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Demagnetization: Home - Education Resources - NDT Course Material - MPI Introduction To Magnetic Particle Inspection

There are several methods to demagnetize a component after magnetic particle inspection. Heating the material above its Curie temperature causes the magnetic domains to become randomly oriented, removing any residual magnetic field. Alternatively, passing the component through a reversing magnetic field from a coil or electromagnetic yoke with alternating current will also demagnetize it by randomizing the magnetic domains. A field meter is used to verify the magnetic flux has been reduced below 3 gauss, as required by industry standards.

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209 views2 pages

Demagnetization: Home - Education Resources - NDT Course Material - MPI Introduction To Magnetic Particle Inspection

There are several methods to demagnetize a component after magnetic particle inspection. Heating the material above its Curie temperature causes the magnetic domains to become randomly oriented, removing any residual magnetic field. Alternatively, passing the component through a reversing magnetic field from a coil or electromagnetic yoke with alternating current will also demagnetize it by randomizing the magnetic domains. A field meter is used to verify the magnetic flux has been reduced below 3 gauss, as required by industry standards.

Uploaded by

zaw lin oo
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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-
Introduction to
Magnetic Particle
Demagnetization Inspection
After conducting a magnetic particle inspection, it is usually necessary to
Introduction
demagnetize the component. Remanent magnetic fields can:
Introduction
affect machining by causing cuttings to cling to a component. Basic Principles
interfere with electronic equipment such as a compass. History of MPI
create a condition known as "arc blow" in the welding process. Arc
Physics
blow may cause the weld arc to wonder or filler metal to be repelled
Magnetism
from the weld.
Magnetic Mat'ls
cause abrasive particles to cling to bearing or faying surfaces and
Magnetic Domains
increase wear.
Magnetic Fields
Removal of a field may be accomplished in several ways. This random Electromag. Fields
orientation of the magnetic domains can be achieved most effectively by Field From a Coil
heating the material above its curie temperature. The curie temperature for Mag Properties
Hysteresis Loop
a low carbon steel is 770oC or 1390oF. When steel is heated above its
Permeability
curie temperature, it will become austenitic and loses its magnetic
Field Orientation
properties. When it is cooled back down, it will go through a reverse
Magnetization of
transformation and will contain no residual magnetic field. The material
Mat'ls
should also be placed with it long axis in an east-west orientation to avoid
Magnetizing Current
any influence of the Earth's magnetic field.
Longitudinal Mag
It is often inconvenient to heat a Fields
material above its curie temperature to Circular Mag Fields
demagnetize it, so another method that Demagnetization
returns the material to a nearly Measuring Mag
unmagnetized state is commonly used. Fields
Subjecting the component to a
Equipment &
reversing and decreasing magnetic
field will return the dipoles to a nearly Materials
random orientation throughout the Portable Equipment
material. This can be accomplished by Stationary
pulling a component out and away Equipment
from a coil with AC passing through it. Multidirectional
The same can also be accomplished Equipment
using an electromagnetic yoke with AC Lights
selected. Also, many stationary Field Strength
magnetic particle inspection units come with a demagnetization feature Indicators
that slowly reduces the AC in a coil in which the component is placed. Magnetic Particles
Suspension Liquids
A field meter is often used to verify that the residual flux has been
removed from a component. Industry standards usually require that the Testing Practices
magnetic flux be reduced to less than 3 gauss after completing a magnetic Dry Particles
particle inspection. Wet Suspension
Magnetic Rubber
Continuous &

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Demagnetization https://www.nde-ed.org/EducationResources/CommunityCollege/MagPa...

Residual Mag
Field Direction &
Intensity
L/D Ratio

Process Control
Particle
Concentration
Suspension
Contamination
Electrical System
Lighting
Eye Considerations

Example
Indications
Visible Dry Powder
Fluorescent Wet

Quizzes

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Terms & Conditions About the Site
Partial support for this work was provided by the NSF-ATE (Advanced Technological
Education) program through grant #DUE 0101709. Opinions expressed are those of the
authors and not necessarily those of the National Science Foundation.

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