains.

Question 10.3

Demagnetization is not required when:

A. welding will be done following the inspection.
B. the residual field is less than allowed by the specification.
C. the material has extremely high retentitivty.
D. the material will be heat-treated at a temperature below 1400 °F
(760 °C).

 Please turn to the end of the chapter for the answer.

Measuring Residual Magnetic Fields

Following the demagnetization process, it’s important to verify that the
residual field has been adequately removed. This is accomplished by using a
magnetic field indicator, either analog or digital, as shown in Figure 10.2.
These types of meters are the most common gages used to measure residual
magnetic fields.

Units are in gauss (traditional) or tesla (SI). To convert from gauss to tesla,
multiply by 10–4 (0.0001). To convert a tesla measurement to gauss,
multiply it by 10 000. For instance, 5 tesla (T) equals 50 000 gauss (G).

184 Programmed Instruction Series

 (a) (b)

Figure 10.2: Field indicator: (a) analog; (b) digital.

Question 10.4

Which of the following is a unit of measurement for residual magnetism?

A. Gauss.
B. Voltage.
C. Webers per meter.
D. Amperage.

 Please turn to the end of the chapter for the answer.

Removing Residual Fields

Removing residual magnetic fields is a fairly
simple process if done correctly. Ultimately,
the goal of demagnetization is to change the
orientation of the magnetic domains from
an aligned to a random orientation. (See (a)
Figure 10.3.)

Remember, prior to a magnetic field being

introduced into the part/material, the
magnetic domains are randomly oriented.
The goal after the inspection is to get these
(b)
domains back, as much as possible, to being
randomly oriented. There are several Figure 10.3: Magnetic domains:
methods used to accomplish this. Some are (a) random; (b) aligned.

Magnetic Particle Testing 185

 very simple and others are a little more complicated. The techniques for
removing a residual field differ based on the type of field involved. Just as
when you are magnetizing a ferromagnetic part, demagnetizing
techniques use either longitudinal or circular magnetic fields.

Longitudinal versus Circular Residual Fields

Unlike circular fields, longitudinal residual magnetic fields are easily
measured and tend to be more easily removed. Most NDT procedures
require that when circular and longitudinal fields are used in conjunction,
the longitudinal field must be done last. Why? A circular magnetic field is
nearly impossible to measure in some parts, especially bar stock or round
cylinders. The circular magnetic field travels around the circumference of
the part, but unless there is a leakage field present, there may be no way to
measure a residual circular field. On the other hand, longitudinal fields,
which run parallel with the part, almost always have an end (pole) from
which flux leakage takes place and are easily measured.

Techniques for Removing a Residual Magnetic Field

Using a Coil or Yoke
By far, the easiest technique for removing a residual magnetic field is to
demagnetize the part/material using an AC coil and/or yoke by slowly
passing the part/material through the coil or between the legs of the yoke.
This is an extremely effective and simple way to remove a residual field
when an MT inspection has been performed using a yoke or coil.
However, some residual magnetic fields induced using rectified alternating
current, such as cable wraps, head shots, and prods, can also be removed
with a coil or yoke. The main limitation when using this technique is the
size of the part versus the width of the yoke legs. The part must be small
enough to fit inside of a coil or between the legs of a yoke.
Step-Down Technique or Downcycling
Another way to remove residual magnetic fields is by using the step-down
technique, also referred to as downcycling. The step-down technique can be
performed using a coil, prod machine, or horizontal bench unit. The step-
down technique involves an initial magnetic field (coercive force) that is higher
than the one that was used to perform the inspection. Once the step-down
process begins, the magnetic field switches polarity as it is slowly reduced to
zero. The combination of alternating polarity and a reduction in the magnetic
field intensity removes the remaining magnetic field. (See Figure 10.4.)

186 Programmed Instruction Series

 B+ +

Flux density
H– H+
Time

FLUX CURVE

–
H– H+

CURRENT CURVE
Time

B–

Figure 10.4: Demagnetization hysteresis loops with current and flux intensity curves
during downcycling.

Question 10.5

The step-down technique of demagnetizing is also referred to as:

A. retentivity.
B. coercity.
C. downcycling.
D. saturation.

 Please turn to the end of the chapter for the answer.

Demagnetizing Procedures
A variety of equipment is available to perform demagnetization
procedures. From the most common to the more difficult, below is a
listing of the various methods for demagnetizing following magnetic
particle testing.

Magnetic Particle Testing 187

 AC Demagnetization
The simplest and most common
process for demagnetization is by
using an AC coil. Coils come in
many sizes and shapes, including
rectangular, square, or round. For
large quantities of parts, a coil is
built around the conveyor belt
and demagnetizes the parts as
they pass through. A freestanding
AC demagnitizer is shown in
Figure 10.5: AC demagnetization unit.
Figure 10.5.

Alternating current has a natural cyclic characteristic, which, in

combination with the part slowly moving away from the coil, has a
demagnetizing effect very similar to that of the step-down technique. The
farther the part moves away from the coil, the less intense the magnetic
field becomes, which is just like stepping down or lowering the current
used to create a magnetic field. The combination of the alternating current
and the part moving away from the coil is very effective in removing
residual magnetic fields. The same is true for the AC yoke. When you pass
a part between and away from the legs of a yoke, the part will be
demagnetized in the same way as when it is demagnetized using a coil.

Direct Demagnetization
When using a horizontal bench unit or prods to demagnetize, the part
must be in contact with either the heads or the prods in order for the
technique to be effective. When this technique is used, the current must
be slowly reduced, from high to low, in order to remove the residual
magnetic field. Typically, the current would be set just above the current
used to inspect the part and then reduced to zero. Most of the newer
horizontal bench units have an automated demagnetizing control that
makes the procedure quite simple for the technician. Once the button is
depressed, the demagnetizing process automatically begins.

188 Programmed Instruction Series

Cable Wrap
In cases where the part is too big
for a coil or other demagnetizing
equipment, a cable wrap can be
used for demagnetization. This
technique allows the inspector to
create a coil that is large enough to
accommodate the size of the part
needing to be demagnetized. (See
Figure 10.6.) Figure 10.6: Coil for circumferential
demagnetization improvised to fit crimper
A portable power source, similar to connector of railroad car.
the one used for a prod unit, can be
used to provide the high current
necessary to properly perform an
adequate demagnetization.

Question 10.6

A cable wrap is typically used to demagnetize:

A. large parts that cannot fit into typical MT equipment.
B. small parts.
C. diamagnetic parts.
D. nonferromagnetic parts.

 Please turn to the end of the chapter for the answer.

Demagnetizing Do’s and Don’ts

For the most part, demagnetizing is fairly simple and straightforward, but
there are some factors you must keep in mind.

Current
You should always try to demagnetize using the same type of current
used to inspect the part. Rectified alternating current should be used to
demagnetize a field created with this type of current, whether half-wave or
full-wave, and the same applies to alternating current. If for some reason
that’s not possible, following the demagnetization process, verify that the
residual field has been removed by using a gauss (tesla) meter.

Magnetic Particle Testing 189

 Demagnetizing Requirements
All inspectors need to know what the requirements are for the residual
magnetic field. The inspection procedure or specification should provide
the necessary information for determining the allowable residual field. If it
does not, consult with your NDT Level III.

Demagnetizing Field Strength

Always ensure that the demagnetizing field strength begins at a strength
above that used to perform the inspection. When using a coil, you should
keep the part as close to the inside edge of the coil as possible, as the
strongest magnetic field is located here.

Direction
The part being demagnetized should be oriented correctly as it is passed
through the coil or yoke. Its longest axis should be parallel to the edge of
the coil. Parts with complex geometries should be rotated as they go
through the coil. It is sometimes necessary to pass them through more
than once to remove the residual field in all areas of the part.

Verifying the Residual Field

Following the demagnetization process, it’s important to verify that the
residual field has been adequately removed. A calibrated magnetic field
indicator, as previously discussed, is the most common tool used to
measure residual magnetic fields.

Question 10.7

Which of the following is a good practice to follow when demagnetizing

a part?
A. Begin demagnetizing with a field strength less than that used to
magnetize the part.
B. Always use unrectified alternating current to demagnetize a part.
C. Use a different type of current to demagnetize than was used to
magnetize the part.
D. Use the same type of current to demagnetize than was used to
magnetize the part.
 Please turn to the end of the chapter for the answer.

190 Programmed Instruction Series

Summary
Demagnetizing parts and other materials following an MT inspection is
almost always required. Not performing a thorough demagnetization
process can have extremely adverse effects for processes that follow an MT
inspection. That is why it is critical for NDT inspectors to understand the
process and know how to properly demagnetize the material they are
inspecting.

Magnetic Particle Testing 191

Chapter 10 Summary
r Following a magnetic particle inspection, it’s usually necessary to
demagnetize ferromagnetic materials.
r The amount of the residual magnetic field remaining in a material
after inspection is determined by the magnetic properties of the
test object and the type of current used to create a magnetic field.
r The magnetic properties of the material being inspected include
retentivity and coercive force.
r Alternating current does not usually leave a strong residual
magnetic field and is much easier to remove when compared to a
direct-current residual field.
r Circular fields are more difficult to remove than longitudinal fields.
r The most important considerations for demagnetization are the
subsequent manufacturing processes and the end use of the part
or material.
r Residual magnetic fields can detrimentally affect the machining
process by causing machining chips to cling to the material.
r Residual fields can also interfere with electronic gages and cause
arc blow in the welding process.
r Metals with low retentivity, such as low carbon steels, often do not
retain a great deal of magnetism and may not require
demagnetization.
r When the part/material will be heat-treated above 1400 °F
(760 °C), no demagnetization is necessary.
r Following the demagnetization process, it’s important to verify
that the residual field has been adequately removed by using a
magnetic field indicator.
r Most NDT procedures require that when circular and longitudinal
fields are used in conjunction, the longitudinal field must be done
last.
r The easiest technique for removing a residual magnetic field is to
slowly pass the part/material through an AC coil or between the
legs of a yoke.
r Another way to remove residual magnetic fields is by using the
step-down technique, also referred to as downcycling, or by using
a coil, prod machine, or horizontal bench unit.
r In cases where the part is too big for a coil or other demagnetizing
equipment, a cable wrap can be used for demagnetization.
r You should always try to demagnetize using the same type of
current used to inspect the part.
r The inspection procedure or specification should provide the
necessary information for determining the allowable residual field.

Magnetic Particle Testing 193

 Answers to Chapter 10 Questions
Question 10.1
Answer: D – Retentivity refers to the ability of ferromagnetic material to
retain magnetism.

Question 10.2

Answer: C – A part magnetized with a circular field using rectified

alternating current would be more difficult to demagnetize than a part
magnetized with a longitudinal field using unrectified alternating current.

Question 10.3

Answer: B – Most specifications clearly define the maximum residual

magnetic field allowed.

Question 10.4

Answer: A – There are two units of measure for magnetism: gauss and
tesla. These measurements may also be converted into webers per square
meter.

Question 10.5

Answer: C – Step-down demagnetization is also called downcycling.

Question 10.6

Answer: A – When parts are too big to fit into a yoke or coil, a cable wrap
can be used to fit the size of the part.

Question 10.7

Answer: D – The same type of current used to magnetize a part should be

used to demagnetize it, except at a higher field strength than was used
during the inspection.

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Chapter 10 Review
1. The easiest technique used to demagnetize parts is by using a(n):
A. cable wrap.
B. horizontal bench unit.
C. AC yoke or coil.
D. prod machine.

2. Longitudinal MT should always be performed:

A. after circular MT.
B. prior to circular MT.
C. either before or after circular MT.
D. before and after circular MT.

3. Ferromagnetic materials with high coercive force:

A. are easy to magnetize.
B. are difficult to magnetize and harder to demagnetize.
C. are difficult to demagnetize.
D. cannot be demagnetized.

4. A measurement of 7 T is equivalent to:

A. 7 G.
B. 70 G.
C. 7000 G.
D. 70 000 G.

5. Material is __________ when magnetic domains are aligned.

A. magnetized
B. demagnetized
C. ready to be magnetized
D. considered nonferromagnetic

Magnetic Particle Testing 195

 Chapter 10 Review Key
1. C

2. A

3. C

4. D

5. A

196 Programmed Instruction Series

 Chapter 11
Materials and Accessories

In this chapter:
Characteristics of dry and wet ferromagnetic particles
Size of dry versus wet particles for MT
Importance of shape and color of dry and wet particles
Permeability and retentivity of ferromagnetic particles
Role of lighting in the accuracy of MT inspection results
Use of LED lights for white light and ultraviolet light testing conditions
UV-A range for ultraviolet magnetic particle testing
Intensity requirements of ultraviolet light for MT

197
 MT Particle Types and Characteristics
The ferromagnetic particles used when performing an MT inspection are
not just steel shavings collected from the bottom of a grinding wheel.
Instead, they are manufactured to very specific requirements. Their size,
shape, and color are very important to the effectiveness of an MT
inspection. There are two main types of MT particles: dry and wet. Both
types of particles can be made from a variety of materials, which include
iron, black and brown oxides, ferrites, nickel, and nickel alloys. The
requirements for their physical characteristics are summarized below.

Dry Particles
Dry ferromagnetic particles are made up of iron particles with a variety of
sizes and shapes. The size and shape of MT particles play an important
role in how they function during the inspection. For instance, small
particles of 0.002 in. (50 µm) are better for finding smaller discontinuities
that have weaker leakage fields and would be less likely to attract larger
MT particles of 0.006 in. (150 µm). The benefit of larger particles is that
they do a much better job of filling in or bridging the space in larger
discontinuities. When MT particles vary in size and are combined
together, they create an effective magnetic particle powder that not only
increases sensitivity for finding smaller discontinuities but is equally
effective for finding larger discontinuities. Dry particles come in a variety
of colors. (See Figure 11.1.)

Figure 11.1: Dry ferromagnetic particles.

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Question 11.1

What is the size range of dry MT particles?

A. From 0.001 in. to 0.004 in. (25 µm to 100 µm).
B. From 0.0002 in. to 0.0006 in. (5 µm to 15 µm).
C. From 0.002 in. to 0.006 in. (50 µm to 150 µm).
D. Size is random.

 Please turn to the end of the chapter for the answer.

Wet Particles
The main difference between dry and wet ferromagnetic particles is that
the wet particles are much smaller, ranging from 0.0002 to 0.0006 in.
(5 to 15 μm) for sensitive wet technique particles in commonly used
formulations. Wet particles must be able to stay suspended in the oil or
water bath for an extended period of time, thus their small size. If dry
particles, which are larger, were used, they would settle out of suspension
very quickly and the concentration of particles in the solution being
sprayed onto the part would be lower than necessary for an effective
inspection process.

Specific Characteristics of Particles

The specific characteristics that MT particles must have are described below:

Size
Dry Particles: Of all of the
characteristics of dry MT particles,
size is probably one of the more
important ones. If the particles are
too large, the sensitivity of the test
will be lessened. If they are too small,
they could remain airborne and not
collect on the surface being
inspected. The perfect dry particle
mix is made up from particles of
different sizes. The smaller particles Figure 11.2: Dry ferromagnetic particles.
allow for the proper sensitivity,
which is needed to locate small discontinuities, and the larger particles
help to fill the gap between larger discontinuities. See Figure 11.2 for an
actual look at dry ferromagnetic particles.

Magnetic Particle Testing 199

 The ideal size of the smallest dry MT particles is approximately 0.002 in.
(50 µm). Particles much smaller would probably not reach the surface of
the part. Particles this size and slightly larger are more mobile (move
around the surface of the part more readily) than larger particles and are
able to be attracted to weaker leakage fields, which allows for a more
sensitive test (ability to detect smaller discontinuities). The largest particles
should be approximately 0.006 in. (150 µm). Unlike the smaller particles,
particles this size do a much better job of finding larger discontinuities and
help to prevent the particle mixture from staying airborne.

Question 11.2

Dry MT particles that are smaller than 0.002 in. (50 µm) would be:
A. optimal for locating discontinuities.
B. too small to be of use in an MT inspection.
C. one size of MT particle used to make dry MT powder.
D. too large to be of use in an MT inspection.

 Please turn to the end of the chapter for the answer.

Wet Particles: Wet particles are

considerably smaller than dry
particles in large part because they
must have the ability to stay
suspended in the carrier (water
or oil). (See Figure 11.3.)
Typically, wet particles measure
approximately 0.0005 in.
(12.7 µm) in diameter, which is
much smaller than the size of dry
particles but allows them to
remain suspended in the carrier Figure 11.3: Wet MT particles.
for a longer time period than
larger particles.

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Question 11.3

Wet MT particles:
A. are larger than dry particles.
B. do not vary in size.
C. range in size from 0.0001 in. to 0.0003 in. (2.5 µm to 7.6 µm).
D. must be small enough to stay suspended in the MT bath.

 Please turn to the end of the chapter for the answer.

Shape
Dry Particles: Since dry MT powder is made from particles of different
sizes, the shape of the particles is different as well. Some of the particles
have a long body and a small diameter, while others are shorter with a
larger diameter. This combination provides improved sensitivity (smaller
particles) along with an increased ability to form indications along linear
discontinuities (longer particles).

Wet Particles: The shape of wet MT particles is the same as dry particles,
combining long slender and round globular particles in one mixture.
Shape and size may vary depending on the material from which they are
made. For instance, synthetic iron oxide particles are very small, with
diameters as little as 0.000004 in. (0.1 µm). This is equivalent to 100 nm,
which is the upper limit for a particle to be considered a nanoparticle.

Wet particles rely on each other to work. A single wet particle is not very
effective, but as they combine and attach to one another, as a magnetic
field is applied, they become much more effective.

