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CSWIP 3.1 - Welding lnspector - Level 2

WISS

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Training & Examination Services
Granta Park, Great Abington
Cambridge CB21 6AL, UK

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 Rev 2 April 2013
Contents
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CSWIP 3.1 .. Welding Inspector-

Level2
Contents
Section Subject

1 Typical Duties of Welding Inspectors

1.1 General
2 Terms and Definitions
2.1 Types of weld
2.2 Types of Joints (see BS EN ISO 15607)
2.3 Features of the completed weld
2.4 Weld preparation
2.5 Size of butt welds
2.6 Fillet weld
2.7 Welding position, slope and rotation
2.8 Weaving
3 Welding Imperfections and Materials Inspection
3.1 Definitions
3.2 Cracks
3.3 Cavities
3.4 Solid inclusions
3.5 Lack of fusion and penetration
3.6 Imperfect shape and dimensions
3.7 M iscellaneous imperfections
3.8 Acceptance standards
4 Destructive Testing
4 .1 Test types, pieces and objectives
4 .2 Macroscopic examination
5 Non-destructive Testing
5.1 Introduction
5.2 Radiographic methods
5.3 Ultrasonic methods
5.4 Magnetic particle testing
5.5 Dye penetrant testing
6 WPS/Welder Qualifications
6.1 General
6.2 Qualified welding procedure specifications
6.3 Welder qualification

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7 Materials Inspection
7.1 General
7.2 Material type and weldability
7.3 Alloying elements and their effects
7.4 Material traceability
7.5 Material condition and dimensions
7.6 Summary
8 Codes and Standards
8.1 General
8.2 Definitions
8.3 Summary
9 Welding Symbols
9.1 Standards for symbolic representation of welded joints on drawings
9.2 Elementary welding symbols
9.3 Combination of elementary symbols
9.4 Supplementary symbols
9.5 Position of symbols on drawings
9.6 Relationship between the arrow and jo'int lines
9. 7 Position of the reference line and weld symbol
9.8 Positions of the continuous and dashed lines
9.9 Dimensioning of welds
9.10 Complimentary indications
9.11 Indication of the welding process
9.12 Weld symbols in accordance with AWS 2.4
10 Introduction to Welding Processes
10.1 General
10.2 Productivity
10.3 Heat input
10.4 Welding parameters
10.5 Power source characteristics
11 Manual Metal Arc/Shielded Metal Arc Welding (MMA/SMAW)
11 .1 MMA basic equipment requirements
11 .2 Power requirements
11 .3 Welding variables
11 .4 Summary of MMNSMAW
12 TIG Welding
12.1 Process characteristics
12.2 Process variables
12.3 Filler wires
12.4 Tungsten inclusions
12.5 Crater cracking
12.6 Common applications
12.7 Advantages
12.8 Disadvantages

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13 MIG/MAG Welding
13.1 Process
13.2 Variables
13.3 MIG basic equipment requirements
13.4 Inspection when MIG/MAG welding
13.5 Flux-cored arc welding (FCAW)
13.6 Summary of solid wire MIG/MAG
14 Submerged Arc Welding
14.1 Process
14.2 Fluxes
14.3 Process variables
14.4 Storage and care of consumables
14.5 Power sources
15 Thermal Cutting Processes
15.1 Oxy-fuel cutting
15.2 Plasma arc cutting
15.3 Arc air gouging
15.4 Manual metal arc gouging
16 Welding Consumables
16.1 Consumables for MMA welding
16.2 AWS A 5.1- and AWS 5.5-
16.3 Inspection points for MMA consumables
16.4 Consumables for TIG/GTW
16.5 Consumables for MIG/MAG
16.6 Consumables for SAW welding
17 Weldabifity of Steels
17. 1 Introduction
17.2 Factors that affect weldability
17.3 Hydrogen cracking
17.4 Solidification cracking
17.5 Lamellar tearing
17.6 Weld decay
18 Weld Repairs
18.1 Two specific areas
19 Residual Stresses and Distortions
19.1 Development of residual stresses
19.2 What causes distortion?
19.3 The main types of distortion?
19.4 Factors affecting distortion?
19.5 Prevention by pre-setting, pre-bending or use of restraint
19.6 Prevention by design
19.7 Prevention by fabrication techniques
19.8 Corrective techniques

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20 Heat Treatment
20.1 Introduction
20.2 Heat treatment of steel
20.3 Postweld heat treatment (PWHT)
20.4 PWHT thermal cycle
20.5 Heat treatment furnaces
21 Arc Welding Safety
21 .1 General
21.2 Electric shock
21 .3 Heat and light
21.4 Fumes and gases
21 .5 Noise
21 .6 Summary
22 Calibration
22.1 Introduction
22.2 Terminology
22.3 Calibration frequency
22.4 Instruments for calibration
22.5 Calibration methods
23 Application and Control of Preheat
23.1 General
23.2 Definitions
23.3 Application of preheat
23.4 Control of preheat and interpass temperature
23.5 Summary
24 Gauges

Appendix 1 Homework Multiple Choice Questions

Appendix 2 Plate Reports and Questions
Appendix 3 Pipe Reports and Questions
Appendix 4 Welding Crossword
Appendix5 Macro Practicals

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Examination Contents

30 General multiple choice

Questions 45 Min

60 Technology questions
90Min

20 Macroscopic questions
45Min

20 Plate Butt questions

75 Min

20 Pipe Butt questions

105 Min

70% is required in each section

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 Section 1

Typical Duties of Welding Inspectors

 Rev 2 Apri 2013
Typical Dulles of Welding Inspectors
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1 Typical Duties of Welding Inspectors

1.1 General
Welding inspectors are employed to assist with the quality control (QC)
activities necessary to ensure that welded items meet specified
requirements and are fit for their application.

For employers to have confidence in their work, welding inspectors need to

to understand/interpret the various QC procedures and also have a sound
knowledge of welding technology.

Visual inspection is one of the non-destructive examination (NDE)

disciplines and for some applications may be the only form.

For more demanding service conditions, visual inspection is usually followed

by one or more of the other non-destructive testing (NDT) techniques -
surface crack detection and volumetric inspection of butt welds.

Application Standards/Codes usually specify (or refer to other standards)

that give the acceptance criteria for weld inspection and may be very
specific about the particular techniques to be used for surface crack
detection and volumetric inspection; they do not usually give any guidance
about basic requirements for visual inspection.

Guidance and basic requirements for visual inspection are given by:

ISO 17637 (Non-destructive examination of fusion welds visual

Examination)

1.1.1 Basic requirements for visual inspection (to ISO 17637)

ISO 17637 provides the following:

Requirements for welding inspection personnel.

Recommendations about conditions suitable for visual examination.
Advice on the use of gauges/inspection aids that may be needed/helpful
for inspection.
Guidance about information that may need to be in the inspection
records.
Guidance about when inspection may be required during fabrication .

A summary of each of these topics is given in the following sections.

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1.1.2 Welding inspection personnel

Before starting work on a particular contract, ISO 17637 states that welding
inspectors should:

Be familiar with relevant standards, rules and specifications for the

fabrication work to be undertaken.
Be Informed about the welding procedure(s) to be used.
Have good vision- in accordance with EN 473 and checked every 12
months.

ISO 17637 does not give or make any recommendation about a formal
qualification for visual inspection of welds. However, it has become industry
practice for inspectors to have practical experience of welding inspection
together with a recognised qualification in welding inspection - such as a
CSWIP qualification.

1.1.3 Conditions for visual inspection

Illumination
ISO 17637 states that the minimum illumination shall be 350 lux but
recommends a minimum of 500 lux (normal shop or office lighting).

Access
Access to the surface for direct inspection should enable the eye to be:

Within 600mm of the surface being inspected.

In a position to give a viewing angle of not less than 30.

600mm (max.)

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1.1 .4 Aids to visual inspection

Where access for direct visual inspection is restricted, a mirrored baroscope
or a fibre optic viewing system, may be used - usually by agreement
between the contracting parties.

It may also be necessary to provide auxiliary lighting to give suitable

contrast and relief effect between surface imperfections and the
background.

Other items of equipment that may be appropriate to facilitate visual

examination are:

Welding gauges (for checking bevel angles and weld profile, fillet sizing,
measuring undercut depth).
Dedicated weld gap gauges and linear misalignment (hi-lo) gauges.
Straight edges and measuring tapes.
Magnifying lens (if a magnification lens is used it should be X2 to XS),

ISO 17637 shows a range of welding gauges together with details of what
they can be used for and the precision of the measurements.

1.1.5 Stages when inspection may be required

ISO 17637 states that examination is normally performed on welds in the
as-welded condition. This means that visual inspection of the finished weld
is a minimum requirement.

However, ISO 17637 says that the extent of examination and the stages
when inspection activity is required should be specified by the Application
Standard or by agreement between client and fabricator.

For fabricated items that must have high integrity, such as pressure vessels
and piping or large structures inspection, activity will usually be required
throughout the fabrication process:

Before welding.
During welding .
After welding .

Inspection activities at each of these stages of fabrication can be considered

the duties of the welding inspector and typical inspection checks that may
be required are described in the following section.

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Typ1cal Duties of Welding Inspectors
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1.1.6 Typical duties of a welding inspector

The relevant standards, rules and specifications that a welding inspector
should be familiar with at the start of a new contract are all the documents
he will need to refer to during the fabrication sequence in order to make
judgements about particular details.

Typical documents that may need to be referred to are:

The Application Standard (or Code): For visual acceptance criteria:

Although most of the requirements for the fabricated item should be
specified by National Standards, client standards or various QC
procedures, some features are not easy to define precisely and the
requirement may be given as to good workmanship standard.
Quality plans or inspection check lists: For the type and extent of
inspection.
Drawing: For assembly/fit-up details and dimensional requirements.
QC procedures: Company QC/QA procedures such as those for
document control, material handling, electrode storage and issue,
Welding Procedure Specifications, etc.

Examples of requirements difficult to define precisely are some shape

tolerances, distortion, surface damage or the amount of weld spatter.

Good workmanship is the standard that a competent worker should be able

to achieve without difficulty when using the correct tools in a particular
working environment.

In practice the application of the fabricated item will be the main factor that
influences what is judged to be good workmanship or the relevant client
specification will determine what the acceptable level of workmanship is.

Reference samples are sometimes needed to give guidance about the

acceptance standard for details such as weld surface finish and toe blend,
weld root profile and finish required for welds that need to be dressed, by
grinding or finishing.

A welding inspector should also ensure that any inspection aids that will be
needed are:

In good condition.
Calibrated as appropriate/as specified by QC procedures.

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Safety consciousness is a duty of all employees and a welding inspector

should:

Be aware of all safety regulations for the workplace.

Ensure that safety equipment that will be needed is available and in
suitable condition.

Duties before welding

Check j Action
Material In accordance with drawing/WPS.
Identified and can be traced to a test certificate.
In suitable condition (free from damage and contamination).
WPSs Approved and available to welders (and inspectors).
Welding equipment In suitable condition and calibrated as appropriate.
Weld preparations In accordance with WPS (and/or drawings).
Welder qualifications Identification of welders qualified for each WPS to be used.
All welder qualification certificates are valid (in date).
Welding consumables Those to be used are as specified by the WPSs, are
stored/controlled as specified by the QC procedure.
Joint fit-ups In accordance with WPS/drawings tack welds are to good
workmanship standard and to code/WPS.
Weld faces Free from defects, contamination and damage.
Preheat (if required) Minimum temperature is in accordance with WPS.

Duties during welding

Check Action
Site/field welding Ensure weather conditions ar:e suitable/comply with Code
(conditions will not affect welding).
Welding process In accordance with WPS.
Preheat (if required) Minimum temperature is being maintained in accordance with
WPS.
lnterpass temperature Maximum temperature Is in accordance with WPS.
Welding consumables In accordance with WPS and being controlled as procedure.
Welding parameters Current, volts, travel speed are in accordance with WPS.
Root run Visually acceptable to Code before filling the joint (for single
sided welds).
Gouging/grinding By an approved method and to good workmanship standard.
Inter-run cleaning To good workmanship standard.
Welder On the approval register/qualified for the WPS being used .

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Duties after welding

Check Action
Weld identification Each weld is marked with the welder's identification and is
identified in accordance with drawing/weld map.
Weld appearance Ensure welds are suitable for all NOT (profile, cleanness, etc) .
Visually inspect welds and sentence in accordance with Code.
Dimensional survey Check dimensions are in accordance with drawing/Code.
Drawings Ensure any modifications are included on as-built drawings.
NOT Ensure all NOT is complete and reports are available for records.
Repairs Monitor in accordance with the procedure.
PWHT (if required) Monitor for compliance with procedure (check chart record).
Pressure/load test Ensure test equipment is calibrated.
(if required) Monitor test to ensure compliance with procedure/Code.
Ensure reports/records are available.
Documentation records Ensure all reports/records are completed and collated as
required.

1.1 .7' Examination records

The requirement for examination records/inspection reports varies according
to the contract and type of fabrication and there is frequently no requirement
for a formal record.

When an inspection record is required it may be necessary to show that

items have been checked at the specified stages and have satisfied the
acceptance criteria.

The form of this record will vary , possibly a signature against an activity on
an inspection checklist or quality plan, or it may be an individual inspection
report for each item.

For individual inspection reports, ISO 17637 lists typical details for inclusion
such as:

Name of manufacturer/fabricator.
Identification of item examined.
Material type and thickness.
Type of joint.
Welding process.
Acceptance standard/criteria.
Locations and types of all imperfections not acceptable (when specified,
it may be necessary to include an accurate sketch or photograph).
Name of examiner/inspector and date of examination.

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 Section 2

Terms and Definitions

 Rev 2 Apnl 2013
Terms and Definitions
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2 Terms and Definitions

The following definitions are taken from BS 499-1 : Welding terms and
symbols -glossary for welding, brazing and thermal cutting.

Brazing
A process of joining generally applied to metals in which, during or after
heating, molten filler metal is drawn into or retained in the space between
closely adjacent surfaces of the parts to be joined by capillary attraction. In
general, the melting point of the filler metal is above 450C but always below
the melting temperature of the parent material.

Braze welding
The joining of metals using a technique similar to fusion welding and a filler
metal with a lower melting point than the parent metal, but neither using
capillary action as in brazing nor intentionally melting the parent metal.

Joint
A connection where the individual components, suitably prepared and
assembled, are joined by welding or brazing.

Weld
A union of pieces of metal made by welding.

Welding
An operation in which two or more parts are united by means of heat,
pressure or both, in such a way that there is continuity in the nature of the
metal between these parts.

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Type of
joint Sketch Definition
Butt Connection between the ends or edges
of two parts making an angle to one
{ II ~ another of 135-180 inclusive in the
region of the joint.

T Connection between the end or edge of

D
one part and the face of the other part,
the parts making an angle to one
another of more than 5 up to and
including 90 in the region of the joint.
{ ~
Comer Connection between the ends or edges
of two parts making an angle to one
{ I another of more than 30 but less than

D
135 in the region of the joint.

Edge A connection between the edges of two

~
parts making an angle to one another of
0-30 inclusive in the region of the joint.

L ~
Cruciform A connection in which two flat plates or
two bars are welded to another flat plate
at right angles and on the same axis.

{
D ~

Lap
D Connection between two overlapping
{ I parts making an angle to one another of
I ~ 0-5 inclusive in the region of the weld
or welds.

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2.1 Types of weld

2.1.1 From the configuration point of view (as per 2.2)

Butt weld Fillet weld

In a butt joint

Butt weld In aT joint

In a corner joint

Autogenous weld
A fusion weld made without filler metal by TIG, plasma, electron beam, laser
or oxy-fuel gas welding.

Slot weld
A joint between two overlapping components made by depositing a fillet
weld round the periphery of a hole in one component so as to join it to the
surface of the other component exposed through the hole.

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Plug weld
A weld made by filling a hole in one component of a workpiece with filler
metal so as to join it to the surface of an overlapping component exposed
through the hole (the hole can be circular or oval).

2.1.2 From the penetration point of view

Full penetration weld
A welded joint where the weld metal fully penetrates the joint with comple1e
root fusion. In the US the preferred term is complete joint penetration (CJP)
weld (see AWS 01 .1.}.

Q I
Partial penetration weld
A welded joint without full penetration. In the US the preferred term is partial
joint penetration (PJP) weld.

2.2 Types of joints (see BS EN ISO 15607)

Homogeneous
Welded joint in which the weld metal and parent material have no significant
differences in mechanical properties and/or chemical composition. Example:
Two carbon steel plates welded with a matching carbon steel electrode.

Heterogeneous
Welded joint in which the weld metal and parent material have significant
differences in mechanical properties and/or chemical composition. Example:
A repair weld of a cast iron item performed with a nickel-based electrode.

Dissimilar/Transition
Welded joint in which the parent materials have significant differences in
mechanical properties and/or chemical composition . Example: A carbon
steel lifting lug welded onto an austenitic stainless steel pressure vessel

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2.3 Features of the completed weld

Parent metal
Metal to be joined or surfaced by welding, braze welding or brazing.

Filler metal
Metal added during welding. braze welding, brazing or surfacing.

Weld metal
All metal melted during the making of a weld and retained in the weld.

Heat-affected zone (HAZ)

The part of the parent metal metallurgically affected by the heat of welding or
thermal cutting but not melted.

Fusion line
Boundary between the weld metal and the HAZ in a fusion weld.

Weld zone
Zone containing the weld metal and the HAZ.

Weld face
The surface of a fusion weld exposed on the side from which the weld has been
made.

Root
Zone on the side of the first run furthest from the welder.

Toe
Boundary between a weld face and the parent metal or between runs. This is a
very important feature of a weld since toes are points of high stress concentration
and often are initiation points for different types of cracks (eg fatigue and cold
cracks). To reduce the stress concentration, toes must blend smoothly into the
parent metal surface.

Excess weld metal

Weld metal lying outside the plane joining the toes. Other non-standard terms for
this feature are reinforcement and overfill.

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Weld
Parent zone
Weld
face metal

1----I Parent

metal Root
line weld

Penetration

Parent metal

HAZ

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2.4 Weld preparation

A preparation for making a connection where the individual components,
suitably prepared and assembled, are joined by welding or brazing. The
dimensions below can vary depending on WPS.

2.4.1 Features of the weld preparation

Angle of bevel
The angle at which the edge of a component is prepared for making a weld .

For an MMA weld on carbon steel plates, the angle is:

25-30 for a V preparation.

8-12 for aU preparation.
40-50 for a single bevel preparation.
10-20 for a J preparation.

Included angle
The angle between the planes of the fusion faces of parts to be welded . For
single and double V or U this angle is twice the bevel angle. In the case of
single or double bevel, single or double J bevel, the included angle is equal
to the bevel angle.

Root face
The portion of a fusion face at the root that is not bevelled or grooved. Its
value depends on the welding process used, parent material to be welded
and application; for a full penetration weld on carbon steel plates, it has a
value of 1-2mm (for the common welding processes).

Gap
The minimum distance at any cross-section between edges, ends or
surfaces to be joined. Its value depends on the welding process used and
application; for a full penetration weld on carbon steel plates, it has a value
of 1-4mm.

Root radius
The radius of the curved portion of the fusion face in a component prepared
for a single or double J or U, weld.

Land
Straight portion of a fusion face between the root face and the radius part of
a J or U preparation can be 0. Usually present in weld preparations for MIG
welding of aluminium alloys.

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2.4.2 Types of preparation

Open square butt preparation

II
Used for welding thin components from one or both sides. If the root gap is
zero (ie if components are in contact), this preparation becomes a closed
square butt preparation (not recommended due to problems caused by lack
of penetration)!

Single V preparation
Included angle

Gap-11- Rootf~
One of the most common preparations used in welding and can be
produced using flame or plasma cutting (cheap and fast). For thicker plates
a double V preparation is preferred since it requires less filler material to
complete the joint and the residual stresses can be balanced on both sides
of the joint resulting in lower angular distortion.

Double V preparation

The depth of preparation can be the same on both sides (symmetric double
V preparation) or deeper on one side (asymmetric double V preparation).
Usually, in this situation the depth of preparation is distributed as 2/3 of the
thickness of the plate on the first side with the remaining 1/3 on the
backside. This asymmetric preparation allows for a balanced welding

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sequence with root back gouging, giving lower angular distortions. Whilst a
single V preparation allows welding from one side, double V preparation
requires access to both sides (the same applies for all double sided
preparations).

Single U preparation

Rootfa:-f

U preparations can be produced only by machining (slow and expensive},

however, tighter tolerances give a better fit-up than with V preparations.
Usually applied to thicker plates compared with single V preparation as it
requires less filler material to complete the joint, lower residual stresses and
distortions. Like for V preparations, with very thick sections a double U
preparation can be used.

Double U preparation

Usually this type of preparation does not require a land, (except for
aluminium alloys).

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Single V preparation with backing strip

Backing strips allow production of full penetration welds with increased

current and hence increased deposition rates/productivity without the
danger of burn-through. Backing strips can be permanent or temporary.
Permanent types are made of the same material as being joined and are
tack welded in place. The main problems with this type of weld are poor
fatigue resistance and the probability of crevice corrosion between the
parent metal and the backing strip. It is also difficult to examine by NOT due
to the built-in crevice at the root of the joint. Temporary types include copper
strips, ceramic tiles and fluxes.

Single bevel preparation.

Double bevel preparation.

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Single J preparation.

Double J preparation.
l
All these preparations (single/double bevel and J) can be used on T joints
as well. Double preparations are recommended for thick sections. The main
advantage of these preparations is that only one component is prepared
(cheap, can allow for small misalignments).

For further details regarding weld preparations, please refer to Standard

BS EN ISO 9692.

2.5 Size of butt welds

Full penetration butt weld

Actual throat
thickness

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Partial penetration butt weld

Actual throat 'l '-........_ ___ ~ J Design throat

thickness
t+-----------+-t _ thickness

As a general rule:

Actual throat thickness= design throat thickness+ excess weld metal.

I \)
~----------~-'--------~--~--~
(
)I ~~~~~~;~a~esign
throat thickness

Full penetration butt weld ground flush.

Actual throat thickness Design throat

=maximum thickness thickness =thickness
through the joint t of the thinner plate

Butt weld between two plates of different thickness.

Run (pass)
The metal melted or deposited during one pass of an electrode, torch or
blowpipe.

I
Single run weld. Multi-run weld.

Layer
A stratum of weld metal consisting of one or more runs.

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Tenns and Definitions
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Types of butt weld (from accessibility point of view)

Single side weld.

Double side weld.

2.6 Fillet weld

A fusion weld, other than a butt, edge or fusion spot weld, which is
approximately triangular in transverse cross-section.

2.6.1 Size of fillet welds

Unlike butt welds, fillet welds can be defined using several dimensions.

Actual throat thickness

Perpendicular distance between two lines, each parallel to a line joining the
outer toes, one being a tangent at the weld face and the other being through
the furthermost point of fusion penetration.

Design throat thickness

The minimum dimension of throat thickness used for design purposes, also
known as effective throat thickness. (a on drawings).

Leg length
Distance from the actual or projected intersection of the fusion faces and the
toe of a fillet weld, measured across the fusion face (z on drawings).

Leg
-.,.)o,~-.., length

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2.6.2 Shape of fillet welds

Mitre fillet weld
A flat face fillet weld in which the leg lengths are equal within the agreed
tolerance. The cross-section area of this type of weld can be considered to
be a right angle isosceles triangle with design throat thickness a and leg
length z. The relation between design throat thickness and leg length is:

a =0. 707 x z or z = 1.41 x a

Convex fillet weld

A fillet weld in which the weld face is convex. The above relation between
leg length and design throat thickness for mitre fillet welds is also valid for
this type of weld. Since there is excess weld metal present, the actual throat
thickness is bigger than the design throat thickness.

Concave fillet weld

A fillet weld in which the weld face is concave. The relation between leg
length and design throat thickness specified for mitre fillet welds is not valid
for this type of weld. Also, the design throat thickness is equal to the actual
throat thickness. Due to the smooth blending between the weld face and the
surrounding parent material, the stress concentration effect at the toes of
the weld is reduced compared with the previous type. This is why this type
of weld is highly desired in applications subjected to cyclic loads where
fatigue phenomena might be a major cause for failure.

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Asymmetrical fillet weld

A fillet weld in which the vertical leg length is not equal to the horizontal leg
length. The relation between leg length and design throat thickness is not
valid for this type of weld because the cross-section is not an isosceles
triangle.
Horizontal leg
size

l
Vertical leg
size
-r-
Throat size

Deep penetration fillet weld

A fillet weld with a deeper than normal penetration. It is produced using high
heat input welding processes (ie SAW or MAG with spray transfer). This
type of weld uses the benefits of greater arc penetration to obtain the
required throat thickness whilst reducing the amount of deposited metal
needed thus leading to a reduction in residual stress level. To produce
consistent and constant penetration, the travel speed must be kept constant
at a high value. Consequently this type of weld is usually produced using
mechanised or automatic welding processes. Also, the high depth-to-width
ratio increases the probability of solidification centreline cracking . To
differentiate this type of weld from the previous types, the throat thickness is
symbolised with s instead of a.

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2.6.3 Compound of butt and fillet welds

A combination of butt and fillet welds used for T joints with full or partial
penetration or butt joints between two plates with different thickness. Fillet
welds added on top of the groove welds improve the blending of the weld
face towards the parent metal surface and reduce the stress concentration
at the toes of the weld.

Bevel
weld Fillet
weld

Double bevel compound weld.

2. 7 Welding position, slope and rotation

Welding position
Orientation of a weld expressed in terms of working position, weld slope and
weld rotation (for further details see ISO 6947).

Weld slope
Angle between root line and the positive X axis of the horizontal reference
plane, measured in mathematically positive direction (ie counter-clockwise).

Weld rotation
Angle between the centreline of the weld and the positive Z axis or a line
parallel to the Y axis, measured in the mathematically positive direction (ie
counter-clockwise) in the plane of the transverse cross-section of the weld in
question.

/i
I I

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Definition and symbol according

Welding position Sketch to ISO 6947
Flat Welding position in which the
welding is horizontal with the
centreline of the weld vertical.
PA.
Horizontal- Welding position in which the
vertical welding is horizontal (applicable
in case of fillet welds). PB.

Horizontal Welding position in which the

welding is horizontal, with the
centreline of the weld horizontal.
PC.

Vertical-up Welding position in which the

welding is upwards. PF.

Vertical-down Welding position in which the

welding is downwards. PG.

Overhead A welding position in which the

welding is horizontal and
overhead (applicable in fillet
welds). PE.
Horizontal-
overhead Welding position in which the
welding is horizontal and
overhead with the centreline of
the weld vertical. PD.

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Vertir.al-llfl Verfcal-llown
90" go
1o 11r 10" 10
Flat
Rotation

aff:#::tl!it:
Slope
: j:: 1ao
go
0.180 in
both cases

o
"\}1
'y o
Rotation Slope Slope

t'orizo'1 taf verticdl Overheafl

c;:::;:;;[J~ ow.Al\.,
_115.
so _. ~go

S tope ~
w C':-
\
I
I
. 75j
o- -
1800
kJ

180" Slope Rotation

Rotation

Tolerances for the welding positions.

2.8 Weaving
Transverse oscillation of an electrode or blowpipe nozzle during the
deposition of weld metal, generally used in vertical-up welds.

Stringer bead
A run of weld metal made with little or no weaving motion.

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3 Welding Imperfections and Materials Inspection

3.1 Definitions (see BS EN ISO 6520-1)
Imperfection Any deviation from the ideal weld.
Defect An unacceptable Imperfection.

Classification of tmperfections according to BS EN ISO 6520-1 :

This standard classifies the geometric Imperfections in fusion welding

dividing them into six groups:

1 Cracks.
2 Cavities.
3 Solid inclusions.
4 Lack of fusion and penetration.
5 Imperfect shape and dimensions.
6 Miscellaneous imperfections.

It is important that an imperfection is correctly identified so the cause can be

established and actions taken to prevent further occurrence.

3.2 Cracks
Definition
Imperfection produced by a local rupture in the solid state, which may arise
from the effect of cooling or stresses. Cracks are more significant than other
types of imperfection as their geometry produces a very large stress
concentration at the crack tip making them more likely to cause fracture.

Types of crack:
Longitudinal.
Transverse.
Radiating (cracks radiating from a common point).
Crater.
Branching (group of connected cracks originating from a common crack) .

These cracks can be situated In the:

Weld metal.
HAZ.
Parent metal.

Exception: Crater craeks are found only in the weld metal.

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Depending on their nature, these cracks can be:

Hot (ie solidification or liquation cracks).

Precipitation induced (ie reheat cracks present in creep resisting steels).
Cold (ie hydrogen induced cracks) .
Lamellar tearing.

3.2.1 Hot cracks

Depending on their location and mode of occurrence, hot cracks can be:

Solidification cracks: Occur in the weld metal (usually along the

centreline of the weld) as a result of the solidification process.
Liquation cracks: Occur in the coarse grain HAZ, in the near vicinity of
the fusion line as a result of heating the material to an elevated
temperature, high enough to produce liquation of the low melting point
constituents placed on grain boundaries.

3.2.2 Solidification cracks

---------------20mm
Generally, solidification cracking can occur when:

Weld metal has a high carbon or impurity (sulphur, etc) content.

The depth-to-width ratio of the solidifying weld bead is large (deep and
narrow).
Disruption of the heat flow condition occurs, eg stop/start condition.

The cracks can be wide and open to the surface like shrinkage voids or sub-
surface and possibly narrow.

Solidification cracking is most likely to occur in compositions and result in a

wide freezing temperature range. In steels this is commonly created by a
higher than normal content of carbon and impurity elements such as sulphur
and phosphorus. These elements segregate during solidification, so that
intergranular liquid films remain after the bulk of the weld has solidified. The

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thermal shrinkage of the cooling weld bead can cause these to rupture and
form a crack.

Sulphur-enri ched Solidification

llq11id fllm "\. ___.-:-..;;.
' _crack

It is important that the welding fabricator does not weld on or near metal
surfaces covered with scale or contaminated with oil or grease. Scale can
have a high sulphur content and oil and grease can supply both carbon and
sulphur. Contamination with low melting point metals such as copper, tin ,
lead and zinc should also be avoided.

Hydrogen induced cracks

Root (underbead) crack. Toe crack.

Hydrogen induced cracking occurs primarily in the grain coarsened region of

the HAZ and is also known as cold, delayed or underbead/toe cracking . It
lies parallel to the fusion boundary and its path is usually a combination of
inter and transgranular cracking. The direction of the principal residual
tensile stress can in toe cracks cause the crack path to grow progressively
away from the fusion boundary towards a region of lower sensitivity to
hydrogen cracking . When this happens, the crack growth rate decreases
and eventually arrests.

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Four factors are necessary to cause HAZ hydrogen cracking:

Hydrogen level > 15ml/1 OOg of weld metal deposited

Stress > 0.5 of the yield stress
Temperature < 300oC
Susceptible microstructure > 400HV hardness

If any one factor is not satisfied, cracking is prevented, so can be avoided

through control of one or more factors:

Apply preheat slow down the cooling rate and thus avoid the formation of
susceptible microstructures.
Maintain a specific interpass temperature (same effect as preheat).
Postheat on completion of welding to reduce the hydrogen content by
allowing hydrogen to diffuse from the weld area.
Apply PWHT to reduce residual stress and eliminate susceptible
microstructures.
Reduce weld metal hydrogen by proper selection of welding
process/consumable (eg use TIG welding instead of MMA, basic
covered electrodes instead of cellulose).
Use a multirun instead of a single run technique and eliminate
susceptible microstructures by the selftempering effect, reduce
hydrogen content by allowing hydrogen to diffuse from the weld area.
Use a temper bead or hot pass technique (same effect as above).
Use austenitic or nickel filler to avoid susceptible microstructure
formation and allow hydrogen to diffuse out of critical areas).
Use dry shielding gases to reduce hydrogen content.
Clean rust from joint to avoid hydrogen contamination from moisture
present in the rust.
Reduce residual stress.
Blend the weld profile to reduce stress concentration at the toes of the
weld.

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Lamellar tearing

Lamellar tearing occurs only in rolled steel products (primarily plates) and its
main distinguishing feature is that the cracking has a terraced appearance.

Cracking occurs in joints where:

A thermal contraction strain occurs in the through-thickness direction of

steel plate.
Non-metallic inclusions are present as very thin platelets, with their
principal planes parallel to the plate surface.

Contraction strain imposed on the planar non-metallic inclusions results in

progressive decohesion to form the roughly rectangular holes which are the
horizontal parts of the cracking, parallel to the plate surface. With further
strain the vertical parts of the cracking are produced, generally by ductile
shear cracking . These two stages create the terraced appearance of these
cracks.

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Two main options are available to control the problem in welded joints liable
to lamellar tearing:

Use a clean steel with guaranteed through-thickness properties

(Z grade).
A combination of joint design, restraint control and welding sequence to
minimise the risk of cracking.

Gas cavity: formed Shrinkage cavity:

by entrapped gas Caused by shrinkage
during solidification

lnterdendritic
Gas pore shrinkage

Crater pipe

Interdendritic Transgranular
Elongated cavity
microshrinkage microshrinkage
Worm hole

Surface pore

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3.3 Cavities
3.3.1 Gas pore

A gas cavity of essentially spherical shape trapped within the weld metal.

Gas cavities can be present in various forms:

Isolated.
Uniformly distributed porosity.
Clustered (localised) porosity.
Linear porosity.
Elongated cavity.
Surface pore.

Causes Prevention
Damp fluxes/corroded electrode Use dry electrodes in good condition
(MMA)
Grease/hydrocarbon/water Clean prepared surface
contamination of prepared surface
Air entrapment in gas shield Check hose connections
(MIG/MAG, TIG)
lncorrecVinsufficient deoxidant in Use electrode with sufficient deoxidation activity
electrode, filler or parent metal
Too great an arc voltage or length Reduce voltage and arc length
Gas evolution from priming Identify risk of reaction before surface treatment
paints/surface treatment is applied
Too high a shielding gas flow rate Optimise gas flow rate
results in turbulence (MIG/MAG, TIG)

Comment
Porosity can be localised or finely dispersed voids throughout the weld
metal.

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3.3.2 Worm holes

Elongated or tubular cavities formed by trapped gas during the solidification

of the weld metal which can occur singly or in groups.

Causes Prevention
Gross contamination of preparation Introduce preweld cleaning procedures
surface
Laminated work surface Replace parent material with an unlaminated
piece
Crevices in work surface due to joint Eliminate joint shapes which produce crevices
geometry

Comments
Worm holes are caused by the progressive entrapment of gas between the
solidifying metal crystals (dendrites) producing characteristic elongated
pores of circular cross-section. These can appear as a herringbone array on
a radiograph and some may break the surface of the weld.

3.3.3 Surface porosity

A gas pore that breaks the surface of the weld.

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Causes Prevention
Damp or contaminated surface or Clean surface and dry electrodes
electrode
Low fluxing activity (MIG/MAG) Use a high activity flux
Excess sulphur (particularly free-cutting Use high manganese electrodes to produce
steels) producing sulphur dioxide MnS. Note free-cutting steels (high sulphur)
should not normally be welded
Loss of shielding gas due to long arc or Improve screening against draughts and
high breezes (MIG/MAG) reduce arc length
A shielding gas flow rate that is too high Optimise gas flow rate
results in turbulence (MIGIMAG,TIG)

The origins of surface porosity are similar to those for uniform porosity.

Crater pipe

A shrinkage cavity at the end of a weld run usually caused by shrinkage

during solidification.

Causes Prevention
Lack of welder skill due to using processes Retrain welder
with too high a current
Inoperative crater filler (slope out) (TIG) Use correct crater filling techniques

Crater filling is a particular problem in TIG welding due to its low heat input.
To fill the crater for this process it is necessary to reduce the weld current
(slope out) in a series of descending steps until the arc is extinguished.

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3.4 Solid inclusions

Definition
Solid foreign substances trapped in the weld metal.

Flux Oxide Metallic

inclusion inclusion inclusion

Tungsten

Copper

linear

3.4.1 Slag inclusions

Slag trapped during welding which is an irregular shape so differs in

appearance from a gas pore.

Causes Prevention
Incomplete slag removal from underlying Improve inter-run slag removal
surface of multi-pass weld
Slag flooding ahead of arc Position work to gain control of slag.
Welder needs to correct electrode angle
Entrapment of slag in work surface Dress/make work surface smooth

A fine dispersion of inclusions may be present within the weld metal,

particularly if the MMA process is used. These only become a problem when
large or sharp-edged inclusions are produced.

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3.4.2 Flux inclusions

Flux trapped during welding which is an irregular shape so differs in
appearance from a gas pore. Appear only in flux associated welding
processes (ie MMA, SAW and FCAW).

Causes Prevention
Unfused flux due to damaged coating Use electrodes in good condition
Flux fails to melt and becomes trapped in Change the flux/wire. Adjust welding
the weld (SAW or FCAW) parameters ie current, voltage etc to produce
satisfactory welding conditions

3.4.3 Oxide inclusions

Oxides trapped during welding which is an irregular shape so differs in
appearance from a gas pore.

Cause Prevention
Heavy millscale/rust on work surface Grind surface prior to welding

A special type of oxide inclusion is puckering, which occurs especially in the

case of aluminium alloys. Gross oxide film enfoldment can occur due to a
combination of unsatisfactory protection from atmospheric contamination
and turbulence in the weld pool.

3.4.4 Tungsten inclusions

Particles of tungsten can become embedded during TIG weld ing appears as
a light area on radiographs as tungsten is denser than the surrounding
metal and absorbs larger amounts of X-/gamma radiation.

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Causes Prevention
Contact of electrode tip with weld pool Keep tungsten out of weld pool; use HF start
Contact of filler metal with hot tip of Avoid contact betwegn electrode and filler
electrode metal
Contamination of the electrode tip by Reduce welding current; adjust shielding gas
spatter from the weld pool flow rate
Exceeding the current limit for a given Reduce welding current; replace electrode
electrode size or type with a larger diameter one
Extension of electrode beyond the normal Reduce electrode extension and/or welding
distance from the collet, resulting in current
overheating of the electrode
Inadequate tightening of the collet Tighten the collet
Inadequate shielding gas flow rate or Adjust the shielding gas flow rate; protect the
excessive draughts resulting in oxidation weld area; ensure that the post gas flow after
of the electrode tip stopping the arc continues for at least five
seconds
Splits or cracks in the electrode Change the electrode, ensure the correct
size tungsten is selected for the given
welding current used
Inadequate shielding gas (eg use of Change to correct gas composition
argon-oxygen or argon-carbon dioxide
mixtures that are used for MAG welding)

3.5 Lack of fusion and penetration

3.5.1 Lack of fusion
Lack of union between the weld metal and the parent metal or between the
successive layers of weld metal.

Lack of
fusion

Lack of sidewall Lack of inter- Lack of root

fusion run fusion fusion

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Lack of sidewall fusion

Lack of union between the weld and parent metal at one or both sides of the
weld.

Causes Prevention
Low heat input to weld Increase arc voltage and/or welding current;
decrease travel speed
Molten metal flooding ahead of arc Improve electrode angle and work position;
increase travel speed
Oxide or scale on weld preparation Improve edge preparation procedure
Excessive inductance in MAG dip Reduce inductance, even if this increases
transfer welding spatter

During welding sufficient heat must be available at the edge of the weld pool
to produce fusion with the parent metal.

Lack of inter-run fusion

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Lack of union along the fusion line between the weld beads.

Causes Prevention
Low arc current resulting in low fluidity of Increase current
weld pool
Too high a travel speed Reduce travel speed
Inaccurate bead placement Retrain welder

Lack of inter-run fusion produces crevices between the weld beads and
causes local entrapment of slag.

Lack of root fusion

Lack of fusion between the weld and parent metal at the root of a weld.

Causes Prevention
Low heat input Increase welding current and/or arc voltage;
decrease travel speed
Excessive inductance in MAG dip Use correct induction setting for the parent
transfer welding, metal thickness
MMA electrode too large Reduce electrode size
(low current density)
Use of vertical-down welding Switch to vertical-up procedure
Large root face Reduce root face
Small root gap Ensure correct root opening
Incorrect angle or electrode Use correct electrode angle.
manipulation Ensure welder is fully qualified and competent
Excessive misalignment at root Ensure correct alignment

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3.5.2 Lack of penetration

Lack of
penetration

Incomplete Incomplete root

penetration penetration

Incomplete penetration

The difference between actual and nominal penetration.

Causes Prevention
Excessively thick root face, Insufficient Improve back gouging technique and ensure the
root gap or failure to cut back to sound edge preparation is as per approved WPS
metal when back gouging
Low heat input Increase welding current and/or arc voltage;
decrease travel speed
Excessive inductance in MAG dip Improve electrical settings and possibly switch to
transfer welding, pool flooding ahead spray arc transfer
of arc
MMA electrode too large Reduce electrode size
(low current density)
Use of vertical-down welding Switch to vertical-up procedure

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If the weld joint is not of a critical nature, ie the required strength is low and
the area is not prone to fatigue cracking, it is possible to produce a partial
penetration weld. In this case incomplete root penetration is considered part
of this structure and not an imperfection This would normally be determined
by the design or code requirement.

Incomplete root penetration

Both fusion faces of the root are not melted. When examined from the root
side, you can clearly see both of the root edges unmelted.

Causes and prevention

Same as for lack of root fusion .

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3.6 Imperfect shape and dimensions

3.6.1 Undercut

"--~-
- - - - 2 0 mm

An irregular groove at the toe of a run in the parent metal or previously

deposited weld metal due to welding. Characterised by its depth, length and
sharpness.

I Undercut I

Continuous Intermittent Inter-run

undercut undercut undercut

Causes Prevention
Melting of top edge due to high welding Reduce power input, especially
current (especially at the free edge) or high approaching a free edge where overheating
travel speed can occur
Attempting a fillet weld in horizontal-vertical Weld in the flat position or use multi-run
(PB) position with leg length >9mm techniques
Excessive/incorrect weaving Reduce weaving width or switch to multi-
runs
Incorrect electrode angle Direct arc towards thicker member
Incorrect shielding gas selection (MAG) Ensure correct gas mixture for material type
and thickness (MAG)

Care must be taken during weld repairs of undercut to control the heat input.
If the bead of a repair weld is too small, the cooling rate following welding
will be excessive and the parent metal may have an increased hardness
and the weld susceptible to hydrogen cracking.

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3.6.2 Excess weld metal

- - - -somm
Excess weld metal is the extra metal that produces excessive convexity in
fillet welds and a weld thickness greater than the parent metal plate in butt
welds. It is regarded as an imperfection only when the height of the excess
weld metal is greater than a specified limit.

Causes Prevention
Excess arc energy {MAG, SAW) Reduction of heat input
Shallow edge preparation Deepen edge preparation
Faulty electrode manipulation or build-up Improve welder skill
sequence
Incorrect electrode size Reduce electrode size
Travel speed too slow Ensure correct travel speed is used
Incorrect electrode angle Ensure correct electrode angle is used
Wrong polarity used {electrode polarity Ensure correct polarity ie DC+ve
DC-ve (MMA, SAW) Note DC-ve must be used for TIG

The term reinforcement used to designate this feature of the weld is

misleading since the excess metal does not normally produce a stronger
weld in a butt joint in ordinary steel. This imperfection can become a
problem, as the angle of the weld toe can be sharp leading to an increased
stress concentration at the toes of the weld and fatigue cracking.

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3.6.3 Excess penetration

Projection of the root penetration bead beyond a specified limit, local or

continuous.

Causes Prevention
Weld heat input too high Reduce arc voltage and/or welding current;
increase welding speed
Incorrect weld preparation ie excessive Improve wo.rkpiece preparation
root gap, thin edge preparation, lack of
backing
Use of electrode unsuited to welding Use correct electrode for position
position
Lack of welder skill Retrain welder

The maintenance of a penetration bead of uniform dimensions requires a

great deal of skill, particularly in pipe butt welding. This can be made more
difficult if there is restricted access to the weld or a narrow preparation.
Permanent or temporary backing bars can assist in the control of
penetration.

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3.6.4 Overlap

- --20mm

Imperfection at the toe of a weld caused by metal flowing on to the surface

of the parent metal without fusing to it.

Causes Prevention
Poor electrode manipulation (MMA) Retrain welder
High heat input/low travel speed Reduce heat input or limit leg size to 9mm
causing surface flow of fillet welds maximum for single pass fillets
Incorrect positioning of weld Change to flat position
Wrong electrode coating type resulting Change electrode coating type to a more
in too high a fluidity suitable fast freezing type which is less fluid

For a fillet weld overlap is often associated with undercut, as if the weld pool
is too fluid the top of the weld will flow away to produce undercut at the top
and overlap at the base.llf the volume of the weld pool is too large in a fillet
weld in horizontal-vertical (PB) position, weld metal will collapse due to
gravity, producing both defects (undercut at the top and overlap at the
base), this defect is called ~a~ng.

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3.6.5 Linear misalignment

----50mm

Misalignment between two welded pieces such that while their surface
planes are parallel, they are not in the required same plane.

Causes Prevention
Inaccuracies in assembly procedures or Adequate checking of alignment prior to
distortion from other welds welding coupled with the use of clamps and
wedges
Excessive out of flatness in hot rolled Check accuracy of roned section prior to
plates or sections welding

Misalignment is not a weld imperfection but a structural preparation

problem. Even a small amount of misalignment can drastically increase the
local shear stress at a joint and induce bending stress.

3.6.6 Angular distortion

Misalignment between two welded pieces such that their surface planes are
not parallel or at the intended angle.

Causes and prevention are the same as for linear misalignment.

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3.6.7 Incompletely filled groove

Continuous or intermittent channel in the weld surface due to insufficient

deposition of weld filler metal.

Causes Prevention
Insufficient weld metal Increase the number of weld runs
Irregular weld bead surface Retrain welder

This imperfection differs from undercut, as it reduces the load-bearing

capacity of a weld, whereas undercut produces a sharp stress-raising notch
at the edge of a weld .

3.6.8 Irregular width

Excessive variation in width of the weld.

Causes Prevention
Severe arc blow Switch from DC to AC, keep arc length as short as
possible
Irregular weld bead surface Retrain welder

Although this imperfection may not affect the integrity of the completed weld ,
it can affect the width of HAZ and reduce the load-carrying capacity of the
joint (in fine-grained structural steels) or impair corrosion resistance (in
duplex stainless steels).

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3.6.9 Root concavity

A shallow groove that occurs due to shrinkage at the root of a butt weld.

Causes Prevention
Insufficient arc power to produce positive bead Raise arc energy
Incorrect preparation/fit-up Work toWPS
Excessive backing gas pressure (TIG) Reduce gas pressure
Lack of welder skill Retrain welder
Slag flooding in backing bar groove Tilt work to prevent slag flooding

A backing strip can be used to control the extent of the root bead.

3.6.1 0 Burn-through

A collapse of the weld pool resulting in a hole in the weld.

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Causes Prevention
Insufficient travel speed Increase the travel speed
Excessive welding current Reduce welding current
Lack of welder skill Retrain welder
Excessive grinding of root face More care taken, retrain welder
Excessive root gap Ensure correct fit-up

This is a gross imperfection which occurs due to lack of welder skill but can
be repaired by bridging the gap formed into the joint, but requires a great
deal of attention.

3.7 Miscellaneous imperfections

3.7.1 Stray arc

Local damage to the surface of the parent metal adjacent to the weld ,
resulting from arcing or striking the arc outside the weld groove. This results
in random areas of fused metal where the electrode, holder or current return
clamp have accidentally touched the work.

Causes Prevention
Poor access to the work Improve access (modify assembly sequence)
Missing insulation on electrode Institute a regular inspection scheme for
holder or torch electrode holders and torches
Failure to provide an insulated Provide an insulated resting place
resting place for the electrode holder
or torch when not in use
Loose current return clamp Regularly maintain current return clamps
Adjusting wire feed (MAG welding) Retrain welder
without isolating welding current

An arc strike can produce a hard HAZ which may contain cracks, possibly
leading to serious cracking in service. It is better to remove an arc strike by
grinding than weld repair.

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3.7.2 Spatter

Globules of weld or filler metal expelled during welding adhering to the

surface of parent metal or solidified weld metal.

Causes Prevention
High arc current Reduce arc current
Long arc length Reduce arc length
Magnetic arc blow Reduce arc length or switch to AC power
Incorrect settings for GMAW process Modify electrical settings (but be careful to
maintain full fusion!)
Damp electrodes Use dry electrodes
Wrong selection of shielding gas Increase argon content if possible, however if
(100%C0 2) too high may lead to lack of penetration

Spatter is a cosmetic imperfection and does not affect the integrity of the
weld. However as it is usually caused by an excessive welding current, it is
a sign that the welding conditions are not ideal so there are usually other
associated problems within the structure, ie high heat input. Some spatter is
always produced by open arc consumable electrode welding processes.
Anti-spatter compounds can be used on the parent metal to reduce sticking
and the spatter can then be scraped off.

3.7.3 Torn surface

Surface damage due to the removal by fracture of temporary welded
attachments. The area should be ground off, subjected to a dye penetrant or
magnetic particle examination then restored to its original shape by welding
using a qualified procedure. Some applications do not allow the presence of
any overlay weld on the surface of the parent material.

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3.7.4 Additional imperfections

Grinding mark
Local damage due to grinding.

Chipping mark
Local damage due to the use of a chisel or other tools.

Underflushing
Lack of thickness of the workpiece due to excessive grinding.

Misalignment of opposite runs

Difference between the centrelines of two runs made from opposite sides of
the joint.

Temper colour (visible oxide film)

Lightly oxidised surface in the weld zone, usually occurs in stainless steels.

3.8 Acceptance standards

Weld imperfections can seriously reduce the integrity of a welded structure.
Prior to service of a welded joint, it is necessary to locate them using NDE
techniques, assess their significance and take action to avoid their
reoccurrence.

The acceptance of a certain size and type of defect for a given structure is
normally expressed as the defect acceptance standard, usually incorporated
in application standards or specifications.

All normal weld imperfection acceptance standards totally reject cracks. In

exceptional circumstances and subject to the agreement of all parties,
cracks may remain if it can be demonstrated beyond doubt that they will not
lead to failure. This can be difficult to establish and usually involves fracture
mechanics measurements and calculations.

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It is important to note that the levels of acceptability vary between different

applications and in most cases vary between different standards for the
same application. Consequently, when inspecting different jobs it is
important to use the applicable standard or specification quoted in the
contract.

Once unacceptable weld imperfections have been found they have to be

removed. If the weld imperfection is at the surface, the first consideration is
whether it is of a type shallow enough to be repaired by superficial dressing.
Superficial implies that after removal of the defect the remaining material
thickness is sufficient not to require the addition of further weld metal.

If the defect is too deep it must be removed and new weld metal added to
ensure a minimum design throat thickness.

Replacing removed metal or weld repair (as in filling an excavation or re-

making a weld joint) has to be done in accordance with an approved
procedure. The rigour with which this procedure is qualified depends on the
application standard for the job. In some cases it will be acceptable to use a
procedure qualified for making new joints whether filling an excavation or
making a complete joint. If the level of reassurance required is higher, the
qualification will have to be made using an exact simulation of a welded
joint, which is excavated then refilled using a specified method. In either
case, qualification inspection and testing will be required in accordance with
the application standard.

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Destructive Testing

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4 Destructive Testing
Introduction
European Welding Standards require test coupons made for welding
procedure qualification testing to be subjected to non-destructive and then
destructive testing.

The tests are called destructive tests because the welded joint is destroyed
when various types of test piece are taken from it.

Destructive tests can be divided into two groups, those used to:

Measure a mechanical property - quantitative tests

Assess the joint quality -qualitative tests

Mechanical tests are quantitative because a quantity is measured, a

mechanical property such as tensile strength, hardness or impact
toughness.

Qualitative tests are used to verify that the joint is free from defects, of
sound quality and examples of these are bend tests, macroscopic
examination and fracture tests (fillet fracture and nick-break).

4.1 Test types, pieces and objectives

Various types of mechanical test are used by material manufacturers/
suppliers to verify that plates, pipes, forgings, etc have the minimum
property values specified for particular grades.

Design engineers use the minimum property values listed for particular
grades of material as the basis for design and the most cost-effective
designs are based on an assumption that welded joints have properties that
are no worse than those of the base metal.

The quantitative (mechanical) tests carried out for welding procedure

qualification are intended to demonstrate that the joint properties satisfy
design requirements.

The emphasis in the following sub-sections is on the destructive tests and

test methods widely used for welded joints.

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4.1.1 Transverse tensile tests

Test objective
We!ding procedure qualification tests always require transverse tensile tests
to show that the strength of the joint satisfies the design criterion.

Test specimens
A transverse tensile test piece typical of the type specified by European
Welding Standards is shown below.

Standards, such as EN 895, that specify dimensions for transverse tensile

test pieces require all excess weld metal to be removed and the surface to
be free from scratches.
Parallel
length

I WI !i I
:I
i

~ ~~

Tesl pieces may be machined to represent the full thickness of the joint but
for very thick joints it may be necessary to take seyeral transverse tensile
test specimens to be able to test the full thickness.

Method
Test specimens are accurately measured before testing, then fitted into the
jaws of a tensile testing machine and subjected to a continually increasing
tensile force until the specimen fractures.

The tensile strength (Rm) is calculated by_djyiding the maximum load by the
cross-sectional area of the test specimen, measured before testing.

The test is intended to measure the tensile strength of the joint and thereby
show that the basis for design, the base metal properties, remain the valid
criterion.

Acceptance criteria
If the test piece breaks in the weld metal, it is acceptable provided the
calculated strength is not Jess than the minimum tensile strength specified,
which is usually the minimum specified for the base metal material grade.

In the ASME IX code, if the test specimen breaks outside the weld or fusion
zone at a stress ab.9ve 95% of the minimum base metal strength the test
result is acceptable.

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4.1.2 All-weld tensile tests

Objective
On occasion it is necessary to measure the weld metal strength as part of
welding procedure qualification, particularly for elevated temperature
designs.

The test is to measure tensile strength and also yield (or proof strength) and
tensile ductility.

All-weld tensile tests are regularly carried out by welding consumable

manufacturers to verify that electrodes and filler wires satisfy the tensile
properties specified by the standard to which the consumables are certified .

Specimens
Machined from welds parallel with their longitudinal axis and the specimen
gauge length must be 100% weld metal.

Parallel length

Gauge length

Radius Diameter or the

reduced section

Gripped end

Round cross-section

Round tensile specimen from a Round tensile specimen from an

welding procedure qualification electrode classification test piece.
test piece.

Method
Specimens are subjected to a continually increasing force in the same way
that transverse tensile specimens are tested .

Yield (Re) or proof stress (Rp) are measured by an extensometer attached to

the parallel length of the specimen that accurately measures the extension
of the gauge length as the load is increased.

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Typical load extension curves and their principal characteristics are shown
below.

Am== Ultimate tensile strength

------------~ ~
ReH=Upper yield point
AeL= Lower yield point ~~ ---n-,-------
:.
~~
I
I

"
0,2%

Load extension curve for a steel that Load-extension curve for a steel (or other
shows a distinct yield point at the metal) that does not show a distinct yield
elastic limit. point; proof stress is a measure of the
elastic limit.

Tensile ductility is measured in two ways:

Percent elongation of the gauge length.

Percent reduction of area at the point of fracture.

The figure below illustrates these two ductility measurements.

o . ._ , : ~~ ~:o

g 1 -oog~~~
.J-=: o
gauseleng
Plastic
defoMlatlon
r-------1
Gauge length
(a) at failure

(b)

To calculate elongation: : : : : : : X 100 ltmaatltm cu a%

To calculate UTS: = = Ulimat11 tm.t1U. atrmgt.h

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4.1.3 Impact toughness tests

Objective
Charpy V notch test pieces are the internationally accepted method for
assessing resistance to brittle fracture. by measuring the energy to initiate
and propagate a crack from a sharp notch in a standard sized specimen
subjected to an impact load.

Design engineers need to ensure that the toughness of the steel used for a
particular item will be sufficient to avoid brittle fracture in service and so
impact specimens are tested at a temperature related to the design
temperature for the fabricated component.

C-Mn and low alloy steels undergo a sharp change in their resistance to
brittle fracture as their temperature is lowered so that a steel that may have
very good toughness at ambient temperature may show extreme brittleness
at sub-zero temperatures, as illustrated below.

47 Joules

1---1--.......
:
1
Ductile/Brittle
transition
po1nt
-
I
I
--+-----
:
28 Joules
Energy absorbed
I

-50 -40 -30 -20 -10 0

Testing temperature- Degrees Centigrade
Three specimens are normally tested at each temperature

The transition temperature is defined as the temperature midway between

the upper shelf (maximum toughness) and lower shelf (completely brittle}. In
the above the transition temperature is -20C.

Specimens
Test specimen dimensions have been standardised internationally and are
shown below for full size specimens. There are also standard dimensions
for smaller sized specimens, for example 10 x 7.5mm and 10 x 5mm.

~~
. -------~1 TnTc 025mm
10mm Bmm (0.010")
(0.3941~0.315') red.
10mm
(2.165') (0.394") 45.

Charpy V notch test piece dimensions for full size specimens.

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Specimens are machined from welded test plates with the notch position
located in different positions according to the testing requirements but
typically in the centre of the weld metal and at positions across the HAZ, as
shown below,

Fusion llne 2mm Fusion line

Fusion ~ne + 5mm

Fusion line + 2mm Fusion line

Typical notch positions for Charpy V notch test specimens from double V butt
welds.

Method
Test specimens are cooled to the specified test temperature by immersion in
an insulated bath containing a liquid held at the test temperature.

After allowing the specimen temperature to stabilise for a few minutes it is

quickly transferred to the anvil of the test machine and a pendulum hammer
quickly released so that the specimen experiences an impact load behind
the notch.

The main features of an impact test machine are shown below.

Scale

Impact specimen on
the anvil showing the
hammer position at
point of impact.
Impact testing machine.

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Charpy V notch test pieces after and before testing.

The energy absorbed by the hammer when it strikes each test specimen is
shown by the position of the hammer pointer on the scale of the machine.
Energy values are given in Joules (or ft-lbs in US specifications).

Three Impact test specimens are taken for each notch position as there is
always some degree of scatter in the results, particularly for weldments.

Acceptance criteria
Each test result is recorded and an average value calculated for each set of
three tests. These values are compared with those specified by the
application standard or client to establish whether specified requirements
have been met.

After impact testing, examination of the test specimens provides additional

information about their toughness characteristics and may be added to the
test report:

Percent crystallinity: % of the fracture face that has crystalline

appearance which indicates brittle fracture; 100% indicates completely
brittle fracture.

Lateral expansion: Increase in width of the back of the specimen

behind the notch, as indicated below; the larger the value the tougher
the specimen.

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Lateral--+-+--
contraction

I~ 10mm ~I
Non lateral expansion =
a + b lateral expansion
brittle fracture ductile fracture

A specimen that exhibits extreme brittleness will show a clean break, both
halves of the specimen having a completely flat fracture face with little or no
lateral expansion.

A specimen that exhibits very good toughness will show only a small degree
of crack extension, without fracture and a high value of lateral expansion.

4.1.4 Hardness testing

Objective
The hardness of a metal is its' resistance to plastic deformation, determined
by measuring the resistance to indentation by a particular type of indenter.

A steel weldment with hardness above a certain maximum may be

susceptible to cracking, either during fabrication or in-service and welding
procedure qualification testing for certain steels and applications requires
the test weld to be hardness surveyed to ensure no regions exceed the
maximum specified hardness.

Specimens prepared for macroscopic examination can also be used for

taking hardness measurements at various positions of the weldments,
referred to as a hardness survey.

Methods
There are three widely used methods:

Vickers- uses a square-based ~m_gnd pyramid indenter.,

Rockwell - uses a diamond cone indenter or steel ball.
Brinell - uses a bait indenter.

The hardness value is given by the size of the indentation produced under a
standard load, the smaller the indentation, the harder the metal.