Question 11.4

The longer dry MT particles are best for which type of indications?
A. Small porous.
B. Surface.
C. Subsurface.
D. Large linear.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 201

 Color
Except for fluorescent particles, the color of MT particles is determined
based on the background of the material being inspected. The goal should
be to have as much of a contrast as possible.

As for fluorescent MT particles, the fluorescent pigment is attached to the

particles using a binding resin. The binder and pigment reduce the
permeability of the particles, but they still have high enough permeability
to be effective. Figure 11.4 shows an example of MT powder for a
fluorescent suspension.

Figure 11.4: MT particles for use in a wet inspection.

Ë you
From the Field: For a Level I or II inspector, the characteristic
should understand best, for dry MT particles, and most
important for performing MT inspections, is color. Choosing
the right color can greatly improve the outcome of an
inspection by aiding the inspector’s ability to see indications,
especially very small ones. Choosing a color that provides the
greatest contrast between the part being inspected and MT
particles is one of the few factors that the inspector can control
when it comes to MT particle characteristics.

202 Programmed Instruction Series

Question 11.5

Why do MT particles vary in shape, size, and color?

A. It’s due to the manufacturing process.
B. Combined, they provide the optimal formula for finding
discontinuities.
C. They react differently to magnetism.
D. There is no benefit to having particles with varying shape and size.

 Please turn to the end of the chapter for the answer.

Permeability
The optimal MT particle has high permeability. This enables particles to
be attracted to magnetic leakage fields and, more importantly, the weaker
leakage fields. Because dry particles are larger, it is much easier to measure
their permeability; therefore, the specific permeability of dry particles can
be measured, but the opposite is true for wet particles. Wet particles are
much smaller, and it is more difficult to measure their permeability.
Because of this, it is important that the particles are made from material
with high permeability, even though the final permeability of the particles,
after being formed to their desired shape, cannot be accurately measured.

Retentivity
The ability of an MT particle to retain magnetism is very important.
However, low retentivity is preferable to high retentivity. If the particles
have high retentivity, they will likely cluster together and adhere to the
surface of the part being inspected, which would prevent them from
moving across the surface and make it extremely difficult to locate
discontinuities. High retentivity would also make it very difficult to apply
dry particles, as they would cluster together and clog the opening of the
applicator, which would prevent any particles from being released onto the
inspection area.

All of the characteristics associated with ferromagnetic particles, dry and

wet, play an important role in the MT inspection process. Over the years,
dry and wet MT particles have been refined and improved to optimize the
effectiveness of magnetic particle inspection. For Level I and II inspectors,

Magnetic Particle Testing 203

 control of the characteristics is minimal, but it is still important to
understand the role of each characteristic—size, shape, permeability, and
so on—and how each can affect the results of an MT inspection.

Question 11.6

Which of the following is true regarding the characteristics of MT

particles?
A. They should have low permeability.
B. They should have high retentivity.
C. They should have high permeability and low retentivity.
D. They should have low permeability and high retentivity.

 Please turn to the end of the chapter for the answer.

MT Inspection Lighting: Visible and

Fluorescent
There are three main techniques for performing MT inspections:

• Dry with visible particles.

• Wet with visible particles.
• Wet with fluorescent particles.

For any of these techniques, lighting plays an important role as to the

accuracy of the final inspection results. If the lighting is inadequate,
whether it’s white or fluorescent light, the ability of the inspector to locate
indications and distinguish between relevant and nonrelevant
discontinuities will be diminished. Another important factor for lighting is
brightness. For both white and fluorescent lighting there are minimum
brightness requirements. The minimum brightness for white light is
usually 100 fc (1076 lx). The minimum brightness for fluorescent lighting
is normally between 900 and 1200 µm/cm2 depending on the distance of
the source.

204 Programmed Instruction Series

Question 11.7

How does lighting affect the results of an MT inspection?

A. Lighting enables the inspector to better see the inspection area
and indications as they form.
B. It has no effect.
C. Lighting enhances the color of the MT particles.
D. The particles react when exposed to white light.

 Please turn to the end of the chapter for the answer.

White Light
When performing dry magnetic particle inspections, the inspector must
ensure that the brightness of the light being used to illuminate the
inspection surface is adequate so that any indications that may be present
can be seen, especially smaller indications such as crater cracks. Small or
tight indications are difficult to see in perfect inspection conditions, let
alone whenever the lighting is less than adequate. The one requirement
that an inspector must remember is that it is extremely important to
maintain the brightness of the lighting during the entire inspection
process, especially when the indications are forming. Without adequate
light brightness throughout the inspection, discontinuities may be missed.

Something else that limits an inspector’s ability to see indications is the

contrast between the background of the part and the color of the MT
particles. When the contrast between the background of the part and
particle color is low, the lighting becomes even more important.

White light requirements can be satisfied using flashlights or other types

of portable lighting. The lighting in most MT inspection environments
usually requires some type of supplemental lighting. LED lighting is
becoming more popular for this purpose. LED flashlights tend to be much
brighter and typically last longer than incandescent bulbs. They also use
less energy, which means that the battery lasts longer than older types of
battery-powered flashlights. Most flashlights currently available in stores
use LED bulbs.

Magnetic Particle Testing 205

 To give you an example of white light intensity, a typical office would have
lighting in the range of 50 to 70 fc (538 to 753 lx) at desk or table height.
In order to achieve illumination of 100 fc (1076 lx), you would have to
measure the light at about 6 ft (1.8 m) off of the floor.

Question 11.8

White light intensity is usually required to be:

A. 1 fc (10.76 lx).
B. 10 fc (107.6 lx).
C. 100 fc (1076 lx).
D. 1000 fc (10 760 lx).

 Please turn to the end of the chapter for the answer.

Ë From the Field: Although it is best to choose an MT powder

with optimal contrast between it and the material being
inspected, it is not always possible. That is why adequate
lighting is crucial. When the brightness at the surface of the
part is below what is required, which is typically 100 fc
(1076 lx), the ability of the inspector to see small indications
as they develop can be difficult.

Ultraviolet Light
Fluorescent magnetic particle inspection requires the use of an ultraviolet
light source. Although there are many types to choose from, the
requirements for UV light intensity must be met. There are many UV
lamps available for purchase, especially on the internet, but many of them
do not meet the UV light brightness requirements for most MT inspection
procedures. Fluorescent lights come in a variety of shapes and sizes. Most
are handheld, such as flashlights, but larger, fixed housing lights are
available too.

Ultraviolet light (black light) is defined as light that falls between 4 to

400 nm (40 to 4000 Å) in the light spectrum. The UV range is usually
divided into three bands:

206 Programmed Instruction Series

 1. UV-C range (short ultraviolet radiation) – between 180 and
280 nm (1800 and 2800 Å).
2. UV-B range (medium ultraviolet radiation) – between 280 and
320 nm (2800 and 3200 Å).
3. UV-A range (long ultraviolet radiation) – between 320 and
400 nm (3200 and 4000 Å).

Fluorescent ferromagnetic testing media fluoresce when activated by

ultraviolet radiation at 365 nm. Thus, the ultraviolet light used for NDT
inspection is in the UV-A range, 365 nm ±5 nm. The chart of the
electromagnetic spectrum in Figure 11.5 shows the relatively small band of
ultraviolet radiation used in fluorescent magnetic particle tests.

Visible light
(400 to 700 nm)

X-rays
(10 pm to Infrared Microwave Radio waves
10 nm) (700 nm to 1 mm) (1 mm to 1 m) (10 to 100 000 m)
Ultraviolet
(4 to
400 nm) UHF VHF

10 100 1 10 100 1 10 100 1 10 100 1 10 100 1 10 100

Picometers Nanometers Micrometers Millimeters Meters Kilometers

Radiation used for fluorescent

magnetic particle tests
(365 nm ±5 nm)
WAVELENGTH

Figure 11.5: Ultraviolet radiation range used for fluorescent magnetic particle testing.

For older-style lamps, those using mercury vapor bulbs, a filter is placed
on the outside of the bulb to provide light in this range. Although there
are still many mercury vapor lamps still in use, LED ultraviolet lamps are
quickly becoming the first choice for many NDT laboratories. LED
ultraviolet lamps have a number of advantages when compared to
traditional mercury vapor lamps, including:

Magnetic Particle Testing 207

 • Brighter.
• Extremely portable.
• No warm-up time (on and off instantly).
• Longer lifespan (over 25 000 hours).
• Battery or 110 V powered.
• No filter required.
• Very little white light emitted.
• Uses less power.
• Low heat.
• Available in many shapes and sizes.

Portable battery-powered UV lamps

are quickly making mercury vapor
lamps obsolete, especially with
the introduction of high-intensity
UV-LEDs.

UV-LEDs are available with narrow-

band UV-A (365 nm wavelength)
that emit very little, if any, visible
light, and do not require the need for
UV filters. A handheld ultraviolet
lamp is shown in Figure 11.6.

Ultraviolet Radiation
Intensity Requirements
Lighting is critical when performing
any NDT inspection. Regardless of
whether you’re performing visible
MT or fluorescent MT, choosing the
right light will have a significant
impact on the final result.
Figure 11.6: Portable ultraviolet lamp.
Testing of an object for fluorescent
magnetic particle indications should
always be done under the lowest possible level of ambient light. This
increases the contrast between the light emitted from the indication and
the background, thus the sensitivity of the test. Test booths of a stationary
fluorescent magnetic particle system should not exceed 2 fc (22 lx) per
1 ft2 (0.1 m2) of ambient light.

208 Programmed Instruction Series

At the same time, the ultraviolet radiation intensity must be as high as
possible because the luminance of the indication is directly proportional
to the quantity of ultraviolet radiation exciting it. The adequacy of an
ultraviolet radiation source for fluorescent magnetic particle testing is
determined by measuring the intensity of the ultraviolet radiation at
a distance of 15 in. (380 mm) from the front or outside surface of the
ultraviolet radiation source filter. This intensity should be at least
1000 μW/cm2. Sources providing less than this intensity should not
be used.

Ambient Light Measurements

Visible light is measured with silicon photodiodes. Visible light meters
must have filters to limit the meter response to the 400 to 760 nm range.
The units of measurement are footcandles (fc) or, in SI, lumens per square
meter (lm/m2), where 1 fc = 10.76 lm/m2. Another term often used is lux
(lx), which equals 1 lm/m2. The conversion factor is the same as for
lumens per square meter: 1 fc = 10.76 lx

Ultraviolet Light Measurements

Ultraviolet light is electromagnetic
radiation and must be measured in
units of irradiance, either watts per
square meter or microwatts per
square centimeter. Measurement of
ultraviolet irradiance requires
equipment sensitive in that spectral
region. Such meters are typically
filtered so that they respond only
to the appropriate ultraviolet
wavelengths, as shown in
Figure 11.7.

Figure 11.7: Photometer for measuring UV-A

radiation.

Magnetic Particle Testing 209

 Question 11.9

For fluorescent MT inspections, background white light should not

exceed:
A. 1 fc (10.76 lx).
B. 2 fc (22 lx).
C. 20 fc (215 lx).
D. 1000 μW/cm2.

 Please turn to the end of the chapter for the answer.

210 Programmed Instruction Series

Chapter 11 Summary

r There are two main types of MT particles: dry and wet.

r Wet ferromagnetic particles are much smaller than dry particles,
ranging from 0.0002 to 0.0006 in. (5 to 15 μm).
r The smaller size of wet particles allows them to remain suspended
in the carrier for longer time periods.
r The ideal dry particle mix is made up of small and large particles
for smaller and larger indications.
r The shape of wet and dry particles combines long slender and
round globular particles in one mixture.
r The color of MT particles is determined based on the most
effective contrast with the background of the material being
inspected.
r With fluorescent MT particles, the fluorescent pigment is attached
to the particles using a binding resin.
r The optimal MT particle has high permeability and low retentivity.
r Three main techniques for performing MT inspections are dry with
visible particles, wet with visible particles, and wet with
fluorescent particles.
r For each technique, adequate lighting is essential to ensure the
accuracy of the final inspection results.
r The minimum brightness for white light is usually 100 fc (1076 lx).
r The light intensity for fluorescent MT inspections should be at
least 1000 μW/cm2 at a distance of 15 in. (380 mm) from the
source.
r The lighting for most MT inspections using white light requires
some type of supplemental lighting, with LED lighting becoming
more popular.
r The ultraviolet light used for NDT inspection is the UV-A range,
equivalent to 320 to 400 nm (3200 to 4000 Å).
r Testing of an object for fluorescent magnetic particle indications
should always be done under the lowest possible level of ambient
light, not to exceed 2 fc (22 lx) per 1 ft2 (0.1 m2).

Magnetic Particle Testing 211

 Answers to Chapter 11 Questions
Question 11.1
Answer: C – The optimal size of dry particles is 0.002 in. (50 µm) to
0.006 in. (150 µm).

Question 11.2

Answer: B – Particles less than 0.002 in. (50 µm) would be too small and
could stay airborne too long.

Question 11.3

Answer: D – Wet MT particles must be small enough to stay suspended or

they will settle too quickly.

Question 11.4

Answer: D – Long, slender particles are best for locating larger, more linear
discontinuities.

Question 11.5

Answer: B – Small particles are better for small discontinuities and larger
particles are better for large discontinuities. A combination of globular
and long particles is effective for finding a range of discontinuities. Color
contrast improves visualization of indications.

Question 11.6

Answer: C – A combination of high permeability and low retentivity works

best.

Question 11.7

Answer: A – Adequate lighting, whether white or fluorescent, enhances

the inspector’s ability to detect MT indications.

212 Programmed Instruction Series

Question 11.8

Answer: C – A minimum of 100 fc (1076 lx) is normally required by NDT

specifications.

Question 11.9

Answer: B – A maximum of 2 fc (22 lx) of ambient light is usually

permitted. When ultraviolet lamps are damaged, they can emit white light
in addition to ultraviolet light.

Magnetic Particle Testing 213

 Chapter 11 Review
1. Dry ferromagnetic particles are:
A. made up of particles with different sizes and shapes.
B. the same size as wet particles.
C. nonferromagnetic.
D. made up of particles with the same size and shape.

2. Wet particles are smaller than dry particles because:

A. wet particles are used to find smaller discontinuities.
B. fewer particles are used.
C. they must stay suspended in the bath.
D. larger particles would clog the nozzle.

3. The largest dry particles are approximately:

A. 0.002 in. (50.8 µm).
B. 0.060 in. (1524 µm).
C. 0.020 in. (508 µm).
D. 0.006 in. (152.4 µm).

4. One of the most important characteristics of dry particles for

detecting discontinuities that have weaker leakage fields is:
A. size.
B. shape.
C. color.
D. material.

5. Wet particles are more effective when:

A. fewer particles are attracted to a leakage field.
B. different size particles are used together.
C. particles made from different materials are used.
D. more particles gather at a leakage field.

6. MT particles should have:

A. high permeability and high retentivity.
B. low permeability and low retentivity.
C. no permeability.
D. high permeability and low retentivity.

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7. Lighting brightness is especially important:
A. when performing inspections outdoors.
B. for wet MT inspections.
C. as an indication is forming.
D. just prior to magnetizing the part.

8. The minimum brightness for fluorescent lighting is usually:

A. 1000 µW/cm2..
B. 1200 µW/cm2.
C. 700 µW/cm2.
D. 900 µW/cm2.

9. LED lighting:
A. is quickly becoming obsolete.
B. requires more power than mercury vapor bulbs.
C. has a short lifespan.
D. does not require a warm-up period.

10. Ultraviolet light used for NDT inspection is between:

A. 180 to 280 nm.
B. 360 to 370 nm.
C. 280 to 320 nm.
D. 320 to 360 nm.

Magnetic Particle Testing 215

 Chapter 11 Review Key
1. A

2. C

3. D

4. A

5. D

6. D

7. C

8. A

9. D

10. B

216 Programmed Instruction Series

 Chapter 12
NDT Certification and
Test Standards

In this chapter:
Overview of personnel certification standards and requirements
Types of examinations required for NDT certification
Differences between standards and recommended practices
Inspection standards for magnetic particle testing
Information required by inspection standards for NDT
Differences between inspection and acceptance standards
Acceptance standards for MT versus PT

217
 NDT Personnel Certification Standards
Standards are documents that are intended to improve the consistency of
a process or, in the case of NDT personnel certification, a consistent set of
rules for certifying inspectors. Specifications are made up of detailed
descriptions for how a specific task or a group of tasks is performed. The
main benefit of using standards is that if tasks are performed as specified
in the standard, the outcome is almost always the same. Standards are
usually written by a group of like-minded people who are experts in the
process that the standard will apply to. Another feature of a standard is
that as technology changes, standards can be revised to include new and
better equipment and/or processes.

Some other advantages of standards are:

• Provide industries with consistent processes with repeatable results.

• Make processes more efficient.
• Give customers a higher level of confidence because the standards
are generally made up of “best practices” for the applicable process.

The goal for any standard is for it to become widely used within one or
more industries. This can only happen when a well-written standard proves
itself to be effective for those using it. As a standard becomes more widely
accepted, the greater its impact on the processes that are affected by it.

Certification Requirements
Personnel who perform nondestructive testing, including magnetic
particle testing, must be certified. The requirements for certification can
vary depending on the certification standard that you will be certifying to.
However, in most cases, the processes are very similar. There are typically
five steps required in order to achieve NDT certification, regardless of the
nondestructive testing standard that you’re working with:

1. Training.
2. Experience.
3. Written examinations.
4. Practical examination.
5. Eye examinations.

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A description of each step in the certification process follows.

Question 12.1

How many basic steps are there in the NDT inspector certification process?
A. 2
B. 3
C. 4
D. 5

 Please turn to the end of the chapter for the answer.