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The Vickers method of testing is illustrated below.

fulcrum

loading arm

diamond pyramid specimen

indentor

timing
mechanism
load rigid specimen table

sampl e /
(a) Vickers indentation (b) measurement of impression
diagonals

Both the Vickers and Brinell methods are suitable for carrying out hardness
surveys on specimens prepared for macroscopic examination of weldments.

A typical hardness survey requires the indenter to measure the hardness in

the base metal (on both sides of the weld), the weld metal and across the
HAZ (on both sides of the weld}.

The.Brinell method gives an indentation too large to accurately measure the

hardness in specific regions of the HAZ and is mainly used to measure the
hardness of base metals.

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A typical hardness survey (using Vickers hardness indenter) is shown

below:

Hardness values are shown on test reports as a number followed by letters

indicating the test method, for example:

240HV10 =hardness 240, Vickers method, 10kg indenter load.

22HRC =hardness 22, Rockwell method, diamond cone indenter

(scaleC). -

23BHBW = hardness 238, Brinell method, tungsten ball indenter.

4.1.5 Crack tip opening displacement (CTOD) testing.

Objective
Charpy V notch testing enables engineers to make judgements about the
risk of brittle fracture occurring in steels, but a CTOD test measures a
material property -fracture toughness.

Fracture toughness data enables engineers to carry out fracture mechanics

analyses such as:

Calculating the size of a crack that would initiate a brittle fracture under
certain stress conditions at a particular temperature.
The stress that would cause a certain sized crack to give a brittle fracture
at a particular temperature.

This data is essential for making an appropriate decision when a crack is

discovered during inspection of equipment that is in-service.

Specimens
A CTOD specimen is prepared as a rectangular or square shaped bar cut
transverse to the axis of the butt weld. A V notch is machined at the centre
of the bar, which will be coincident with the test position, weld metal or HAZ.

A shallow saw cut is made at the bottom of the notch and the specimen put
into a machine that induces a cyclic bending load until a shallow fatigue
crack initiates from the saw cut.

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The specimens are relatively large, typically having a cross-section 8 x 28

and length -108 (8 =full thickness of the weld). The test piece details are
shown below.

Machined not.ch

Section through
notch
Method
CTOD specimens are usually tested at a temperature below ambient and
the specimen temperature is controlled by immersion in a bath of liquid
cooled to the required test temperature.

A load is applied to the specimen to cause bending and induce a

concentrated stress at the tip of the crack and a clip gauge, attached to the
specimen across the mouth of the machined notch, gives a reading of the
increase in width of the crack mouth as the load is gradually increased.

For each test condition (position of notch and test temperature) it is usual to
carry out three tests.

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The figures below illustrate the main features of the CTOD test.

Bend
roller

Ttp cpen~ng

Fracture toughness is expressed as the distance the crack tip opens without
initiation of a brittle crack.

The clip gauge enables a chart to be generated showing the increase in

width of the crack mouth against applied load from which a CTOD value is
calculated .

Acceptance criteria
An application standard or client may specify a minimum CTOD value that
indicates ductile tearing. Alternatively, the test may be for information so that
a value can be used for an engineering critical assessment (ECA).

A very tough steel weldment will allow the mouth of the crack to open widely
by ductile tearing at the tip of the crack whereas a very brittle weldment will
tend to fracture when the applied load is quite low and without any extension
at the tip of the crack.

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CTOD values are expressed in millimetres - typical values might be

= =
<<-2.1 mm brittle behaviour; >-1 mm very tough behaviour.

4.1.6 Bend testing

Objective
Bend tests routinely taken from welding procedure qualification test pieces
and sometimes welder qualification test pieces.

Subjecting specimens to bending is a simple way of verifying there are no

significant flaws in the joint. Some degree of ductility is also demonstrated, it
is not measured but shown to be satisfactory if test specimens can
withstand being bent without fracture or fissures above a certain length.

Specimens
There are four types of bend specimen:

Face
Taken with axis transverse to butt welds up to -12mm thickness and bent so that
the face of the weld is on the outside of the bend (face in tension).
Root
Taken with axis transverse to butt welds up to -12mm thickness and bent so that
the root of the weld is on the outside of the bend (root in tension).
Side
Taken as a transverse slice (-10mm) from the full thickness of butt we.lds
>-12mm and bent so that the full joint thickness is tested (side in tension).
Longitudinal bend
Taken with axis parallel to the longitudinal axis of a butt weld; specimen thickness
is -12mm and the face or root of weld may be tested in tension.

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Former

-L
t
1

Root bend

Face bend

Side bend

Method
Guided bend tests are usually used for welding procedure and welder
qualification.

Guided means that the strain imposed on the specimen is uniformly

controlled by being bent around a former with a certain diameter.

The diameter of the former used for a particular test is specified in the code,
having been determined by the type of material being tested and the ductility
that can be expected from it after welding and any PWHT.

The diameter of the former is usually expressed as a multiple of the

specimen thickness (t) and for C~Mn steel is ty,pj,cally~but for materials that
have lower tensile ductility the radius of the former may be greater than 1Ot.

The standard that specifies the test method will specify the minimum bend
angle the specimen must experience and is typically 120-180'.

.
{.(I

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Acceptance criteria
Bend tests pieces should exhibit satisfactory soundness by not showing
cracks or any signs of significant fissures or cavities on the outside of the
bend.

Small indications ~s than a~out 3mm in length may be allowed by some

standards.

4.1.7 Fracture tests

.... Fillet weld fractures
Objective
The quality/soundness of a fillet weld can be assessed by fracturing test
pieces and examining the fracture surfaces.

This method for assessing the quality of fillet welds may be specified by
application standards as an alternative to macroscopic examination.

It is a test method that can be used for welder qualification testing according
to European Standards but is r:'ot used for welding procedure qualification.

Specimens
A test weld is cut into short (typically ~SOmm) lengths and a longitudinal
notch machined into the specimen as shown below. The notch profile may
be square, V or U shape.

Lo ncltudln" l not c h l u nuet w e hh

Method
Specimens are made to fracture through their throat by dynamic strokes
(hammering) or by pressing, as shown below. The welding standard or
application standard will specify the number of tests (typically four).

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Moving press

Acceptance criteria
The standard for welder qualification, or application standard, will specify the
acceptance criteria for imperfections such as lack of penetration into the root
of the joint and solid inclusions and porosity that are visible on the fracture
surfaces.

Test reports should also give a description of the appearance of the fracture
and location of any imperfection.

Butt weld fractures (nick-break tests)

Objective
The same as for fillet fracture tests.

These tests are specified for welder qualification testing to European

Standards as an alternative to radiography and are not used for welding
procedure qualification testing.

Specimens
Taken from a butt weld and notched so that the fracture path will be in the
central region of the weld. Typical test piece types are shown below.

Method

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Test pieces are made to fracture by hammering or three-point bending.

Acceptance criteria
The standard for welder qualification or application standard will specify the
acceptance criteria for imperfections such as lack of fusion, solid inclusions
and porosity that are visible on the fracture surfaces.

Test reports should also give a description of the appearance of the fracture
and location of any imperfection.

4.2 Macroscopic examination

Transverse sections from butt and fillet welds are required by the European
Standards for welding procedure qualification testing and may be required
-
for some welder qualification testing for assess.ing thELqualit)' of the welds.
- -
This is considered in detail in a separate section of these course notes.

4.2.1 European Standards for destructive test methods

The following Standards are specified by the European Welding Standards
for destructive testing of welding procedure qualification test welds and for
some welder qualification test welds.

EN875 Destructive tests on welds in metallic materials impact tests test

specimen location, notch orientation and examination.
EN895 Destructive tests on welds in metallic materials transverse tensile
test.
EN910 Destructive tests on welds In metallic materials bend tests.
EN 1321 Destructive tests on welds in metallic materials - macro and
microscopic examination of welds.
BS EN 10002 Metallic materials Tensile testing. Part 1: Method oftest at ambient
temperature.
BS EN 10002 Tensile testing of metallic materials. Part 5: Method of test at elevated
temperatures.

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Non-destructive Testing
 Rev 2 Aprtl2013
Non-destructive Testing
Copyrigl'ltlWI ltd2013

5 Non-destructive Testing
5.1 Introduction
Radiographic, ultrasonic, dye penetrant and magnetic particle methods are
briefly described below. Their relative advantages and limitations are
discussed in terms of their applicability to the examination of welds.

5.2 Radiographic methods

In all cases radiographic methods as applied to welds involve passing a
beam of penetrating radiation through the test object. The transmitted
radiation is collected by some form of sensor, capable of measuring the
relative intensities of penetrating radiations impinging upon it. In most cases
this sensor is radiographic film; however the use of various electronic
devices is on the increase. These devices facilitate so-called real-time
radiography and examples may be seen in the security check area at
airports. Digital technology has enabled the storing of radiographs using
computers. The present discussion is confined to film radiography since this
is still the most common method applied to welds.

5.2.1 Sources of penetrating radiation

Penetrating radiation may be generated from high energy electron beams
and (X-rays), or from nuclear disintegrations (atomic fission), in which case
they are termed gamma rays. Other forms of penetrating radiation exist but
are of limited interest in weld radiography.

5.2.2 X-rays
X-rays used in the industrial radiography of welds generally have photon
energies in the range 30keV up to 20MeV. Up to 400keV they are generated
by conventional X-ray tubes which, dependent upon output may be suitable
for portable or fixed installations. Portability falls off rapidly with increasing
kilovoltage and radiation output. Above 400keV X-rays are produced using
devices such as betatrons and linear accelerators, not generally suitable for
use outside of fixed installations.

All sources of X-rays produce a continuous spectrum of radiation, reflecting

the spread of kinetic energies of electrons within the electron beam. Low
energy radiations are more easily absorbed and the presence of low energy
radiations within the X-ray beam, gives rise to better radiographic contrast
and therefore better radiographic sensitivity than is the case with gamma-
rays which are discussed below. Conventional X-ray units are capable of
~orming high quality radiography on steel of up to 60mm thickness,
betatrons and linear accelerators in excess of 300mm.

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5.2.3 Gamma rays

Early sources of gamma rays used in industrial radiography were in
generally composed of naturally occurring radium . The activity of these
sources was not very high so they were large by modem standards even for
quite modest outputs of radiation and the radiographs produced were not of
a particularly high standard. Radium sources were also extremely
hazardous to the user due to the production of radioactive radon gas as a
product of the fission reaction. Since the advent of the nuclear age it has
been possible to artificially produce isotopes of much higher specific activity
than those occurring naturally which do not produce hazardous fission
products. Unlike X-ray sources gamma sources do not produce a
continuous distributioh of quantum energies.

Gamma sources produce a number of specific quantum energies unique for

any particular isotope. Four isotopes in common use for the radiography of
welds; are in ascending order of radiation energy: Thulium 90, ytterbium
169, iridium 192 and cobalt 60. In terms of steel thuliuifiSO is useful up to a
thickness of about 7mm, it's energy is similar to that of 90keV X-rays and
due to it's high specific activity useful sources can be produced with physical
dimensions of less than O.Smm. Ytterbium 169 has only fairly recently
become available as an isotope for industrial use, it's energy is similar to
that of 120keV X-rays and is useful for the radiography of steel up to
approximately 12mm thickness.(lridium 192 is probably the most commonly
encountered isotopic source of radiation used in the radiographic
examination of welds. It has a relatively and high specific activity and output
sources with physical dimensions of 2-3mm in common usage, it's energy is
approximately equivalent to that of SOOkeV X-rays and is useful for the
radiography of steel of j0-75mm thickness. Cobalt 60 has an energy
approximating that of 1.2MeV X-rays, so suitable source containers are
large and heavy so Cobalt 60 sources are not fully portable. They are useful
for the radiography of steel in 40-150mm of thickness. The major
advantages of using isotopic sources over X-rays are:

- Increased portability.
- No need for a power source.
- Lower initial equipment costs.

Against this the quality of radiographs produced by gamma ray techniques

is inferior to those produced by X-ray the hazards to personnel may be
increased (if the equipment is not properly maintained or if the operating
personnel have insufficient training) and due to their limited useful lifespan
new isotopes have to be purchased on a regular basis so that the operating
costs may exceed those of an X-ray source.

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5.2.4 Radiography of welds

Radiographic techniques depend on detecting differences in absorption of
the beam, ie changes in the effective thickness of the test object, to reveal
defective areas. Volumetric weld defects such as slag inclusions (except in
special cases where the slag absorbs radiation to a greater extent than does
the weld metal) and various forms of gas porosity are easily detected by
radiographic techniques due to the large negative absorption difference
between the parent metal and the slag or gas. \ Planar defects such as
gracks or lack of siqewall or inter-run fusion are much less likely to be
~ ed_by radiography since they may cause little or no change in the
penetrated thickness, Where defects of this type are likely to occur other
r\JDE techniques such as ultrasonic testing are preferable. Ibis tack of
sensitivity to planar defects makes radiography unsuitable where a fitness-
for-purpose approach Js taken when assessing the acceptability of a weld.
However, film radiography produces a permanent record of the weld
condition which can be archived for future reference and provides an
excellent means of assessing the welder's performance so it is often still the
preferred method for new construction.

X-ray equipment. Gamma ray equipment.

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X-ray of a welded seam showing porosity.

5.2.5 Radiographic testing

Advantages Limitations
Permanent record Health hazard. Safety {important)
Good for sizing non-planar defects/ Classified workers, medicals required
flaws
Can be used on aU materials Sensitive to defect orientation
Direct image of defect/flaws Not good for planar defect detection
Real-time imaging Limited ability to detect fine cracks
Can be positioned inside pipe Access to both sides required
(productivity)
Very good thickness penetration Skilled interpretation required
No power required with gamma Relatively stow
High capital outlay and running costs
Isotopes have a half-life {cost)

5.3 Ultrasonic methods

The velocity of ultrasound in any given material is a constant for that
material and ultrasonic beams travel in straight lines in homogeneous
materials. When ultrasonic waves pass from a given material with a given
sound velocity to a second material with different velocity, refraction and a
reflection of the sound beam will occur at the boundary between the two
materials. The same laws of physics apply to ultrasonic waves as to light
waves. Ultrasonic waves are refracted at a boundary between two materials
having different acoustic properties so probes may be constructed which
can beam sound into a material at (within certain limits) any given angle.
Because sound is reflected at a boundary between two materials having
different acoustic properties ultrasound is a useful tool for the detection of

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weld defects. Since velocity is a constant for any given material and sound
travels in a straight line (with the right equipment) ultrasound can also be
used to give accurat~ positional information about a given reflector. Careful
observation of the echo pattern of a given reflector and its behaviour as the
ultrasonic probe is moved together with the positional information obtained
above and knowledge of the component history enables the experienced
ultrasonic operator to classify thEt!_eflector C!_S sLaQ. lack of fusion or a~cracls.

5.3.1 Equipment for ultrasonic testing

Equipment for manual ultrasonic testing consists of:

A flaw detector:
- Pulse generator.
- Adjustable time base generator with an adjustable delay control.
- Cathode ray tube with fully rectified display.
- Calibrated amplifier with a graduated gain control or attenuator.

An ultrasonic probe:
- Piezo-electric crystal element capable of converting electrical vibrations
into mechanical vibrations and vice versa.
-Probe shoe, normally a Perspex block to which the crystal is firmly
attached using suitable adhesive.
- Electrical and/or mechanical crystal damping facilities to prevent
excessive ringing.

Such equipment is lightweight and extremely portable. Automated or

semi-automated systems for ultrasonic testing the same basic use
equipment although since in general this will be multi-channel it is bulkier
and less portable. Probes for automated systems are set in arrays and
some form of manipulator is necessary to feed positional information about
them to the computer. Automated systems generate very large amounts of
data and make large demands upon the RAM of the computer. Recent
advances in automated UT have led to a reduced amount of data being
recorded for a given length of weld . Simplified probe arrays have greatly
reduced the complexity of setting-up the automated system to carry out a
particular task. Automated UT systems now provide a serious alternative to
radiography on such constructions as pipelines where a large number of
similar inspections allow the unit cost of system development to be reduced
to a competitive level.

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Ultrasonic equipment.

Compression and a shear wave probe.

Example of a scanning technique with a shear wave probe.

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Typical screen display when using a shear wave probe.

5.3.2 Ultrasonic testing

Advantages Limitations
Portable {no mains power) battery No permanent record
Direct location of defect Only ferritic materials {mainly) .:
{3 dimensional)
Good for complex geometry High level of operator skill required
Safe operation (can be done next to Calibration of equipment requi~ed
someone)
Instant results Special calibration blocks requfred
High penetrating capability No good for pin pointing porosity
Can be done from one side only Critical of surface conditions {clean smooth)
Good for finding planar defects Will not detect surface defects
Material thickness >8mm due to dead zone
-
5.4 Magnetic particle testing
Surface breaking or very near surface discontinuities in ferromagnetic
materials give rise to leakage fields when high levels of magnetic flux are
applied. These leakage fields attract magnetic particles (finely divided
magnetite) to themselves leading to the formation of an indication. The
magnetic particles may be visibly or fluorescently pigmented to provide
contrast with the substrate or conversely the substrate may be lightly coated
with a White background lacquer to contrast with the particles. Fluorescent
magnetic particles normally provide the greatest sensitivity in a liquid
suspension, usually applied by spraying. In certain cases dry particles may
be applied by a gentle jet of air. The technique is applicable only to
ferromagnetic materials at a temperature below the Curie point (about
650C). The leakage field will be greatest for linear discontinuities at right
angles to the magnetic field so for a comprehensive test the magnetic field
must normally be applied in two directions, mutually perpendicular. The test
is economical to carry out in terms of equipment cost and rapidity of
inspection and the level of operator training required is relatively low.

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Magnetic particle inspection using a yoke.

Crack found using magnetic particle inspection.

Advantages Limitations
Inexpensive equipment Only magnetic materials
Direct location of defect May need to demagnetise components
Surface conditions not critical Access may be a problem for the yoke
Can be applied without power Need power if using a yoke
Low skill level No permanent record
Sub-surface defects found 1-2mm Calibration of equipment
Quick, instant results Testing in two directions required
Hot testing (using dry powder) Need good lighting - 500 lux minimum
Can be used in the dark (UV light)

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5.5 Dye penetrant testing

Any liquid with good wetting properties will act as a penetrant, which is
attracted into surface-breaking discontinuities by capillary forces. Penetrant
which has entered a tight discontinuity will remain even when the excess is
removed. Application of a suitable developer will encourage the penetrant
within discontinuities to bleed out. If there is a suitable contrast between the
penetrant and the developer an indication visible to the eye will be formed.
Provided by either visible or fluorescent dyes. Use of fluorescent dyes
considerably increases the sensitivity of the technique. The technique is not
applicable at extremes of temperature as at below S~C_Jhe _penetrant
vehicle, normally oil, will become excessively viscous causing an increase in
the penetration time with a consequent decrease in sensitivity. Aj?,QY.EL6.0C
the penetrant will dry out and the technique will not work.

Methods of applying the red dye during dye penetrant inspection.

-~

-
-- ---- . _:....
~

Crack found using dye penetrant inspection.

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5.5.1 Dye penetrant

Advantages limitations
All non porous materials Will only detect defects open to the surface
Portable Requires careful space preparation
Applicable to small parts with complex Not applicable to porous surfaces
geometry
Simple Temperature dependent
Inexpensive Cannot retest indefinitely
Sensitive Potentially hazardous chemicals
Relatively low skill level No permanent record
(easy to interpret)
Tlme lapse between application and results
Messy

Surface crack detection (magnetic particle/dye penetrant)

When considering the relative value of NDE techniques it should not be
forgotten that most catastrophic failures initiate from the surface of a
component, so the value of the magnetic particle and dye penetrant
techniques should not be under-estimated. Ultrasonic inspection may not
detect near-surface defects easily since the indications may be masked by
echoes arising from the component geometry and should therefore be
supplemented by an appropriate surface crack detection technique for
maximum test confidence.

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 Section 6

WPS/Welder Qualifications
 Rev 2 April 2013
WPS/Welder Qualffications
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6 WPS/Welder Qualifications
6.1 General
When structures and pressurised items are fabricated by welding. it is
essential that all the welded joints are sound and have suitable properties
for their application.

Control of welding is by WPSs that give detailed written instructions about

the welding conditions that must be used to ensure that welded joints have
the required properties.

Although WPSs are shopfloor documents to instruct welders, welding

inspectors need to be familiar with them because they will refer to them
when checking that welders are working within the specified requirements.

Welders need to be able to understand WPSs, make non-defective welds

and demonstrate these abilities before being allowed to make production
welds.

6.2 Qualified welding procedure specifications

It is industry practice to use qualified WPSs for most applications.

A welding procedure is usually qualified by making a test weld to

demonstrate that the properties of the joint satisfy the requirements
specified by the application standard and the client/end user.

Demonstrating the mechanical properties of the joint is the principal purpose

of qualification tests, but showing that a defect-free weld can be produced is
also very important.

Production welds made in accordance with welding conditions similar to

those used for a test weld should have similar properties and therefore be fit
for their intended purpose.

Table 6.1 is a typical WPS written in accordance with the European Welding
Standard format giving details of all the welding conditions that need to be
specified.

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Table 6.1 Typical sequence for welding procedure qualification by means of a test
weld.

The welding engineer writes a preliminary Welding Procedure Specification

(pWPS) for each test coupon to be welded.

A welder makes the test coupon in accordance with the pWPS.

A welding inspector records all the welding conditions used to make the test
coupon (the as-run conditions).

An independent examiner/examining body/third party inspector may be

requested to monitor the procedure qualification.

The test coupon is subjected to NOT in accordance with the methods specified
by the Standard - visual inspection, MT or PT and RT or UT.

The test coupon is destructively tested (tensile, bend, macro tests).

The code/application standard client may require additional tests such as
hardness, impact or corrosion tests - depending on the material and
application.

A WPQR is prepared by the welding engineer giving details of:

-As run welding conditions.
- Results of the NOT.
- Results of the destructive tests.
-Welding conditions allowed for production welding.
If a third party inspector is involved he will be requested to sign the WPQR
as a true record of the test

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6.2.1 Welding standards for procedure qualification

European and American Standards have been developed to :give
comprehensive details about:

How a welded test piece must be made to demonstrate joint properties.

Howihe test piece must be tested.
Which welding details need to be included in a WPS.
The range of production welding allowed by a particular qualification test
weld.
The principal European Standards that specify these requirements are:

EN ISO 15614
Specification and qualification of welding procedures for metallic materials,
welding procedure test.

Part 1
Arc and gas welding of steels and arc welding of nickel and nickel alloys.

Part2
Arc welding of aluminium and its alloys.

The principal American Standards for procedure qualification are:

ASME Section IX
Pressurised systems (vessels and pipework).

AWS 01.1
Structural welding of steels.

AWS 01 .2
Structural welding of aluminium.

6.2.~ The qualification process for welding procedures

Although qualified WPSs are usually based on test welds made to
demonstrate weld joint properties; welding standards also allow qualified
WPSs for some applications to be written based on other data.

Some alternative ways that can be used for writing qualified WPSs for some
applications are:

Qualification by adoption of a standard welding procedure - test welds

previously qualified and documented by other manufacturers.
Qualification based on previous welding experience - weld joints that
have been repeatedly made and proved to have suitable properties by
their service record.

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Procedure qualification to European Standards by a test weld (similar in

ASME Section IX and AWS) requires a sequence of actions typified by
those shown by Table 6.1.

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A successful procedure qualification test is completed by the production of a

WPQR, an example of which is shown in Figure 6.2.

ID'ftna.-.ltlldri)~J ~I'Wflt~ UJI

~"-Otts~mtcr ':'11l.riii''WISKC
-a Welds~

.,.,. Ulf

COCIIIT~- IN Ult4ot ~

asnnw as f't'lt-n, nee rc : 1"4u: .. ,. J"UC':Cif : "wt DfOrC

---
G.=nt-1
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rla.l'ft"aatf\~
...... ,.._
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a,.,.,...,... (m...:
!!lot : ....
tc. 1:1
ic ~

EXTENTOFAPPtiOY..U.Il'IIOCIUI
'N l!t::lltt P'S':.II
W r.i:lftl I"'CZJS !ret
'Neu-J~
., 100M o.tl'ldlt d....,. OtaJar llte~t sn.o
,. . . ... . ~01
aft:t!d"'IOU.11ua
'N........- .
fl.r:Metzm~rt rc
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Ale:lllnltl. . ""~'
llelllfAiaa

Figure 6.2 Example ofWPQR (qualification range) to N15614 format

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6.2.3 Relationship between a WPQR and a WPS

Once a WPQR has been produced, the welding engineer can write
qualified WPSs for the various production weld joints that need to be made.

The welding conditions that are allowed to be written on a qualified WPS are
referred to as the qualification range and depend on the welding conditions
used for the test piece (as-run details) and form part of the WPQR.

Welding conditions are referred to as welding variables by European and

American Welding Standards and are classified as either essential or non-
essential variables and can be defined as:

Essential variable
Variable that has an effect on the mechanical properties of the weldment and if
changed beyond the limits specified by the standard will require the WPS to be re-
qualified.
Non-essential variable
Variable that must be specified on a WPS but does not have a significant effect on
the mechanical properties of the weldment and can be changed without the need
for re-qualification but will require a new WPS to be written .

Because essential variables can have a significant effect on mechanical

properties they are the controlling variables that govern the qualification
range and determine what can be written in a WPS.

If a welder makes a production weld using conditions outside the range

given on a particular WPS there is a danger that the welded joint will not
have the required properties and there are two options:

7 Make another test weld using similar welding conditions to those used
for the affected weld and subject this to the same tests used for the
relevant WPQR to demonstrate that the properties still satisfy specified
requirements.

8 Remove the affected weld and re-weld the joint strictly in accordance
with the designated WPS.

Most of the welding variables classed as essential are the same in both the
European and American Welding Standards but their qualification ranges
may differ.

Some application standards specify their own essential variables and it is

necessary to ensure these are considered when procedures are qualified
and WPSs written.

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Examples of essential variables (according to European Welding Standards)

are given in Table 6. 2.

Table 6.2 Typical examples of WPS essential variables according to EU Welding

Standards

Variable Range for procedure qualification

Welding process No range - process qualified must be used in production.
PWHT Joints tested after PWHT only qualify PWHT production joints.
Joints tested as-welded only qualify as-welded production joints.
Parent material type Parent materials of similar composition and mechanical properties
are allocated the same Material Group No; qualification only
allows production welding of materials with the same Group No.
Welding Consumables for production welding must. have the same
consumables European designation ~eneral rule.
Material thickness A thickness range Is allowed - below and above the test coupon
thickness.
Type of current AC only qualifies for AC; DC polarity (+ve or -ve) cannot be
changed; pulsed current only qualifies for pulsed current
production welding.
Preheat temperature The preheat temperature used for the test is the minimum that
must be applied.
lnterpass The highest interpass temperature reached in the test is the
temperature maximum allowed.
Heat input (HI) When impact requirements apply the maximum HI allowed Is 25%
above test HI.
When hardness requirements apply the minimum HI allowed is
25% below test HI.

6.3 Welder qualification

The use of qualified WPSs is the accepted method for controlling production
welding but will only be successful if the welders understand and work in
accordance with them.

Welders also need to have the skill to consistently produce sound (defect-
free) welds.

Welding Standards have been developed to give guidance on which test

welds are required to show that welders have the required skills to make
certain types of production welds in specified materials.

6.3.1 Welding standards for welder qualification

The principal EU Standards that specify requirements are:

EN 287-1
Qualification test of welders- Fusion welding.
Part 1: Steels.

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EN ISO 9606-2
Qualification test of welders- Fusion welding.
Part 2: Aluminium and aluminium t:~lloys .

EN 1418
Welding personnel - Approval testing of welding operators for fusion
welding and resistance weld setters for fully mechanised and automatic
welding of metallic materials.

The principal American Standards that specify requirements for welder

qualification are:

ASME Section IX
Pressurised systems (vessels & pipework).

AWS 01.1
Structural welding of steels.

AWS 01.2
Structural welding of aluminium.

6.3.2 The qualification process for welders

Qualification testing of welders to European Standards requires test welds
to be made and subjected to specified tests to demonstrate that the welder
is able to understand the WPS and produce a sound weld.

For manual and semi-automatic welding tests demonstrate the ability to

manipulate the electrode or welding torch .

For mechanised and automatic welding the emphasis is on demonstrating

the ability to control particular types of welding equipment.

American Standards allow welders to demonstrate they can produce sound

welds by subjecting their first production weld to NOT.

Table 6.3 shows the steps required for qualifying welders in accordance with
EU Standards.

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Table 6.3 The stages for qualification of a welder.

The welding engineer writes a WPS for a welder qualification test piece.

The welder makes the test weld in accordance with the WPS.
A welding inspector monitors the welding to ensure that the welder is
working in accordance with the WPS.

An independent examiner/examining body/third party inspector may be

requested to monitor the test.

The test coupon is subjected to NOT in accordance with the methods

specified by the Standard (visual inspection, MT or PT and RT or UT).
For certain materials, and welding processes, some destructive testing may
be required (bend tests or macros).

~
1
ij ~
A welder's Qualification Certificate is prepared showing the welding
conditions used for the test piece and the range of qualification allowed by
the Standard for production welding.
If a third party is involved they would endorse the Qualification Certificate as
a true record of the test.

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Figure 6.3 shows a typical welder qualification certificate in accordance with

EU Standards.
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Figure 6.3 Example of a WPQR document (test weld details) to EN15614 format

6-10
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WPSfWelder Qualifications
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6.3.3 Welder qualification and production welding allowed

The welder is allowed to make production welds within the range of
qualification recorded on his Welder Qualification Certificate.

The range of qualification is based on the limits specified by the Welding

Standard for welder qualification essential variables - defined as:

A variable that if changed beyond the limits specified by the Welding

Standard may require greater skill than has been demonstrated by the
test weld.

Some welding variables classed as essential for welder qualification are the
same types as those classified as essential for welding procedure
qualification, but the range of qualification may be significantly wider.

Some essential variables are specific to welder qualification.

Examples of welder qualification essential variables are given in Table 6.4.