Training
Training includes attending formal classes specifically for the NDT
method that you’re certifying for. The number of training hours will
depend on the standard. For example, 20 hours are typical for an ASNT
Level II MT certification. NAS 410 requires 32 hours for the same
certification.

Question 12.2

The number of training hours for a Level II certification in MT:

A. is the same regardless of the applicable standard.
B. is greater for ASNT than NAS 410.
C. excludes formal classroom training.
D. depends on the standard to which an instructor is certifying.

 Please turn to the end of the chapter for the answer.

Experience
Accumulating experience hours while performing MT testing, including
sample preparation, testing, and evaluation, is usually the most difficult
part of the certification process. Not because it’s physically difficult, but
the experience must be gained while working directly with personnel with
Level II or III certification. A Level II MT inspector needs to accumulate
several hundred hours of experience. In SNT-TC-1A, ASNT recommends
530 hours (130 hours for Level I plus 400 hours for Level II) in NDT with
a minimum of 280 hours (70 hours for Level I plus 210 hours for Level II)
in the MT method.

Magnetic Particle Testing 219

 Written Examinations
The next step in the process is to take two written examinations. The first
is a general exam. This will test your knowledge of the theory of magnetic
particle testing. It is a closed-book exam, and most standards require a
minimum of 40 questions. The second is a specific exam. This exam is
used to determine your competence level with specific processing
requirements, such as the evaluation of MT indications, including
correctly determining their disposition based on the acceptance criteria
and calculating amperages for head shots, prods, and coils. This is usually
a 20-question test. For example, ANSI/ASNT CP-189: ASNT Standard for
Qualification and Certification of Nondestructive Testing Personnel specifies
a minimum of 20 questions for Level I or Level II specific examinations.
However, many specific examinations comprise as many as 30 questions.

Practical Examination
Once you’ve taken the written exams, you will be tested on your ability to
actually perform a magnetic particle test. It’s usually required that one or
two practical tests be taken for each technique that the inspector will use
once he or she is certified. Each test consists of preparing, testing, and
evaluating the test sample. SNT-TC-1A only recommends one practical
exam for each test technique: yoke, coil, and prods.

Question 12.3

The written examinations that are required for certifying personnel are:
A. general and practical.
B. general and specific.
C. theory and practical.
D. practical and specific.

 Please turn to the end of the chapter for the answer.

Eye Examinations
The last part of the certification process is the eye exam. There are two
eye exams. The first eye exam is used to check your near vision, which
requires that you be able to read the J1 or J2 section of a jaeger/snellen
reading card. (See Figure 12.1.) For example, NAS 410 requires a snellen
test result of 20/25 at 16 in. (41 cm) or equivalent. The second eye exam is
used to check your color perception. This test utilizes an ishihara color
blindness booklet. (See Figure 12.2.)

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Figure 12.1: Jaeger/snellen reading card for Figure 12.2: Ishihara 14 color plate test.
performance examination of near vision acuity.

Question 12.4

Which sections of the jaeger/snellen near-vision tests are used for

certification?
A. J3 or J4
B. J1 or J3
C. J1 or J2
D. J1 or J5

 Please turn to the end of the chapter for the answer.

Certification Standards
Once the candidate successfully completes all five steps of the certification
process, he or she may be certified as a Level I, II, or III, whichever is
applicable. The various certification standards determine the specific
number of training and experience hours that are required for each level of
certification and how many written examination questions must be asked,
how many practical specimens are required to be tested and evaluated, and
the eye exam requirements.

Magnetic Particle Testing 221

 Below is a list of some of the NDT personnel certification standards used
in the United States:

• The American Society for Nondestructive Testing – ANSI/ASNT

CP-189: ASNT Standard for Qualification and Certification of
Nondestructive Testing Personnel.
• Aerospace Industries Association – NAS 410, NAS Certification and
Qualification of Nondestructive Test Personnel.
• American National Standards Institute/The American Society for
Nondestructive Testing – ANSI/ASNT CP-189: ASNT Standard for
Qualification and Certification of Nondestructive Testing Personnel.
• Air Transport Association of America – ATA SPEC105, Training and
Qualifying Personnel in Nondestructive Testing Method.
• International Organization for Standardization – ISO 9712,
Non-destructive Testing – Qualification and Certification
of NDT Personnel.
• Military and Government Specs & Standards (Naval Publications
and Form Center) – MIL-STD-2132, Nondestructive Examination
Requirements for Special Applications.

Question 12.5

NAS 410 is a standard used by the:

A. aerospace industry.
B. nuclear industry.
C. petroleum industry.
D. military.

 Please turn to the end of the chapter for the answer.

Ë From the Field: Although most NDT personnel certification

standards do not offer much, if any, flexibility with their
requirements, ASNT Recommended Practice No. SNT-TC-1A:
Personnel Qualification and Certification in Nondestructive
Testing is a little different. It’s a recommended practice, which
means that the user has some flexibility in how the requirements
are applied. For instance, if a company only utilizes yoke-type
MT examination using dry powder, it can choose to reduce
the number of training and experience hours required for
certification because knowledge of the other MT techniques,
such as prods, a bench unit, and coils, is not required.
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Although there are many different standards available for certifying
NDT personnel, in large part they are all very similar, with only minor
differences in their requirements. The differences are usually related to
hours of training or experience, or the number of questions asked on the
written exams. Regardless of which standard is used to certify NDT
personnel, the basic requirements for NDT certification are the same.

Question 12.6

Which of these documents is considered “flexible”?

A. ASTM E 1444/E 1444M
B. NAS 410
C. MIL-STD 2132
D. SNT-TC-1A

 Please turn to the end of the chapter for the answer.

NDT Inspection Standards

In order to perform any NDT inspection, the inspector must acquire
certain information about the examination. For example, if you were asked
to perform an MT inspection on a part using a prod machine, how would
you know what to do? You might think that after completing many hours
of training and accumulating hundreds of hours of experience that an
inspector would be able to perform inspections without any guidance
whatsoever, but that is far from the reality of NDT examinations. This is
where standards come into play. (Note: The terms inspection and
examination are used interchangeably in this section.)

For each MT examination performed, there is a standard that details

exactly how to perform it. For a prod examination, the standard would
provide the information that would allow the inspector to calculate the
amperage range to be used. Although most examination standards have
many similarities, there are almost always some differences. Whether it’s a
formula for calculating the amperage range, prod spacing requirements, or
equipment calibration frequencies, each standard will be a little different.
For this reason, an inspector can never assume that the requirements for
every single examination are identical.

Magnetic Particle Testing 223

 Examples of Inspection Standards
There are many examination standards written for each type of NDT
examination method, as well as all of the techniques within each method.
Just as with NDT certification standards, many industries have written
their own examination standards for the specific methods and techniques
used to inspect their parts and assemblies. One example of a standard
written for a specific industry is ASTM E 1444/E 1444M, Standard Practice
for Magnetic Particle Testing. Although it is now widely used in many
industries, it was written specifically for the aerospace industry.

There are other standards that have been written for a much broader
group of users, such as ASTM E 709, Standard Guide for Magnetic Particle
Testing. This MT examination standard is a basic MT method standard
that is intended for use by those who only need to meet the basic
requirements of MT examinations.

Question 12.7

Which of the standards listed below is used primarily by the aerospace

industry?
A. ASTM E 1444/E 1444M
B. NAS 410
C. MIL-STD 2132
D. SNT-TC-1A

 Please turn to the end of the chapter for the answer.

Information Contained in Inspection Standards

All inspection standards, even the most basic ones, require certain
information. For MT and other NDT methods, the “must have”
information includes, in part, the following:

• Personnel certification requirements or reference to other standards.

• Types of equipment covered.
• Pre- and post-test cleanliness requirements.
• Formulas and guides for developing minimum and maximum
amperages.

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 • Specific examination steps, from the start to the end of the
examination process.
• Reporting requirements.

The main difference between the various NDT examination standards is

the level of detail included. As mentioned previously, a standard such as
ASTM E 709 contains basic information and requirements for performing
MT. On the other end of the standards spectrum is E 1444/E 1444M,
which goes into greater detail and has many more requirements that its
users must adhere to. There are also exclusions of certain types of
techniques, equipment, and materials.

Another important difference can be the calibration requirements, which

can vary greatly among the standards. For instance, one standard may
require the frequency for calibration of a gauss (tesla) meter to be
semiannual, whereas another will require it to be annual. It is because of
these types of seemingly minor differences between the standards that we,
as NDT inspectors, must familiarize ourselves with any examination
standard that is new to us. Significant problems can arise if an inspector
signs off on an examination report that is performed to the requirements
of a reference with which he or she is unfamiliar.

Question 12.8

Examination standards typically include each of the following

requirements except:
A. calibration.
B. acceptance.
C. examination process.
D. personnel certification.

 Please turn to the end of the chapter for the answer.

NDT Acceptance Standards

Of the three categories of standards that are involved with MT
examinations—personnel certification, inspection, and acceptance—there
are a greater number of individual acceptance standards than there are
certification and inspection standards. This is because for each part

Magnetic Particle Testing 225

 examined, there is a unique acceptance standard written specifically for it,
or its family of parts, which is based on various factors. These include:

1. Type of material.
2. End use of part.
3. Processing characteristics:
a. casting
b. ingot
c. rolled
d. forged
e. welded
f. machined
4. Inspection points:
a. finished machined
b. as-welded
c. following load testing
d. inservice
e. maintenance

Acceptance standards that cover a part category or family, such as pump

parts for a specific application or end user, or brake rotors used by a
specific automobile manufacturer, may have acceptance standards that are
applicable to all parts that fall within the same category. Usually, when this
is the case, the parts are made of similar materials that have been
processed similarly too.

Although most MT acceptance standards are similar, there are normally

some differences, as with other types of standards. Because each standard
is different, it is important that inspectors become familiar with each one
that they will be working with. Assuming that all discontinuities are
handled the same by all standards can lead to serious problems.

Interpreting Inspection Standards

Interpreting acceptance standards is a very important part of being an
NDT inspector. The end result for all NDT examinations is a final
disposition; that is, when the item being examined is either accepted or
rejected. In some ways, this is the most critical part of the examination
process because if an inspector does not understand or misinterprets the
acceptance criteria, a discontinuity that should have been cause for
rejecting the part may be accepted. If a part that has a rejectable

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 discontinuity is placed into service, it could ultimately cause a failure to
occur, which could place people or equipment at risk.

Acceptance standards typically include limits for the types of

discontinuities that would likely be found during the NDT examination
being performed. For instance, acceptance criteria for an MT examination
would likely be a little different than they would for a liquid penetrant
examination. This is because an MT indication and a PT indication
represent the discontinuity differently. An MT indication represents, for
the most part, the actual size of the discontinuity, whereas the size of a PT
indication represents the amount of liquid penetrant that was trapped and
released from the discontinuity.

So, if we were developing acceptance criteria for an MT examination of a

part where we could find laminations, the acceptance criteria would list
the maximum dimension of the actual discontinuity that would be
allowed. For the same discontinuity discovered during a PT examination,
assuming it was open to the surface, the acceptance criteria would need to
be based on the amount of penetrant that the largest lamination allowed
could hold. Once that was determined, we would need to be able to
estimate the size of the indication once the evaluation time began.
Although the discontinuity in both of these scenarios is the same, the
dimensions referenced by the acceptance standard would be slightly
different. An example of the difference between PT and MT indications
is presented in Figure 12.3.

(a) (b)

Figure 12.3: Centerline cracking in a weld: (a) liquid penetrant indication; (b) magnetic particle
indication.

Magnetic Particle Testing 227

 Question 12.9

Acceptance standards are used to:

A. evaluate the inspector.
B. determine what kind of discontinuity a particular indication is.
C. determine if an indication exceeds allowable limits.
D. determine how to repair discontinuities.

 Please turn to the end of the chapter for the answer.

Variations in Acceptance Standards

Acceptance standards for MT inspections vary depending on many
factors, including:

• Material used to make the part.

• Processes used to form the part.
• Type of examination being performed.
• End use of the part.
• Environmental conditions the part is exposed to.

Although there is some variability among acceptance standards, there are

also some similarities too. Here are a few examples:

1. In most cases, acceptance standards allow for indications that are

nonrelevant. Nonrelevant discontinuities are there by design, for
example, keyways, sharp radii, or other geometric features of the
test object. An indication needs to be determined if it is of a
relevant or nonrelevant discontinuity. Also, a nonrelevant
indication could mask a true indication of a relevant discontinuity.
An example of nonrelevant MT indications is shown in
Figure 12.4. Note, however, that there are conflicting definitions
and uses of the terms nonrelevant and false indications. In some
industry parlance, false indications are caused by geometric
features of the test object, whereas nonrelevant indications are
those that can be disregarded due to size. However, in this
publication, false indications are defined as indications caused by
an aspect of the magnetic particle testing procedure without
relation to a discontinuity. For example, a false indication may be
caused when ferromagnetic particles are accumulated and held

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Figure 12.4: Nonrelevant MT indications Figure 12.5: Clustered porosity in a weld
on threads. using fluorescent PT.

onto the surface of the part by material obstructions, such as dirt,

fingerprints, rust, scale, or paint drip lines. A false indication may
also be caused by improper processing, for instance, of drain lines.
Also, particles may be held by gravity in shallow depressions.
2. Although for PT examinations, rounded indications are common,
as shown in Figure 12.5, for MT, rounded indications are not
nearly as prevalent. Yes, they are included in acceptance standards,
but often the leakage field of smaller rounded discontinuities is not
strong enough for them to show up during an MT examination.
3. The most common type of indication that a technician will
discover when performing MT inspections is a linear indication, as
shown in Figure 12.6. This type of discontinuity is usually the most
detrimental to the integrity
of any part. Although many
acceptance standards do not
allow any linear indications,
regardless of size, some do.
Therefore, make sure you
know the acceptance criteria
for the inspection you’re
performing. Do not assume
the acceptance standard you
used during a previous Figure 12.6: Through-wall fracture visible
inspection applies to the part with fluorescent MT.
you’re examining today.

Magnetic Particle Testing 229

 Summary
For NDT inspections, standards provide all of the necessary information
for personnel certification, developing effective inspection procedures, and
evaluating the indications found during MT examinations. As long as their
requirements are followed correctly, any parts that have rejectable
discontinuities should be identified and corrective measures taken to
ensure that they are not released into service.

Question 12.10
Which of the following is not typically addressed in a standard for NDT?
A. Personnel certification and near-vision acuity.
B. Acceptance criteria for linear indications.
C. Material specification of concrete and steel.
D. Techniques to be used in an inspection method.

 Please turn to the end of the chapter for the answer.

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Chapter 12 Summary

r NDT personnel certification standards are intended to improve the

consistency of the process for certifying inspectors.
r NDT certification typically requires five steps: training, experience,
written examinations, practical examination, and eye
examinations.
r Training includes attending formal classes specifically for the NDT
method that you’re certifying for.
r Experience in MT testing includes sample preparation, testing, and
evaluation.
r Written examinations comprise a general exam on MT theory and
a specific exam on MT processing requirements.
r A practical exam tests your ability to actually perform a magnetic
particle test.
r Certification requires two eye exams: one for near vision and the
other for color perception.
r Various certification standards have different requirements per
industry; SNT-TC-1A is a recommended practice allowing more
flexibility than a standard.
r For each MT examination performed, there is a standard that
details exactly how to perform it.
r Many industries have written their own examination standards for
the specific methods and techniques used to inspect their parts
and assemblies.
r The main difference between various NDT examination standards
is the level of detail included.
r Acceptance standards govern a final disposition as to whether a
part or component is accepted or rejected following an MT
inspection.
r Acceptance criteria for MT may differ from those for liquid
penetrant testing (PT) because of the way indications are formed
for each method.
r Acceptance standards for MT inspections vary depending on such
factors as the end use of the part and the environmental
conditions the part is exposed to.
r Acceptance standards enable discrimination between relevant
and nonrelevant as well as linear and rounded indications.

Magnetic Particle Testing 231

 Answers to Chapter 12 Questions
Question 12.1
Answer: D – The process for certification includes five steps.

Question 12.2

Answer: D – For example, NAS 410 requires 32 hours of training, which

equates to 16 hours for Level I and an additional 16 for Level II, whereas
ASNT Level II certification in MT requires only 20 hours.

Question 12.3

Answer: B – Both a general and specific written examination are required.

Question 12.4

Answer: C – Either J1 or J2 is used, depending on the standard.

Question 12.5

Answer: A – The Aerospace Industry Association oversees this standard.

Question 12.6

Answer: D – SNT-TC-1A allows the user to be flexible with its certification

recommendations.

Question 12.7

Answer: A – ASTM E 1444/E 1444M has been developed for use within the
aerospace industry.

Question 12.8

Answer: B – Most examination standards do not include acceptance

criteria.

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Question 12.9

Answer: C – Acceptance standards provide the inspector with criteria for

evaluating indications found during MT examinations.

Question 12.10

Answer: C – Material specification standards would normally not be used

for developing NDT inspection techniques, certifying personnel, or
providing acceptance criteria.

Magnetic Particle Testing 233

 Chapter 12 Review
1. Acceptance standards are created based on all of the following except:
A. material the part is made of.
B. processes used to form the part.
C. end use of the part.
D. type of equipment being used during the examination.

2. Inspection standards provide specific details for:

A. evaluating indications.
B. certifying personnel.
C. performing NDT examinations.
D. choosing the correct NDT inspection method.

3. SNT-TC-1A is an:
A. MT inspection standard.
B. NDT personnel certification recommended practice.
C. NDT personnel certification standard.
D. MT equipment calibration standard.

4. A nonrelevant indication is defined as a(n):

A. rejectable MT indication.
B. indication that is detrimental to the part.
C. indication that may be ignored.
D. indication that must be removed.