Table 6.4 Typical examples of welder qualification essential variables according to

EU Welding Standards

Variable Range for welder qualification

Welding process No range - process qualified is the process that a welder can use
in production.
Type of weld Butt welds cover any type of joint except braheh welds.
Fillet welds only qualify fillets.
Parent material type Parent materials of similar composition and mechanical
properties are allocated the same Material Group No; qualification
only allows production welding of materials with the same Group
No. but the Groups allow much wider composition ranges than
the procedure Groups.
Filler material Electrodes and filler wires for production welding must be of the
same form as the test (solid wire, flux-cored, etc); for MMA
coating type is essential.
Material thickness A thickness range is allowed; for test pieces above 12mm allow ~
5mm.
Pipe diameter Essential and very restricted for small diameters:
Test pieces above 25mm allow~ 0.5 x diameter used (minimum
25mm).
Welding positions Position of welding very important; H-L045 allows all positions
except PG.

6.3.4 Period of validity for a Welder Qualification Certificate

A welder's qualification begins from the date of welding the test piece.

The European Standard allows a qualification certificate to remain valid for

two years, provided that:

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The welding coordinator or other responsible person can confirm that

the welder has been working within the initial range of qualification.
Working within the initial qualification range is confirmed every six
months.

6.3.5 Prolongation of welder qualification

A welder's qualification certificate can be extended every two years by an
examiner/examining body but certain conditions need to be satisfied:

Records/evidence are available that can be traced to the welder and the
WPSs used for production welding.
Supporting evidence must relate to volumetric examination of the
welder's production welds (RT or UT) on two welds made during the six
months prior to the extension date.
Supporting evidence welds must satisfy the acceptance levels for
imperfections specified by the EU welding standard and have been
made under the same conditions as the original test weld.

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 Rev 2 April2013
WPS/INelder Qualifications
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Weldspee

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Figure 6.4 Example of WPQR document (details of weld test) to EN15614 format.

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 Rev 2 Apri12013
WPS/Welder Qualificatfons
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TWI
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Figure 6.5 Example of a welder qualification test certificate (WPQ) to EN287 format.

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 Section 7

Materials Inspection
 Rev 2 Apnl2013
Materials Inspection
Copyright @TWI Ltd 2013

7 Materials Inspection
7.1 General
One of the duties of the visual/welding inspector is materials inspection and
there are a number of situations where this will be required:

At the plate or pipe mill.

During fabrication or construction of the material.
After installation of material, usually during a planned maintenance
programme, outage or shutdown.

A wide range of materials can be used in fabrication and welding and

include, but is not limited to:

Steels.
Stainless steels.
Aluminium and its alloys.
Nickel and its alloys.
Copper and its alloys.
Titanium and its alloys.
Cast iron.

These materials are all widely used in fabrication, welding and construction
to meet the requirements of a diverse range of applications and industry
sectors.

There are three essential aspects to material inspection that the Inspector
should consider:

Material type and weld ability.

Material traceability.
Material condition and dimensions.

7.2 Material type and weldability

A welding inspector must understand and interpret the material designation
to check compliance with relevant normative documents. For example
materials standards such as BS EN, API , ASTM , the WPS, purchase order,
fabrication drawings, quality plan/contract specification and client
requirements.

A commonly used material standard for steel designation is BS EN 10025 -

Hot rolled products of non"alloy structural steels.

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A typical steel designation to this standard, S355J2G3, would be classified

as follows:

S Structural steel.
35S Minimum yield strength: N/mm2 at t s 16mm.
J2 Longitudinal Charpy, 27Joules 20C.
G3 Normalised or normalised rolled.

Commonly used materials and most of the alloys can be fusion welded
using various welding processes, in a wide range of thickness and where
applicable, diameters.

Reference to other standards such as ISO 15608 Welding - Guidelines for a

metallic material grouping system and steel producer and welding
consumable data books can also provide the inspector with guidance as to
the suitability of a material and consumable type for a given application.

7.3 Alloying elements and their effects

Iron Fe
Carbon c Strength
Manganese Mn Toughness
Silicon Si < 0.3% deoxidiser
Aluminium AI Grain refiner, <0.008% deoxidiser +toughness
Chromium Cr Corrosion resistance
Molybdenum Mo 1% is for creep resistance
Vanadium v Strength
Nickel Ni Low temperature applications
Copper Cu Used for weathering steels (Corten)
Sulphur s Residual element (can cause hot shortness)
Phosphorus p Residual element
Titanium Ti Grain refiner, used as a micro-alloying element
(strength and toughness)
Niobium Nb Grain refiner, used as a micro-alloying element
(strength and toughness)

7.4 Material traceability

Traceability is defined as the ability to trace the history, application or
location of that which is under consideration. With a welded product,
traceability may require the inspector to consider the:

Origin of both parent and filler materials.

Processing history -for example before or after PWHT.
Location of the product - this usually refers to a specific part or sub-
assembly.

To trace the history of the material, reference must be made to the

inspection documents. BS EN 10204 Metallic products- Types of inspection
documents is the standard which provides ;guidance on these types of

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document. According to BS EN 10204 inspection documents fall into two

types:

Non-specific inspection
Carried out by the manufacturer in accordance with his own procedures
to assess whether products defined by the same product specification
and made by the same manufacturing process, comply with the
requirements of the order.

- Type 2.1 are documents in which the manufacturer declares that the
products supplied comply with the requirements of the order without
inclusion of test results.

-Type 2.2 are documents in which the manufacturer declares that the
products supplied comply with the requirements of the order and
includes test results based on non-specific inspection.

Specific inspection
Inspection carried out before delivery according to the product
specification on the products to be supplied or test units of which the
products supplied are part, to verify that these products comply with the
requirements of the order.

- Type 3. 1 are certificates in which the manufacturer declares that the

products supplied comply with the requirements of the order and in
which test results are supplied.

-Type 3.2 are certificates prepared by both the manufacturer's

authorised inspection representative independent of the manufacturing
department and either the purchaser's authorised representative or the
inspector designated by the official regulations and in which they
declare that the products supplied comply with the requirements of the
order and in which test results are supplied.

Application or location of a particular material can be carried out through a

review of the WPS, fabrication drawings, quality plan or by physical
inspection of the material at the point of use.

In certain circumstances the inspector may have to witness the transfer of

cast numbers from the original plate to pieces to be used in production.

On pipeline work it is a requirement that the inspector records all the

relevant information for each piece of linepipe. On large diameter pipes this
information is usually stencilled on the inside of the pipe. On smaller
diameter pipes it may be stencilled along the outside of the pipe.

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BS EN 10204: Metallic materials

Summary of types of inspection documents.

Non-specific inspection
May be replaced by specific
inspection if specified in the material
standard or the order.

I
Inspection document Type 2.1 Inspection document Type 2.2
Declaration of compliance with Test report.
the order. Statement of compliance with
Statement of compliance with the order, with Indication of
the order. results of non-specific
Validated by manufacturer. inspection.
Validated by manufacturer.

Specific inspection
Quality management system of the material
manufacturer certified by a competent body
established within the community and having
undergone a specific assessment for materials.

,
Inspection certificate Type 3.1 Inspection certificate Type 3.2
Statement of compliance with the Statement of compliance with the
order with indication of results of order with indication of results of
specific inspection. specific inspection.
Validated by manufacturer's Validated by manufacturer's
authorised inspection authorised inspection
representative independent of the representative independent of the
manufacturing department. manufacturing department and
either the purchaser's authorised
inspection representative or the
inspector designated by the official
regulations.

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7.5 Material condition and dimensions

The condition of the material could have an adverse effect on the service life
of the component so is an important inspection point. The points for
inspection must include:

General inspection.
Visible imperfections.
Dimensions.
Surface condition.

General inspection
This takes account of storage conditions, methods of handling, number of
plates or pipes and distortion tolerances.

Visible imperfections
Typical visible imperfections are usually attributable to the manufacturing
process and include cold laps which break the surface or laminations if they
appear at the edge of the plate. Ultrasonic testing using a compression
probe may be required for laminations which may be present in the body of
the material.

Cold lap. Plate lamination.

Dimensions
For plates this includes length, width and thickness.

For pipes this includes length and wall thickness and also inspection of
diameter and ovality. At this stage of inspection the material cast or heat
number may be recorded for validation against the material certificate.

Surface condition
The surface condition is important and must not show excessive millscale or
rust, be badly pitted or have unacceptable medhanical damage.

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There are four grades of rusting which the inspector may have to consider:

Rust Grade A: Steel surface largely covered with adherent millscale with
little or no rust.

Rust Grade B: Steel surface which has begun to rust and from which mill
scale has begun to flake.

Rust Grade C: Steel surface on which the mill scale has rusted away or
from which it can be scraped. Slight pitting visible under normal vision.

Rust Grade 0 : Steel surface on which mill scale has rusted away. General
pitting visible under normal vision .

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7.6 Summary
Material inspection is an important part of the inspector's duties and an
understanding of the documentation involved is key to success.

Material inspection must be approached in a logical and precise manner if

material verification and traceability are to be achieved. This can be difficult
if the material is not readily accessible, access may have to be provided,
safety precautions observed and authorisation obtained before material
inspection can be carried out. The quality plan should identify the level of
inspection required and the point at which inspection takes place. A
fabrication drawing should provide information on the type and location of
the material.

If material type cannot be determined from the inspection documents

available or the inspection document is missing, other methods of identifying
the material may need to be used.

These methods may include but are not limited to: Spark test, spectroscopic
analysis, chemical analysis, scleroscope hardness test, etc. These types of
test are normally conducted by an approved test house but sometimes on-
site and the inspector may be required to witness them to verify compliance
with the purchase order or appropriate standard(s).

*EN ISO 9000 Quality management systems - Fundamentals and

vocabulary.

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Codes and Standards

 Rev 2 April 2013
Codes and Stamdards
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8 Codes and Standards

8.1 General
It is not necessary for the inspector to carry a range of codes and standards
In the performance of his duties, normally the contract specification is the
only document required. However this may reference supporting codes and
standards and the inspector should know where to access these normative
documents.

The following is a list of definitions relating to codes and standards the

inspector may come across whilst carrying out his duties.

8.2 Definitions
Normative document
Document that provides rules, guidelines or characteristics for activities or
their results. The term normative document is generic, covering documents
such as standards, technical specifications, codes of practice and
regulations.*

Standard
Document established by consensus and approved by a recognised body. A
standard provides, for common and repeated use, guidelines, rules,
characteristics for activities or their results, aimed at achieving the optimum
degree of order in a given context.

Harmonised standards
Standards on the same subject approved by different standardising bodies,
that establish inter~changeability of products, processes and services, or
mutual understanding of test results or information provided according to
these standards.*

Code of practice
Document that recommends practices or procedures for the design,
manufacture, installation, maintenance and utilisation of equipment,
structures or products. A code of practice may be a standard, part of a
standard or independent of a standard.*

Regulation
Document providing binding legislative rules adopted by an authority.*

Authority
A body (responsible for standards and regulations legal or administrative
entity that has specific tasks and composition) that has legal powers and
rights.*

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Regulatory authority
Authority responsible for preparing or adopting regulations.*

Enforcement authority
Authority responsible for enforcing regulations.*

Specification
Document stating requirements, meaningful data and its supporting medium
stating needs or expectations that are stated, generally implied or
obi igatory.**

Procedure
Specified way to carry out an activity or process*. Usually a written
description of all essential parameters and precautions to be observed when
applying a technique to a specific application following an established
standard, code or specification.

Instruction
Written description of the precise steps to be followed based on an
established procedure. standard, code or specification.

Quality plan
Document specifying which procedures and associated resources shall be
applied by whom and when to a specific project, product, process or
contract.*

ISO IEC Guide 2 - Standardisation and related activities - General

vocabulary.
- EN ISO 9000 - 2000 - Quality management systems - Fundamentals
and vocabulary .

8.3 Summary
Application standards and codes of practice ensure that a structure or
component will have an acceptable level of quality and be fit-for-purpose.

Applying the requirements of a standard, code of practice or specification

can be a problem for the inexperienced inspector. Confidence in applying
the requirements of one or all of these documents to a specific application
only comes with use.

If in doubt the inspector must always refer to a higher authority in order to

avoid confusion and potential problems.

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Comments
Standard Number Year Status =
AMD amended
=
COR corrected
BS 499-1 2009 Current

BS709 1983 Superseded Superseded by:

BS EN ISO 9016:2011
BS EN ISO 5178:2011
BS EN ISO 4136:2011
BS EN ISO 5173:2010 + A1 2011
BS EN ISO 9015-1 :2011
BS EN ISO 9015-2:2011
BS EN 1320:1997
BS EN 1321 :1997 AMD 14972
BS EN ISO 9018: 2003 AMD 15061

BS 1113(1999) 1999 Superseded Superseded By:

BS EN 12952-1 :2001 COR 2010
BS EN 12952-2:2011
BS EN 12952-3:2011
BS EN 12952-5:2011
BS EN 12952-6:2011
BS EN 12952-7:2010
BS EN 12952-10:2002 COR 2010
BS EN 12952-11:2002 COR 2010

BS1453 (1972) 1972 (amended 2001) Partly Partly superseded by:

Superseded BS EN 12536:2000

BS 1821 1982 (amended 1998) Cancelled

BS 2493(1985) 1985 Superseded Superseded by:

BS EN ISO 18275:2012 &
BS EN ISO 3580:2011

BS 2633(1987) 1987 (amended 1998) Current

BS 2640(1982) 1982 (amended 1998) Cancelled .

BS 2654(1989) 1989 (amended 1997) Superseded Superseded by: BS EN 14015:2005

BS 2901-3(1990) 1990 Superseded Superseded by:

BS EN ISO 24373:2009

BS2926 1984 Superseded Superseded by:

BS EN ISO 3581 :2012

BS 3019 1984 Superseded Superseded by:

BS EN 1011-4:2000 AMD 2004

BS 3604-1 (1990) 1990 Superseded Superseded by:

BS EN 10216-2:2002 AM D 2007 &
BS EN 10217-2:2002 AMD 2006

BS 3605 1991 AMD 1997 Superseded Superseded by:

BS EN 10216-5:2004 COR 2008

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Comments
Standard Number Year Status AMD =amended
COR = corrected
BS 4515-1 (2009) 2009 Current
BS 4570 (1985) 1985 Partly Partly superseded by:
Superseded BS EN 1011-8:2004 AMD

BS 4677 (1984) 1985 Current

BS 4872-1 (1982) 1982 Current

BS 4872-2 (1976) 1976 Current

1982 Superseded There are multiple parts to this

standard, part 1 was superseded by:
BS EN 10305-5:2010
BS EN 10305-1 :2010
BS EN 10305-2:2010
BS EN 10305-3:2010
BS EN 10305-4:2011
BS EN 10305-6:2005 AMD 2007
BS EN 10296-1:2003
BS EN 10296-2:2005 AMD 2007
BS EN 10297-1 :2003AMD2003

BS 6693-1(1986) 1986 Superseded There are multiple parts to this

standard, part 1 was superseded by:
BS EN ISO 3690:2012

BS 6990(1989) 1989AMD 1998 Current

BS 7191(1989) 1989 AMD 1991 Superseded Superseded By: BS EN 10225:2009

BS 7570(2000) 2000 Superseded Superseded By: BS EN 50504:2009

BS EN 287-1:2011 2011 Current

BS EN 440:1995 1995 Superseded Superseded By:

BS EN ISO 14341 :2011

BS EN 499:1995 1995 Superseded Superseded By:

BS EN ISO 2560:2009

BS EN 383

BS EN 756:2004 2004 Superseded Superseded By:

BS EN ISO 14171:2010

BS EN 760:1996 1996 Superseded Superseded By:

BS EN ISO 14174:2012

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Comments
Standard Number Year Status AMD = amended
=
COR corrected
BS EN 910:1996 1996 Superseded Superseded By: BS EN ISO
5173:2010 + A1 2011

BS EN 970:1997 1997 Superseded Superseded By: BS EN ISO

17637:2011

BS EN 12072:2000 2000 Superseded Superseded By: BS EN ISO

14343:2009

BS EN ISO 18274:2011 2011 Current

BS EN 1011-1 :2009 2009 Current

BS EN 1011-2:2001 AMD 2001 Current

2004
BS EN 1011-3:2000 AMD 2000 Current
2004
BS EN 1011-4:2000 AMD 2000 Current
2004
BS EN 1320:1997 1997 Current

BS EN 1435:1997 AMD 2004 1997 Current

BS EN 10002-1:2001 2001 Superseded There are multiple parts for this

standard, part one Is superseded by:
BS EN ISO 6892-1:2009

BS EN 10020:2000 2000 Current

BS EN 10027:200S 2005 Current

BS EN 10045-1:1990 1990 Superseded

BS EN 10204:2004 2004 Current

BS EN 22SS3:1995 1995 Current

BS EN 24063:1992 1992 Superseded Superseded By: BS EN ISO 4063:2010

BS EN 25817:1992 1992 Superseded Superseded By: BS EN ISO 5817:2007

85 EN 26520:1992 1992 Superseded Superseded By: BS EN ISO 6520-1:2007

85 EN 26848:1991 1991 Superseded Superseded By: BS EN ISO 6848:2004

ISO 8S7-1:1998 1998 Current

BS EN ISO 6947:2011 2011 Current

BS EN ISO 15607:2003 2003 Current

8-5

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BS PO CR ISO 15608:2000 2000 Superseded Superseded By: BS PO CEN

ISO/TR 15608:2005

BS EN ISO 15609-1:2004 2004 Current

BS EN ISO 15610:2003 2003 Current

BS EN ISO 15611:2003 2003 Current

BS EN ISO 15613:2004 2004 Current

BS EN ISO 15614-1: 2004 A2 2012 2004 Current

BS EN ISO 15614-2:2005 2005 Current

BS EN ISO 15614-3:2008 2008

BS EN ISO 15614-4:2005 2008

COR 2008
BS EN ISO 15614-5:2004 2004

BS EN ISO 15614-6:2006 2006

BS EN ISO 15614-7:2007 2007

BS EN ISO 15614-8:2002 2002

BS EN ISO 15614-9 None available

BS EN ISO 15614-10:2005 2005

BS EN ISO 15614-11:2002 2002

BS EN ISO 15614-12:2004 2004

BS EN ISO 15614-13:2005 2005

8-6
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 Section 9

Welding Symbols
 Rev 2 Apri12013
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9 Welding Symbols
A weld joint can be represented on an engineering drawing by a detailed
sketch showing every detail and dimension of the joint preparation, as
shown below.

Single U preparation.

While this method of representation gives comprehensive information, it can

be time-consuming and overburden the drawing.

An alternative is to use a symbolic representation to specify the required

information, as shown below for the same joint detail.

Advantages of symbolic representation:

Simple and quick to add to the drawing.
Does not overburden the drawing.
No need for an additional view, all welding symbols can be put on the
main assembly drawing.

Disadvantages of symbolic representation:

Can only be used for standard joints (eg BS EN ISO 9692).
No way of giving precise dimensions for joint details.
Some training is necessary to correctly interpret the symbols.

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9.1 Standards for symbolic representation of welded joints on

drawings
Two principal standards are used for welding symbols:

European Standard
EN 22553 - Welded, brazed & soldered joints, Symbolic representation on
drawings.

American Standard
AWS A2.4, standard symbols for welding, brazing and oon~destructive
examination.

These standards are very similar in many respects, but there are also some
major differences that need to be understood to avoid misinterpretation.

Details of the European Standard are given in the following sub-sections

with only brief information about how the American Standard differs.

Elementary welding symbols

Various types of weld joint are represented by a symbol that is intended to
help interpretation by being similar to the shape of the weld to be made.

Examples of symbols used by EN 22553 are shown on the following pages.

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9.2 Elementary welding symbols

Designation Illustration of joint preparation Symbol

Square butt weld

r .II ~ II

Single V butt weld

~ \~ ~t v
Single bevel butt weld
~ I/ ~-' i ~ v
Single V butt weld with
broad root face ~-. :~ ~( s y
Single bevel butt weld
with broad root face ~l I( 'J r
Single U butt weld
f ~J ~
;~
y
Single J butt weld
~~ /, *l) ,I ~
Fillet weld

~ ~

Surfacing (cladding)
r:-=l.:;:; '" rY"\
_1:
l

~ ~c. : ~
Backing run
(back or backing weld) ~
if

) {{'fj ., ~ ~
- ,y ~

Backing bar ~ fi~,q;ly ~ I ~i I I

It ':4'8 ' ' . I 'l)o<!'i

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9.3 Combination of elementary symbols

For symmetrical welds made from both sides, the applicable elementary
symbols are combined, as shown below.

DesiQnation Illustration of joint preparation Symbol

Double V butt weld

(X weld)
EUO X
Double bevel butt
weld
(Kweld) DO K
Double U butt weld
DG ~
Double J butt weld
0{] ~

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9.4 Supplementary symbols

Weld symbols may be complemented by a symbol to indicate the required
shape of the weld.

Examples of supplementary symbols and how they are applied are given
below.

Designation Illustration of joint preparation Symbol

~
Flat (flush) single
V butt weld
{
~
~----~D-------~
)
)
v
Convex double V
butt weld

Concave fillet weld

Flat (flush) single

V butt weld with
flat (flush) backing
run

Single V butt weld ~

with broad root ( 'i )
face and backing
run
~ ~ _ l
'---=::,:.;_______...,.'-------~

Fillet weld with

both toes blended
smoothly

Note: If the weld symbol does not have a supplementary symbol then the shape of
the weld surface does not need to be indicated precisely.

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9.5 Position of symbols on drawings

To be able to provide comprehensive details for weld joints, it is necessary
to distinguish the two sides of the weld joint.

This is done, according to EN 22553, by:

An arrow line.
A dual reference line consisting of a continuous and a dashed line.

The figure below illustrates the method of representation .

3
2a

1 \
--- ~- - - ~a ~ ~:~.;~~:(continuous line)
2b = Identification line (dashed line)
= Welding symbol (single V joint)

Joint line

9.6 Relationship between the arrow and joint lines

One end of the joint .line is called the arrow side and the opposite end is
called other side.

The arrow side is always the end of the joint line that the arrow line points to
(and touches).

It can be at either end of the joint line and 1t Is the draughtsman who decides
which end to make the arrow side.

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The figure below illustrates these principles.

Arrow si L Arrow line

Other side

I
Other side
Arrow sid";'; Arrow line

Other side Arrow side Other side

Arrow side

Arrow line Arrow line

There are some conventions about the arrow line:

It must touch one end of the joint line.

It joins one end of the continuous reference line.
In case of a non-symmetrical joint, such as a single bevel joint, the
arrow line must point towards the joint member that will have the weld
preparation put on to it (as shown below).

An example of how a single bevel butt joint should be represented .

I!
9.7 Position of the reference line and weld symbol
The reference line should, wherever possible, be drawn parallel to the
bottom edge of the drawing (or perpendicular to it).

For a non-symmetrical weld it is essential that the arrow side and other side
of the weld are distinguished. The convention for doing this is:

Symbols for the weld details required on the arrow side must be placed
on the continuous line.
Symbols for the weld details on the other side must be placed on the
dashed line.

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9.8 Positions of the continuous and dashed lines

EN22553 allows the dashed line to be either above or below the continuous
line- as shown below.

or

If the weld is symmetrical it is not necessary to distinguish between the two

sides and EN22553 states that the dashed line should be omitted. Thus, a
single V butt weld with a backing run can be shown by any of the four
symbolic representations shown below.

Single V weld with backing run.

Other side

Other side Arrow side\'-___,...,

____
____A ____ _
___ _Q ____ _

Arrow side ( 1\ ,...._ _ _ _o_t_he..,...r_s_id_e_ _ _--,

Olher side
11
AITOw side\ ____ \l_~---
c::>
This flexibility of the position of the continuous and dashed lines is an
interim measure that EN22553 allows so that old drawings (to the obsolete
BS 499 Part 2, for example) can be easily converted to show the EN method
of representation .

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9.9 Dimensioning of welds

General rules
Dimensions may need to be specified for some types of weld and EN 22553
specifies a convention for this.

Dimensions for the cross-section of the weld are written on the lefthand
side of the symbol.
Length dimensions for the weld are written on the righthand side of the
symbol.
In the absence of any indication to the contrary, all butt welds are full
penetration welds.

9.9.1 Symbols for cross-section dimensions

The following letters are used to indicate dimensions:

a Fillet weld throat thickness.

Z Fillet weld leg length.
s Penetration depth (applicable to partial penetration butt welds and
deep penetration fillets) .

Some examples of how these symbols are used are shown below.

Partial penetration
1omm1 single V butt weld

\ ~---T l ~ '---'------'

Fillet weld with 8mm leg

Fillet weld with 6mm throat

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9.9.2 Symbols for length dimensions

To specify weld length dimensions and, for intermittent welds the number of
individual weld lengths (weld elements), the following letters are used:

I Length of weld.
(e) Distance between adjacent weld elements.
n Number of weld elements.

The use of these letters is shown for the intermittent double-sided fillet weld
shown below.

10Dmm
I I

:+-+~
I I

I I
:4
I
~:t Plan view End view
150mm

~ n xI (e) za ~ 3 x 1so (100)

z 17' n xI (e) zav 3 x 150 (100)

Note: Dashed line is not required because it is a symmetrical weld,

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The convention for an intermittent double-sided staggered fillet weld is

shown below.

(e)

Plan view End view

~...........~__,~(e)
(e)

9.10 Complimentary indications

Complementary indications may be needed to specify other characteristics
ofwelds, eg:

Field or site welds are indicated by a flag.

A peripheral weld to be made all around a part is indicated by a circle.

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9.11 Indication of the welding process

If required, the welding process is symbolised by a number written between
the two branches of a fork at the end of the reference line.

Some welding process designations:

111 = MMA
121 =SAW
131 =MIG
135 =MAG

9.11.1 Other information in the tail of the reference line

Information other than the welding process can be added to an open tail
such as the NDT acceptance level, the working position and filler metal type
and EN22553 defines the sequence that must be used for this information.

A closed tail can also be used into which reference to a specific instruction
can be added.

/-----<1 \W$0141

9.12 Weld symbols in accordance with AWS 2.4

Many of the symbols and conventions specified by EN22553 are the same
as those for AWS.

The major differences are:

Only one reference line is used (a continuous line).

Symbols for weld details on the arrow side go underneath the reference
line.
Symbols for weld details on the other side go on top of the reference
line.

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These differences are illustrated by the following example.

1\
Arrow side
/
Other side

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 Section 10

Introduction to Welding Processes

 Rev 2 April 2013
Introduction to Welding Processes
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10 Introduction to Welding Processes

10.1 General
Common characteristics of the four main arc welding processes, MMA, TIG,
MIG/MAG and SAW are:

An arc is created when an electrical discharge occurs across the gap

between the electrode and parent metal.
The discharge causes a spark to form causing the surrounding gas to
ionise.
The ionised gas enables a current to flow across the gap between the
electrode and base metal thereby creating an arc.
The arc generates heat for fusion of the base metal.
With the exception of TIG welding, the heat generated by the arc also
causes the electrode surface to melt and molten droplets can transfer to
the weld pool to form a weld bead or run.
Heat input to the fusion zone depends on the voltage, arc current and
welding/travel speed.

10.2. Productivity
With most welding processes, welding in the PA (flat or 1G) position results
In the highest weld metal deposition rate and therefore productivity.

For consumable electrode welding processes the rate of transfer of molten

metal to the weld pool is directly related to the welding current density (ratio
of the current to the diameter of the electrode).

For TIG welding, the higher the current, the more energy there is for fusion
so the higher the rate at which filler wire can be added to the weld pool.

10.3 Heat input

Arc energy is the amount of heat generated in the welding arc per unit
length of weld and is usually expressed in kilojoules per millimetre length of
weld (kJ/mm) Heat input (HI) for arc welding is calculated from the following
formula:

Vo/tsxAmps
Arcenergy(kJ I mm) =-------':...______
Travel speed (mm I sec) x 1000

Heat input is the energy supplied by the welding arc to the workpiece and is
expressed in terms of arc energy x thermal efficiency factor.

The thermal efficiency factor is the ratio of heat energy into the welding arc
to the electrical energy consumed by the arc.

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Heat input values into the weld for various processes can be calculated from
the arc energy by multiplying by the following thermal efficiency factors:

SAW (wire electrode) 1.0

MMA (covered electrode) 0.8
MIG/MAG 0.8
FCAW (with or without gas shield) 0.8
TIG OB
Plasma 0.6

Example

A weld is made using the MAG welding process and the following welding
conditions were recorded:

Volts: 24
Amps: 240
Travel speed: 300mm per minute

VoltsxAmps
Arc energy (k.J I mm)
T1avel speed (mm I sec) x 1000

24 x 240
= 300/60 X 1000

5760
= 5000

Arc energy = 1.152 or 1.2kJ/mm

Heat input = 1.2 x 0.8 =0.96kJ/mm

Heat input is mainly influenced by the travel speed.

Welding position and the process have a major influence on the travel
speed that can be used.

For manual and semi-automatic welding the following are general principles:

Vertical-up progression tends to give the highest heat input because

there is a need to weave to get a suitable profile and the forward travel
speed is relatively slow.
Vertical-down welding tends to give the lowest heat input because of the
fast travel speed that can be used.
Horizontal-vertical welding is a relatively low heat input welding position
because the welder cannot weave in this position.

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Overhead welding tends to give low heat input because of the need to
use low current and relatively fast travel speed.
Welding in the flat position (downhand) can be a low or high heat input
position because the welder has more flexibility about the travel speed
that can be used.
Of the arc welding processes, SAW has the potential to give the highest
heat input and deposition rates and TIG and MIG/MAG can produce very
low heat input.
Typical heat input values for controlled heat input welding will tend to be
-1 .0--3.5kJ/mm.

10.4 Welding parameters

Arc voltage
Arc voltage is related to the arc length. For processes where the arc voltage
is controlled by the power source (SAW, MIG/MAG and FCAW) and can be
varied independently from the current, the voltage setting will affect the
profile of the weld,

As welding curren~ is raised, the voltage also needs to be raised to spread

the weld metal and produce a wider and flatter deposit.