5. Standards used to certify personnel define all of the following except:

A. inspection process requirements.
B. training hours required.
C. experience hours required.
D. recertification requirements.

6. How many basic steps are involved with certifying NDT personnel?
A. 1
B. 3
C. 5
D. 10

234 Programmed Instruction Series

7. MT acceptance criteria are based on which of the following?
A. MT technique used.
B. The size of the indication after it is allowed to develop.
C. The actual size of the discontinuity.
D. Type of MT particles used.

8. An inspection standard provides:

A. definitions for types of indications to be evaluated.
B. the number of questions required for the written examinations.
C. eye examination requirements.
D. formulas for calculating amperages used for a prod inspection.

9. Standards:
A. are not used in NDT.
B. help to ensure examination processes work properly.
C. are all the same, regardless of the inspection technique.
D. can be changed without justification.

10. NAS 410 requires how many training hours for Level II MT personnel
certification?
A. 12
B. 16
C. 24
D. 32

Magnetic Particle Testing 235

 Chapter 12 Review Key
1. D

2. C

3. B

4. C

5. A

6. C

7. C

8. D

9. B

10. D

236 Programmed Instruction Series

 Chapter 13
Discontinuities Detectable by MT
Photo taken by Michael Schultz Photography, courtesy of Scot Forge.

In this chapter:
Difference between discontinuities and defects
Discontinuities associated with inherent processes
Difference between primary and secondary processes
Discontinuities resulting from forging, rolling, and extruding
Discontinuities arising from welding operations
Secondary processing discontinuities
Inservice discontinuities caused by stress and environment

237
 What Is a Discontinuity?
Discontinuities come in all shapes and sizes. The cause for each type of
discontinuity can be attributed to a specific process, such as pouring of an
ingot, grinding, welding, and machining, or the environment to which it is
exposed, for example, salt water, extreme cold, tension cycling, and others.
A discontinuity is defined as the quality or state of not being continuous,
lack of continuity, or a change or break in a process. This definition can also
be used to describe a discontinuity in ferromagnetic metals. Per
ASTM E 1316, Standard Terminology for Nondestructive Examinations,
a discontinuity is “a lack of continuity or cohesion; an intentional or
unintentional interruption in the physical structure or configuration of
a material or component.”

When there is a break or interruption in the material, regardless of how

it got there, it is a discontinuity. However, it is important to remember
that a discontinuity is not yet a defect. It is only a defect when it has been
evaluated and determined to be outside of the limits of the applicable
acceptance criteria. Once it is determined to be over the limits of the
acceptance criteria, the discontinuity can now be described as a defect.
For example, a keyway or section change represents an intentional
interruption of the physical structure of a part, and the indication it
produces is a type of nonrelevant indication that is not rejectable.

Question 13.1

A discontinuity is defined as:

A. a defect.
B. a crack in all cases.
C. lack of continuity.
D. any indication discovered during an MT examination.

 Please turn to the end of the chapter for the answer.

Although inspectors may not think it is important to know why a

discontinuity is present in the part or material that they are inspecting, the
truth is that it is important. Knowing the types of discontinuities that may
be discovered during an inspection can be very useful. For instance, if an
inspector were examining a part made from a rolled plate and discovered
an intermittent linear type of indication in the middle of the plate, it could

238 Programmed Instruction Series

be concluded that it was likely to be caused by a lamination located under
the surface of the material being inspected. This is very common in rolled
steel and can occur during coinage, as shown in Figure 13.1.

The reason that this is important to know is

that if the inspector was not knowledgeable
of the types of discontinuities in rolled plate,
this indication may be dismissed as being
nonrelevant and the part would be accepted.
This is why inspectors should be
knowledgeable of the most common types of
discontinuities found in ferromagnetic steel
as well as the causes of discontinuities in the
materials that they will be examining. Figure 13.1: Example of a lami-
nation in a coin.
Question 13.2

When are discontinuities considered to be defects?

A. They are always defects.
B. When they are discovered during an MT examination.
C. When they exceed the limits of the acceptance criteria.
D. When they are greater than 1 in. (25.4 mm) long.

 Please turn to the end of the chapter for the answer.

Inherent Discontinuities
Discontinuities in steel come in all shapes and sizes and can be attributed
to a wide variety of causes. In the NDT world, there are typically three
categories of discontinuities: inherent, processing (both primary and
secondary), and service-induced (referred to as inservice in this publication)
based on the process during which they occur.

Inherent processes include:

• Steelmaking.
• Casting.

Magnetic Particle Testing 239

 The majority of all steel is manufactured using some type of casting
process, which includes ingot, centrifugal, continuous, die, and sand
castings.

Question 13.3

The majority of steel is manufactured using which process?

A. Casting.
B. Cooling.
C. Vacuum.
D. Cold-formed.

 Please turn to the end of the chapter for the answer.

Inherent discontinuities can be attributed to the processes of making steel.

There are a number of discontinuities unique to these processes; however,
the majority of inherent discontinuities will not be discovered until
magnetic particle examinations are performed following primary or
secondary processing. Inherent discontinuities are a result of problems
that occur during the initial melting and refining processes (used to
produce ingots, for instance) and during solidification from the molten
state (in the form of castings).

Although there are some discontinuities unique to specific casting

processes, we will focus on the most common types, which can result from
most, if not all, of the casting processes. These discontinuities are likely to
produce magnetic particle indications if they are not removed prior to the
MT examination.

Question 13.4

Inherent discontinuities may be attributed to which of the following

manufacturing processes?
A. Mold forming.
B. Casting.
C. Metal shaping.
D. Machining.

 Please turn to the end of the chapter for the answer.

240 Programmed Instruction Series

What follows are some examples of common casting discontinuities.

Shrinkage
Three types of shrinkage can occur in castings: liquid, solidification,
and patternmaker. (See Figure 13.2.) Shrinkage in castings occurs when
the volume of molten metal is reduced at various stages during the
solidification process. During cooling of the solidified metal, contractions
may cause tensile stresses that result in ruptures.

(a) (b)

Figure 13.2: Shrinkage cracks: (a) visual testing; (b) magnetic particle testing.

The location of shrinkage cracks can be surface or subsurface, with cracks

open to the surface more readily detectable using MT. Near-surface cracks
may go undetected. Shrinkage cracks are sharp, clean-cut indications that
usually appear in groups, in some cases with fine, branchlike patterns.

Porosity
Porosity is caused by gas that forms
bubbles inside of a casting after it has
solidified and has cooled. Liquid metal
retains gas, which forms gas pockets
within the material. Porosity can
appear on the surface of the casting (as
blowholes) or inside the casting.
Either way, the strength of the material
is reduced. Porosity is not always
detected or well-defined using MT
Figure 13.3: Surface porosity.
since indications are neither strong
nor pronounced. In general, only
surface porosity will be evident, as shown in Figure 13.3.

Magnetic Particle Testing 241

 Blowholes
Blowholes, a type of porosity, are the
result of trapped gas pockets that are
close to the surface of a casting. As the
molten metal cools and solidifies, the
trapped gas is able to escape, but the
material is too solid to fill in the voids
left behind. (See Figure 13.4.)
Blowholes appear as rounded
cavities—flattened, elongated, or Figure 13.4: Blowholes. (Courtesy,
spherical—and may appear as seams American Foundry Society.)
in a rolled ingot. Deep blowholes that
are not rolled shut may take the form of laminations.

Question 13.5

The difference between porosity and blowholes is:

A. porosity is larger than blowholes.
B. blowholes are subsurface.
C. blowholes and porosity are the same except the blowholes are
created as the trapped gas escapes.
D. porosity is not a casting discontinuity.

 Please turn to the end of the chapter for the answer.

Cold Shuts
Cold shuts occur due to the material freezing before completely filling
the cavity of the mold, resulting in imperfect fusion between the two
streams of converging metal. Cold shuts significantly weaken the part in
the area where they occur. This discontinuity produces magnetic particle
indications similar to those of cracks or seams, with smooth or rounded
edges. (See Figure 13.5.)

242 Programmed Instruction Series

 Figure 13.5: Cold shut.

Question 13.6

When molten metal flows into a casting but does not fuse together, it is
called:
A. a lamination.
B. porosity.
C. a cold shut.
D. shrinkage.

 Please turn to the end of the chapter for the answer.

Nonmetallic Inclusions
Nonmetallic inclusions result
from a variety of physical and
chemical effects occurring in the
liquid metal. (See Figure 13.6.)
There are two categories of
nonmetallic inclusions: natural
and foreign. Natural inclusions
result from oxides, nitrides, and
sulfides. Foreign inclusions
include slag, refractory bricks
Figure 13.6: Inclusion.
material, and mold materials.

Magnetic Particle Testing 243

 Nonmetallic inclusions can become stress risers because of their
discontinuous shape and incompatibility with surrounding material.
Typically, inclusions are mechanically worked during rolling or forming,
which deforms them into elongated shapes that appear in longitudinal
sections as stringers or seams. MT can be used to evaluate surface and
near-surface nonmetallic inclusions in ferromagnetic materials.

Question 13.7

There are two categories of inclusions:

A. primary and natural.
B. foreign and secondary.
C. natural and unnatural.
D. natural and foreign.

 Please turn to the end of the chapter for the answer.

Hot Tears
Hot tears are a type of crack
caused by nonuniform cooling
resulting in stresses that
rupture the surface of the
metal while its temperature is
still in the brittle range. (See
Figure 13.7.) Tears may
Figure 13.7: Hot tear in a casting.
originate where stresses are
built up by more rapid cooling
of thin sections adjoined to heavier masses of metal. With MT, hot tears
present sharp, jagged, linear indications, either continuous or intermittent,
with possible branching. Subsurface cracks may not be detectable until
after machining.

Primary Processing Discontinuities

Many of the indications that you will find during an MT examination
result from discontinuities introduced during primary and secondary
processing.

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Primary manufacturing processes include:

• Rolling.
• Forging.
• Extruding.
• Welding.

Secondary manufacturing processes include:

• Machining.
• Grinding.
• Heat-treating.
• Plating.

Just about every piece of manufactured steel will require some type of
processing. Forging, rolling, and welding are just three of the primary
processes that are used to form and shape raw steel. Secondary processes,
such as machining, are used to further refine the processed product into a
more finished form. In this section, we will consider discontinuities
arising from primary processes.

Forging, Rolling, and Extruding Discontinuities

Forging Cracks and Laps

Forging cracks, shown in Figure 13.8, are discontinuities formed during
the mechanical shaping of metal. Typically, they are caused by too much
stress when the part is forged, which can result from the incorrect design
of dies, temperature inconsistencies—for instance, forging at too low of a

Figure 13.8: Forging cracks.

Magnetic Particle Testing 245

 temperature, rupturing the metal—and too much material being forced
through the die. In addition to cracks, an unfilled section may result when
a portion of the die cavity is not completely filled by the flowing metal.
Forging cracks typically occur at the juncture of light and heavy sections
or in thin fins of turbine blades.

Forging laps result from metal being folded over, forming an area that is
squeezed tightly but not welded together. (See Figure 13.9.) Usually open
to the surface, forging laps typically enter the surface at a small angle.
However, forging laps may be smeared closed by further processing or
rolling operations. MT of forging laps may produce straight, spiral, or
slightly curved indications. Magnetic buildup of laps may be small, thus
requiring a magnetizing current greater than that used for the detection
of cracks. Correct magnetizing technique should be used because the
discontinuity may not be parallel to the surface.

Figure 13.9: Ferromagnetic particles attracted to the flux leakage of a forging lap, proba-
bly caused by poor die design.

Question 13.8

Forging cracks can be caused by all of the following except:

A. folding over of die metal.
B. uneven temperature.
C. incorrect dies.
D. excessive material.

 Please turn to the end of the chapter for the answer.

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Forging Bursts
External forging bursts
typically occur when the
forming section is too severe
or where sections are thin.
(See Figure 13.10.) Internal
bursts, by contrast, are found
in bars and forgings and
result from excessive hot-
working temperatures. Bursts Figure 13.10: External forging burst.
form straight or irregular
cavities varying in size from
wide open to very tight. MT produces a straight indication or irregularly
shaped rounded indication. Results using MT are limited to surface and
near-surface evaluation.

Laminations
A lamination results from an abnormality within steel that is expanded
linearly during the rolling process. (See Figure 13.11.) It is usually aligned
parallel to the rolled surface and appears as separated layers within the
material. Laminations can be caused by seams, inclusions, or pipe
(shrinkage in the shape of an inverted cone in the top portion of a
casting).

(a) (b)

Figure 13.11: Laminations indicated with: (a) fluorescent MT; (b) visible MT.

Magnetic Particle Testing 247

 Laminations can be detected by MT at an end or at a transverse cross
section taken through rolled plate. They are subsurface, generally flat,
and extremely thin. MT indications are straight and intermittent.

Question 13.9

A lamination typically appears as a(n):

A. curved linear indication.
B. rounded indication.
C. straight linear indication.
D. indication only found using X-rays.

 Please turn to the end of the chapter for the answer.

Seams
Seams originate from
blowholes, cracks, and tears
introduced during earlier
processing and elongated in
the direction of forging,
rolling, or extruding. With
the use of MT on a typically
oxidized surface, a seam may
appear to form a very tight,
usually straight, cracklike Figure 13.12: Seam in extruded rod (thin horizontal
indication. Figure 13.12 line).
shows a seam in an extruded
rod used in the manufacture of aircraft components. A seam (thinner of
the two lines) was created as the rod was extruded and is the total length of
the rod—approximately 3 ft (0.9 m).

Hydrogen Flakes
Hydrogen flakes are formed while cooling after forging or rolling
operations. (See Figure 13.13.) They are produced by decreased solubility
of hydrogen during cooling after hot-working. The flakes are usually
found deep in heavy steel forgings, are extremely thin, and are aligned
parallel with the grain. Because of their positioning, hydrogen flakes are
not detectable by MT unless machining brings them near the surface.
If MT indications are formed, hydrogen flakes appear as short
discontinuities sometimes referred to as chrome checks or hairline cracks.

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Figure 13.13: Cross section of hand forging showing hydrogen flakes in the center of the
material.

Welding Discontinuities

Cold Cracking (Heat-Affected Zone Cracks)

Cold cracking produces sharply defined, heavy magnetic particle
indications if they are open to the test object surface, as shown in
Figure 13.14. Indications are also produced by underbead cracks that
extend to the weld toe. Weld metal cracks may be oriented in any
direction and are often
associated with nonmetallic
inclusions. Subsurface
indications may be harder to
detect with MT.

Along the edge of every weld is

an area in the base metal called
the heat-affected zone (HAZ).
The temperature in the heat-
affected zone is below the
melting point but high enough
to affect grain structure,
hardness, ductility, strength, Figure 13.14: Cross section of a weld joint
and other material and exhibiting hydrogen-induced cold cracking in
the weld metal.
microstructures of the metal.

Magnetic Particle Testing 249

 The width of the HAZ varies depending on the amount of heat in the
weld, interpass temperature, and cooling rate, although as much as 1 in.
(25 mm) of the base metal on either side of a weld may be included.

A heat-affected zone crack is a cold crack that occurs adjacent to the fusion
line. The exact cause of an HAZ crack is unknown; however, dissolved
hydrogen must be present and internal stresses are thought to be another
contributor. (See Figure 13.15.) Residual stresses caused by weld shrinkage
or externally applied
tensile stresses result in
hydrogen-induced cracks
in hydrogen-rich areas of
the HAZ. Cold cracking
is also known as
underbead or delayed
cracking that occurs upon
cooling over a period of
time from hours up to
several days. Figure 13.15: HAZ crack.

Question 13.10

A HAZ crack usually occurs in the:

A. center of the weld.
B. stops and starts of a weld.
C. base metal.
D. zone next to the toe of a weld.

 Please turn to the end of the chapter for the answer.

Hot Cracking (Crater Cracks)

Hot cracking in the HAZ is caused by high restraint of the joints and
improper electrode control. Also, hot cracking of the HAZ weldment
increases in severity with increasing carbon content. Steels containing
more than 0.3% carbon are prone to this type of failure. This kind of
cracking is often quite deep and very tight. HAZ cracking on the surface
of the part, usually parallel to the weld bead, is easily detected by MT.

250 Programmed Instruction Series

Figure 13.16: Crater crack indication using Figure 13.17: Crater crack indication
visible MT. using fluorescent MT.

Crater cracks, a type of hot cracking, occur in the weld crater and are
caused when the crater is not filled when the weld arc is removed. The
crack is caused by stresses in the crater formed at the termination of a
weld pass. The stresses are introduced when the edges of the crater cool
too quickly. Crater cracks are typically star-shaped on the surface.
Examples of crater cracks are shown in Figures 13.16 and 13.17.

Another form of solidification cracking is called centerline hot cracking

because it follows the longitudinal centerline of the weld bead. (Refer back
to Figure 12.3.) The likelihood of this type of discontinuity is increased by
high travel speed, high depth-to-
width ratio of the weld, and a small
weld bead, particularly at the root
pass.

Discontinuties caused by hot

cracking are also referred to as
surface shrinkage cracks or star cracks.
Cracks range in size from very small,
tight, and shallow to open and deep.
Cracks may run parallel or transverse
to the direction of welding. A large Figure 13.18: Surface shrinkage crack.
shrinkage crack with porosity in
welded tube stock is shown in
Figure 13.18.

Magnetic Particle Testing 251

 Question 13.11

Which type of hot cracking appears as a longitudinal discontinuity along

the weld bead?
A. Surface shrinkage crack.
B. Centerline hot cracking.
C. Crater crack.
D. Star crack.

 Please turn to the end of the chapter for the answer.