For MIG/MAG, arc voltage has a major influence on droplet transfer across
the arc.

Welding current
Welding current has a major influence on the depth of fusion/penetration
into the base metal and adjacent weld runs.

As a rule, the higher the current the greater the penetration depth.

Penetration depth affects dilution of the weld deposit by the parent metal
and it is particularly important to control this when dissimilar metals are
joined.

Polarity
Polarity determines whether most of the arc energy (heat) is concentrated at
the electrode surface or at the surface of the parent material.

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The location of the heat with respect to polarity is not the same for all
processes and the effects/options/benefits for each of the main arc welding
processes are summarised below.

Polarity
Process
DC+ve DC-ve AC
Best penetration Less penetration but higher Not suitable for
deposition rate (used for some electrodes.
MMA
root passes and weld Minimises arc blow
overlaying)
Rarely used due Used for aU metals except Required for AVAI
to tungsten AVAI alloys and Mg/Mg alloys to break-up
TIG overheating the refractory
alloys
oxide film
GMAWsolid Used for all metals Rarely used Not used
wires and virtually all
(MIG/MAG) situations
Most common Some positional basic fluxed Not used
FCAW/MCAW
wires are designed to run on
gas-shielded -ve; some metal cored wires
and self- may also be used on -ve
shielded cored particularly for positional
wires welding
Best penetration Less penetration but higher Used to avoid arc
deposition rate (used for blow, particularly
SAW
root passes and overlaying) for multi-electrode
systems

10.5 Power source characteristics

To strike an arc, a relatively high voltage is required to generate a spark
between the electrode and base metal. This is known as the open circuit
voltage (OCV) and is typically -S0--90V.

Once an arc has been struck and stabilised there is a relationship between
the arc voltage and current flowing through the welding circuit that depends
on the electrical characteristics of the power source.

This relationship is known as the power source static characteristic and

power sources are manufactured to give a constant current or voltage
characteristic.

10.5.1 Constant current power source

This is the preferred type of power source for manual welding (MMA and
manual TIG).

The volt-amp relationship for a constant current power source is shown in

Figure 10.1 and shows the no current position (the OCV) and from this point
there are arc voltage/current curves that depend on the power source for the
various current settings.

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100

ocv

>
a)

->
0>
t'O
0

t:
0
50
:.p
t'O
c:

~
t'O
>
Q)
0
.....
t'O
0
>
...u
<(

Current, A
\{9
Small change in current
Figure 10.1 Typical volt-amp curves for a constant current power source.

For manual welding (MMA and manual TIG) the welder sets the required
current on the power source but arc voltage is controlled by the arc length
the welder uses.

A welder has to work within a fairly narrow range of arc length for a
particular current setting, if it is too long the arc will extinguish, too short and
the electrode may stub into the weld pool and the arc extinguish.

For the operating principle of this type of power source see Figure 10.1.

The welder tries to hold a fairly constant arc length (B in Figure 10.1) for the
current (Y) that has been set. However, he cannot keep the arc length
constant and it will vary over a small working range (A-C) due to normal
hand movement during welding.

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The power source is designed to ensure that these small changes in arc
voltage during normal welding will give only small changes in current (X to
Z). Thus the current can be considered to be essentially constant and this
ensures that the welder is able to maintain control of fusion.

The drooping shape of the volt-amp curves has led to constant current
power sources sometimes being said to have a drooping characteristic.

10.5.2 Constant voltage power source

This is the preferred type of power source for welding processes that have a
wire feeder (MIG/MAG, FCAW and SAW).

Wire feed speed and current are directly related so that as the current
increases, so does the feed speed and there is a corresponding increase in
the burn-off rate to maintain the arc length/voltage.

The operating principle of this type of power source is shown in Figure 10.2.

A welder sets voltage B and current Y on the power source. If the arc length
is decreased to C (due to a variation in weld profile or as the welder's hand
moves up and down during semi-automatic welding) there will be a
momentary increase in welding current to Z. The higher current Z gives a
higher bum-off rate which brings the arc length (and arc voltage) back to the
pre-set value.

Similarly, if the arc length increases the current quickly falls to X and the
bum-off rate is reduced so that the arc length is brought back to the pre-set
level B.

Thus, although the arc voltage does vary a little during welding the changes
in current that restore the voltage to the pre-set value happen extremely
quickly so that the voltage can be considered constant.

The straight-line relationship between voltage and current and the relatively
small gradient is why this type of power source is often referred to as having
a flat characteristic.

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Current, A
~
Large (momentary) change in current

Figure 10.2 Typical volt-amp curves for a constant voltage power source.

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 Section 11

MMA Welding
 Rev 2 April 201 3
Manual Metal Arc/Shielded Metal Arc Welding (MMAJSMAW)
Copyright e TWI Ltd 2013

11 Manual Metal Arc/Shielded Metal Arc Welding

(MMA/SMAW)
Manual metal arc (MMA) welding was invented in Russia in 1888 and
involved a bare metal rod with no flux coating to give a protective gas shield.
Coated electrodes weren't developed until the early 1900s when the
Kjellberg process was invented in Sweden and the quasi-arc method was
introduced in the UK.

The most versatile welding process, MMA is suitable for most ferrous and
non-ferrous metals, over a wide range of thicknesses. It can be used in all
positions, with reasonable ease of use and relatively economically. The final
weld quality is primarily dependent on the skill of the welder.

When an arc is struck between the coated electrode and workpiece, both
surfaces melt to form a weld pool. The average temperature of the arc is
approximately 6000C, sufficient to simultaneously melt the parent metal,
consumable core wire and flux coating. The flux forms gas and slag which
protect the weld pool from oxygen and nitrogen in the surrounding
atmosphere. The molten slag solidifies, cools and must be chipped off the
weld bead once the weld run is complete (or before the next weld pass is
deposited). The process allows only short lengths of weld to be produced
before a new electrode needs to be inserted in the holder.

Electrode angle
75-80 to the horizontal

mable electrode
Filler metal core

................~ "'"l)!!ol-Direction of electrode travel

-Parent
metal
Weld metal
MMA welding.

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11.1 MMA basic equipment requirements

9 Power source transformer/rectifier (constant current type).

10 Holding oven (holds at temperatures up to 150C).
11 Inverter power source (more compact and portable).
12 Electrode holder (of a suitable amperage rating).
13 Power cable (of a suitable amperage rating).
14 Welding visor (with correct rating for the amperage/process).
15 Power return cable (of a suitable amperage rating).
16 Electrodes (of a suitable type and amperage rating).
17 Electrode oven (bakes electrodes at up to 350C).
18 Control panel (on/off/amperage/polarity/OCV).

11.2 Power requirements

MMA welding can be carried out using either DC or AC current. With DC
welding current either positive (+ve) or negative (-ve) polarity can be used,
so current is flowing in one direction. AC welding current flows from negative
to positive and is two directional.

Power sources for MMA welding are transformers (which transform mains
AC-AC suitable for welding), transformer-rectifiers (which rectify AC-DC),
diesel or petrol driven generators (preferred for site work) or inverters (a

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more recent addition to welding power sources). A power source with a

constant current (drooping) output must be used.

The power source must provide:

An OCV.
Initiate the arc.
Welding voltage between 20 and 40V to maintain the arc during welding .
Suitable current range, typically 30-350 amps.
Stable arc-rapid arc recovery or arc re-ignition without current surge.
Constant welding current. The arc length may change during welding ,
but consistent electrode burn-off rate and weld penetration
characteristics must be maintained.

11.3 Welding variables

Other factors or welding variables which affect the final quality of the MMA
weld, are:

Current (amperage)
Voltage
Travel speed Affects heat input
Polarity
Type of electrode

Examples of the MMA welding process.

11.3.'1 Current (amperage)

The flow of electrons through the circuit is the welding current measured in
amperes (1). Amperage controls burn-off rate and depth of penetration.
Welding current level is determined by the size of electrode and
manufacturers recommend the normal operating range and current.

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Amperage too low

Poor fusion or penetration. irregular weld bead shape, slag inclusion
unstable arc, arc stumble, porosity and potential arc strikes.

Amperage too high

Excessive penetration, burn-through, undercut, spatter, porosity, deep
craters, electrode damage due to overheating, high deposition making
positional welding difficult.

11.3.2 Voltage
The welding potential or pressure required for current to flow through the
circuit is the voltage (U). For MMA welding the voltage required to initia te
the arc is OCV, the voltage measured between the output terminals of the
power source when no current is flowing through the welding circuit

For safety reasons the OCV should not exceed 90V and is usually 50-90V.
Arc voltage that is required to maintain the arc during welding and is usually
20-40V and is a function of arc length. With MMA the welder controls the arc
length and therefore the arc voltage which in turn controls weld pool fluidity.

Arc voltage too low

Poor penetration, electrode stubbing, lack of fusion defects, potential for arc
strikes, slag inclusion, unstable arc condition, irregular weld bead shape.

Arc voltage too high

Excessive spatter, porosity, arc wander, irregular weld bead shape, slag
inclusions, fluid weld pool making positional welding difficult.

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OCV90V

Normal Normal arc length

arc voltage
range

Welding amperage

Constant current (drooping) output characteristic.

Large change in arc voltage =small change in welding amperage,

11.3.3 Travel speed
The rate of weld progression, the third factor that affects heat input and
therefore metallurgical and mechanical conditions.

Travel speed too fast

Narrow thin weld bead, fast cooling, slag inclusions, undercut, poor fusion,
penetration.

Travel speed too slow

Cold lap, excess weld deposition, irregular bead shape undercut.

11.3.4 Polarity (type of current)

Polarity will determine the distribution of heat energy at the welding arc. The
preferred polarity of the MMA system depends primarily on the electrode
being used and the desired properties of the weld.

Direct current (DC)

Direct current is the flow of current in one direction and for MMA welding it
refers to the polarity of the electrode.

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Direct current/electrode positive (DCEP/DC+ve)

When the electrode is on the positive pole of the welding circuit, the
workpiece becomes the negative pole. Electron flow direction is from the
workpiece to the electrode.

When the electrode is positively charged (DC+ve) and the workpiece is

negatively charged the two thirds of the available heat energy is at the tip of
the electrode, with the remaining third being generated in the parent
material, resulting in an increase in weld penetration.

Direct current/electrode negative (DCEN/DC-ve)

When the electrode is on the negative pole of the welding circuit, the
workpiece becomes the positive pole, electron flow direction is from the
electrode to the workpiece. The distribution of energy is now reversed. One
third of the available heat energy is generated at the tip of the electrode, the
remaining two thirds in the parent material.

Direct current with a negatively charged electrode (DC-ve) causes heat to

build up on the electrode, increasing the electrode melting rate and
decreasing the depth of the weld penetration depth.

When using DC the welding arc can be affected by arc blow, the deflection
of the arc from its normal path due to magnetic forces.

Alternating current (AC)

The current alternates in the welding circuit, flowing first in one direction
then the other. With AC, the direction of flow changes 100-120 times per
second, 50-60 cycles per second (cps). AC is the flow of current in two
directions.

Therefore, distribution of heat energy at the arc is equal, 50% at the

electrode, 50% at the workpiece.

11.3.5 Type of consumable electrode

For MMA welding there are three generic types of flux covering:

Rutile electrodes
Contain a high proportion of titanium oxide (rutile) in the coating which
promotes easy arc ignition, smooth arc operation and low spatter. These
electrodes are general purpose with good welding properties and can be
used with AC and DC power sources and in all positions. The electrodes are
especially suitable for welding fillet joints in the horizontal/vertical (HV)
position.

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Features:
Moderate weld metal mechanical properties.
Good bead profile produced through the viscous slag.
Positional welding possible with a fluid slag (containing fluoride).
Easily removable slag.

Basic electrodes
Contain a high proportion of calcium carbonate (limestone) and calcium
fluoride (fluorspar) in the coating, making the slag coating more fluid than
rutile coatings. This is also fast freezing which assists welding in the vertical
and overhead positions. These electrodes are used for welding medium and
heavy section fabrications where higher weld quality, good mechanical
properties and resistance to cracking due to high restraint are required.

Features
Low hydrogen weld metal.
Requires high welding currents/speeds.
Poor bead profile (convex and coarse surface profile).
Slag removal difficult.

Cellulosic electrodes
Contain a high proportion of cellulose in the coating and are characterised
by a deeply penetrating arc and rapid burn-off rate giving high welding
speeds. Weld deposit can be coarse and with fluid slag, deslagging can be
difficult. These electrodes are easy to use in any position and are noted for
their use in the stovepipe welding technique.

Features
Deep penetration in all positions.
Suitable for vertical-down welding .
Reasonably good mechanical properties.
High level of hydrogen generated, risk of cracking in the HAZ.

Within these three generic groups sub-groups of covered electrodes provide

a wide range of electrode choice.

MMA electrodes are designed to operate with AC and DC power sources.

Although AC electrodes can be used on DC, not all DC electrodes can be
used with AC power sources.

Operating factor: (0/F) The percentage of arc on time in a given time.

When compared with semi-automatic welding processes MMA has a low

0/F of approximately 30%. Manual semi-automatic MIG/MAG 0/F is about
60% with fully automated in the region of 90%. A welding process 0/F can
be directly linked to productivity.

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Operating factor should not be confused with the tenn duty cycle which is a
safety value given as the % of time a conductor can carry a current and is
given as a specific current at 60 and 100% of 10 minutes, ie 350A 60% and
300A 100%

11.4 Summary of MMAISMAW

Equipment requirement:
Transformer/rectifier, generator, inverter (constant amperage type).
Power and power return cable (of a suitable amperage rating).
Electrode holder (of a suitable amperage rating).
Electrodes (of a suitable type and amperage rating).
Correct visor/glass, safety clothing and good extraction.
I -
Parameters and inspection points:
Amperage .
ocv.
AC/DC and polarity.
Speed of travel.
Electrode type and diameter.
Duty cycles.
Electrode condition.
Connections.
Insulation/extraction.
Any special electrode treatment.

Typical welding imperfections

Slag inclusions caused by poor welding technique or insufficient inter-run
cleaning .
Porosity from using damp or damaged electrodes or when welding
contaminated or unclean material.
Lack of root fusion or penetration caused by incorrect settings of the
amps, root gap or face width .
Undercut caused by amperage too high for the position or by a poor
welding technique. eg travel speed too fast or slow, arc length (therefore
voltage) variations particularly during excessive weaving .
Arc strikes caused by incorrect arc striking procedure or lack of skill.
These may also be caused by incorrectly fitted/secured power return
lead clamps.
Hydrogen cracks caused by the use of incorrect electrode type or baking
procedure and/or control of basic coated electrodes.

Successful welding with the MMA process is reliant on a number of factors,

not least of which is the skill required to produce a sound weld. This is
dependent on the welder's ability to match the arc length (distance from the
tip of the electrode to the workpiece), to the bum-off rate (rate at which the
electrode is consumed).

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Advantages:
Field or shop use.
Range of consumables.
All positional.
Very portable.
Simple equipment.

Disadvantages:
High skill factor required.
Arc strikes/slag inclusions.
Low operating factor.
High level of generated fumes.
Hydrogen control.

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TIG Welding
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12 TIG Welding
12.1 Process characteristics
In the US the TIG process is also called gas tungsten arc welding {GTAW).
Melting is produced by heating with an arc struck between a non-
consumable tungsten electrode and the workpiece. An inert gas shields the
electrode and weld zone to prevent oxidation of the tungsten electrode and
atmospheric contamination of the weld and hot filler wire (as shown below).
Current
con ductor

Gas nozzle

Manual TIG welding.

Tungsten is used because it has a melting point of 3370C, well above any
other common metal.

12.2 Process variables

The main variables in TIG welding are:

Welding current.
Current type and polarity.
Travel speed.
Shape of tungsten electrode tip and vertex angle.
Shielding gas flow rate.
Electrode extension.

Each of these is consjdered in more detail in the following sub-sections.

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12.2.1 Welding current

Weld penetration is directly related to welding current.
If the welding current is too low, the electrode tip will not be properly
heated and an unstable arc may result.
If the welding current is too high, the electrode tip might overheat and
melt, leading to tungsten inclusions.

12.2.2 Current type and polarity

Best welding results are usually obtained with DC~ve.
Refractory oxides such as those of aluminium or magnesium can hinder
fusion but can be removed by using AC or DC electrode positive.
With a DC positively connected electrode, heat is concentrated at the
electrode tip so the electrode needs to be of greater diameter than when
using DC~ve if overheating of the tungsten is to be avoided. A water
cooled torch is recommended if DC positive is used.
The current carrying capacity of a DC positive electrode is about one
tenth that of a negative one so it is limited to welding sections.

+ -
(AC.)

Current
DC-ve AC DC+ve
type/polarity
Heat 70% at work 50% at work 30% at work
balance 30% at electrode 50% at electrode 70% at electrode
Weld profile Deep, narrow Medium Shallow, Wide
Cleaning No Yes -every half cycle Yes
action
Electrode Excellent Good Poor
capacity (3.2mm/400A) {3.2mm/225A) {6.4mmi120A)

12.2.3 Travel speed

Affects both weld width and penetration but the effect on width is more
pronounced.
Increasing the travel speed reduces the penetration and width.
Reducing the travel speed increases the penetration and width.

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12.2.4 Tungsten electrode types

Different types of tungsten electrodes suit different applications:

Pure tungsten electrodes are used when welding light metals with AC
because they maintain a clean balled end, but possess poor arc initiation
and stability in AC mode compared with other types.
Thoriated electrodes are alloyed with thorium oxide (thoria) to improve
arc initiation and have higher current carrying capacity than pure
tungsten electrodes and maintain a sharp tip for longer. Unfortunately,
thoria is slightly radioactive (emitting a radiation) and the dust generated
during tip grinding should not be inhaled. Electrode grinding machines
used for thoriated tungsten grinding should be fitted with a dust
extraction system.
Ceriated and lanthaniated electrodes are alloyed with cerium and
lanthanum oxides, for the same reason as thoriated electrodes and
operate successfully with DC or AC and as cerium and lanthanum are
not radioactive, they have been used as replacements for thoriated
electrodes.
Zirconiated electrodes are alloyed with zirconium oxide with operating
characteristics between the thoriated types and pure tungsten. They are
able to retain a balled end during welding, so are recommended for AC
welding . They have a high resistance to contamination so are used for
hi_gh integrity welds where tungsten inclusions must be avoided.

12.2.5 Shape of tungsten electrode tip

With DC-ve, thoriated , ceriated or lanthanated tungsten electrodes are
used with the end ground to a specific angle (the electrode tip or vertex
angle, shown below).
As a general rule the length of the ground portion of the electrode tip
should have a length equal to approximately 2-2.5 times the electrode
diameter.
When using AC the electrode tip is ground flat to minimise the risk of it
breaking off when the arc is initiated or during welding (shown on the
next page).
If the vertex angle is increased, the penetration increases.
If the vertex angle is decreased, bead width increases.
Pure or zirconiated tungsten electrodes are used for AC welding with a
hemispherical (balled) end (as shown below). To produce a balled end
the electrode is ground, an arc initiated and the current increased until it
melts the tip of the electrode.

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Electrode tip Electrode tip Electrode tip with

(or vertex angle). with flat end. a balled end.

12.2.6 Shielding gases

The following inert gases can be used as shielding gases for TIG welding:

Argon.
Helium.
Mixtures of argon and helium .

Note: For austenitic stainless steels and some cupro-nickel alloys, argon
with up to -5% hydrogen improves penetration and reduces porosity.

Characteristics of argon and helium s hielding gases for TIGweld1ng.

Argon Performance item Helium
Lower than with helium which Arc voltage Higher than with argon. Arc is hotter
can be helpful when welding which is helpful in welding thick
thin sections. Less change in sections and viscous metals, (eg
arc voltage with variations in nickel.
arclength.
Lower than with helium which Heating power High, advantageous when welding
gives reduced penetration. of the arc metals with high thermal
conductivity and thick materials.
Argon is heavier than air so Protection Helium is lighter than air and
requires less gas to shield in of weld requires more gas to properly shield
the flat and horizontal the weld. Exception: Overhead
positions. Better draught welding.
resistance.
Obtained from the Availability Obtained by separation from natural
atmosphere by the separation and cost gas - lower availability and higher
of liquefied air - lower cost cost.
and greater availability.

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Shielding gas flow rate

Too low and the shielding gas cannot remove the air from the weld area
resulting in porosrty and contamination.
Too high and turbulence occurs at the base of the shielding gas column,
air tends to be sucked in from the surrounding atmosphere and this may
also lead to porosity and contamination.
Typically in the range -10--121/min.

Flow rate too low

9; , Flow rate too high

Back purging
It is necessary to protect the back of the weld from excessive oxidation
during TIG welding, achieved by using a purge gas, usually pure argon.

For pipe welding spools it is relatively easy to purge the pipe bore, but for
plate/sheet welding irt is necessary to use a purge channel or sometimes
another operator positions and moves a back purge nozzle as the weld
progresses. For purging large systems soluble dams or bungs are required
and can it can be a complex operation.

The initial stage of back purging is to exclude all the air at the back of the
weld and having allowed sufficient time for this the flow rate should be
reduced prior to starting to weld so there is positive flow (typically
-41/min).

Back purging should continue until two or more layers of weld have been
deposited.

For C and C-Mn steels it is possible to make satisfactory welds without a

back purge.

12.2.7 Electrode extension

The distance from the contact tube to the tungsten tip.
Because the contact tube is recessed inside the gas nozzle this
parameter can be checked indirectly by measuring the stickout length, as
shown below.

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Electrode
Stickout extension

t >

If the electrode extension is too short, the electrode tip will not be
adequately heated leading to an unstable arc. 0
If the electrode extension is too long, the electrode tip might overheat,
causing melting and lead to tungsten inclusions.
As a general rule stickout length should be 2-3 times the electrode
diameter.

12.3 Filler wires

Filler wires usually have a similar composition to the parent metal but
contain small additions of elements that will combine with any oxygen and
nitrogen present.

12.4 Tungsten inclusions

Small fragments of tungsten that enter a weld will always show up on
radiographs because of the relatively high density of this metal and for most
applications will not be acceptable.

Thermal shock to the tungsten causing small fragments to enter the weld
pool is a common cause of tungsten inclusions and is why modern power
sources have a current slope-up device to minimise this risk.

This device allows the current to rise to the set value over a short period so
the tungsten is heated more slowly and gently.

12.5 Crater cracking

One form of solidification cracking which some filler metals are sensitive to.
Modern power sources have a current slope-out device so that at the end of
a weld when the welder switches off the current it reduces gradually and the
weld pool gets smaller and shallower. The weld pool will have a more

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favourable shape when it finally solidifies and crater cracking can be

avoided.

12.6 Common applications

Include autogenous welding of longitudinal seams in thin walled pipes and
tubes in stainless steel and other alloys on continuous forming mills.

Using filler wires, TIG is used for making high quality joints in heavier gauge
pipe and tubing for the chemical, petroleum and power generating
industries.

It is also used in the aerospace industry for items such as airframes and
rocket motor cases.

12.7 Advantages
Produces superior quality welds with very low levels of diffusible
hydrogen so there is less danger of cold cracking .
No weld spatter or slag inclusions which makes it particularly suitable for
applications that require a high degree of cleanliness, eg pipework for
the food and drinks industry, manufacturing semiconductors, etc.
Can be used with filler metal and on thin sections without filler and can
produces welds at relatively high speed.
Enables welding variables to be accurately controlled and is particularly
good for controlling weld root penetration in all welding.
Can weld almost all weldable metals including dissimilar joints but
welding in position is not generally used for those with low melting points
such as lead and tin . Especially useful in welding reactive metals with
very stable oxides such as aluminium, magnesium, titanium and
zirconium.
The heat source and filler metal additions are controlled independently
so it is very good for joining thin base metals.

12.8 Disadvantages
Gives low deposition rates compared with other arc welding processes.
Need higher dexterity and welder co-ordination than with MIG/MAG or
MMA welding .
Less economical than MMA or MIG/MAG for sections thicker than
-10mm.
Difficult to fully shield the weld zone in draughty conditions so may not
be suitable for site/field welding .
Tungsten inclusions can occur if the electrode contacts the weld pool.
No cleaning action so low tolerance for contaminants on filler or base
metals.

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13 MIG/MAG Welding
13.1 Process
Known in the US as gas metal arc welding (GMAW), the MIG/MAG welding
process (Figure 13.1) is a versatile technique suitable for both thin sheet
and thick section components in most metallic materials. An arc is struck
between the end of a wire electrode and the workpiece, melting both to form
a weld pool. The wire serves as the source of heat (via the arc at the wire
tip) and filler metal for the joint and is fed through a copper contact tube
(also called a contact tip) which conducts welding current into the wire. The
weld pool is protected from the surrounding atmosphere by a shielding gas
fed through a nozzle surrounding the wire. Shielding gas selection depends
on the material being welded and the application. The wire is fed from a reel
by a motor drive and the welder or machine moves the welding gun or torch
along the joint line. The process offers high productivity and is economical
because the consumable wire is continuously fed.

Figure 13.1 MIG/MAG welding.

The MIG/MAG process uses semi-automatic, mechanised or automatic

equipment. In semi-automatic welding, the wire feed rate and arc length are
controlled automatically, but the travel speed and wire position are under
manual control. In mechanised welding, all parameters are under automatic
control but can be varied manually during welding, eg steering of the
welding head and adjustment of wire feed speed and arc voltage. With
automatic equipment there is no manual intervention during welding. Figure
13.2 shows equipment required for the MIG/MAG process.

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Wir. electrode
1upply

Stllefdlng g~~s
supply
Safety earth
(Seek HSE
guidance) 1. Return lead 5. Wire electrode In liner
2. Welding current cable 6. Primary Input power
3. CooQng water In and out 7. Gun switch c:ircuit
... Stlteldina gas rrom ey1inder

Figure 13.2 MIG/MAG welding equipment.

Advantages:
Continuous wire feed.
Automatic self-regulation of the arc length.
High deposition rate and minimal number of stop/start locations.
High consumable efficiency.
Heat inputs in the range 0.12kJ/mm.
Low hydrogen potential process.
Welder has good visibility of weld pool and joint line.
Little or no post-weld cleaning.
Can be used in all positions (dip transfer).
Good process control possibilities.
Wide range of applications.

Disadvantages:
No independent control of filler addition.
Difficult to set up optimum parameters to minimise spatter levels.
Risk of lack of fusion when using dip transfer on thicker weldments.
High level of equipment maintenance.
Lower heat input can lead to high hardness values.
Higher equipment cost than MMA welding.
Site welding requires special precautions to exclude draughts which may
disturb the gas shield.
Joint and part access is not as good as MMA or TIG welding.
Cleanliness of base metal, slag processes tolerate greater contamination.

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13.2 Primary variables

Welding current/wire feed speed.
Voltage.
Gases.
Travel speed and electrode orientation.
Inductance.
Contact tip to work distance (CTWD).
Nozzle to work distance.
Shielding gas nozzle.
Type of metal transfer.

13.2.1 Wire feed speed

Increasing wire feed speed automatically increases the current in the wire.
Wires are generally produced in 0.6, 0.8, 1, 1.2, 1.4 and 1.6mm diameter.

13.2.2 Voltage
The most important setting in spray transfer as it controls the arc length. In
dip transfer it also affects the rise of current and the overall heat input into
the weld. Increase both wire feed speed/current and voltage will increase
heat input. Welding connections need to be checked for soundness as any
loose ones will result in resistance and cause a voltage drop in the circuit
and will affect the characteristic of the welding arc. The voltage will affect
the type of transfer achievable but this is also highly dependent on the type
of gas being used.

a) 22V b) 23V c) 24V

Figure 13.3 The effect of arc voltage:

a Increasing arc voltage.
b Reduced penetration. increased width.
c Excessive voltage can cause porosity, spatter and undercut.

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0)---0J
13.2.3 Gases

[V ~
Ar Ar-He He

Figure 13.4 Gas composition effect on weld bead profile.

For non-ferrous metals and their alloys (such as AI, Ni and Cu) an inert
shielding gas must be used, usually pure argon or an argon rich gas with a
helium addition. The use of a fully inert gas is why the process is also called
metal inert gas (MIG) welding and for precise use of terminology this should
only be used when referring to the welding of non-ferrous metals.

The addition of some helium to argon gives a more uniform heat

concentration within the arc plasma which affects the shape of the weld
bead profile. Argon-helium mixtures give a hotter arc so are beneficial for
welding thicker base materials, those with higher, thermal conductivity, eg
copper or aluminium.

For welding all grades of steels, including stainless steels, a controlled

addition of oxygen or carbon dioxide (C02) to generate a stable arc and give
good droplet wetting. Because these additions react with the molten metal
they are referred to as active gases, hence metal active gas (MAG) welding
is the technical term when referring to welding steels.

100%C02
C02 gas cannot sustain spray transfer as the ionisation potential of the gas
is too high it gives very good penetration but promotes globular droplet
transfer also a very unstable arc and lots of spatter.

Argon +15-20%C02
The percentage of C02 or oxygen depends on the type of steel being
welded and the mode of metal transfer used. Argon has a much lower
ionisation potential and can sustain spray transfer above 24 welding volts.
Argon gives a very stable arc, little spatter but lower penetration than C02.
Argon and 5-20%C02 gas mixtures give the benefit of both gases ie good
penetration with a stable arc and very little spatter. C02 gas is much
cheaper than argon or its mixtures and is widely used for carbon and some
low alloy steels.

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Argon +1-5%C02
Widely used for stainless steels and some low alloy steels.

Matenal lhrckness (mm). o , 2 3 s 6 1 a g to 11 12 13 14 ts~

AlgonCOz mixtures

ArgonC02 milrt\Jres

Figure 13.5 Active shielding gas mixtures for MAG welding of carbon, C-Mn and
low alloy steels. Blue is a cooler and red a hotter mixture gas.

Gas mixtures with helium instead of argon give a hotter arc, more fluid weld
pool and better weld profile. These quaternary mixtures permit higher
welding speeds but may not be suitable for thin sections.

Stainless steels
Austenitic stainless steels are typically welded with argon-C02/02 mixtures
for spray transfer or argon-helium-C02 mixtures for all modes of transfer.
The oxidising potential of the mixtures is kept to a minimum (2-2.5%
maximum C02 content) to stabilise the arc but with minimum effect on
corrosion performance. Because austenitic steels have a low thermal
conductivity, the addition of helium helps to avoid lack of fusion defects and
overcome the high heat dissipation into the material. Helium additions are
up to 85%, compared with -25% for mixtures used for carbon and low alloy
steels. C02-containing mixtures are sometimes avoided to eliminate
potential carbon pick-up.

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Matenal thickness (mm) o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15+

A ustenitic and rerrilic
Argon - Helium - 0.5-2% C~

Argon - C>.2IC0:1 mixtures

(spray transf er onfy}

Figure 13.6 Active shieldjng gas mixtures for MAG welding of stainless s teels. Blue
is a cooler and; red a hotter gas mixture.

For martensitic and duplex stainless steels, specialist advice should be

sought. Some Ar-He mixtures containing up to 2.S%N2 are available for
welding duplex stainless steels.

Light alloys (aluminium magnesium, titanium, copper and nickel and

their alloys)
Inert gases are used for light alloys and those sensitive to oxidation.
Welding grade inert gases should be purchased rather than commercial
purity to ensure good weld quality.

Argon
Can be used for aluminium because there is sufficient surface oxide
available to stabilise the arc. For materials sensitive to oxygen, such as
titanium and nickel alloys, arc stability may be difficult to achieve with inert
gases in some applications. The density of argon is approximately 1.4 times
that of air so in the downhand position, the relatively heavy argon is very
effective at displacing air. A disadvantage is when working in confined
spaces there is a risk of argon building up to dangerous levels and
asphyxiating the welder.

Argon-helium mixtures
Argon is most commonly used for MIG welding of light alloys but an
advantage can be gained by use of helium and argon/helium mixtures.
Helium possesses a higher thermal conductivity than argon and the hotter
weld pool produces improved penetration and/or an increase in welding
speed. High helium contents give a deep broad penetration profile but
produce high spatter levels. With less than 80% argon a true spray transfer
is not possible. With globular-type transfer the welder should use a buried
arc to minimise spatter. Arc stability can be problematic in helium and
argon-helium mixtures, since helium raises the arc voltage so there is a
larger change in arc voltage with respect to arc length. Helium mixtures
require higher flow rates than argon shielding to provide the same gas
protection.

There is a reduced risk of lack of fusion defects when using argon-helium

mixtures particularly on thick section aluminium. Ar-He gas mixtures will
offset the high heat dissipation in material over about 3mm thickness.

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Matenalthtckness (mm) o 1 :1 4 ~ b 7 ~ I) 10 II 1.2 13 14 15 1

Argon

Argon- Helium mxtures

Figure 13.71nert shielding gas mixtures for MIG welding of aluminium,

magnesium, titanium, nickel and copper alloys. Blue is a cooler and red a hotter
gas mixture.

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Summary of shielding gases and mixtures used for different base materials
for MIG/MAG welding

Shielding Reaction
Metal Characteristics
gas behaviour
Carbon Argon-C02 Slightly Increasing C~ content gives hotter arc,
steel oxidising improved arc stability, deeper penetration,
transition from finger-type to bowl-shaped
penetration profile, more fluid weld pool giving
flatter weld bead with good wetting, increased
spatter levels, better toughness than C02.
Minimum 80% argon for axial spray transfer.
General purpose mixture: Argon-1 0-15%C02.
Argon-02 Slightly Stiffer arc than Ar-C0 2 mixtures minimises
oxidising undercutting, suited to spray transfer mode,
lower penetration than Ar-C02 mixtures, finger-
type weld bead penetration at high current
levels. General purpose mixture: Argon-3%
C02.
Ar-He-C02 SlightlY Substituting of helium for argon gives hotter arc,
oxidising higt)er arc voltage, more fluid weld pool, flatter
bead profile, more bowl-shaped and deeper
penetration profile and higher welding speeds,
compared with Ar-C02 mixtures. High cost.
C02 Oxidising Arc voltages 2-3V higher than Ar-C02 mixtures,
best penetration, higher welding speeds, dip
transfer or buried arc technique only, narrow
working range, high spatter levels, low cost.
Stainless He-Ar-C02 Slightly Good arc stability with minimum effect on
steels oxidising corrosion resistance (carbon pick-up), higher
helium contents designed for dip transfer, lower
helium contents designed for pulse and spray
transfer. General purpose gas: He-Ar-2%C02 .
Argon-02 Slightly Spray transfer only, minimises undercutting on
oxidising heavier sections, good bead profile.
Aluminium, Argon Inert Good arc stability, low spatter and general-
copper, purpose gas. Titanium alloys require Inert gas
nickel, backing and trailing shields to prevent air
titanium contamination.
alloys Ar-He Inert Higher heat input offsets high heat dissipation
on thick sections, lower risk of lack of fusion
defects, higher spatter and higher cost than
argon.

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13.2.4 Travel speed and electrode orientation

The faster the travel speed the less penetration, narrower bead width and
the higher risk of undercut.

Figure 13.8 The effect of travel speed. As travel speed increases, reducing
penetration and width, undercut.

Penetration Deep Moderate Shallow

Excess weld metal Maximum Moderate Minimum
Undercut Severe Moderate Minimum

Figure 13.9 Effect of torch angle.

13.2.5 Effect of contact tip to workpiece distance

ClWD has an influence over the welding current because of resistive
heating in the electrode extension (Figure 13.1 0). The welding current
required to melt the electrode at the required rate to match the wire feed
speed reduces as the CTWD is increased. Long electrode extensions can
cause lack of penetration, for example, in narrow gap joints or with poor
manipulation of the welding gun. Conversely, the welding current increases
when the CTWD is reduced. This provides the experienced welder with a
means of controlling the current during welding but can result in variable
penetration in manual welding with a constant voltage power source.

As the electrode extension is increased the burn-off rate increases for a

given welding current due to increased resistive heating. Increasing the
electrode extension, eg in mechanised applications, is therefore one way of

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increasing deposition rates, as the wire feed speed is increased to maintain

the required welding current.

Contact tip

Gas nozzle

Contact ti
setback
Contact
Nozzle-to-
tip to work
work (stand-
distance

Figure 13.10 Contact tip to workpiece distance; electrode extension and nozzle to
workpiece distance.

Resistive heating depends on the resistivity of the electrode, the electrode

extension length and wire diameter so is more pronounced for welding
materials which have high resistivity, such as steels. The electrode
extension should be kept small when small diameter wires are being used to
prevent excessive heating in the wire and avoid the resulting poor bead
shape.

Stable condition Sudden change in gun position

Arc length L = 6,4mm

Arc voltage= 24V
Welding current = 250A

Arc length remains

the same, but
welding current
decreases

Figure 13.11 Effect of increasing the contact tip to workpiece distance. Arc length
remains same length.

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Increased extension

Figure 13.12 Effect of increasing electrode extension.

At short CTWDs, radiated heat from the weld pool can cause overheating of
the contact tube and welding torch which can lead to spatter adherence and
increased wear of the contact tube.

The electrode extension should be checked when setting-up welding

conditions or fitting a new contact tube. Normally measured from the contact
tube to the workpiece (Figure 13.13) suggested CTWDs for the principal
metal transfer modes are:

Metal transfer mode CTWD,mm

Dip 10-15
Spray 20-25
Pulse 15-20

Contact tip
recessed
(3-5mm

Figure 13.13 Suggested contact tip to work distance.

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13.2.6 Effect of nozzle to work distance

Nozzle to work distance (Figure 13.13) has a considerable effect on gas
shielding efficiency with a decrease stiffening the column. The nozzle to
work distance is typically 12."15mm. If the CTWD is simultaneously reduced,
however, the deposition rate at a given current is decreased and visibility
and accessibility are affected; so in practice a compromise is necessary.
The following gives suggested settings for the mode of metal transfer being
used.

Metal transfer mode Contact tip position relative to nozzle

Dip 2mm inside to 2mm protruding
Spray 4-Smm inside
Spray (aluminium) 6-10mm inside

13.2.7 Shielding gas nozzle

The purpose of the shielding gas nozzle is to produce a laminar gas flow to
protect the weld pool from atmospheric contamination. Nozzle diameters
range from 13-22mm and should be increased in relation to the size of the
weld pool. Therefore, larger diameter nozzles are used for high current,
spray transfer application and smaller diameter for dip transfer. The flow
rate must also be tuned to the nozzle diameter and shielding gas type to
give sufficient weld pool coverage. Gas nozzles for dip transfer welding tend
to be tapered at the outlet of the nozzle.

Joint access and type should also be considered when selecting the
required gas nozzle and flow rate. Too small a nozzle may cause it to
become obstructed by spatter more quickly and if the wire bends on leaving
the contact tube, the shielding envelope and arc location may not coincide.

13.2.8 Types of metal transfer

Ofp lransfer Spray transfer

Arc Voltage, V

Transition region
Globular transfer

Welding Current, A
Figure 13.14 Arc characteristic cuNe.

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Key characteristics of dip transfer

Metal transfer by wire dipping or short-circuiting into the weld pool.
Relatively low heat input process.
Low weld pool fluidity.
Used for thin sheet metal above 0.8mm and typically Jess than 3.2mm,
positional welding of thicker section and root runs in open butt joints.
Process stability and spatter can be a problem if poorly tuned .
Lack of fusion of poorly set-up and applied.
Not used for non-ferrous metals and alloys.

Figure 13.15 Dip transfer.

In dip transfer the wjre short-circuits the arc 50-200 times/second and this
type of transfer is normally achieved with C02 or mixtures of C02 and argon
gas + low amps and welding volts <24V.

Key characteristics of spray transfer

Free-flight metal transfer.
High heat input.
High deposition rate.
Smooth stable arc.
Used on steels above 6mm and aluminium alloys above 3mm thickness.

Figure 13.16 Spray transfer.

Spray transfer occurs at high currents and voltages. Above the transition
current, metal transfer is a fine spray of small droplets projected across the

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arc with low spatter levels. The high welding current produces strong
electromagnetic forces (pinch effect) that cause the molten filament
supporting the droplet to neck down. Droplets detach from the tip of the wire
and accelerate across the arc gap. The frequency with which the droplets
detach increases with the current. The droplet size equates to the wire
diameter at the threshold level but decreases significantly as the welding
current increases. At very high currents (wire feed speeds), the molten
droplets can start to rotate (rotating transfer). The arc current is flowing
during the drop detachment resulting in maximum penetration and a high
heat input. When the correct arc voltage to give spray transfer is used, the
arc is short with the wire tip 1-3mm from the surface of the plate.

With steels it can be used only in downhand butts and HN fillet welds but
gives higher deposition rate, penetration and fusion than dip transfer
because of the continuous arc heating. It is mainly used for steel plate
thicknesses >3mm but has limited use for positional welding due to the
potential large weld pool involved.

Key characteristics pulsed transfer:

Free-flight droplet transfer without short-circuiting over the entire working
range.
Very low spatter.
Lower heat input than spray transfer.
Reduced risk of lack of fusion compared with dip transfer.
Control of weld bead profile for dynamically ,loaded parts.
Process control/flexibility.
Enables use of larger diameter, less expensive wires with thinner plates
- more easily fed (particular advantage for aluminium welding).

Pulsing the welding current extends the range of spray transfer operation
well below the natural transition from dip to spray transfer. This allows
smooth, spatter-free spray transfer at mean currents below the transition
level, eg 50-150A and at lower heat inputs. Pulsing was jntroduced originally
to control metal transfer by imposing artificial cyclic operation on the arc
system by applying alternately high and low currents.

A typical pulsed waveform and the main pulse welding variables are shown
in Figure 13.17. A low background current (typically 20-BOA) is supplied to
maintain the arc, keep the wire tip molten, give stable anode and cathode
roots and maintain average current during the cycle. Droplet detachment
occurs during a high current pulse at current levels above the transition
current level. The pulse of current generates very high electromagnetic
forces which cause a strong pinch effect on the metal filament supporting
the droplet the droplet is detached and projected across the arc gap. Pulse
current and current density must be sufficiently high to ensure that spray
transfer (not globular) always occurs so that positional welding can be used.

Pulse transfer uses pulses of current to fire a s,ingle globule of metal across
the arc gap at a frequency of 50-300 pulses/second. It is a development of

13 14
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spray transfer that gives positional welding capability for steels, combined
with controlled heat input, good fusion and high productivity and may be
used for all sheet steel thickness >1mm, but is mainly used for positional
welding of steels >6mm.

1/F'requency

Time, ms

Figure 13.17 Pulsed welding waveform and parameters.

Key characteristics of globular transfer:

Irregular metal transfer.
Medium heat input
Medium deposition rate.
Risk of spatter.
Not widely used in the UK can be used for mechanised welding of
medium thickness (typically 36mm) steel in the flat (PA) position.

The globular transfer range occupies the transitional range of arc voltage
between free-flight and fully short-circuiting transfer. Irregular droplet
transfer and arc instability are inherent, particularly when operating near the
transition threshold. In globular transfer a molten droplet several times the
electrodediameter forms on the wire tip, gravity eventually detaches it when
its weight overcomes surface tension forces and transfer takes place often
with excessive spatter. Before transfer the arc wanders and its cone covers
a large area, dissipating energy.

There is a short duration short-circuit when the droplet contacts with the
molten pool but rather than causing droplet transfer it occurs as a result of it.
Although the short-circuit is of very short duration, some inductance is
necessary to reduce spatter, although to the operator the short-circuits are
not discernible and the arc has the appearance of a free-flight type.

To further minimise spatter levels, it is common to operate with a very short

arc length and in some cases a buried arc technique is adopted. Globular
transfer can only be used in the- flat position and is often associated with
lack of penetration, fusion defects and uneven weld beads because of the
irregular transfer and tendency for arc wander.

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13.2.9 Inductance
When MIG/MAG welding in the dip transfer mode, the welding electrode
touches the weld pool causing a short-circuit during which the arc voltage is
nearly zero. If the constant voltage power supply responded instantly, very
high current would immediately begin to flow through the welding circuit and
the rapid rise in current to a high value would melt the short-circuited
electrode free with explosive force, dispelling the weld metal and causing
considerable spatter.

Inductance is the property in an electrical circuit that slows down the rate of
current rise (Figure 13.18). The current travelling through an inductance coil
creates a magnetic field which creates a current in the welding circuit in
opposition to the welding current. Increasing inductance will also increase
the arc time and decrease the frequency of short-circuiting.

No inductance

~ Inductance added

Time
Figure 13.18 Relationship between inductance and current rise.
There is an optimum value of inductance for each electrode feed rate,. Too
little results in excessive spatter, too much and current will not rise fast
enough and the molten tip of the electrode is not heated sufficiently causing
the electrode to stub into the base metal. Modern electronic power sources
automatically set inductance to give a smooth arc and metal transfer.

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13.3 MIG basic equipment requirements

19 Power source-transformer/rectifier (constant voltage type).

20 Inverter power source.
21 Power hose assembly (liner, power cable, water hose, gas hose).
22 Liner.
23 Spare contact tips.
24 Torch head assembly.
25 Power-return cable and clamp.
26 15kg wire spool (copper coated and uncoated wires).
27 Power control panel.
28 External wire feed unit.

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The MIG/MAG wire drive assembly

Internal wire drive system.

Groove half bottom drive roller. Wire guide.

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The MIG torch head assembly

1 Torch body.
2 On/off or latching switch.
3 Spot welding spacer attachment.
4 Contact tips.
5 Gas diffuser.
6 Gas shrouds.
7 Torch head assembly (minus the shroud).

13.4 Inspection when MIG/MAG welding

13.4.1 Welding equipment
Visual check to ensure the welding equipment is in good condition.

13.4.2 Electrode wire

The diameter, specification and quality of wire are the main inspection
headings. The level of de-oxidation of the wire is an important factor with
single, double and triple de-oxidised wires being available.

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The higher the level of de--oxidants in the wire, the lower the chance of
porosity in the weld . The quality of the wire winding, copper coating and
temper are also important factors in minimising wire feed problems.
Quality of wire windings and increasing costs
a) Random wound. b) Layer wound. c) Precision layer wound .

13.4.3 Drive rolls and liner

Check the drive rolls are the correct size for the wire and that the pressure is
hand tight or just sufficient to drive the wire. Excess pressure will deform the
wire to an ovular shape making it very difficult to drive through the liner,
resulting in arcing in the contact tip and excessive wear of the contact tip
and liner.

Check that the liner is the correct type and size for the wire. One size of liner
generally fits two sizes of wire, ie 0.6 and 0.8, 1 and 1.2, 1.4 and 1.6mm
diameter. Steel liners are used for steel wires and Teflon for aluminium
wires.

13.4.4 Contact tip

Check the contact tip is the right size for the wire being driven and the
amount of wear frequently. Any loss of contact between the wire and contact
tip will reduce the efficiency of current pick. Most steel wires are copper
coated to maximise the transfer of current by contact between two copper
surfaces at the contact tip but this also inhibits corrosion. The contact tip
should be replaced regularly.

13.4.5 Connections
The electric arc length in MIG/MAG welding is controlled by the voltage
settings, achieved by using a constant voltage voiVamp characteristic inside
the equipment. Any poor connection in the welding circuit will affect the
nature and stability of the electric arc so is a major inspection point.

13.4.6 Gas and gas flow rate

The type of gas used is extremely important to MIG/MAG welding, as is the
flow rate from the cylinder which must be adequate to give good coverage
over the solidifying and molten metal to avoid oxidation and porosity.

13.4.7 Other variable welding parameters

Checks should be made for correct wire feed speed, voltage, speed of travel
and all other essential variables of the process given on the approved
welding procedure.

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13.4.8 Safety checks

Checks should be made on the current carrying capacity or duty cycle of
equipment and electrical insulation. Correct extraction systems should be
used to avoid exposure to ozone and fumes.

A check should always be. made to ensure that the welder is qualified
to weld the procedure being used.

Typical welding imperfectio.ns

Silica inclusions on ferritic steels only caused by poor inter~run cleaning.
Lack of sidewall fusion during dip transfer welding of thick section
vertically down.
Porosity caused by loss of gas shield ~nd low tolerance to contaminants.
Bum~through from using the incorrect metal transfer mode on sheet
metal.

13.5 Flux-cored arc welding (FCAW)

In the mid-1980s the development of self~and gas-shielded FCAW was a
major step in the successful application of on-site semi-automatic welding
and has enabled a much wider range of materials to be welded.

The cored wire consists of a metal sheath containing a granular flux which
can contain elements normally used in MMA electrodes so the process has
a very wide range of applications.

In addition, gas producing elements and compounds can be added to the

flux so the process can be independent of a separate gas shield, which
restricts the use of conventional MIG/MAG welding in many field
applications.

Most wires are sealed mechanically and hermetically with various forms of
joint. The effectiveness of the were joint is an inspection point of cored wire
welding as moisture can easily be absorbed into a damaged or poor seam.

Wire types commonly used are:

Rutile which give good positional capabilities.

Basic also positional but good on dirty material.
Metal-cored higher productivity, some having excellent root run
capabilities.
Self-shielded no external gas needed.

Baking of cored wires is ineffective and will not restore the condition of a
contaminated flux within a wire.

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Note: Unlike MMA electrodes the potential hydrogen levels and mechanical
properties of welds with rutile wires can equal those of the basic types.

13.6, Summary of solid wire MIG/MAG

Equipment requirements
Transformer/rectifier (constant voltage type) .
Power and power return cable.
Inert, active or mixed shielding gas (argon or C02).
Gas hose, flow meter and gas regulator.
MIG torch with hose, liner, diffuser, contact tip and nozzle.
Wire feed unit with correct drive rolls.
Electrode wire to correct specification and diameter.
Correct visor/glass, safety clothing and good extraction.

Parameters and inspection points

Wire feed speed/amperage.
Open circuit and welding voltage.
Wire type and diameter.
Gas type and flow rate.
Contact tip size and condition .
Roller type, size and pressure.
Liner size.
Inductance settings.
Insulation/extraction.
Connections (voltage drops).
Travel speed, direction and angles.

Typical welding imperfections

Silica inclusions.
Lack of fusion (dip transfer).
Surface porosity.

Advantages and disadvantages

Advantages Disadvantages
High productivity Lack of fusion {dip transfer)
Easily automated Small range of consumables
All positional {dip, pulse and FCAW) Protection for site working
Material thickness range Complex equipment
Continuous electrode High ozone levels

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Submerged Arc Welding

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14 Submerged Arc Welding

14.1 Process
In submerged arc welding (SAW) an arc is ~truck between a continuous
bare wire and the parent plate. The arc, electrode end molten pool are
submerged in an agglomerated or fused powdered flux, which turns into a
gas and slag in its lower layers when subjected to the heat of the arc thus
protecting the weld from contamination. The wire electrode is fed
continuously by a feed unit of motor driven rollers which are usually voltage-
controlled to ensure an arc of constant length. A hopper fixed to the welding
head has a tube which spreads the flux in a continuous elongated mound in
front of the arc along the line of the intended weld and of sufficient depth to
submerge the arc completely so there is no spatter. The weld is shielded
from the atmosphere and there are no ultraviolet or infrared radiation effects
(see below). Unmelted flux is reclaimed for use. The use of powdered flux
restricts the process to the flat and horizontal-vertical welding positions.

Contact tube

Parent Material
Slag

Submerged arc welding is able to use where high weld currents (owing to
the properties and functions of the flux) which give deep penetration and
high deposition rates. Generally DC+ve is used up to about 1OOOA because
it produces deep penetration. On some applications (ie cladding operations)
DC-ve is needed to reduce penetration and dilution. At higher currents or
with multiple electrode systems, AC is often preferred to avoid arc blow
(when used with multiple electrode systems, DC+ve is used for the lead arc
and AC for the trail arc).

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+
(A. C.)

Difficulties sometimes arise in ensuring conformity of the weld with a pre-

determined line owing to the obscuring effect of the flux. Where possible, a
guide wheel to run in the joint preparation is positioned in front of the
welding head and flux hoppers.

Submerged arc welding is widely used in the fabrication of ships, pressure

vessels, linepipe, railway carriages and where long welds are required. It
can be used to weld thicknesses from Smm upwards.

Materials joined
Welding of carbon steels.
Welding low alloy steels (eg fine grained and creep resisting).
Welding stainless steels.
Welding nickel alloys.
Cladding to base metals to improve wear and corrosion resistance.

14.2 Fluxes
Flux is granular mineral compounds mixed to various formulations.

Welding characteristics: More stable arc, improved weld

appearance, easier slag removal, higher welding speeds.

Weld metal mechanical properties (YS, UTS and CEN)

amount of Mn and Si

I Neutral II Basic Highly basic

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Fused fluxes are produced by the constituents being dry mixed, melted in an
electric furnace then granulated by pouring the molten mixture into water or
on to an ice block. Subsequently these particles are crushed and screened
to yield a uniform glass-like product.

Advantages of fused fluxes

Good chemical homogeneity.
Less hygroscopic so handling and storage are easier.
Fines (fine powders) can be removed without changes in composition.
Easily recycled through the system without significant change in particle
size or composition.

Disadvantages of fused fluxes

Limitations in composition as some components, such as basic
carbonates unable to withstand the melting process.
Difficult to add deoxidisers and ferro-alloys (due to segregation or
extremely high loss).

In agglomerated fluxes constituents may be bonded by m1xrng the dry

constituents with potassium or sodium silicate and the wet mixture is then
pelletised, dried, crushed and screened to size.

Advantages of agglomerated fluxes

Deoxidisers and alloying elements can easily be added to the flux to
adjust the weld metal composition .
Allow a thicker flux layer when welding.
Can be identified by colour and shape.

Disadvantages of agglomerated fluxes

Generally more hygroscopic (baking hardly practical).
Gas may evolve from the slag as it is melted, leading to porosity.
May be changes in weld metal chemical composition from the
segregation of fine particles produced by the mechanical handling of the
granulated flux.

14.3 Process variables

Several variables when changed can have an effect on the weld
appearance and mechanical properties:

Welding current.
Type of flux and particle distribution.
Arc voltage.
Travel speed.
Electrode size.
Electrode extension.
Type of electrode.
Width and depth of the layer of flux.

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Electrode angle (leading, trailing}.

Polarity.
Single, double or multi-wire system.

14.3.1 Welding current

Increasing current increases penetration and wire melt-off rate.

350A SODA 650A

Welding current effect on weld profile (2.4mm electrode diameter. 35V arc voltage
and 61 em/min travel speed).

Excessively high current produces a deep penetrating arc with a

tendency to burn-through, undercut or a high, narrow bead prone to
solidification cracking.
Excessively low current produces an unstable arc, lack of penetration
and possibly a lack of fusion.

14.3.2 Arc voltage

Arc voltage adjustment varies the length of the arc between the electrode
and the molten weld metal. As it increases, the arc length increases and
vice versa. Voltage principally determines the shape of the weld bead cross-
section and its external appearance.

25V 35V 45V

Arc voltage effect on weld profile 2.4mm electrode diameter, SOOA welding
current and 61cm/min travel speed.

Increasing the arc voltage with constant current and travel speed will:

Produce a flatter, wider bead.

Increase flux consumption .
Tend to reduce porosity caused by rust or scale on steel.
Help to bridge excessive root opening when fit-up is poor.
Increase pick-up of alloying elements from the flux if present.

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Excessively high arc voltage will:

Produce a wide bead shape subject to solidification cracking.

Make slag removal difficult in groove welds.
Produce a concave-shaped fillet weld that may be subject to cracking.
Increase undercut along the edge(s) of fillet welds.
Over-alloy the weld metal via the flux.

Reducing the arc voltage with constant current and travel speed will produce
a stiffer arc which improves penetration in a deep weld groove and resists
arc blow.

Excessively low arc voltage will:

Produce a high, narrow bead.

Cause difficult slag removal along the weld toes.

14.3.3 Travel speed

If travel speed is increased:

Heat input per unit length of weld decreases.

Less filler metal is applied per unit length of weld therefore Jess excess
weld metal.
Penetration decreases so the weld bead becomes smaller.

305mm/min 610mm/min 1220mm/min

Travel speed effect on weld profile (2.4mm electrode diameter. 500A welding
current and 35V arc voltage).

14.3.4 Electrode size affects

Weld bead shape and depth of penetration at a given current

A high current density results in a stiff arc that penetrates into the base
metal. Conversely, a lower current density in the same size electrode
results in a soft arc that is less penetrating.

Deposition rate
At any given amperage setting, a small diameter electrode will have a
higher current density and deposition rate of molten metal than a larger
diameter electrode. However, a larger diameter electrode can carry more
current than a smaller one, so can ultimately produce a higher deposition
rate at higher amperage.

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3.2mm 4mm 5mm

Electrode size effect on weld profile (600A welding current, 30V arc voltage and
76cmlmin travel speed).

14.3.5 Electrode extension

The electrode extension is the distance the continuous electrode protrudes
beyond the contact tip. At high current densities resistance heating of the
electrode between the contact tip and the arc can be to increase the
electrode melting rate (as much as 25-50%). The longer the extension, the
greater the amount of heating and the higher the melting rate but
drecreases penetration and weld bead width (see below).

30mm 45mm 60mm 80mm

14.3.6 Type of electrode

An electrode with low electrical conductivity, such as stainless steel, can
with a normal electrode extension, experience greater resistance heating.
Thus for the same size electrode and current, the melting rate of a stainless
steel electrode will be higher than that of a carbon steel electrode.

14.3.7 Width and depth of flux

The width and depth of the layer of granular flux influence the appearance
and soundness of the finished weld as well as the welding action. If the
granular layer is too deep, the arc is too confined and a rough weld with a
rope-like appearance is likely to result and it may produce local flat areas on
the surface often referred to as gas flats. The gases generated during
welding cannot readily escape and the surface of the molten weld metal Is
irregularly distorted. If the granular layer is too shallow the arc will not be
entirely submerged in flux, flashing and spattering will occur and the weld
will have a poor appearance and may show porosity.

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14.4 Storage and care of consumables

Care must be taken with fluxes supplied for SAW which, although they may
be dry when packaged, may be exposed to high humidity during storage. In
such cases they should be dried as per the manufacturer's
recommendations before use or porosity or cracking may result.

Ferrous wire coils supplied as continuous feeding electrodes are usually

copper-coated which provides some corrosion resistance, ensures good
electrical contacts and helps in smooth feeding. Rust and mechanical
damage should be avoided in such products as they interrupt smooth
feeding of the electrode. Rust is detrimental to weld quality generally since it
is hygroscopic (may contain or absorb moisture) so can lead to hydrogen
induced cracking .

Contamination by carbon-containing materials such as oil, grease, paint and

drawing lubricants, is especially harmful with ferrous metals. Carbon pick~up
in the weld metal can cause a marked and usually undesirable change in
properties. Such contaminants may also result in hydrogen being absorbed
in the weld pool.

Welders should always follow the manufactureris recommendations for

consumables storage and handling.

14.5 Power sources

In arc welding it is principally the current which determines the amount of
heat generated and this controls the melting of the electrode and parent
metal and also such factors as penetration and bead shape and size.
Voltage and arc length are also important factors with increasing voltage
leading to increasing arc length and vice/versa. Usually in SAW a constant
voltage or flat characteristic power source is used.

Power can be supplied from a welding generator with a flat characteristic or

a transformer/rectifier arranged to give output voltages of approximately 14~
SOV and current according to the output of the unit, can be in excess of
1000A

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 Section 15

Thermal Cutting Processes

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Thermal Cutting Processes
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15 Thermal Cutting Processes

15.1 Oxy-fuel cutting
Oxy-fuel cutting cuts or removes metal by the chemical reaction of oxygen
with the metal at elevated temperatures. Temperature is provided by a gas
flame which preheats and brings the material up to the burning temperature
(approximately 850C}. Once this is achieved, a stream of oxygen is
released which rapidly oxidises most of the metal and performs the actual
cutting operation. Metal oxides, together with molten metal, are expelled
from the cut by the kinetic energy of the oxygen stream. Moving the torch
across the workpiece produces a continuous cutting action.
Oxygen

Heating flame

" - Slagjet

Oxy-fuel cutting.