Lamellar Tearing
A lamellar tear is a base metal crack
that occurs in plates and shapes of
rolled steel exhibiting a high
nonmetallic inclusion content. During
manufacturing, the inclusions are rolled
flat in the steel plate, severely reducing
(a)
strength and ductility in the through-
thickness direction. Shrinkage stresses
caused by solidification of an adjacent
weld bead may induce separation
between lamellar planes, resulting
in a terraced fracture, as shown in
Figure 13.19. Lamellar tearing is
readily detectable by magnetic (b)
particle techniques.
Figure 13.19: Lamellar tearing:
(a) diagram showing weld joints in
Lack of Fusion steel plate; (b) typical visual appear-
Lack of fusion occurs when some ance and location.
portion of the weld filler metal
fails to coalesce with the adjacent
base metal or the weld metal from
a previous pass. Lack of fusion
may or may not occur near the
outside surface of the weld joint.
The closer it is to the surface, the
sharper the magnetic particle Figure 13.20: Magnetic particle indication of
indication, as shown in lack of fusion in a high-frequency resistance-
welded tube.
Figure 13.20.

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Lack of Penetration
Sometimes confused with
lack of fusion, lack of
penetration is inadequate
(less than specified)
penetration of the weld joint
root. (See Figure 13.21.)
This condition can be
caused when an insufficient
root gap is provided during
fit-up operations or when
residual welding stresses Figure 13.21: Voids and discontinuities created by
close the gap. Other causes lack of penetration in a tube stock weld using fluo-
include too fast of a welding rescent MT.
rate, too large of a welding
rod, or too cold of a weld bead. Lack of penetration appears as an irregular
magnetic particle indication of varying width, similar to a subsurface
longitudinal crack, and follows the centerline of the weld. MT is normally
used where the backside of the weld is visible.

Inclusions
Inclusions may be metallic or nonmetallic and may appear individually
or be linearly distributed or scattered throughout the weld. The most
common are slag and tungsten inclusions. Slag inclusions are generally
created by failure to remove slag between passes in a multilayer operation.
Tungsten inclusions typically occur when particles of the tungsten
electrode are transferred into the
weld deposit. Metallic inclusions
are shown in Figure 13.22.

Slag inclusions may be straight

intermittent or continuous
indications usually running along
the toe of the weld. However, a
magnetic particle indication of a
slag inclusion is weak and poorly
defined, requiring a high-strength
magnetizing field for detection.
Tungsten inclusions are typically
rounded discontinuities that may
Figure 13.22: Metallic inclusions in a weld.

Magnetic Particle Testing 253

 be hard to detect with MT. Although MT is not normally used for
detecting inclusions in welds, the magnetizing technique should be such
that surface and near-surface inclusions may be satisfactorily detected
when their axes are in any direction.

Question 13.12

Which of the following discontinuities does not appear in the weld per se
but in the base metal?
A. Crater crack.
B. Lamellar tear.
C. Slag inclusion.
D. Centerline hot cracking.

 Please turn to the end of the chapter for the answer.

Arc Strikes
Arc strikes caused by welding practices are found in ferrous and nonferrous
welded material. They are situated on the surface of the base metal where
the welder has momentarily touched an arc-welding electrode to start the
arc, resulting in a localized weld. Arc strikes often harbor minute cracks
and porosity that can cause failure of the affected material. MT indications
of arc strikes may appear fuzzy or porous. Arc strikes should also be
detectable by visual testing. (See Figure 13.23.)

Figure 13.23: Surface arc strike.

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Secondary Processing Discontinuities
Machining Tears
Machining tears are caused by poor machining practices, such as improper
or dull tools or lack of proper coolant. For example, a dull machining tool
shears off metal in a manner that produces rough, torn surfaces. Soft or
ductile materials, such as low-carbon steel, are more susceptible to
machining tears than harder steels. Heavy cuts and residual tool marks
from rough machining act as stress risers. Machining tears are surface
discontinuities that are reliably detected with MT.

Grinding Cracks
Grinding cracks can be attributed to the use of glazed wheels, inadequate
coolant, excessive feed rate, or attempts to remove too much material in
one pass. They are typically at right angles to the grinding direction and
are very shallow. Often, grinding cracks are forked and sharp at the
root. They are critical stress risers that form a potential for failure. Wet
continuous MT is the best choice for detecting grinding cracks. Examples
are shown in Figures 13.24 and 13.25.

Figure 13.24: Magnetic particle indications Figure 13.25: Grinding cracks along a weld
of cracks on a rotor disk due to localized bead in tube stock using fluorescent MT.
overheating.

Heat-Treating and Quench Cracks

To obtain a specific hardness and microstructure, materials are
customarily heat-treated. During this operation, the metal is heated and
cooled under controlled conditions. However, in some cases, the process
produces stresses that exceed the material’s tensile strength, causing it to
form heat-treating cracks.

Magnetic Particle Testing 255

 When a test object is quenched or cooled following heat-treating, the
initial transformation occurs at the test object’s surface. When the interior
cools and transforms, volumetric expansion takes place, but the interior
expansion is restrained by the solidified layer on the surface. If the solid
layer does not expand enough or if the internal expansion is great enough,
cracking through the outer layer results. The cracks that occur during this
cooling cycle are called quench cracks.

Magnetic particle indications of heat-treating cracks are usually deep and

may be straight, forked, or curved. Seldom following a definite pattern,
they can be in any direction in the part. They originate in areas with rapid
change of material thickness, sharp machining marks, fillets, nicks, and
discontinuities that have been exposed to the surface of the material. Heat-
treating cracks are serious and usually warrant unconditional rejection of
the part.

Question 13.13

Which of the following discontinuities is not due to secondary processing

operations?
A. Arc strike.
B. Quench crack.
C. Machining tear.
D. Grinding cracks.

 Please turn to the end of the chapter for the answer.

Inservice Discontinuities
Inservice discontinuities that occur when the material is in use are usually
caused by repeated stresses. These stresses may be constant, random, or
cyclic. Unfortunately, stress discontinuities are usually not discovered until
they are large enough to be seen visually or after the material has failed.
If MT is used to inspect parts or components in service, it is essential for
testing personnel to know the service conditions of the component to
accurately perform a magnetic particle test. When stress discontinuities
are identified by visual or magnetic particle inspection, steps can be taken
to repair the affected area or, when possible, the affected part can be
replaced.

256 Programmed Instruction Series

Discontinuities that occur during the service of a part or component
include:

• Fatigue cracks.
• Creep and overstress cracks.
• Stress-corrosion cracking.
• Hydrogen cracking.

Fatigue Cracks
Fatigue cracks occur when a
material is subjected to repeated
stresses. (See Figure 13.26.) When
the loads are taken beyond a
certain limit, cracks begin to
form. Following further stresses
being applied, the cracks will
grow and eventually the material Figure 13.26: Fatigue crack.
will fail. MT produces fine, sharp,
jagged lines with a good buildup of particles that normally follow the
stress lines of the part. Caution: Over-magnetization can mask fatigue
cracks in their early stages.

Creep Cracking
At temperatures greater than half
the melting point and at stresses
below the yield strength of the
material, deformation can occur
by action of grains gradually
separating over an extended
Figure 13.27: Photomicrograph of fracture
period of time. This can and creep in various stages in the heat-
eventually lead to cracking and affected zone near the fusion zone interface.
failure. This deformation or
failure mechanism is called creep.

Creep can be detected and controlled. Periodic tests, particularly those

involving field metallography and circumferential measurement can be
used to monitor creep, as shown in Figure 13.27.

Magnetic Particle Testing 257

 Stress-Corrosion Cracking
Stress-corrosion cracking is a fracture mechanism that results from the
combined effects of a static tensile load and a corrosive environment. The
stress involved can either be from
actual applied loads or from
residual stress. Common examples
of materials in corrosive
environments include aluminum
and austenitic stainless steels
exposed to salt water; copper and
copper alloys exposed to ammonia;
and mild steel exposed to sodium
hydroxide.

Stress-corrosion cracking produces

brittle failure, either intergranular
or transgranular, depending on the
type of alloy or the corrosive
environment. In most cases, while
fine cracks penetrate into the cross
section of a component, the surface
shows little evidence of corrosion.
Cracks range from shallow to very
deep and usually follow the grain of Figure 13.28: Photomicrograph showing a
typical stress-corrosion crack, including a
the material, although transverse small pit produced by corrosive attack acting
cracks are also possible. An as a stress riser.
example of stress-corrosion
cracking is shown in Figure 13.28.

Hydrogen Cracking (Embrittlement)

Hydrogen cracking, also referred to as hydrogen embrittlement, is
a small, nondimensional interface with no orientation or direction.
This discontinuity results from the corrosive environment produced by
hydrogen media and usually occurs in conjunction with an applied
tensile stress or residual stress. If no stress riser is present on the surface,
hydrogen can diffuse into the metal and initiate subsurface cracks. In
low-strength alloys, this condition can lead to what is called hydrogen
blistering.

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 Magnetic indications appear as a fractured pattern. Hydrogen
embrittlement follows grain boundaries and rarely shows signs of
branching. It can be found in highly heat-treated material that has been
plated, as shown in Figure 13.29.

(a) (b)

Figure 13.29: Hydrogen embrittlement: (a) under chrome plate; (b) detailed crack pattern.

Ë From the Field: Although we would prefer to never have

inservice discontinuities occur, the good news for NDT
inspectors performing magnetic particle examinations is that
they almost always appear as linear indications and are usually
easy to find. Understanding each type of discontinuity, their
cause(s), and how they appear during an NDT examination is
very useful to NDT inspectors. It allows an inspector to
anticipate where indications are likely to occur.

Magnetic Particle Testing 259

 Question 13.14

Fatigue cracks result from __________ stress on a material.

A. hydrogen-induced
B. corrosive
C. repeated
D. high-temperature

 Please turn to the end of the chapter for the answer.

Summary
The purpose of performing magnetic particle examinations is to identify
discontinuities at various stages during manufacturing or while the
material is in service. To ensure the best examination results, it is
important that NDT inspectors have a thorough understanding of the
processes that create discontinuities, where in the material they are likely
to be found, and how the discontinuities would appear as an MT
indication. The more you know about each type of indication, the more
likely you are to find it.

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Chapter 13 Summary

r Per ASTM E 1316, a discontinuity is “an intentional or unintentional

interruption in the physical structure or configuration of a material
or component.”
r A discontinuity is only considered a defect when it has been
evaluated and determined to be outside of the limits of the
applicable acceptance criteria.
r Three categories of processes cause discontinuities: inherent,
processing (both primary and secondary), and service-induced (or
inservice).
r Inherent discontinuities are attributable to steelmaking, including
initial melting and refining processes and during solidification
from the molten state.
r Inherent discontinuities include shrinkage cracks and porosity in
castings, along with cold shuts, nonmetallic inclusions, and hot
tears.
r Many of the indications discovered during an MT examination
result from discontinuities introduced during primary and
secondary processing.
r Primary processing discontinuities arise from forging, rolling, and
extruding operations, as well as welding processes.
r Welding discontinuities include both cold cracking in the heat-
affected zone (HAZ) and hot cracking such as crater cracks.
r Linear discontinuities in welds include lamellar tears, lack of
fusion, and lack of penetration.
r The most common types of inclusions in welds are slag and
tungsten inclusions.
r Arc strikes often contain small cracks and porosity that can cause
failure of the affected material.
r Secondary processing discontinuities include machining tears,
grinding cracks, and heat-treating cracks.
r Inservice discontinuities such as fatigue cracks are caused by
repeated stresses when the material is in use.
r Stress-corrosion cracking is a fracture mechanism that results from
the combined effects of a static tensile load and a corrosive
environment.
r Hydrogen embrittlement results from a corrosive hydrogen-rich
environment and appears as a fractured pattern.

Magnetic Particle Testing 261

 Answers to Chapter 13 Questions
Question 13.1
Answer: C – A discontinuity is anything that interrupts the continuity of
the metal or material.

Question 13.2

Answer: C – Discontinuities are acceptable until they exceed acceptance

criteria.

Question 13.3

Answer: A – Casting is the most common process used to make steel.

Question 13.4

Answer: B – Casting is an inherent process.

Question 13.5

Answer: C – Porosity can be internal or external; pores caused by escaping

gas are called blowholes.

Question 13.6

Answer: C – Cold shuts are caused by molten material cooling before

fusing together.

Question 13.7

Answer: D – Natural inclusions include oxides and sulfides. Foreign

inclusions, such as refractory bricks and slag, are contaminants.

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Question 13.8

Answer: A – Forging cracks can be caused by uneven temperatures,

improper dies, and excessive material in the workpiece. Too little material
may result in cavities or unfilled sections. The folding over of metal
produces a forging lap.

Question 13.9

Answer: C – Laminations normally appear as a straight line, which may be

intermittent.

Question 13.10

Answer: D – HAZ cracks occur next to the HAZ in the fusion line. They can
be caused by internal stresses within the material.

Question 13.11

Answer: B – A centerline hot crack follows the weld bead. Crater cracks are
typically star-shaped.

Question 13.12

Answer: B – A lamellar tear occurs in the base metal near a weld. However,
this type of separation may be caused by shrinkage stresses of an adjacent
weld bead.

Question 13.13

Answer: A – Arc strikes result from improper welding procedures.

Question 13.14

Answer: C – Fatigue stress is caused by excessive stress over time, not

necessarily related to corrosion, hydrogen, or temperature.

Magnetic Particle Testing 263

 Chapter 13 Review
1. Cold shuts occur as a result of molten metal cooling too much as
it comes together and:
A. do not affect material strength.
B. do not usually cause discontinuities that are rejectable.
C. significantly reduce the strength of the material.
D. slightly affect material strength.

2. Inservice discontinuities may be caused by each of the following

stresses except:
A. fatigue.
B. cyclic.
C. welding.
D. creep.

3. Crater cracks are caused by:

A. premature removal of shielding gas.
B. stresses in the termination of a weld pass.
C. grinding of the crater to remove it.
D. slow cooling of the molten weld metal.

4. All of the following are categories of processes that cause

discontinuities except:
A. primary.
B. secondary.
C. mishandling.
D. inservice.

5. Knowing how and where discontinuities are formed for a particular

material:
A. allows the inspector to inspect only critical areas.
B. is not important to know.
C. allows the inspector to determine if the discontinuity is acceptable
or not.
D. provides the inspector with a better understanding of where
discontinuities may be found during the inspection.

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6. Shrinkage occurs as a result of:
A. reduction in the volume of molten metal.
B. too much molten metal.
C. material that is too hot.
D. premature solidification.

7. Which type of discontinuity is most likely due to exposure to

environmental conditions?
A. Stress-corrosion cracking.
B. Heat-treating crack.
C. Porosity in a casting.
D. Cold cracking.

8. Discontinuities become defects when:

A. they are linear.
B. they are rounded.
C. they exceed the limits of the acceptance criteria.
D. the material fractures.

9. Primary discontinuities result from:

A. casting.
B. welding.
C. drilling.
D. galvanizing.

10. Porosity:
A. has no effect on material strength.
B. reduces the strength of material.
C. adds strength to material.
D. is a cosmetic discontinuity.

Magnetic Particle Testing 265

 Chapter 13 Review Key
1. C

2. C

3. B

4. C

5. D

6. A

7. A

8. C

9. A

10. B

266 Programmed Instruction Series

 Chapter 14
Process Quality Assurance

In this chapter:
Importance of equipment calibration for MT inspections
Use of a hall effect meter to determine magnetic field strength
Calibration requirements for yoke, light meters, and other MT equipment
Difference between calibration and system checks
Specifications of system checks using pie gage or ketos steel ring
Water bath maintenance and settling test
Factors documented in an MT inspection report
Purpose and priorities of an MT examination report

267
 Calibration
Calibration of the equipment and accessories that you will use to perform
magnetic particle examinations is extremely important. Inspectors must be
able to rely on their test equipment, amp, and gauss (tesla) meters to
provide a magnetic field with adequate strength, accurate amperage
readings, and a true residual magnetic field measurement. Without having
requirements for equipment calibration, there would be no way for an
inspector to have complete confidence in the equipment or the
examination results.

Let’s use an amperage and hall effect meter to show how critical
calibrations are. When using MT equipment fitted with an amperage
meter, the inspector would rely on the reading from the amperage meter
to know that the amperage induced into the part being examined is at or
close to what the amperage control was set to.

Question 14.1

It’s important to calibrate MT equipment to ensure that:

A. the equipment is accurate.
B. the equipment is pressurized properly.
C. the inspection is performed correctly.
D. the equipment is not outdated.

 Please turn to the end of the chapter for the answer.

Additionally, the inspector might use a

hall effect meter to verify the magnetic
field strength. (See Figure14.1.) If the
amperage and hall effect meters were
not calibrated at the intervals required
or at all, it’s possible that the amperage
being induced and magnetic field
strength being measured by the hall
effect meter could be incorrect, which Figure 14.1: Checking external field
could lead to an inspection being level with calibrated hall effect meter
performed using much lower or much to obtain gauss (tesla) reading.

higher amperages than required. This

could have a negative impact on the final examination results.

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Yes, this is a worst-case scenario, but it is still possible. That is why
equipment calibration is included in just about every NDT inspection
specification.

Calibration Requirements
Depending on the equipment you have, and the inspection specification(s)
that your shop commonly works with, the calibration requirements can
vary somewhat. Below is a list of the most common types of MT
equipment and accessories, including the typical calibration accuracy
requirements specified for each:

• Yoke – dead weight test (10, 30, or 50 lbs [4.5, 13.6, or 22.7 kg]).
• Ammeter – gage accuracy.
• Timers – timer accuracy.
• White and black (ultraviolet) light meter – frequency of output.
• Gauss (tesla) meter – accuracy.