A material must simultaneously fulfil two conditions to be cut by the oxy-fuel

cutting process:

Burning temperature must be below the parent material melting point.

Melting temperature of the oxides formed during the cutting process
must be below the parent material melting point.

These conditions are fulfilled by carbon steels and some low alloy steels.
However, the oxides of many of the alloying elements in steels, such as
aluminium and chromium have melting points higher than those of iron
oxides. These high melting point oxides (which are refractory in nature!)
may shield the material in the kerf so that fresh iron is not continuously
exposed to the cutting oxygen stream, leading to a decrease of the cutting
speed and ultimately an unstable process. In practice the process is

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effectively limited to low alloy steels containing <0.25%C, <5%Cr, <5%Mo,

<5%Mn and <9%Ni.

Advantages
Steels can generally be cut faster than by most machining methods.
Section shapes and thicknesses difficult to produce by mechanical
means can be cut economically.
Basic equipment costs are low compared with machine tools.
Manual equipment is very portable so can be used on site.
Cutting direction can be changed rapidly on a small radius.
Large prates can be cut rapidly in place by moving the torch rather than
the plate.
Economical method of plate edge preparation.

Disadvantages
Dimensional tolerances significantly poorer than machine tool
capabflities.
Process essentially limited to cutting carbon and low alloy steels.
Preheat flame and expelled red hot slag present fire and bum hazards to
plant and personnel.
Fuel combustion and oxidation of the metal require proper fume control
and adequate ventilation.
Hardenable steels may require pre and/or post-heat adjacent to the cut
edges to control their metallurgical structures and mechanical properties.
Special process modifications are needed for cutting high alloy steels
and cast irons (ie iron powder or flux addition).
Being a thermal process, expansion and shrinkage of the components
during and after cutting must be taken into account.

15.1.1 Requirements for gases

Oxygen for cutting operations should be 99.5% or higher purity: Lower will
result in a decrease in cutting speed and an increase in consumption of
cutting oxygen thus reducing the efficiency of the operation. With purity
below 95% cutting becomes a melt-and-wash action that is usually
unacceptable.

The preheating flame has the following functions in the cutting operation:

Raises the temperature of the steel to the ignition point.

Adds heat energy to the work to maintain the cutting reaction.
Provides a protective shield between the cutting oxygen stream and the
atmosphere.
Dislodges from the upper surface of the steel any rust, scale, paint or
other foreign substance that would stop or retard the normal forward
progress of the cutting action.

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Factors to be considered when selecting a fuel gas include:

Preheating time.
Effect on cutting speed and productivity.
Cost and availability.
Volume of oxygen required per volume of fuel gas to obtain a neutral
flame.
Safety in transporting and handling.

Some of the more common fuel gases used are acetylene, natural gas
(methane), propane, propylene and methylacetylene propadiene (MAPP)
gas.

Fuel gas characteristics and applications

Fuel gas Main characteristics Applications

Highly focused, high temperature Cutting of thin plates
flame Bevel cuts
Acetylene
Rapid preheating and piercing Short, multi-pierce cuts
Low oxygen requirement
Low temperature flame, high heat Cutting of thicker sections (1 00-
content 300mm), long cuts
Propane
Slow preheating and piercing
High oxygen requirement
MAPP Medium temperature flame Cutting underwater
Propylene Medium temperature flame Cutting of thicker sections
Methane Low temperature flame Cutting of thicker sections

15.1 .2 Oxy-fuel gas cutting quality

Generally, oxy-fuel cuts are characterised by:

Large kerf (>2mm)'.

Low roughness values (Ra <SOJ,Jm).
Poor edge squareness (>0.7mm).
Wide HAZ (>1mm).

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The face of a satisfactory cut has a sharp top edge, drag lines, which are
fine and even, little oxide and a sharp bottom edge. Underside is free of
slag.

A satisfactory cut is shown in the centre. If the cut is too slow (left) the top
edge is melted, there are deep grooves in the lower portion of the face,
scaling is heavy and the bottom edge may be rough, with adherent dross. If
the cut is too fast (right) the appearance is similar, with an irregular cut
edge. Plate thickness 12mm.

With a very fast travel speed the drag lines are coarse and at an angle to
the surface with an excessive amount of slag sticking to the bottom edge of
the plate, due to the oxygen jet trailing with insufficient oxygen reaching the
bottom of the cut.

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A satisfactory cut is shown in the centre. If the preheating flame is too low
(left) the most noticeable effect on the cut edge is deep gouges in the lower
part of the cut face. If the preheating flame is too high (right) the top edge is
melted, the cut irregular and there is an excess of adherent dross. Plate
thickness 12mm.

A satisfactory cut is shown in the centre. If the blowpipe nozzle is too high
above the work (left) excessive melting of the top edge occurs with much
oxide. If the torch travel speed is irregular (right) uneven spacing of the drag
lines can be observed together with an irregular bottom surface and
adherent oxide. Plate thickness 12mm.

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15.2 Plasma arc cutting

Plasma arc cutting uses a constricted arc which removes the molten metal
with a high velocity jet of ionised gas issuing from the constricting orifice. A
pilot arc is struck betwsen a tungsten electrode .and a water-cooled nozzlel
the arc is then transferred to the workpiece, thus being constricted by the
orifice downstream of the electrode. As plasma gas passes through this arc,
it is heated rapidly to a high temperature, expands and is accelerated as it
passes through the constricting orifice towards the workpiece. The orifice
directs the super heated plasma stream from the electrode toward the
workpiece. When the arc melts the workpiece, the high velocity jet blows
away the molten metal. The cutting arc attaches or transfers to the
workpiece, known as the transferred arc method. Where materials are non-
electrical conductors there is a method known as non-transferred arc where
the positive and negative poles are inside the torch body creating the arc
and the plasma jet stream travels toward the workpiece.

Advantages
Not limited to materials which are electrical conductors so is widely used
for cutting all types of stainless steels, non-ferrous materials and non-
electrical conductive materials.
Operates at a much higher energy level compared with oxy-fuel cutting
resulting in faster cutting speeds.
Instant start-up is particularly advantageous for interrupted cutting as it
allows cutting without preheat.

Disadvantages
Dimensional tolerances significantly poorer than machine tool
capabilities.
Introduces hazards such as fire, electric shock (due to the high OCV),
intense light, fumes, gases and noise levels that may not be present with
other processes. However, in underwater cutting the level of fumes, UV
radiation and noise are reduced to a low level.
Compared with oxy-fuel cutting , plasma arc cutting equipment tends to
be more expensive and requires a fairly large amount of electric power.
Being a thermal process, expansion and shrinkage of the components
during and after cutting must be taken into consideration.
Cut edges slightly tapered .

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a) b)

Plasma arc cutting,

a) Transferred arc.
b) Non-transferred.

15.3 Arc air gouging

During arc air gouging the metal to be gouged or cut is melted with an
electric arc and blown away by a high velocity jet of compressed air. A
special torch directs the compressed air stream along the electrode and
underneath it. The torch is connected to an arc welding machine and
compressed air line, which delivers approximately 690MPa (1 OOpsi) of
compressed air and since pressure is not critical, a regulator is not
necessary. The electrode is made of graphite and copper-coated to increase
the current pick-up and operating life. This process is usually used for
gouging and bevelling, being able to produce U and J preparations and can
be applied to both ferrous and non-ferrous materials.

electrode

~air
always under
the electrode

workpiece(-)

Arc air gouging.

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Advantages
Approximately five times faster than chipping.
Easily controllable, removes defects with precision as they are clearly
visible and may be followed with ease. The depth of cut is easily
regulated and slag does not deflect or hamper the cutting action.
Low equipment cost no gas cylinders or regulators necessary except on-
site.
Economical to operate as no oxygen or fuel gas required. The welder
may also do the gouging (no qualification requirements for this
operation).
Easy to operate as the equipment is similar to MMA except the torch and
air supply hose.
Compact with the torch not much larger than an MMA electrode holder,
allowing work in confined areas.
Versatile.
Can be automated.

Disadvantages
Other cutting processes usually produce a better and quicker cut.
Requires a large volume of compressed air.
Increases the carbon content leading to an increase in hardness irn of
cast iron and hardenable metals. In stainless steels it can lead to carbide
precipitation and sensitisation so grinding the carbide layer usually
follows arc air gouging.
Introduces hazards such as fire (due to discharge of sparks and molten
metal), fumes, noise and intense light.

15.4 Manual metal arc gouging

The arc is formed between the tip of the electrode and are required
workpiece and special purpose electrodes with thick flux coatings to
generate a strong arc force and gas stream. Unlike MMA welding where a
stable weld pool must be maintained, this process forces the molten metal
away from the arc zone to leave a clean cut surface.

The gouging process is characterised by the large amount of gas generated

to eject the molten metal. However, because the arc/gas stream is not as
powerful as a gas or a separate air jet, the surface of the gouge is not as
smooth as an oxy-fuel or air carbon arc gouge.

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Manual metal arc gouging.

Although DC-ve is preferred, an AC constant current power source can also

be used.

MMA gouging is used for localised gouging operations, removal of defects

for example and where it is more convenient to switch from a welding to a
gouging electrode rather than use specialised equipment. Compared with
alternative gouging processes, metal removal rates are low and the quality
of the gouged surface is inferior.

When correctly applied, MMA gouging can produce relatively clean gouged
surfaces. For general applications, welding can be carried out without the
need to dress by grinding, however when gouging stainless steel, a thin
layer of higher carbon content material will be produced which should be
removed by grinding.

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 Section 16

Welding Consumables
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Welding Consumables
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16 Welding Consumables
Welding consumables are defined as all that is used up during the
production of a weld.

This list could include all things used up in the production of a weld;
however, we normally refer to welding consumables as those items used up
by a particular welding process.

These are:

Electrodes Wires Fluxes Gases

When inspecting welding consumables arriving at site it is important to

check for the following:

Size.
Type or specification.
Condition.
Storage.

The checking of suitable storage conditions for all consumables is a

critical part of the welding inspector's duties.

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Welding Consumables
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16.1 Consumables for MMA welding

Welding consumables for MMA consist of a core wire typically 350-450mm
length and 2.5-Smm diameter but other lengths and diameters are available.
The core wire is generally of low quality rimming steel as the weld can be
considered a casting so can be refined by the addition of cleaning or refining
agents in the extruded flux coating. This coating contains many elements
and compounds which have a variety of jobs during welding. Silicon is
mainly added as a de-oxidising agent (in the form of ferro-silicate), which
removes oxygen from the weld metal by forming the oxide silica.
Manganese additions of up to 1.6% will improve the strength and toughness
of steel. Other metallic and non-metallic compounds are added that have
many functions, including:

Aid arc ignition.

Improve arc stabilisation.
Produce a shielding gas to protect the arc column.
Refine and clean the solidifying weld metal.
Form a slag which protects the solidifying weld metal.
Add alloying elements.
Control hydrogen content of the weld metal.
Form a cone at the end of the electrode which directs the arc.

Electrodes for MMNSMAW are grouped by the main constituent in their flux
coating, which in turn has a major effect on the weld properties and ease of
use. The common groups are:

Group Constituent Shield gas Uses AWSA5.1

Rutile Titania Mainly C02 General purpose E6013
Basic Calcium Mainly C02 High quality E 7018
compounds
Cellulosic Cellulose Hydrogen + C02 Pipe root runs E6010

Some basic electrodes may be tipped with a carbon compound which eases
arc ignition.

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E 46 3 lNi B 5 4 H_

Symbol Tensile Strength Yield S~ength Elongation Symbol Welding posiUon

N/~ min. Nimfil2 mi n.~
1 All positions
35 44()..570 355 22 2 All poslllons. extept ver11cal down
38 470-600 980 20 3 Flat bull weld, flat fillet weld, horizontal-vertical fillet weld
42 ~0 420 20 4 Flat bull welct, flat fillet weld
46 530..80 460 20 5 Vet11cal down and positions according to symbol 3
50 56()..720 500 18

Symbol Metal recovery.~ Type ol current

1 <105 AC+DC
2 <105 DC
3 > 105 "125 AC+DC
4 >105125 DC
5 > 125"' 160 AC+ DC
6 > 125 s 160 DC
7 >160 AC+DC
8 >160 DC

I
E 46 3 1Ni B 5 4 H5
I
Covered electrode
I
Symbol Coaling type Symbol
IHydrogen contenl.
for manual metal arc ml/100 g deposhed
welding. A Acid
8 Basic weld metal. ma.
c Cellulosic HS 5
R Rutile H10 10
RR Rutile (thick coated) H1S 15
RC Rutile-Cellulosic
RA Rutile-Acid
RB Rutile-Basic

Symbol Chemical composllion ol all-weld metal %

Mn Mo Nl
Symbol lt11Jt.ICI Energy
Charpy-V
No symbol
Mo
2.0
14
-
0.3 - 0.6
--
Temp c lor 47J min. MnMo > 1.4 - 2.0 0.3-0.6 -
z tNi 1.4 - 06 1.2

--
No requlremants
A +20 2NI 1.4 1,8-2.6
0 0 3Ni 14 >2.6-3.8
2 -20 Mn1NI
1NIMo
> 1.4- 2.0
1.4
-
0.3 - 0.6
0.6 - 1.2
0.612
3 -30
4 -40 z Any other agreed composllron
5 -50 II nol specified Mo < 02. Nl < 03. Cr < 0.2. V <005, Nb <0.05. Cu -: 0 3
6 -60 Single values shown In 111e table mean maximum values.

The electrode classification system of EN ISO 2560

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EN ISO 2560 (supersedes BS EN 499 1994)

Classification of Welding Consumables for Covered Electrodes for

Manual Metal Arc (111) Welding of Non-alloy and Fine Grain Steels.

This standard applies a dual approach to classification of electrodes using

methods A and B as indicated below:

Classification of electrode mechanical properties of an all weld metal

specimen:

Method A: Yield strength and average impact energy at 47J

Example ISO 2560 -A- E XX X XXX X ,.-
X rX -r-
HX
1 I
I
I
I
Mandatory I
I
I
designation:
I

Classified for impacts

at 47J + yield strength

Covered electrode

Minimum
yield strength
Charpy V notch
minimum test
temperature oc

Chemical composition

Electrode covering

Optional designation:

Weld metal recovery

and current type

Positional designation
--- ----- -------------------

II
I
Diffusible hydrogen I
I
I
ml/1 OOg weld metal I
----------------------------------1
Typical example: I SO 2560 -A - E 43 2 1Ni RR 6 3 H 15

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Method 8: Tensile strength and average impact energy at 27J

Example ISO 2560- 8- E XX XX XXX X ,....

X HX
-~-
I 1
I I
I I
Mandatory I
I
I
I
I
designation: I
I
I
I
I
I I
I I
I I
Classified for impacts I I
I
at 27J + tensile strength I
I
I
I
I
I
Covered electrode I
I
I
I
I
M inimum I
I
tensile strength I
t
I
I
I
I
Electrode covering I
I
I
I
I
I
I
I
Chemical composition I
I
I
I
I
Heat treatment I
I
condition I
I
I
I
I
I
Optional designation: I
I

I

Optional supplemental
I

I
impact test at 47Jat I
same test temperature
I

given for 27J test

I

I
Diffusible hydrogen
I
ml/1 OOg weld metal
I

Typical example: ISO 2560 - 8 - E 55 16 -N7 A U HS

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Classification of tensile characteristics

Method A
Minimum yield Tensile strength, Minimum E% b,
Symbol 2
N/mm2
N/mm N/mm2
35 355 440-570 22
38 380 470-600 20
42 420 500-640 20
46 460 530-680 20
50 500 560-720 18
Lower yield Rei shall be used. b Gauge length =5 x 0

Method 8
Symbol Minimum tensile strength, N/mm2
43 430
49 490
55 550
57 570

Other tensile characteristics ie yield strength and elongation % are

contained within a tabular form in this standard (Table 88) and are
determined by classification of tensile strength, electrode covering and
alloying elements, ie E 5516-N7.

Classification of impact properties

Method A
Symbol Temperature for the minimum
average impact energy of 47J
z No requirement
A +20
0 0
2 -20
3 -30
4 -40
5 -50
6 -60

Method 8
Impact or Charpy V notch testing temperature at 27J temperature in
method 8 is determined through the classification of tensile strength,
electrode covering and alloying elements (Table 88) ie E 55 16-N7 which
must reach 27J at -75C .

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Classification of electrode characteristics and electrical requirements varies

between classification methods A and B as follows:

Method A
Uses an alpha/numerical designation from the tables as listed below:

Symbol Electrode covering type Symbol Efficiency, % Type of current

A Acid 1 < 105 AC or DC
c Cellulosic 2 <105 DC
R Rutile 3 >105-<125 AC or DC
RR Rutile thick covering 4 >105-<125 DC
RC Rutile/cellulosic 5 >125-<160 AC or DC
RA Rutile/acid 6 >125-<160 DC
RB Rutile/basic 7 >160 AC or DC
B Basic 8 >160 DC

Method B
This method uses a numerical designation from the table as listed
below

Symbol Covering type Positions Type of current

03 Rutile/basic All AC and DC+/-
10 Cellulosic All DC+
11 Cellulosic All AC and DC+
12 Rutile All AC and DC-
13 Rutile All AC and DC+/-
14 Rutile + Fe powder All AC and DC+/-
15 Basic All DC+
16 Basic All AC and DC+
18 Basic+ Fe powder All ACand DC+
19 Rutile + Fe oxide (Ilmenite) All AC and DC+/-
20 Fe oxide PA/PB AC and DC-
24 Rutile+ Fe powder PA/PB AC and DC+/-
27 Fe oxide+ Fe powder PA/PB only AC and DC-
28 Basic+ Fe powder PA/PB/PC AC and DC+
40 Not specified As per manufacturer's
recommendations
48 Basic All AC and DC+
All positions may or may not include vertical-down welding
Further guidance on flux type and applications is given in the standard in
Annex 8 and C.

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Hydrogen scales
Diffusible hydrogen is indicated in the same way in both methods, where
after baking the amount of hydrogen is given as ml/1 OOg weld metal ie H 5
= 5ml/100g weld metal.

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16.2 AWS A 5.1- and AWS 5.5-

A typical AWS A5.1 and A5.5 Speciftcation E 80 1 8 G
Reference given in box letter: A 8 C D (A.5 only)
A Tensile+ yield strength and E% B Welding position
Code Min yield Min tensile MinE% 1 All positional
PSix 1000 PSI x 1000 In 2" min 2 Flat butt and HN fillet welds
General 3 Flat only
E60xx 48,000 60,000 17-22 Note: Not all Category 1 electrodes
can weld in the vertical-down
E 70xx 57,000 70,000 17-22
position.
E BOxx 68-80,000 80,000 19-22
E 100xx 87,000 100,000 13-16
V notch impact Radiographic
Specific electrode information for E BOxx and 70xx lzod test (ft.lbs) standard
E6010 48,000 60,000 22 20 ft.rbs at-20F Grade 2
E 6011 48,000 60,000 22 20 ft.lbs at-20"F Grade 2
E6012 48,000 60,000 17 Not required Not required
E6013 48,000 60,000 17 Not required Grade 2
E6020 48,000 60,000 22 Not required Grade 1
E6022 Not required 60,000 Not required Not required Not required
E6027 48,000 60,000 22 20 ft.lbs at-20"F Grade 2
E 7014 58,000 70,000 17 Not required Grade 2
E 7015 58,000 70,000 22 20 ft.lbs at-20F Grade 1
E7016 58,000 70,000 22 20 ft.lbs at-20F Grade 1
E 7018 58,000 70,000 22 20 ft.lbs at-20F Grade 1
E 7024 58,000 70,000 17 Not required Grade 2
E 7028 58,000 70,000 20 20 ft.lbs at oaF Grade 2

C Electrode coating and electri cal characteristic D AWS A5.51ow alloy steels
Code Coating Current type Symbol Approximate alloy deposit
A1 0.5%Mo
Exx10 Cellulosic/organic DC+ only
81 0.5%Cr + 0.5%Mo
Exx11 Cellulosic/organic AC or DC+ 82 1.25%Cr + 0.5%Mo
Exx12 Rutile AC or DC- 83 2.25%Cr + 1.0%Mo
Exx13 Rutile+ 30% Fe powder AC or DC+/- 84 2.0%Cr+ 0.5%Mo
Exx14 Rutile AC or DC+/- 85 0.5%Cr + 1.0%Mo
Exx15 Basic DC+ only C1 2.5%Ni
Exx16 Basic AC or DC+ C2 3.25%Ni
Exx18 Basic + 25% Fe powder AC or DC+ 1%Ni + 0.35%Mo +
Exx20 High Fe oxide content AC or DC+/- C3
0.15%Cr
Exx24 Rutile+ 50% Fe powder AC or DC+/- 0112 0.25-0.45%Mo + 0.15%Cr
Exx27 Mineral + 50% Fe powder AC or DC+/- 0.5%Ni or/and 0.3%Cr
Exx28 Basic+ 50% Fe powder AC or DC+ G or/and 0.2%Mo or/and
0.1%V
For G only 1 element is reauired

16-9
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16.3 Inspection points for MMA consumables

Size Wire diameter and length
I
1-4--
-
..... - -=
:~

h )
t
Condition Cracks, chips and concentricity

All electrodes showing signs of the effects of corrosion should be

discarded.

Type (specification)

Correct specification/code

Storage Suitably dry and warm (preferably 0% humidity)

Checks should also be made to ensure that basic electrodes have been
through the correct pre-use procedure. Having been baked to the correct
temperature (typically 300-350C) for 1 hour then held in a holding oven
(150C max) basic electrodes are issued to welders in heated quivers.
Most electrode flux coatings deteriorate rapidly when damp so care must be
taken to inspect storage facilities to ensure they are adequately dry and that
all electrodes are stored in controlled humidity.

Vacuum packed electrodes may be used directly from the carton only if the
vacuum has been maintained. Directions for hydrogen control are always
given on the carton and should be strictly adhered to. The cost of each
electrode is insignificant compared with the cost of any repair, so basic
electrodes left in the heated quiver after the day's shift may be rebaked but
would normally be discarded to avoid the risk of H2 induced problems.

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16.4 Consumables for TIG/GTAW

Consumables for TIG/GTAW welding consist of a wire and gas, though
tungsten electrodes may also be grouped in this, Though it is considered a
non-consumable electrode process, the electrode is consumed by erosion in
the arc and by grinding and incorrect welding technique. The wire needs to
be of a very high quality as normally no extra cleaning elements can be
added to the weld. The wire is refined at the original casting stage to a very
high quality where it is then rolled and finally drawn down to the correct size.
It is then copper-coated and cut into 1m lengths and a code stamped on the
wire with a manufacture~s or nationally recognised number for the correct
identification of chemical composition. A grade of wire is selected from a
table of compositions and wires are mostly copper-coated which inhibits the
effects of corrosion. Gases for TIG/GTA are generally inert and pure argon
or helium gases are generally used for TIG welding. The gases are
extracted from the air by liquefaction and as argon is more common in air is
generally cheaper than helium.

In the US helium occurs naturally so it is the gas more often used. It

produces a deeper penetrating arc than argon but is less dense (lighter)
than air and needs 2-3 times the flow rate of argon to produce sufficient
cover to the weld area when welding downhand. Argon is denser (heavier)
than air so less gas needs to be used in the downhand position. Mixtures of
argon and helium are often used to balance the properties of the arc and the
shielding cover ability of the gas. Gases for TIG/GTAW need to be of the
highest purity (99.99%) so careful attention and inspection must be given to
the purging and condition of gas hoses as contamination of the shielding
gas can occur due to a worn or withered hose.

Tungsten electrodes for TIG welding are generally produced by powder

forging technology and contain other oxides to increase their conductivity
and electron emission and also affect the characteristics of the arc. They are
available off-the-shelf 1.6-1 Omm diameter. Ceramic shields may also be
considered a consumable item as they are easily broken, the size and
shape depending mainly on the type of joint design and the diameter of the
tungsten.

A particular consumable item that may be used during TIG welding of

pipes is a fusible insert often referred to as an EB insert after the Electric
Boat Co of USA who developed it. The insert is normally made of material
matching the pipe base metal composition and is fused into the root during
welding as shown below.
Before welding Inserted After welding Fused

..._____"/_____.! I \!J
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16.5 Consumables for MIG/MAG

Consumables for MIG/MAG consist of a wire and gas. The wire
specifications used for TIG are also used for MIG/MAG as a similar level of
quality is required in the wire.

The main purpose of the copper coating of steel MIG/MAG welding wire is to
maximise current pick-up at the contact tip and reduce the level of
coefficient of friction in the liner with protection against the effects of
corrosion being secondary.

Wires are available that have not been copper coated as copper flaking in
the liner can cause many wire feed problems. These wires may be coated in
a graphite compound, which again increases current pick-up and reduces
friction in the liner. Some wires, including many cored wires, are nickel
coated.

Wires are available from 0.6-1.6mm diameter wi th finer wires available on a

1 kg reel, though most are supplied on a 15kg drum.

Common gases and mixtures used for MIG/MAG welding include:

Gas type Process Used for Characteristics

Pure argon MIG Spray or pulse welding Very stable arc with poor
aluminium alloys penetration and low spatter
levels.
Pure C02 MAG Dip transfer welding of Good penetration, unstable arc
steels and high levels of spatter.
Argon+ MAG Dip spray or pulse Good penetration with a stable
5-20% C02 welding of steels arc and low levels of spatter.

Argon+ MAG Spray or pulse welding Active additive gives good fluidity
1-2%02 or of austenitic or ferritic to the molten stainless and
C02 stainless steels only improves toe blend.

16.6 Consumables for SAW

Consumables for SAW consist of an electrode wire and flux. Electrode wires
are normally high quality and for welding C-Mn steels are generally graded
on their increasing carbon and manganese content level of de-oxidation.

Electrode wires for welding other alloy steels are generally graded by
chemical composition in a table in a similar way to MIG and TIG electrode
wires. Fluxes for SAW are graded by their manufacture and composition of
which there are two normal methods, fused and agglomerated.

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Fused fluxes
Mixed together and baked at a very high temperature (>1 ,000C} so all the
components fuse. When cooled the resultant mass resembles a sheet of
coloured glass, which is then pulverised into small particles.

These particles are hard, reflective, irregularly shaped and cannot be

crushed in the hand. It is impossible to incorporate certain alloying
compounds, such as ferro-manganese, as these would be destroyed in the
high temperatures of manufacturing. Fused fluxes tend to be of the acidic
type and are fairly tolerant of poor surface conditions, but produce
comparatively low quality weld metal in terms of tensile strength and
toughness. They are easy to use and produce a good weld contour with an
easily detachable slag.

Agglomerated fluxes
A mixture of compounds baked at a much lower temperature and bonded
together by bonding agents into small particles. They are dull, generally
round granules that are friable (easily crushed) and can also be coloured.
Many agents and compounds may be added during manufacture unlike the
fused fluxes. Agglomerated fluxes tend to be of the basic type and produce
weld metal of an improved quality in terms of strength and toughness, at the
expense of usability they are much less tolerant of poor surface conditions
and generally produce a slag much more difficult to detach and remove.

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The weld metal properties result from using a particular wire with a particular
flux in a particular weld sequence so the grading of SAW consumables is
given as a function of a wire/flux combination and welding sequence.

A typical grade will give values for:

Tensile strength.
Elongation, %.
Toughness; Joules.
Toughness testing temperature.

All consumables for SAW (wires and fluxes) should be stored in a dry,
humid-free atmosphere. The flux manufacturer's handling/storage
instruction and conditions must be very strictly followed to minimise any
moisture pick-up. Any re-use of fluxes is totally dependent on applicable
clauses within the application standard.

On no account should different types of fluxes be mixed.

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17 Weldability of Steels
17.1 Introduction
Weldability simply means the ability to be welded and many types of
weldable steel have been developed for a wide range of applications.

The ease or difficulty of making a weld with suitable properties and free from
defects determines whether steels are considered as having good or poor
weldability. A steel is usually said to have poor weld ability if it is necessary
to take special precautions to avoid a particular type of imperfection.
Another reason for poor weldabllity may be the need to weld within a very
narrow range of parameters to achieve properties required for the joint.

17.2 Factors that affect weldability

A number of inter-related factors determine whether a steel has good or
poor weldabllity:

Actual chemical composition.

Weld joint configuration.
Welding process to be used.
Properties required from the weldment.

For steels with poor weldability it is particularly necessary to ensure that

WPSs give welding conditions that do not cause cracking but achieve
the specified properties.
Welders work strictly in accordance with the specified welding
conditions.
Welding inspectors regularly monitor welders to ensure they are working
strictly in accordance with the WPSs.

Having a good understanding of the characteristics, causes and ways of

avoiding imperfections in steel weldments should enable welding inspectors
to focus attention on the most influential welding parameters when steels
with poor weldability are used.

17.3 Hydrogen cracking

During fabrication by welding, cracks can occur in some types of steel due
to the presence of hydrogen, the technical name is hydrogen induced cold
cracking {HICC) but is often referred to by names that describe various
characteristics of hydrogen cracks:

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Cold cracking Cracks occur when the weld has cooled down.
HAZ cracking Cracks occur mainly in the HAZ.
Delayed cracking Cracks may occur some time after welding has
finished (possibly up to -72h).
Underbead cracking Cracks occur in the HAZ beneath a weld bead.

Although most hydrogen cracks occur in the HAZ, there are circumstances
when they may form in weld metal.

Figure 17.1 Typical locations of hydrogen induced cold cracks.

Figure 17.2 Hydrogen induced cold crack that initiated at the HAZ at the toe of a
fillet weld.

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17.3.1 Factors influencing susceptibility to hydrogen cracking

Hydrogen cracking in the HAZ of a steel occurs when four conditions exist at
the same time:

Hydrogen level > 15ml/1 OOg of weld metal deposited

Stress >0.5 of the yield stress
Temperature <300C
Susceptible microstructure >400HV hardness

Susceptible
microstructure

Tensile Temperature
stress

High hydrogen
concentration

These four factors are mutually interdependent so the influence of one

condition (its active level) depends on how active the other three are.
17.3.2 Cracking mechanism
Hydrogen (H) can enter the molten weld metal when hydrogen-containing
molecules are broken down into H atoms in the welding arc.