The frequency of calibration for these items is dependent on the

procedure and/or specification that the inspector is working to. Some
require six-month calibration cycles, and others 12-month. In addition,
the quick-break circuit should be checked for functionality, if applicable to
the technique.

Calibration should also be performed following major repairs, whenever a

malfunction is suspected, when specified by the cognizant engineering
organization, or whenever electrical maintenance that might affect
equipment accuracy is performed.

Question 14.2

A typical calibration frequency specified for MT equipment is:

A. three months.
B. 18 months.
C. after major repairs.
D. at random.

 Please turn to the end of the chapter for the answer.

The requirements governing calibration should be spelled out in the

inspection procedure of the company performing MT testing.

Magnetic Particle Testing 269

 Ë From the Field: It’s important to remember that calibration is not
only the responsibility of the individual in charge of your company’s
calibration program, inspectors are also responsible to ensure that
they do not use equipment that is beyond the calibration due date
or not calibrated at all. You should check the calibration status of
the equipment prior to beginning your examination.

Question 14.3

Equipment calibration requirements are determined by the:

A. customer.
B. type of material being examined.
C. equipment manufacturer.
D. examination specification or procedure.

 Please turn to the end of the chapter for the answer.

System Checks
The only way for an inspector to know for sure if the inspection process—
which includes the equipment, accessories, and materials used to perform
the examination—is working correctly is to periodically perform system
checks to verify that all of the components of the specific MT process are
operational. System checks are normally performed each day and/or prior
to each shift by the inspector. Some shops may only use MT equipment
once per week or less and only need to perform the system check the day
that the examination is performed; however, some shops use MT
equipment on several shifts every day. For these situations, a system check
would need to be performed prior to each shift.

Question 14.4

System checks are used to:

A. ensure that the entire MT examination system is working correctly.
B. calibrate MT equipment.
C. ensure inspectors properly perform MT examinations.
D. verify the accuracy of the MT equipment.

 Please turn to the end of the chapter for the answer.

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Don’t mistake the system check with equipment calibration. Calibrations
are performed periodically to verify the accuracy of MT equipment and
accessories. Unlike a calibration, a system check only ensures that the
system is working and is able to identify the type of discontinuities that
your system is designed for. These could be related to size or location,
depending on what is being examined.

Question 14.5

Making sure that MT equipment is functional prior to an examination is

the responsibility of the:
A. lab manager.
B. inspector.
C. calibration supervisor.
D. company owner.

 Please turn to the end of the chapter for the answer.

System Check Specifications

The type of system check that you would perform is largely dependent
on the specification that you’re working with. The system check can be
as simple as using a pie gage to verify the magnetic field when performing
a yoke examination. (See Figures 14.2 and 14.3.) Or it could be as complex
as using a ketos or AS 5282 ring to verify that a wet MT bench
is working correctly. Regardless of the equipment being used, all MT
equipment requires some type of system check to be completed at
specified intervals.

Figure 14.2: Pie gage on carbon steel curved surface using full-wave current.

Magnetic Particle Testing 271

 Eight low carbon steel pie
sections furnace-brazed
together and copper plated

3/4 to 1 in.
(19.1 to 25.4 mm)

Nonferrous
handles

1/32 in. max Nonferrous trunions

(0.79 mm)
Copper shim

1/8 in. (3.2 mm)

Figure 14.3: Pie gage dimensions.

Ketos Steel Ring

One of the more commonly known discontinuity standards used in MT is
a ketos (or AS 5282) ring. Images of a ketos ring are shown in Figure 14.4.

Ketos rings are the same as AS 5282 rings, except that they are heat-
treated differently. Each type of ring has holes drilled through it at various

(a) (b)

Figure 14.4: Ketos ring: (a) in use; (b) showing holes.

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 3/4 in. Distance from
(1.9 cm) Diameter Edge to Center
Hole
inch of Hole
Number
(centimeter) inch
1 2
(centimeter)
3
4
5 1 0.07 (0.18) 0.07 (0.18)
5 in. 2 0.07 (0.18) 0.14 (0.36)
6 (12.7 cm)
D 3 0.07 (0.18) 0.21 (0.53)
12 7 4 0.07 (0.18) 0.28 (0.71)
8 5 0.07 (0.18) 0.35 (0.89)
11 1–1/4 in.
10 9 (3.2 cm) 6 0.07 (0.18) 0.42 (1.07)
7 0.07 (0.18) 0.49 (1.24)
8 0.07 (0.18) 0.56 (1.42)
9 0.07 (0.18) 0.63 (1.6)
7/8 in. 10 0.07 (0.18) 0.70 (1.78)
(2.2 cm) 11 0.07 (0.18) 0.77 (1.96)
12 0.07 (0.18) 0.84 (2.14)

Figure 14.5: Tool steel ring dimensions. Table 14.1: Comparative dimensions of a tool
steel ring standard.

thickness depths, with the first hole drilled close to the outside surface and
the last hole farthest away. These drilled holes represent discontinuities
and are used to verify that a wet MT bench is working properly at various
amperage settings.

The tool steel ring is a commonly used and universally recognized

reference standard for magnetic particle testing systems, as shown in
Figure 14.5. It essentially indicates only particle efficacy. It appears in
virtually all U.S. codes and specifications as the means for checking
magnetic particle performance. Table 14.1 lists comparative dimensions of
the tool ring standard.

Question 14.6

The difference between a ketos and AS 5282 ring is:

A. the number of holes drilled in the side.
B. the heat-treatment.
C. no difference; they are the same.
D. the diameter of the ring.

 Please turn to the end of the chapter for the answer.

Magnetic Particle Testing 273

 Using the Ring Standard
The ring standard is used by passing a specified direct current through a
conductor that in turn passes through the ring’s center. The magnetic particle
testing procedure (or system) is evaluated based on the number of holes
detected at various current levels. The number of holes that should be
detected at a particular current level is provided by written specifications.

Standard test objects like the ring have proven to be valuable aids in
controlling magnetic particle test system parameters. However, in addition
to magnetizing current level, other factors influence test results, including
the properties of the particles, technician skill, magnetization level,
direction of the magnetic fields produced, and particle concentration.

Water Bath Quality

Wet baths should be carefully controlled to prevent corrosion and provide
wettability of testing components. This requires regular chemical analysis
of corrosive inhibitor and wetting agent concentration. The use of water
bath suspension is not recommended for field testing operations unless
facilities exist to test the serviceability of the wetting agents, dispersing
agents, rust inhibitors, antifoam agents, and other additives that are
required in the water suspension.

Water baths without auxiliary heating can be used only in shop areas where
the temperature is above freezing. Use of antifreeze liquids is not feasible
because the quantities needed raise the viscosity of the bath above the
maximum allowable. Use of detergents to ensure the wetting of oily surfaces
causes foaming of the bath. Circulation systems must be designed to avoid
air entrapment or other conditions that produce foam. Antifoaming agents
are used to minimize this tendency but are not 100% effective.

Water Bath Safety

Since water is a conductor of electricity, units in which it is to be used are
designed to isolate all high-voltage circuits in such a way to prevent all
possibility of a technician receiving a shock. The equipment should be
thoroughly grounded using ground fault interrupter circuitry. Electrolysis
of test objects or entire units can occur if proper precautions are not
followed. Units designed for use with water as a suspension are, however,
safe for the technician and minimize the corrosion on the test objects if
the proper chemistry is maintained during use.

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Water Bath Concentration
The recommended range for any selected concentration is:

• Fluorescent particles: 0.1 to 0.4 mL/100 mL

• Visible particles: 1.2 to 2.4 mL/100 mL

For example, a laboratory may select:

• 0.3 mL/100 mL for optimum fluorescent particle concentration.

• 2.4 mL/100 mL for optimum visible particle concentration.

These concentrations would be maintained within a tolerance of ±0.05 or

±0.5 mL/100 mL, respectively.

It is important that the proportion of magnetic particles in the bath be

maintained uniformly after a satisfactory concentration is obtained. If the
concentration varies, the strength of the indications will also vary, and
interpretation of an indication may be erroneous. Fine indications may be
missed entirely with a weak bath.

Settling Test
The settling test is intended to measure the concentration of magnetic
particles in a suspension. The test is easily and quickly performed at the
magnetic particle testing unit. It is not as accurate as a laboratory test but
is reasonable, quantitative, and reproducible for the same type and brand
of magnetic particles. It can be easily standardized with the material in use
and is satisfactory as a daily guide for the technician.

The bath strength for both water and oil bath suspensions should be
checked daily before use and after every 8 h of continuous operation.
More frequent checks may be required, depending on test object surface
area and texture.

The following procedure should be used in performing the settling

or concentration test. Equipment required is a 100 mL pear-shaped,
graduated centrifuge tube and stand. A settling centrifuge test device is
shown in Figure 14.6.

Magnetic Particle Testing 275

 1. Thoroughly agitate the suspension to ensure particle distribution.
2. Run the suspension through the hand hose and nozzle for at least
1 min. This ensures the suspension in the hose is fresh and
agitated.
3. Fill the 100 mL centrifuge tube with agitated suspension using the
hand hose.
4. Demagnetize the suspension in the tube to reduce clumping.
5. Place the centrifuge tube in its stand and allow it to settle on a
vibration-free surface for 30 min.
6. Illuminate the suspension in the centrifuge tube with ultraviolet
radiation in a darkened area.

Only the particle layer should

fluoresce. Fluorescence in the liquid
indicates bath breakdown, fluorescent
pigmentation being stripped from the
magnetic particles, or fine magnetic
particles remaining suspended in
media.

Fine particles can be removed from

the holding tank’s magnetic particle
bath with the use of magnets. The
bath in the magnetic particle
machine’s holding tank should be
allowed to rest (that is, without being
agitated) for 30 min. Place the Figure 14.6: Typical centrifuge tube used
for magnetic particle settling test.
magnets in the bath, ensuring not to
place them so deep that they will
attract the particles that have settled out of suspension.

The length of time or number of times that the magnets will have to be
cleaned of particles and resubmerged depends on the seriousness of the
problem. After the removal of as many suspended particles as possible, the
bath should be able to pass the system effectiveness check or be replaced.

Bath Maintenance
Maintenance of fluorescent particles is identical to that used for wet visible
nonfluorescent particles. There are, however, three additional sources of
deterioration in a bath of fluorescent particles that must be monitored:

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 • The separation of the fluorescent pigment from the magnetic
particles.
• The accumulation of magnetic dust or dirt in the bath.
• The accumulation of fluorescent oils and greases from the surfaces of
tested objects.

Surface Preparation for Wet Fluorescent Method

One precaution in the preparation of the surface of test objects before
testing with the wet fluorescent method must be given special attention.
This is the removal of surface oil and grease. Most petroleum distillates,
lubricating oils, and greases fluoresce with various degrees of brightness.
Such materials must be kept out of the testing bath because of the increase
in background fluorescence that they produce.

Question 14.7

The settling test is performed once for new mixtures of suspensions and:
A. before each use.
B. in the first shift of every week.
C. in the first shift of every month.
D. in the first and last shift of every week.

 Please turn to the end of the chapter for the answer.

Recording the Examination

Following all NDT examinations, including MT, the inspector is normally
required to complete an examination report, as shown in Figure 14.7. The
examination report is used to document many factors related to the
examination, which would include:

1. Examination specification.
2. Procedure.
3. MT technique used.
4. Equipment type and calibration dates.
5. Inspector’s ID.
6. Part information.
7. Examination results.
8. Length and width of indications.

Magnetic Particle Testing 277

 9. Map of area examined.
10. Other required information.

These are just examples. Your customer would normally dictate the
specific information that needs to be reported. Most NDT shops have a
generic form that can be used when there is nothing specified. Once the
report has been completed and reviewed by the NDT Level III, it is
forwarded to the customer. The examination report provides the customer
with a copy of the examination results, including a list of indications
found and whether they are acceptable or not.

Figure 14.7: MT examination report form.

278 Programmed Instruction Series

An examination report may also include images and drawings that show
the exact location of the examination area and any indications identified.
The inspector may also insert images of the area and any discontinuities
found during the examination. It’s very important that the report be able
to accurately map out the location of indications so that if a repair is
allowed, those who are performing the repair can easily find the
indications.

Question 14.8
What information is not typically documented on the examination report?
A. Equipment calibration.
B. Examination procedure
C. Inspector’s home address.
D. Results of the examination.

 Please turn to the end of the chapter for the answer.

Examination Report Priorities

The three main priorities when reporting MT examinations are:

1. Ensure all of the information required by your customer is

documented.
2. Record, as precisely as possible, the location of all of the
discontinuities found.
3. Evaluate and record the disposition of all of the indications
identified during the examination.

When implemented, process quality assurance, which includes calibration,

system checks, and records, can help to ensure that MT examinations are
performed using equipment that is operating correctly and is accurate, and
that the results are clear and concise for the customer.

Magnetic Particle Testing 279

 Chapter 14 Summary
r Calibration of MT equipment and accessories is extremely important
to ensure adequate magnetic field strength and amperage readings.
r A hall effect meter that has not been calibrated at the required
intervals could have a negative impact on the final MT
examination results.
r MT equipment that needs to be periodically calibrated or checked
includes yokes, ammeters, quick break circuits, timers, light
meters, and gauss (tesla) meters.
r Requirements governing calibration, including proper time
intervals, should be spelled out in the inspection procedure of the
company performing MT testing.
r System checks are done to verify that all of the components of the
specific MT process are properly functioning.
r System checks are normally performed each day and/or prior to
each shift by the inspector.
r The type of system check performed depends on the specification and
may involve simply using a pie gage to verify the magnetic field.
r One of the more commonly known discontinuity standards used
in MT is a ketos (or AS 5282) ring to indicate particle efficacy.
r The magnetic particle testing procedure (or system) is evaluated
based on the number of holes detected at various current levels in
the ring standard.
r The recommended range for a magnetic particle bath
concentration is 0.1 to 0.4 mL/100 mL for fluorescent particles
and 1.2 to 2.4 mL/100 mL for visible particles with tolerances of
±0.05 mL/100 mL and ±0.5 mL/100 mL respectively.
r The settling test uses a pear-shaped graduated centrifuge to
measure the concentration of magnetic particles in a suspension.
r Following an MT inspection, the inspector is normally required to
complete an examination report to document several factors
related to the examination.
r An examination report may include diagrams that show the exact
location of indications in the examination area to facilitate repairs.

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r A main priority when reporting MT examinations is to evaluate
and record the disposition of all of the indications identified
during the examination.
r Process quality assurance, which includes calibration, system checks,
and records, can help to ensure that MT examinations are accurate.

Magnetic Particle Testing 281

 Answers to Chapter 14 Questions
Question 14.1
Answer: A – Knowing that the equipment is accurate and working properly is
very important to ensure a reliable MT inspection.

Question 14.2

Answer: C – Normally MT equipment is calibrated every six or 12 months and

after major repairs.

Question 14.3

Answer: D – The frequency of calibration is usually detailed by the

examination specification or procedure.

Question 14.4

Answer: A – System checks are done to ensure that the entire system is
functioning properly.

Question 14.5

Answer: B – In addition to system checks, inspectors must ensure that the

calibration of the equipment they are using has not expired.

Question 14.6

Answer: B – The only difference between the rings is how they are heat-treated.

Question 14.7

Answer: A – The settling test should be performed at the beginning of every

shift and once for every 8 h of continuous use.

Question 14.8

Answer: C – The inspector’s ID appears on the form, but not ordinarily his or
her home address.

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Chapter 14 Review
1. System checks are performed:
A. at the end of each shift.
B. once per week.
C. prior to each shift.
D. monthly.

2. The system check for a yoke includes:

A. ketos ring check.
B. 50 lb (13.6 kg) dead weight test.
C. AS 5282 ring check.
D. pie gage check.

3. An examination report would typically include all of the following

except:
A. ID of the inspector.
B. map of the indications.
C. length and width of the indications.
D. time of inspection.

4. Which of the following would not be calibrated?

A. Yoke.
B. Pie gage.
C. Timers.
D. Tesla (gauss) meter.

5. A common calibration frequency for NDT equipment is:

A. six months.
B. three months.
C. nine months.
D. daily.

6. The wet suspension concentration should be __________ mL/100 L

for visible and __________ mL/100 L for fluorescent.
A. 0.1 to 0.4; 1.2 to 2.4
B 0.1 to 1.2; 1.2 to 2.4
C. 1.2 to 2.4; 0.1 to 0.4
D. 1.2 to 2.4; 0.1 to 1.2

Magnetic Particle Testing 283

 Chapter 14 Review Key
1. C

2. B

3. D

4. B

5. A

6. C

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Magnetic Particle Testing Self-Test
1. Modern magnetic particle inspection techniques began to be
developed:
A. during the late 1700s.
B. in ancient Greece.
C. at the start of the Cold War.
D. during World War I.

2. Ferromagnetic materials are:

A. slightly magnetic.
B. highly magnetic.
C. negatively magnetic.
D. nonmagnetic.

3. Like magnetic poles:

A. repel each other.
B. are nonmagnetic.
C. attract each other.
D. do not affect each other.

4. Magnetic lines of force:

A. always cross each other.
B. exit the north pole and enter the south pole.
C. exit both the north and south poles.
D. seek the path of greatest resistance.

5. Prods are most commonly used to examine:

A. castings.
B. forgings.
C. welds.
D. extrusions.

6. The current used for MT examinations is almost always:

A. rectified AC.
B. true direct current.
C. modified DC.
D. modified AC.

Magnetic Particle Testing 285

 7. Which technique comprises a specialized form of small contact
plates for field use?
A. Coil.
B. Yoke.
C. Bench unit.
D. Prods.

8. Which current is most like DC?

A. Three-phase FW.
B. AC.
C. HW.
D. Single-phase FW.

9. The right-hand rule assists inspectors with remembering:

A. the strength of a magnetic field.
B. the direction of a magnetic field.
C. the location of magnetic poles.
D. the positive or negative charge of a magnetic field.

10. Longitudinal magnetic fields are produced by which of the

following techniques?
A. Prods.
B. Central conductor.
C. Coil.
D. Head shot.

11. The most effective technique for testing hollow parts is a:

A. coil.
B. copper central conductor.
C. brass central conductor.
D. head shot using direct magnetization.

12. Inservice discontinuities may be caused by:

A. forming and bending operations.
B. overuse.
C. welding procedures.
D. rolling operations.

286 Programmed Instruction Series

13. Nonrelevant discontinuities are discontinuities:
A. that are longer than allowed by the acceptance criteria.
B. caused by under-cleaning of the part.
C. caused by procedural error.
D. caused by part geometry.

14. Magnetic leakage fields are created when:

A. ferromagnetic particles are applied on a part.
B. electricity is induced into a part.
C. a magnetic field encounters a discontinuity.
D. a yoke is energized.

15. Which of the following does not necessarily determine the type
of MT technique to be used for a specific inspection?
A. The need to find surface discontinuities only.
B. The need to find subsurface discontinuities.
C. Available power.
D. Density of the test object.

16. Oil-based carriers used for wet MT techniques:

A. require no additives.
B. require antifoaming additives.
C. are highly flammable.
D. do not require any system checks be performed.

17. An AC yoke is considered:

A. unreliable.
B. best for deep subsurface discontinuities.
C. one of the more difficult MT techniques to use.
D. best for surface discontinuities.

18. An MT bench unit is used for which of the following?

A. Examining brass parts.
B. Examining long sections of pipeline.
C. Field examinations.
D. Shop examinations.

Magnetic Particle Testing 287

 19. Which MT accessory is primarily used to verify field direction?
A. Gauss meter.
B. Pie gage.
C. Tesla meter.
D. Ammeter.

20. Residual magnetism is removed because it:

A. may affect welding that follows the MT examination.
B. can cause discontinuities to develop in the material.
C. weakens the material.
D. will likely cause discontinuities to enlarge when the material is in
service.

21. Which of the following is true with regard to demagnetization?

A. Circular MT should be done last.
B. Circular MT should be done first.
C. Longitudinal MT should be done first.
D. It does not matter what order the MT examination is performed.

22. In most cases, when the gauss level is above __________ the
part should be demagnetized.
A. 1
B. 2
C. 3
D. 5

23. NDT personnel certification requirements are found in the:

A. quality manual.
B. safety procedure.
C. written practice.
D. MT procedure.

24. Requirements for MT examinations vary due to:

A. experience of the inspector.
B. the inspector’s training.
C. equipment manufacturer’s suggestions.
D. industry specification requirements.

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25. The total number of experience hours recommended by SNT-TC-1A
for an NDT Level II in MT is:
A. 400
B. 70
C. 210
D. 280

26. Which of the following is not a part of the “experience” portion of the
certification process for Trainees qualifying to Level I?
A. Evaluation of inspection results.
B. Sample preparation/cleaning.
C. Training other NDT personnel.
D. Documenting the inspection results.

27. The NDT certification process requires all of the following except:
A. practical exam.
B. eye exam.
C. experience.
D. associate’s degree.

28. Ferromagnetic material is __________ when magnetic domains

are aligned.
A. magnetized
B. demagnetized
C. diamagnetic
D. paramagnetic

29. Magnetic lines of flux:

A. are formed only with electromagnets.
B. are concentrated around the poles of a magnet.
C. are visible even without ferromagnetic particles.
D. do not exist except theoretically.

30. Direct magnetization is created by passing electricity through a

part using a:
A. central conductor.
B. coil.
C. yoke.
D. head shot or prods.

Magnetic Particle Testing 289

 31. A hall effect meter measures:
A. permeability.
B. the depth of a magnetic field.
C. the strength of a magnetic field.
D. residual magnetism only.

32. Which of the following is not a main type of current used for
magnetic particle inspection?
A. Alternating current.
B. Battery-supplied direct current.
C. Half-wave current.
D. Full-wave current.

33. Permeability refers to:

A. the hardness of a material.
B. a material that cannot be magnetized.
C. residual magnetism.
D. how easy it is to magnetize ferromagnetic material.

34. A circular magnetic field created using a head shot is:

A. weaker at the ends of the part.
B. stronger at the ends of the part.
C. the same strength along the entire length of the part.
D. weaker on the positive side of the part.

35. The minimum prod spacing is usually:

A. 3 in. (76 mm).
B. 6 in. (152 mm).
C. 8 in. (203 mm).
D. 10 in. (254 mm).

36. Which technique induces localized overheating that may result

in arc strikes?
A. Yoke.
B. Prod.
C. Permanent magnet.
D. Coil.

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37. The most effective way to induce a circular field in a large plate
is to use a:
A. yoke.
B. head shot.
C. coil.
D. prod technique.

38. When magnetizing a hollow part using a central conductor, the

magnetic field is strongest:
A. on the inside surface of the part.
B. on the outside surface of the part.
C. at the ends of the part.
D. just below the surface of the part.

39. Which of the following NDT techniques is considered the most

portable for inducing a longitudinal magnetic field?
A. Yoke.
B. Prods.
C. Cable wrap.
D. Head shot.

40. A longitudinal magnetic field can be created using all of the

following except a:
A. coil.
B. cable wrap.
C. central conductor.
D. yoke.

41. When using a coil or wrapped cables, the L/D ratio of the test part
should be at least __________ for effective longitudinal
magnetization.
A. 100:1
B. 5:1
C. 10:1
D. 2:1

Magnetic Particle Testing 291

 42. Retentivity refers to the:
A. ability of ferromagnetic material to retain magnetism.
B. permeability of a material.
C. depth that a magnetic force can penetrate.
D. strength of the magnetic field.

43. Demagnetization of a part is not required when:

A. the part will undergo further machining.
B. the material will be heat-treated below 1400 °F (760 °C).
C. the material will be heat-treated above 1400 °F (760 °C).
D. the material is highly retentive.

44. Which demagnetization technique switches the polarity of

a magnetic field as it slowly reduces the field strength to zero?
A. Saturation.
B. Downcycling.
C. Residual demagnetization.
D. AC coil.

45. A cable wrap is typically used to demagnetize:

A. small parts that can easily fit into typical MT equipment.
B. large parts that cannot fit into typical MT equipment.
C. diamagnetic parts.
D. irregularly shaped parts.

46. Fluorescent wet particles:

A. are dyed with a fluorescent coating.
B. naturally emit fluorescent light.
C. have low permeability.
D. react to white lighting.

47. Wet ferromagnetic particles are:

A. larger than dry particles.
B. different in shape than dry particles.
C. smaller than dry particles.
D. the same size as dry particles.

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48. MT particles should not have:
A. high permeability.
B. low retentivity.
C. high retentivity.
D. high permeability and low retentivity.

49. White light brightness with an LED is most important:

A. when performing MT examinations outdoors.
B. when the contrast between the background of the part and
the particle color is low.
C. for finding large visible indications.
D. during the demagnetization process.

50. Fluorescent lighting should normally have a minimum brightness of:

A. 800 µW/cm2.
B. 900 µW/cm2.
C. 1000 µW/cm2.
D. 1200 µW/cm2..

51. A common recommended practice for personnel certification is:

A. ASTM E 1444/E 1444M.
B. ASTM E 1417.
C. MIL-STD-2032.
D. SNT-TC-1A.

52. Inspection procedures provide specific details for all of the

following except:
A. applying MT particles.
B. magnetizing the part.
C. cleaning the part.
D. requirements for certifying personnel.

53. A relevant indication is usually a(n):

A. rejectable MT indication.
B. indication that must be evaluated.
C. indication that is detrimental to the part.
D. indication that must be removed.

Magnetic Particle Testing 293

 54. Discontinuities are rejectable when:
A. they are linear.
B. they exceed the limits of the acceptance criteria.
C. they are rounded.
D. the inspector considers them to be too large.

55. Which of the following is an inherent process?

A. Casting.
B. Rolling.
C. Forging.
D. Extruding.

56. Which type of discontinuity results in the imperfect fusion

between two streams of converging metal in a mold?
A. Blowhole.
B. Inclusion.
C. Cold shut.
D. Shrinkage.

57. A lamination is a discontinuity that typically results from the

__________ process.
A. rolling
B. forging
C. machining
D. casting

58. A weld crater crack is usually located:

A. in the toe of a weld.
B. in the termination of a weld bead.
C. in the HAZ of a weld.
D. between weld beads.

59. An example of a processing discontinuity is:

A. lack of fusion in a weld.
B. a fatigue crack.
C. a hot tear.
D. a stress-corrosion crack.

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60. Which of the following is a secondary processing discontinuity?
A. Stress-corrosion crack.
B. Crater crack.
C. Quench crack.
D. Heat-affected zone crack.

61. Which type of inservice discontinuity results in separation of grains

at temperatures greater than half the melting point and at stresses
below the yield strength of the material?
A. Creep cracking.
B. Stress-corrosion cracking.
C. Hydrogen cracking.
D. Fatigue cracking.

62. Calibration of MT equipment is typically performed:

A. every three months.
B. at the start of each workshift.
C. every two years.
D. as required by applicable specifications.

63. A ketos ring system would not be used to:

A. verify that a wet bench unit is working properly.
B. indicate wet particle efficacy.
C. verify the direction of a magnetic field.
D. indicate the size hole detectable at a certain amperage.

64. A settling test is used to measure:

A. residual magnetism.
B. magnetic particle concentration in a bath.
C. accumulation of dust and dirt in a bath.
D. amount of fluorescence.

65. What information is not typically documented on the

examination report?
A. Map of the physical location of the magnetic particle test.
B. Map of the area of interest.
C. Equipment specification.
D Examination results.

Magnetic Particle Testing 295

 Self-Test Answer Key

Question Answer Reference Page(s) Question Answer Reference Page(s)

1. . . . . . . . . . .D . . . . . . . . . . . . . . . . . . . . . .3
34. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .159
2. . . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . . .4
35. . . . . . . . .A . . . . . . . . . . . . . . . .164-165
3. . . . . . . . . . .A . . . . . . . . . . . . . . . . . . . . . .8
36. . . . . . . . .B . . . . . . . . . . . . . . . . . . . .165
4. . . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . . .9
37. . . . . . . . .D . . . . . . . . . . . . . . . .162-163
5. . . . . . . . . . .C . . . . . . . . . . . . . . . . .21-22
38. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .161
6. . . . . . . . . . .A . . . . . . . . . . . . . . . . . . . .23
39. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .166
7. . . . . . . . . . .D . . . . . . . . . . . . . . . . .21-22
40. . . . . . . . .C . . . . . . . . . . .160, 165-169
8. . . . . . . . . . .A . . . . . . . . . . . . . . . . . . . .26
41. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .172
9. . . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . .35
42. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .180
10. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .37
43. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .184
11. . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . .41
44. . . . . . . . .B . . . . . . . . . . . . . . . . . . . .186
12. . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . .50
45. . . . . . . . .B . . . . . . . . . . . . . . . . . . . .189
13. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .59
46. . . . . . . . .A . . . . . . . . . . . . . . . . .75, 202
14. . . . . . . . .C . . . . . . . . . . . . . . . . .55-56
47. . . . . . . . .C . . . . . . . . . . . . . . . .199-200
15. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .72
48. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .203
16. . . . . . . . .A . . . . . . . . . . . . . . . . .75-76
49. . . . . . . . .B . . . . . . . . . . . . . . . . . . . .205
17. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .78
50. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .209
18. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .78
51. . . . . . . . .D . . . . . . . . . . . . . . .110, 222
19. . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . .82
52. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .223
20. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .97
53. . . . . . . . .B . . . . . . . . . . . . . . . . .63, 228
21. . . . . . . . . .B . . . . . . . . . . . . . . . . . . . . .98
54. . . . . . . . .B . . . . . . . . . . . . . . . .226-227
22. . . . . . . . .C . . . . . . . . . . . . . . . . . . .100
55. . . . . . . . .A . . . . . . . . . . . . . . . .239-240
23. . . . . . . . .C . . . . . . . . . . . . . . . . . . .111
56. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .242
24. . . . . . . . .D . . . . . . . . . . . . . . .109-110
57. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .247
25. . . . . . . . .D . . . . . . . . . . . . . . . . . . .113
58. . . . . . . . .B . . . . . . . . . . . . . . . . . . . .251
26. . . . . . . . .C . . . . . . . . . . . . . . . . . . .114
59. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .252
27. . . . . . . . .D . . . . . . . . . . . . . . .112-117
60. . . . . . . . .C . . . . . . . . . . . . . . . . . . . .256
28. . . . . . . . .A . . . . . . . . . . . . . . . . . . .127
61. . . . . . . . .A . . . . . . . . . . . . . . . . . . . .257
29. . . . . . . . . .B . . . . . . . . . . . . . . . . . . .129
62. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .269
30. . . . . . . . .D . . . . . . . . . . . . . . .135-136
63. . . . . . . . .B . . . . . . . . . . . . . . . .272-274
31. . . . . . . . .C . . . . . . . . . . . . . . . . . . .132
64. . . . . . . . .C . . . . . . . . . . . . . . . .275-276
32. . . . . . . . .B . . . . . . . . .18, 138-139
65. . . . . . . . .A . . . . . . . . . . . . . . . .277-279
33. . . . . . . . .D . . . . . . . . . . . . . . . . . . . .142

296 Programmed Instruction Series

References
The following publications are training references for further study as
recommended in ANSI/ASNT CP-105: ASNT Standard Topical Outlines for
Qualification of Nondestructive Testing Personnel (2011).
Annual Book of ASTM Standards: Vol. 03.03, Nondestructive Testing.
Philadelphia, PA: American Society for Testing and Materials. Latest
edition.*
ASM Handbook: Volume 17: Nondestructive Evaluation and Quality
Control. Metals Park, OH: ASM International. 1989.*
ASNT Level II Study Guide: Magnetic Particle Testing Method. Columbus,
OH: The American Society for Nondestructive Testing, Inc. Latest
edition.*
ASNT Level III Study Guide: Magnetic Particle Testing Method. Columbus,
OH: The American Society for Nondestructive Testing, Inc. Latest
edition.*
ASNT Questions & Answers Book: Magnetic Particle Testing Method.
Columbus, OH: The American Society for Nondestructive Testing,
Inc. Latest edition.*
Betz, C.E. Principles of Magnetic Particle Testing. Chicago, IL: Magnaflux
Corp. 2000.*
Mix, P.E. Introduction to Nondestructive Testing: A Training Guide, second
edition. New York: John Wiley & Sons. 2005.
Moore, D.G., tech. ed., P.O. Moore, ed. Nondestructive Testing Handbook,
third edition: Volume 8, Magnetic Testing. Columbus, OH: The
American Society for Nondestructive Testing, Inc. 2008.*
Welding Handbook, Volume 1. Miami, FL: American Welding Society.
Latest edition.*

Additional References
Nondestructive Testing Classroom Training Handbook, second edition:
Magnetic Particle. Fort Worth, TX: General Dynamics, Convair
Division. 1977.
Nonrelevant and False Indications. Columbus, OH: The American
Society for Nondestructive Testing, Inc. 2010.*
Programmed Instruction Handbook, fourth edition: Magnetic Particle.
Fort Worth, TX: General Dynamics, Convair Division. 1977.
Relevant Discontinuities: Magnetic Particle and Liquid Penetrant Testing.
Columbus, OH: The American Society for Nondestructive Testing,
Inc. 2010.*
Smith, Gordon E. Magnetic Particle Testing Classroom Training Book,
second edition. Columbus, OH: The American Society for
Nondestructive Testing, Inc. 2015.*

Magnetic Particle Testing 297

 Workman, G.L., tech. ed., P.O. Moore, ed. Nondestructive Testing
Handbook, third edition: Volume 10, Nondestructive Testing
Overview. Columbus, OH: The American Society for Nondestructive
Testing, Inc. 2012.*
* Available from the American Society for Nondestructive Testing, Inc., Columbus, OH.

Note: Additional references for the advanced study of magnetic

particle testing may be found at the end of each chapter of the
Nondestructive Testing Handbook: Magnetic Testing (see listing
above).

298 Programmed Instruction Series

Figure Sources
The following figures have been reprinted with permission, courtesy of
the individuals, companies, and organizations listed.
Figure 1.1 – Peter Kuiper, Wikimedia Commons.
Figures 1.2, 4.3, 4.4, 5.4, 5.8, 6.1–6.3, 8.9, 9.4, 9.7, 9.9, 9.10, 10.3,
10.6, 11.6, 11.7, 13.14, 13.20, 13.28, and 14.1 – Nondestructive
Testing Handbook, third edition: Volume 8, Magnetic Particle
Testing. ASNT.
Figures 1.3, 1.4, 2.1, 2.4, 3.7, 5.10, 8.17, 9.8, 9.11, 9.13, 10.1,10.4,
11.5, 14.3, 14.5, and 14.6 – Magnetic Particle Testing Classroom
Training Book, second edition. ASNT.
Figures 2.2, 2.6, 2.8, 2.10, 3.5, 5.2, 5.3, 8.6, 8.7, 10.2(a), and 14.4(b)
– Magnaflux, Glenview, Illinois.
Figures 3.1, 3.2, 3.4, 4.5, 4.6, 8.5, 8.10, 8.16, and 9.2, – NDT
Education Resource Center, The Collaboration for NDT Education,
Center for Nondestructive Evaluation, Iowa State University,
www.ndt-ed.org.
Figures 3.3, 3.6, 5.1, 9.12 – Parker Research Corporation.
Figures– Magnetic Particle Testing Classroom Training Book, second
edition. ASNT.
Figure 3.8 – Heartland Precision Fasteners, Inc.
Figure 4.7 – Industrial Inspection Services.
Figures 5.5, 5.6, and 5.9 – Solid State Systems.
Figures 5.7, 8.8, and 14.2 – George Hopman, NDE Solutions Inc.
Figure 5.11 – Spectronics Corporation.
Figure 8.1 – Electronics Tutorials, http://www.electronics-tutorials.ws.
Figure 8.2 – Provided by Boom, boomeria.org.
Figure 8.3 – Gorchy, Wikimedia Commons.
Figure 8.4 – Geek3, Wikimedia Commons.
Figure 8.11 – Image courtesy of The National Board of Boiler and
Pressure Vessel Inspectors.
Figure 10.2(b) – NDT Consultants Ltd, Coventry, England.
Figures 10.5 and 11.1 – Magwerks Corporation.
Figure 11.4 – Met-L-Chek Company.
Figure 12.1 – Nondestructive Testing Handbook, third edition: Volume
10, Nondestructive Testing Overview. ASNT.
Figure 12.2 – Latham & Phillips Ophthalmic.
Figures 12.3, 12.5, 12.6, 13.17, and 13.24 – Amy E. Krauser, Edwards
& Associates.
Figure 12.4 – TEAM Industrial Services.
Figure 13.1 – Jim’s Coins and Precious Metals.
Figures 13.2, 13.6, 13.8, 13.11, 13.15, 13.16, 13.26, and 14.4(a) –
Michael A. Kowatch.

Magnetic Particle Testing 299

 Figures 13.3, 13.7, 13.9, 13.10, 13.13, 13.22, 13.27, and 13.29 –
Nondestructive Testing Classroom Training Handbook, second
edition: Magnetic Particle. General Dynamics.
Figure 13.4 – Courtesy, American Foundry Society.
Figure 13.5 – Chem-Trend.
Figures 13.12, 13.18, 13.21, and 13.25 – David G. Moore, Sandia
National Laboratories.
Figures 13.19(a) and 14.7 – The American Welding Society (AWS),
Miami, FL.
Figure 13.19(b) – The American Iron and Steel Institute.
Figure 13.23 – EPRI NP-1590-SR, NDE Characteristics of Pipe Weld
Defects. Palo Alto, CA: Electric Power Research Institute. 1980.

Cover Image:
Machine Specialty & Manufacturing, Inc.

Chapter Title Page Figures:

Chapter 1 – MR® Chemie GmbH.
Chapter 2 – Machine Specialty & Manufacturing, Inc.
Chapter 3 – Parker Research Corporation.
Chapter 4 – Michael A. Kowatch.
Chapters 5 – Nondestructive Testing Handbook, third edition: Volume
8, Magnetic Particle Testing. ASNT.
Chapter 6 – Magnaflux, Glenview, Illinois.
Chapter 7 – U.S. Department of Transportation, Federal Highway
Administration. “Guidelines for the Installation, Inspection,
Maintenance and Repair of Structural Supports for Highway Signs,
Luminaries, and Traffic Signals.” Bridges and Structures. 2 June 2015.
Available at: http://www.fhwa.dot.gov/bridge/signinspection03.cfm
Chapter 8 – Shutterstock.
Chapter 9 – Magnetic Particle Testing Classroom Training Book, second
edition. ASNT.
Chapter 10 – Magwerks Corporation.
Chapter 11 – Met-L-Chek Company.
Chapter 12 – ASNT.
Chapter 13 – Photo taken by Michael Schultz Photography, courtesy
of Scot Forge.
Chapter 14 – George Hopman, NDE Solutions Inc.

300 Programmed Instruction Series

Glossary
Air gap: When a magnetic circuit contains a small gap, which the
magnetic flux must cross; the space is referred to as an air gap.
Cracks produce small air gaps on the surface of a test object.
Alternating current: Electric current periodically reversing in
polarity or direction of flow.
Ampere: The unit of electrical current. One ampere is the current that
flows through a conductor having a resistance of one ohm at a
potential of one volt.
Ampere turns: The product of the number of turns in a coil and the
number of amperes flowing through it. A measure of the
magnetizing or demagnetizing strength of the coil.
Bath: The suspension of iron oxide particles in a liquid vehicle (light
oil or water).
Black light: See Ultraviolet radiation.
Carbon steel: Steel which does not contain significant amounts of
alloying elements other than carbon and manganese.
Carrier fluid: The fluid in which fluorescent and nonfluorescent
magnetic particles are suspended to facilitate their application in
the wet method. Also referred to as media.
Central conductor: An electrical conductor that is passed through
the opening in a ring or tube, or any hole in a test object, for the
purpose of creating a circular field in the ring or tube, or around
the hole.
Circular field: See Field, circular magnetic.
Circular magnetization: A method of inducing a magnetic field in
a test object so that the magnetic lines of force take the form of
concentric rings about the axis of the current. This is accomplished
by passing the current directly through the test object or through
a conductor that passes into or through a hole in the test object.
The circular method is applicable for the detection of
discontinuities with axes about parallel to the axis of the current
through the test object.
Coercive force: The reverse magnetizing force necessary to remove
residual magnetism in demagnetizing a test object.
Coil shot: A pulse of magnetizing current passed through a coil
surrounding a test object for the purpose of longitudinal
magnetization.
Contact head: The electrode, fixed to the magnetic particle testing
unit, through which the magnetizing current is drawn.
Contact plates: Replaceable metal plates, usually of copper braid,
placed on contact heads to give good electrical contact, thereby
preventing damage to the test object.
Continuous method: A testing method in which ample amounts of
magnetic particles are applied, or are present on the test object,
during the time the magnetizing current is applied.

Magnetic Particle Testing 301

 Core: That part of the magnetic circuit which is within the electrical
winding.
Curie point: The temperature at which ferromagnetic materials can
no longer be magnetized by outside forces, and at which they lose
their residual magnetism: about 1200 to 1600 °F (649 to 871 °C) for
many metals.
Current flow method: A method of circular magnetization by
passing a current through a test object via prods or contact heads.
The current may be alternating, half-wave rectified, full-wave
rectified, or direct.
Current induction method: A method of magnetization in which a
circulating current is induced in a ring-shaped component by a
fluctuating magnetic field.
Defect: A discontinuity that interferes with the usefulness of a test
object or exceeds acceptability limits established by applicable
specifications. A fault in any material or object that is detrimental
to its serviceability. Note that not all cracks, seams, and laps are
necessarily defects, as they may not affect the usefulness of the
object in which they exist.
Demagnetization: The reduction in the degree of residual
magnetism in ferromagnetic materials to an acceptable level.
Diffuse indications: Indications that are not clearly defined as, for
example, indications of subsurface discontinuities.
Direct current: An electric current which flows steadily in one
direction.
Discontinuity: A lack of continuity or cohesion; an intentional or
unintentional interruption in the physical structure or
configuration of a material or component.
Distorted field: The direction of a magnetic field in a symmetrical
object will be substantially uniform if produced by a uniformly
applied magnetizing force. But if the test object being magnetized
is irregular in shape, the field is distorted and does not follow a
straight path or have a uniform distribution.
Dry method: Magnetic particle testing in which the particles used
are in the dry powder form.
Dry powder: Finely divided ferromagnetic particles suitably selected
and prepared for magnetic particle testing by the dry method.
Electromagnet: A magnet created by inserting a suitable metal core
within, or near, a magnetizing field formed by passing electric
current through a coil of insulated wire.
Etching: The process of exposing subsurface conditions of metal test
objects by removal of the outside surface through the use of
chemical agents. Because of the action of the chemicals in eating
away the surface, various surface or subsurface conditions are
exposed or exaggerated and made visible to the eye. For example,
forging flow lines and discontinuities.
Ferromagnetic: A term applied to materials that can be magnetized
and strongly attracted by a magnetic field.

302 Programmed Instruction Series

Field, bipolar longitudinal: Magnetic field within a test object that
creates two poles.
Field, circular magnetic: The magnetic field in and surrounding
any electrical conductor or test object resulting from a current
being passed through the conductor or test object or from
contact pads or prods.
Field, magnetic leakage: The magnetic field that leaves or enters
the surface of a test object at a magnetic pole.
Field, longitudinal magnetic: A magnetic field wherein the flux
lines traverse the component in a direction essentially parallel
with the axis of the magnetizing coil or to a line connecting the
two poles of the magnetizing yoke.
Field, magnetic: The condition of space within and surrounding a
magnetized test object, or a conductor-carrying current,
characterized by the presence of a magnetic force.
Field, residual magnetic: The field that remains in magnetizable
material after the magnetizing force has been removed.
Field, resultant magnetic: A magnetic field that is the result of two
magnetic forces impressed on the same area of a magnetizable
object at the same time, sometimes called a vector field.
Field, vector: See Field, resultant magnetic.
Flash magnetization: Magnetization by a current flow of very brief
duration.
Fluorescence: The emission of visible light by a substance as the
result of, and only during, the absorption of ultraviolet radiation.
Fluorescent magnetic particle testing: The magnetic particle
testing process using a finely divided fluorescent ferromagnetic
testing medium that fluoresces when activated by ultraviolet
radiation of 365 nm.
Flux density: The flux-per-unit area through an element that cuts
the unit area at right angles to the direction of the flux. Flux
density is usually designated by the letter B, and its unit is the
gauss (tesla).
Flux leakage: Magnetic lines of force that leave and enter a test
object at poles on the surface.
Flux lines: Imaginary magnetic lines used as a means of explaining
the behavior of magnetic fields. Based on the pattern of lines
produced when iron filings are sprinkled over a piece of paper laid
over a permanent magnet. Also called lines of force, the unit is a
single line of force called the maxwell designated by the Greek
letter phi (Φ). The SI equivalent of the maxwell is the weber.
1 Wb = 108 Mx = 100 MMx. 1Mx = 10–8 Wb = 0.01 MWb.
Flux penetration, magnetic: The depth to which a magnetic flux is
present in a test object.
Furring: Buildup or bristling of magnetic particles caused by
excessive magnetization of the test object under examination
resulting in a furry appearance. Also referred to as fur or grass.

Magnetic Particle Testing 303

 Gauss: The unit of flux density. Numerically, one gauss is one line
of flux per square centimeter of area, and is designated by the
letter B. The comparable SI unit is the tesla. 1 G = 10-4 T = 0.1 mT.
See Tesla.
Heads: The clamping contacts on a stationary magnetizing unit.
Head shot: A short pulse of magnetizing current passed through a
test object or a central conductor while clamped between the
head contacts of a stationary magnetizing unit for the purpose of
circularly magnetizing the test object.
Henry (H): The unit of inductance of a coil.
Horseshoe magnet: A bar magnet, bent into the shape of a
horseshoe so that the two poles are adjacent. Usually the term
applies to a permanent magnet.
Hysteresis: (a) The lagging of the magnetic effect when the
magnetic force acting on a ferromagnetic body is changed. (b) The
phenomenon exhibited by a magnetic system wherein its state is
influenced by its previous magnetic history.
Hysteresis loop: A curve showing the flux density B plotted as a
function of magnetizing force H. As the magnetizing force is
increased to the saturation point in the positive, negative, and
positive directions sequentially, the curve forms a characteristic
S-shaped loop. Intercepts of the loop with the B and H axes and
the points of maximum and minimum magnetizing force define
important magnetic characteristics of the material.
Indication: Any magnetically held magnetic particle pattern on the
surface of a test object.
Inductance: The magnetism produced in a ferromagnetic body by
some outside magnetizing force. The magnetism is not the result
of passing current through the test object.
Interpretation: The determining of the cause and significance of
indications of discontinuities from the standpoint of whether they
are detrimental discontinuities or false or nonrelevant indications.
Leakage field: The magnetic field forced out into the air by the
distortion of the field within a test object.
Lines of force: See Flux lines.
Longitudinal field: See Field, longitudinal magnetic.
Longitudinal magnetization: The process of inducing a magnetic
field into the test object such that the magnetic lines of force
extending through the test object are about parallel to the axis of
the magnetizing coil or to a line connecting the two poles when
yokes (electromagnets) are used.
Magnet, permanent: A highly retentive metal that has been
strongly magnetized.
Magnetic field: See Field, magnetic.
Magnetic field meter: An instrument designed to detect and/or
measure the flux density and polarity of magnetic fields.

304 Programmed Instruction Series

Magnetic field strength: The measured intensity of a magnetic
field at a point always external to the magnet or conductor;
usually expressed in oersteds or amperes per meter.
Magnetic material: Some materials are attracted by a magnet,
whereas others are repelled. From the definition of magnetism, it
follows that magnetic materials are those that are attracted by
magnetism. These materials are known as paramagnetic materials,
whereas materials that are repelled are known as diamagnetic
materials. The subdivision of paramagnetic, called ferromagnetic,
is a main concern as only ferromagnetic materials can be strongly
magnetized.
Magnetic particle testing: A nondestructive testing method for
locating discontinuities in ferromagnetic materials. It uses flux
leakage that forms magnetic poles to attract finely divided
magnetic particles that mark the discontinuity.
Magnetic particle testing indications: The accumulation of
ferromagnetic particles that may be either true indications of
discontinuities, or may be false or nonrelevant indications.
Magnetic saturation: In a specific material, the degree of
magnetization where an increase in H produces no further
increase in magnetization.
Magnetic writing: A form of nonrelevant indication caused when
the surface of a magnetized object comes in contact with another
piece of ferromagnetic material that is magnetized to a different
value.
Magnetizing current: The flow of alternating, rectified alternating,
or direct current used to induce magnetism into the test object.
Magnetizing force: The total force tending to set up a magnetic
flux by a magnetizing current. Usually designated by the letter H,
its unit is the oersted or amperes per meter.
Media: See Carrier fluid.
Nonrelevant indication: A magnetic particle indication caused by
a leakage magnetic field which is not caused by an actual
discontinuity in the magnetized material but by some other
condition that does not affect the usefulness of the test object
(such as a change of section).
Oersted: A unit of field strength that produces magnetic induction,
and is designated by the letter H. The SI equivalent unit is ampere
per meter. One ampere per meter equals about 1/80 of an oersted.
Consequently, 1 A/m = 13 mOe.
Paramagnetic: Materials that are slightly attracted by a magnetic
field. Examples are chromium, manganese, and aluminum.
Paste: Finely divided ferromagnetic particles in paste form used in
preparing wet suspensions.
Permeability: (a) The ease with which a material can become
magnetized. (b) The ratio between field strength produced and
the magnetizing force (B/H). (c) The ratio of flux density produced
to magnetizing force.

Magnetic Particle Testing 305

 Pole: The area on a magnetized test object from which the magnetic
field is leaving or returning to the test object.
Prods: Handheld electrodes attached to cables used to transmit the
magnetizing current from the source to the test object.
Rectified alternating current: Alternating current that has been
converted into direct current.
Reluctance: The opposition of a magnetic material to the
establishment of magnetic flux. The reluctance of the material
determines the magnitude of the flux produced by a given
magnetic force. Reluctance is analogous to the resistance in an
electric circuit.
Residual field: See Field, residual magnetic.
Residual magnetism: The amount of magnetism that a magnetic
material retains after the magnetizing force is removed, also called
residual field.
Residual method: A procedure in which the indicating material is
applied after the magnetizing force has been discontinued.
Resultant field: See Field, resultant magnetic.
Retentivity: The ability of a material to retain a portion of the
magnetic force induced in it after the magnetizing force has been
removed.
Saturation: The point in the magnetization of a magnetizable test
object at which an increase in the magnetizing force produces no
increase in the magnetic field within the test object.
Sensitivity: The capacity or degree of responsiveness to magnetic
particle testing.
Solenoid (Coil): An electric conductor formed into a coil; often
wrapped around a central core of highly permeable material.
Subsurface discontinuity: Any discontinuity that does not open
onto the surface of the test object in which it exists.
Suspension: A liquid bath in which is suspended the ferromagnetic
particles used in the wet magnetic particle testing method.
Swinging field magnetization: Magnetic fields induced in two
different directions alternately and quickly to more accurately
detect discontinuities oriented in different directions in a test
object.
Testing: The process of examining and checking materials and
objects for possible discontinuities or for deviation from
established standards.
Toroidal field: An induced magnetic field occurring in a ring test
object when current is induced.
Ultraviolet radiation: Near-ultraviolet radiation (UV-A) with
wavelengths in the range of 320 to 400 nm. Near-ultraviolet
sources used for nondestructive testing have a predominant
wavelength of 365 nm.
Ultraviolet radiation filter: A filter that transmits near-ultraviolet
radiation (UV-A) while suppressing the transmission of visible light
and harmful ultraviolet radiation.

306 Programmed Instruction Series

Vector field: See Field, resultant magnetic.
Vibratory demagnetization: The removal of magnetization by
impulse energies that distribute the orientation of magnetic
domains in the test object.
Wet method: The testing method that uses ferromagnetic particles
suspended in a liquid (oil or water) as a media.
Yoke: A U- or C-shaped piece of highly permeable magnetic material,
either solid or laminated, sometimes with adjustable pole pieces,
around which is wound a coil carrying the magnetizing current.
Yoke magnetization: A longitudinal magnetic field induced in a
test object, or in an area of a test object, by means of an external
electromagnet shaped like a yoke.

Magnetic Particle Testing 307

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Programmed Instruction Series: Magnetic Particle Testing

PROGRAMMED INSTRUCTION SERIES

Magnetic Particle
Testing
Written for ASNT by
Michael A. Kowatch
®

Catalog No.: 1533 The American Society for

ISBN: 978-1-57117-363-8 Nondestructive Testing, Inc.