Because H atoms are very small they can move about (diffuse) in solid steel
and while weld metal is hot can diffuse to the weld surface and escape into
the atmosphere.

However, at lower temperatures H cannot diffuse as quickly and if the

weldment cools down quickly to ambient temperature H will become
trapped, usually in the HAZ.

If the HAZ has a susceptible microstructure, indicated by being relatively

hard and brittle and there are also relatively high tensile stresses in the
weldments, then H cracking can occur.

The precise mechanism that causes cracks to form is complex but H is

believed to cause embrittlement of regions of the HAZ so that high localised
stresses cause cracking rather than plastic straining.

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17.3.3 Avoiding HAZ hydrogen cracking

Because the factors that cause cracking are interdependent and each must
be at an active level at the same time, cracking can be avoided by ensuring
that at least one of the factors is not active during welding.

Methods to minimise the influence of each of the four factors are considered
in the following sub-sections.

Hydrogen
The main source of hydrogen is moisture (H20) and the principal source is
being welding flux. Some fluxes contain cellulose and this can be a very
active source of hydrogen.

Welding processes that do not require flux can be regarded as low hydrogen
processes.

Other sources of hydrogen are moisture present in rust or scale and oils and
greases (hydrocarbons).

Reducing the influence of hydrogen is possible by:

Ensuring that fluxes (coated electrodes, flux-cored wires and SAW

fluxes) are low in H when welding commences.
Either baking then storing low H electrodes in a hot holding oven or
supplying them in vacuum-sealed packages.
Basic agglomerated SAW fluxes in a heated silo until issue to maintain
their as-supplied, low moisture, condition.
Checking the diffusible hydrogen content of the weld metal (sometimes
specified on the test certificate).
Ensuring that a low H condition is maintained throughout welding by not
allowing fluxes to pick up moisture from the atmosphere.
Issuing low hydrogen electrodes in small quantities and limiting exposure
time; heated quivers do this.
Covering or returning flux covered wire spools that are not seamless to
suitable storage condition when not in use.
Returning basic agglomerated SAW fluxes to the heated silo when
welding is not continuous.
Checking the amount of moisture present in the shielding gas by
checking the dew point (must be below -60C}.
Ensuring the weld zone is dry and free from rust/scale and oil/grease.

Tensile stress
There are always tensile stresses acting on a weld because there are
always residual stresses from welding.

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The magnitude of the tensile stresses is mainly dependent on the thickness

of the steel at the joint, heat input, joint type and the size and weight of the
components being welded.

Tensile stresses in highly restrained joints can be as high as the yield

strength of the steel and this is usually the case in large components with
thick joints and is not a factor that can easily be controlled.

The only practical ways of reducing the influence of residual stresses may
be by:

Avoiding stress concentrations due to poor fit-up.

Avoiding poor weld profile (sharp weld toes).
Applying a stress relief heat treatment after welding.
Increasing the travel speed as practicable to reduce the heat input.
Keeping weld metal volume as low as possible.

These measures are particularly important when welding some low alloy
steels that are particularly sensitive to hydrogen cracking.

Susceptible HAZ microstructure

A susceptible HAZ microstructure is one that contains a relatively high
proportion of hard brittle phases of steel, particularly martensite.

The HAZ hardness is a good indicator of susceptibility and when it exceeds

a certain value that steel is considered susceptible. For C and C-Mn steels
this value is -350HV and susceptibility to H2 cracking increases as hardness
increases above this value.

The maximum hardness of an HAZ is influenced by:

Chemical composition of the steel.

Cooling rate of the HAZ after each weld run .

For C and C-Mn steels a formula has been developed to assess how the
chemical composition will influence the tendency for significant HAZ
hardening -the carbon equivalent value (CEV) formula.

The CEV formula most widely used (and adopted by IIW) is:

_ ,.. C %Mn %Cr + %Mo + %V %Ni + %Cu

CEVmy - 070 + 6 + 5 +--1-5--

The CEV of a steel is calculated by inserting the material test certificate

values shown for chemical composition into the formula. The higher the
CEV the greater its susceptibility to HAZ hardening therefore the greater the
susceptibility to H2 cracking.

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The element with most influence on HAZ hardness is carbon. The faster the
rate of HAZ cooling after each weld run, the greater the tendency for
hardening.

Cooling rate tends to increase as:

Heat input decreases (lower energy input).

Joint thickness increases (bigger heat sink).

Avoiding a susceptible HAZ microstructure (for C and C-Mn steels) requires:

Procuring steel with a CEV at the low end of the range for the steel
grade (limited scope of effectiveness).
Using moderate welding heat input so that the weld does not cool quickly
and give HAZ hardening.
Applying preheat so that the HAZ cools more slowly and does not show
significant HAZ hardening; in multi-run welds maintain a specific
interpass temperature.

The CEV formula is not applicable to low alloy steels, with additions of
elements such as Cr, Mo and V. The HAZ of these steels will always tend to
be relatively hard regardless of heat input and preheat and so this is a factor
that cannot be effectively controlled to reduce the risk of H cracking. This is
why some of the low alloy steels have a greater tendency to show hydrogen
cracking than in weldable C and C-Mn steels which enable HAZ harclness to
be controlled.

Weldment at low temperature

Weldment temperature has a major influence on susceptibility to cracking
mainly by influencing the rate at which H can move (diffuse) through the
weld and HAZ. While a weld is relatively warm (>-300"C) H will diffuse quite
rapidly and escape into the atmosphere rather than be trapped and cause
embrittlement.

The influence of low weldment temperature and risk of trapping H in the

weldment can be reduced by:

Applying a suitable preheat temperature (typically 50 to -250C}.

Preventing the weld from cooling down quickly after each pass by
maintaining the preheat and specific interpass temperatures during
welding.
Maintaining the preheat temperature (or raising it to -250C} when
welding has finished and holding the joint at this temperature for a
minimum of two hours to facilitate the escape of H (post-heat).
Post-heat in accordance with specification, (not to be confused with
PWHn at a temperature ~-600C.

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17.3.4 Hydrogen cracking in weld metal

Hydrogen cracks can form in steel weld metal under certain circumstances.
Tha mechanism of cracking and identification of all the influencing factors is
less clearly understood than for HAZ cracking but can occur when welding
conditions cause H to become trapped in weld metal rather than in HAZ. It is
recognised that welds in higher strength materials, thicker sections and
using large beads are the most common problem areas.

Hydrogen cracks in weld metal usually lie at 45 to the direction of principal

tensile stress in the weld metal, usually the longitudinal axis of the weld
(Figure 17.3).

x!

a)
~ , ,. . " jY

... .......,.... ....... ..

.= ............... -=..
~

Weld layers with

.: 7 7 .:
............ !
cracks 45 to X-Y
axis

~---~-~-~J
b)

Figure 17. 3:
a) Plan view of a plate butt weld showing subsurface transverse cracks;
b) Longitudinal section X-Y of the above weld showing how the transverse cracks
lie at 48' to the surface. They tend to remain within an individual weld run and may
be in weld several/ayers.

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Their appearance in this orientation gives the name chevron cracks (arrow-
shaped cracks) . There are no well defined rules for avoiding weld metal
hydrogen cracks apart from:

Use a low hydrogen welding process.

Apply preheat and maintain a specific interpass temperature.

BS EN 1011-2 Welding - Recommendations for welding of metallic materials

- Part 2: Arc welding of ferritic steels gives in Annex C practical guidelines
about how to avoid H cracking. These are based principally on the
application of preheat and control of potential H associated with the welding
process.

17.4 Solidification cracking

The technical name for cracks that form during weld metal solidification is
solidification cracks but other names are sometimes used:

Hot cracking : Occur at high temperatures while the weld is hot.

Centreline cracking: Down the centreline of the weld bead.
Crater cracking : Small cracks in weld craters are solidification cracks.

A weld metal particularly susceptible to solidification cracking may be said to

show hot shortness because it is short of ductility when hot so tends to
crack.

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Figure 17.4 shows a transverse section of a weld with a typical centreline

solidification crack.

I
t~ )
.
*

~-
~--;
. "'

b)

Figure 17.4:
a) Solidification crack at the weld centre where columnar dendrites have trapped
some lower melting point liquid;
b) The weld bead does not have an ideal shape but has solidified without the
dendrites meeting end-on and trapping lower melting point liquid thereby resisting
solidification cracking.

17.4.1 Factors influencing susceptibility to solidification cracking

Solidification cracking occurs when three conditions exist at the same time:

Weld metal has a susceptible chemical composition.

Welding conditions used give an unfavourable bead shape.
High level of restraint or tensile stresses present in the weld area.

17.4.2 Cracking mechanism

All weld metals solidify over a temperature range and since solidification
starts at the fusion line towards the centreline of the weld pool, during the
last stages of weld bead solidification there may be enough liquid to form a
weak zone in the centre of the bead. This liquid film is the result of low
melting point constituents being pushed ahead of the solidification front.

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During solidification, tensile stresses start to build up due to contraction of

the solid parts of the weld bead and these stresses can cause the weld
bead to rupture. These circumstances result in a weld bead showing a
centreline crack as soon as the bead has been deposited.

Centreline solidification cracks tend to be surface-breaking at some point in

their length and can be easily seen during visual inspection because they
tend to be relatively wide.

17.4.3 Avoiding solidification cracking

Avoiding solidification cracking requires the influence of one of the factors to
be reduced to an inactive level.

Weld metal composition

Most C and C-Mn steel weld metals made by modem steelmaking methods
do not have chemical compositions particularly sensitive to solidification
cracking . However, they can become sensitive to this type of cracking if they
are contaminated with elements or compounds that produce relatively low
melting point films in weld metal.

Sulphur and copper can make steel weld metal sensitive to solidification
cracking if present in the weld at relatively high levels. Sulphur
contamination may lead to the formation of iron sulphides that remain liquid
when the bead has cooled down as low as -980C, whereas bead
solidification started above 1400C.

The source of sulphur may be contamination by oil or grease or it could be

picked up from the less refined parent steel being welded by dilution into the
weld.

Copper contamination in weld metal can be similarly harmful because it has

low solubility in steel and can form films that are still molten at -1100C.

Avoiding solidification cracking (of an otherwise non-sensitive weld metal)

requires the avoidance of contamination with potentially harmful materials
by ensuring:

Weld joints are thoroughly cleaned immediately before welding.

Any copper containing welding accessories are suitable/in suitable
condition, such as backing-bars and contact tips used for GMA, FCA and
SAW.

Unfavourable welding c.onditions

Encourage weld beads to solidify so that low melting point films become
trapped at the centre of a solidifying weld bead and become the weak zones
for easy crack formation.

Figure 17.5 shows a weld bead that has solidified under unfavourable
welding conditions associated with centreline solidification cracking.

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i W/01 :2

r
Figure 17.5 Weld bead with an unfavourable width-to-depth ratio. This is
responsible for liquid metal being pushed into the centre of the bead by the
advancing columnar dendrites and becoming the weak zone that ruptures.

The weld bead has a cross-section that is quite deep and narrow - a width-
to-depth ratio greater than 1:2 and the solidifying dendrites have pushed the
lower melting point liquid to the centre of the bead where it has become
trapped. Since the surrounding material is shrinking as a result of cooling,
this film would be subjected to tensile stress, whtch leads to cracking.

In contrast, Figure 17.6 shows a bead with a width-to-depth ratio less than
1:2. This bead shape shows lower melting point liquid pushed ahead of the
solidifying dendrites but it does not become trapped at the bead centre,
thus, even under tensile stresses resulting from cooling, this film is self-
healing and cracking avoided.

t
0
~

~irect~=> of travel

Figure 17.6 Weld bead with favourable width-to-depth ratio. The dendrites push
the lowest melting point metal towards the surface at the centre of the bead centre
so it does not form a weak central zone.

SAW and spray-transfer GMA are the arc welding processes most likely to
give weld beads with an unfavourable width-to-depth ratio. Also, electron

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beam and laser weldi'ng processes are extremely sensitive to this kind of
cracking as a result of the deep, narrow beads produced.

Avoiding unfavourable welding conditions that lead to centreline

solidification cracking (of weld metals with sensitive compositions) may
require significant changes to welding parameters, such as reducing:

Welding current (give a shallower bead).

Welding speed (give a wider weld bead).

Avoiding unfavourable welding conditions that lead to crater cracking of a

sensitive weld metal requires changes to the technique used at the end of a
weld when the arc is extinguished, such as:

TIG welding when using a current slope-out device so that the current
and weld pool depth gradually reduce before the arc is extinguished
(gives more favourable weld bead width-to-depth ratio). It is also a
common practice to backtrack the bead slightly before breaking the arc
or lengthen the arc gradually to avoid the crater cracks.

Modify weld pool solidification mode by feeding the filler wire into the
pool until solidification is almost complete and avoiding a concave crater.

When MMA welding modify the weld pool solidification mode by

reversing the direction of travel at the end of the weld run so that the
crater is filled.

17.5 Lamellar tearing

A type of cracking that occurs only in steel plate or other rolled products
underneath a weld.

Characteristics of lamellar tearing are:

Cracks only occur in the rolled products eg plate and sections.

Most common in C-Mn steels.
Cracks usually form close to but just outside, the HAZ.
Cracks tend to lie parallel to the surface of the material and the fusion
boundary of the weld having a stepped aspect.

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~-; .
Fusion ~
boundary -Ji;.;,.,,,.
HAZ{ t'~- -~~
!tt1(~-

Crack propagation by
tearing of ligaments between
Through-thickness De-cohesion of de-cohesloi inclusion stringers
residual slresses inclusion stringers
from welding /
Inclusion
stringer - - - - .

Figure 17.7
a) Typical lamellar tear located just outside the visible HAZ;
b) Step-like crack a characteristic of a lamellar tear.

17.5.1 Factors influencing susceptibility to lamellar tearing

Lamellar tearing occurs when two conditions exist at the same time:

Susceptible rolled plate is used to make a weld joint.

High stresses act in the through-thickness direction of the susceptible
material (known as the short-transverse direction).

Susceptible rolled plate

A material susceptible to lamellar tearing has very low ductility in the
through-thickness (short transverse) direction and is only able to
accommodate the residual stresses from welding by tearing rather than by
plastic straining.

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Low through-thickness ductility in rolled products is caused by the presence

of numerous non-metallic inclusions in the form of elongated stringers. The
inclusions form in the ingot but are flattened and elongated during hot rolling
of the material.

Non-metallic inclusions associated with lamellar tearing are principally

manganese sulphides and silicates.

High through-thickness stress

Weld joints that are T, K andY configurations end up with a tensile residual
stress component in the through-thickness direction.

The magnitude of the through-thickness stress increases as the restraint

(rigidity) of the joint increases. Section thickness and size of weld are the
main influencing factors and lamellar tearing is more likely to occur in thick
section, full penetration T, K andY joints.

17.5.2 Cracking mechanism

High stresses in the through-thickness direction present as welding residual
stresses cause the inclusion stringers to open-up (de-cohese) and the thin
ligaments between individual de-cohesed inclusions then tear and produce
a stepped crack.

17.5.3 Avoiding lamellar tearing

Lamellar tearing can be avoided by reducing the influence of one or both of
the factors.

Susceptible rolled plate

EN 10164 (Steel products with improved deformation properties
perpendicular to the surface of the product, Technical delivery conditions)
gives guidance on the procurement of plate to resist lamellar tearing.

Resistance to lamellar tearing can be evaluated by tensile test pieces taken

with their axes perpendicular to the plate surface (through-thickness
direction). Through-thickness ductility is measured as the % reduction of
area (%R of A) at the point of fracture of the tensile test piece (Figure 17.8).

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Plate surface

Reduction of
diameter at
point of
fracture

Plate surface

Figure 17.8 Round tensile test piece taken with its axis in the short-transverse
direction (through-thickness of plate) to measure the %R of A and assess
resistance to lamellar tearing.

The greater the measured %R of A, the greater the resistance to lamellar

tearing . Values in excess of -20% indicate good resistance even in very
highly constrained joints.

Reducing the susceptibility of rolled plate to lamellar tearing can be

achieved by ensuring that it has good through-thickness ductility by:

Using clean steel that has low sulphur content (<-0.015%) and
consequently relatively few inclusions.
Procuring steel plate that has been subjected to through-thickness
tensile testing to demonstrate good through-thickness ductility {as
EN 10164).

Through-thickness stress
Through-thickness stress in T, K and Y joints is principally the residual
stress from welding, although the additional seNice stress may have some
influence.

Reducing the magnitude of through-thickness stresses for a particular weld

joint would require modification to the joint so may not be practical because
of the need to satisfy design requirements. However, methods that could be
considered are:

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Reducing the size of the weld by:

Using a partial butt weld instead of full penetration.
Using fillet welds instead of a full or partial penetration butt weld
(Figure 17.9).
Applying a buttering layer of weld metal to the surface of a susceptible
plate so that the highest through-thickness strain is located in the weld
metal and not the susceptible plate (Figure 17.10).
Changing the joint design, such as using a forged or extruded
intermediate piece so that the susceptible plate does not experience
through-thickness stress (Figure 17.1 1).

Susceptible plate Susceptible plate

Figure 17.9 Reducing the effective size of a weld will reduce the thro(lgh-thickness
stress on the susceptible plate and may be sufficient to reduce the risk of lamellar
tearing.

Figure 17.10 Lamellar tearing can be avoided by changing the joint design.

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Figure 17.11 Two layers of weld metal applied usually by MMA to susceptible plate
before the T butt is made.

17.6 Weld decay

Occurance
Service failure problem, caused by corrosion, may be long time in service
(associated with welding but is not a fabrication defect, like H cracking, etc).

Appearance
Called weld decay because a narrow zone in the HAZ can be severely
corroded but surrounding areas (weld and parent metal} may not be
affected.

Only affects certain types of austenitic stainless steels.

Requires two factors at the same time:

Sensitive HAZ.
Corrosive liquid in contact with the sensitive HAZ, in service.

17.6.1 Avoiding weld d~cay

Characteristics of sensitive HAZ and failure mechanism

Occurs in stainless steels which are not low carbon grades (L grades) eg
304 and 316.
HAZ becomes sensitive to preferential corrosion because chromium
carbides form at grain boundaries in HAZ thereby locally reducing the
corrosion resistance of the HAZ.
More prone to HAZ degradation the more weld runs put in the joint,
thicker sections (more thermal cycles and HAZ spends more time in
temperature range where carbides form}.

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To reduce sensitivity of HAZ

Use low C grades of austenitic stainless steel (304 and 316L) not
enough carbon to form a large number of Cr carbides during welding,
corrosion resistance thus not degraded.
Use stabilised grades of stainless steel (321 and 347) which contain
titanium 321 and niobium 347 which combine with carbon rather than Cr
so no local reduction in corrosion resistance}.
If welding higher carbon grades use low heat input welding and fewest
weld runs possible (less time available in temperature range when Cr
carbides form).
It is possible to remove HAZ sensitivity by solution heat treatment of the
welded item. As this requires temperature of 1050-11 00C (this has
practical difficulties).

Service environment
Corrosion of HAZ determined by service conditions, type of chemicals
and temperature.
Problem not solved by trying to address service conditions but by
selection of material, taking account of effects of welding/welding
parameters.

Weld decay, also called sensitisation or intercrystalline corrosion, occurs

within the susceptible temperature range of approximately 600-850C, ie in
the HAZ or during high temperature service. At this temperature, carbon
diffuses to the grain boundaries and combines with chromium to form
carbides, leaving a Cr-depleted layer susceptible to corrosion along the
grain boundaries, therefore corrosion cracking occurs along grain
boundaries in the HAZ. Weld decay can be avoided by keeping the carbon
low, eg using low carbon grades like 304L and the heat input low by
avoiding preheat or PWHT. It is also possible to use grades with added
elements which combine with the carbon eg 321 (which contains Ti) or 347
(which contains Nb).

When heated to 600-ssoc Grain bounda!ies become depleted

carbides form at the grain of chromium and lose their
boundaries corrosion resistance

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Weld Repairs
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18 Weld Repairs
18.1 Two specific areas
Production.
In-service.

The reasons for making a repair are many and varied, from the removal of
weld defects induced during manufacture to a quick and temporary running-
repair to an item of production plant. The subject of welding repairs is also
wide and varied and often confused with maintenance and refurbishment
where the work can be scheduled.

With planned maintenance and refurbishment, sufficient time enables the

tasks to be completed without production pressures being applied. In
contrast, repairs are usually unplanned and may result in shortcuts being
taken to allow production to continue so it is advisable for a fabricator to
have an established policy on repairs and to have repair methods and
procedures in place.

The manually controlled welding processes are the easiest to use,

particularly if it is a local repair or carried out onsite. Probably the most
frequently used is MMA as it is versatile, portable and readily applicable to
many alloys because of the wide range of off-the-shelf consumables.
Repairs almost always result in higher residual stresses and increased
distortion compared with first time welds. With C-Mn and low/medium alloy
steels, pre- and postweld heat treatments may be required.

A number of key factors need to be considered before any repair, the most
important being it is financially worthwhile. Before this judgement can be
made, the fabricator needs to answer the following questions:

Can structural integrity be achieved if the item is repaired?

Are there any alternatives to welding?
What caused the defect and is it likely to happen again?
How is the defect to be removed and which welding process is to be
used?
Which NOT method is required to ensure complete removal of the
defect?
Will the welding procedures require approval/re-approval?
What will be the effect of welding distortion and residual stress?
Will heat treatment be required?
What NOT is required and how can acceptability of the repair be
demonstrated?
Will approval of the repair be required, if yes, how and by whom?

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Weld repairs may be relatively straightforward or quite complex and various

engineering disciplines may need to be involved to ensure a successful
outcome.

It is recommended that ongoing analysis of the types of de-fect is carried out

by the QC department to discover the likely reason for their occurrence
(material/process or skill related).

In general terms, a welding repair involves:

Detailed assessment to find out the extremity of the defect possibly using
a surface or sub-surface NOT method.
Cleaning the repair area (removal of paint grease, etc).
Once established the excavation site must be clearly identified and
marked out.
An excavation procedure may be required (method used ie grinding,
arc/air gouging, preheat requirements, etc).
NOT to locate the defect and confirm its removal.
A welding repair procedure/method statement with the appropriate
(suitable for the alloys being repaired and may not apply in specific
situations.) welding process, consumable, technique, controlled heat
input and interpass temperatures, etc will need to be approved.
Use of approved welders.
Dressing the weld and final visual.
NOT procedure/technique prepared and carried out to ensure that the
defect has been successfully removed and repaired.
Any post repair heat treatment requirements.
Final NOT procedure/technique prepared and carried out after heat
treatment requirements.
Applying protective treatments (painting, etc as required}.

Production repairs
Repairs are usually identified during production inspection. Evaluation of the
reports is by the Welding Inspector or NOT operator. Discontinuities in the
welds are only classed as defects when they are outside the range
permitted by the applied code or standard.

Before any repair a number of elements need to be fulfilled.

Analysis
As this defect is surface-breaking and at the fusion face the problem could
be cracking or lack of sidewall fusion. The former may be to do with the
material or welding procedure, if it is done the latter can be apportioned to
the welder's lack of skill.

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Assessment
As the defect is open to the surface, magnetic particle inspection (MPI) or
dye penetrant inspection (DPI) may be used to gauge the length of the
defect and ultrasonic testing (Ul) to gauge the depth.

A typical defect is shown below:

Plan view of defect.

Excavation
If a thermal method of excavation is to be used, ie arc/air gouging it may be
a requirement to qualify a procedure as the heat generated may affect the
metallurgical structure, resulting in the risk of cracking in the weld or parent
material.

To prevent cracking it may be necessary to apply a preheat. Depth to width

ratio shall not be less than 1 (depth) to 1 (width), ideally depth 1 to width 1.5.

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Side view of excavation for slight sub-surface defect

Side view of excavation for deep defect.

Side view of excavation for full root repair.

Cleaning the excavation

At this stage grinding the repair area is important due to the risk of carbon
becoming impregnated into the weld metal/parent material and it should be
ground back typically 3-4mm to bright metal.

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Confirmation of excavation
NDT must confirm that the defect has been completely excavated from the
area.

Rewelding of the excavation

Prior to rewelding a detailed repair welding procedure/method statement
shall be approved.

Typical side view of weld repair.

NOT confirmation of successful repair

After the excavation has been filled the weldment should undergo a
complete retest using the NDT techniques previously used to establish the
original repair to ensure no further defects have been introduced by the
repair welding process. NDT may need to be further applied after any
additional PWHT.

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ln ...service repairs
Most in-service repairs are very complex as the component is likely to be in
a different welding position and condition than during production. It may
have been in contact with toxic or combustible fluids so a permit to work will
be needed prior to any work. The repair welding procedure may look very
different to the original production procedure due to changes.

Other factors may be taken into consideration such as the effect of heat on
surrounding areas of the component, ie electrical components, or materials
that may be damaged by the repair procedure. This may also include
difficulty in carrying out any required pre- or post-welding heat treatments
and a possible restriction of access to the area to be repaired. For large
fabrications it is likely that the repair must also take place on-site without a
shutdown of operations which may bring other considerations.

Repair of in-service defects may require consideration of these and many

other factors so are generally considered more complicated than production
repairs.

Joining technologies often play a vital role in the repair and maintenance of
structures. Parts can be replaced, worn or corroded parts can be built up
and cracks repaired .

When a repair is required it is important to determine two things: The reason

for failure and, can the component be repaired? The latter infers that the
material type is known. For metals, particularly those to be welded, chemical
composition is vitally important. Failure modes often indicate the approach
required to make a sound repair. When the cause-effect analysis, is not
followed through the repair is often unsafe, sometimes disastrously so.

In many instances, the Standard or Code used to design the structure will
define the type of repair that can be carried out and give guidance on the
methods to be followed. Standards imply that when designing or
manufacturing a new product it is important to consider a maintenance
regime and repair procedures. Repairs may be required during manufacture
and this situation should also be considered.

Normally there is more than one way of making a repair, for example, cracks
in cast iron might be held together or repaired by pinning, bolting, riveting,
welding or brazing. The choice will depend on factors such as the reason for
failure, material composition and cleanliness, environment and the size and
shape of the component.

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It is very important that repair and maintenance welding are not regarded
as activities which are simple or straightforward. A repair may seem
undemanding but getting it wrong can result catastrophic failure with
disastrous consequences.

Is welding the best method of repair?

If repair is needed because a component has a local irregularity or a shallow
defect, grinding out any defects and blending to a smooth contour might be
acceptable. It is if the steel has poor weldability or fatigue loading is severe.
It is often better to reduce the so-called factor of safety slightly than risk
putting defects, stress concentrations and residual stresses into a brittle
material.

Brittle materials, including some steels (particularly in thick sections) as well

as cast irons, may not withstand the residual stresses imposed by heavy
weld repairs, particularly if defects are not tota~lyl removed, leaving stress
concentrations to initiate cracking.

Is the repair like earlier repairs?

Repairs of one sort may have peen routine for years but it is important to
check that the next one is not subtly different, the section thickness may be
greater; the steel to be repaired may be different and less weldable or the
restraint higher. If there is any doubt, answer the remaining questions.

What is the composition and weldability of the base metal?

The original drawings usually give some idea of the steel involved although
the specification limits may then have been less stringent and the
specification may not give enough detail to be helpful. If sulphur-bearing
free-machining steel is involved, it could give hot cracking problems during
welding.

If there is any doubt about the composition, a chemical analysis should be

carried out, to analyse for all elements which may affect weldability (Ni, Cr,
Mo, Cu, V, Nb and B) as well as those usually, specified (C, S, P, Si and
Mn).

The small cost of analysis could prevent a valuable component being ruined
by ill-prepared repairs or save money by reducing or avoiding the need for
preheat if the composition is leaner than expected. Once the composition is
known, a welding procedure can be devised.

What strength is required from the repair?

The higher the yield strength of the repair weld metal the greater the
residual stress level on completion of welding, risk of cracking, clamping
needed to avoid distortion and more difficulty in formulating the welding
procedure. The practical limit for the yield strength of conventional steel
weld metals is about 1000N/mm2 .

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Can preheat be tolerated?

A high level of preheat makes conditions more difficult for the welder and
the parent steel can be damaged if it has been tempered at a low
temperature. The steel being repaired may contain items damaged by
excessive heating. Preheat levels can be reduced by using consumables of
ultra-low hydrogen content or non-ferritic weld metals. Of these, austenitic
electrodes may need some preheat, but the more expensive nickel alloys
usually do not but may be sensitive to high sulphur and phosphorus
contents in the parent steel if diluted into the weld metal.

Can softening or hardening of the HAZ be tolerated?

Softening of the HAZ is likely in very high strength steels, particularly if they
have been tempered at low temperatures. It cannot be avoided but its extent
can be minimised. Hard HAZs are particularly vulnerable where service
conditions can lead to stress corrosion. Solutions containing H2S (hydrogen
sulphide) may demand hardness below 248HV (22HRC) although fresh
aerated seawater appears to tolerate up to about 450HV. Excessively hard
HAZs may, require PWHT to soften them provided cracking has been
avoided.

Is PWHT practicable?
Although desirable, PWHT may not be possible for the same reasons
preheating is not. For large structures local PWHT may be possible but care
should be taken to abide by the relevant codes because it is easy to
introduce new residual stresses by improperly executed PWHT.

Is PWHT necessary?
PWHT may be needed for several reasons and the reason must be known
before considering whether it can be avoided.

Will the fatigue resistance of the repair be adequate?

If the repair is in an area highly stressed by fatigue and particularly if it is of
a fatigue crack, inferior fatigue life can be expected unless the weld surface
is ground smooth and no surface defects left. Fillet welds in which the root
cannot be ground smooth are not tolerable in areas of high fatigue stress.

Will the repair resist its environment?

Besides corrosion it is important to consider the possibility of stress
corrosion, corrosion fatigue, thermal fatigue and oxidation in-service.

Corrosion and oxidation resistance usually requires filler metal to be at least

as noble or oxidation resistant as the parent metal for corrosion fatigue the
repair weld profile may need to be blended.

To resist stress corrosion, PWHT may be necessary to restore the correct

microstructure, reduce hardness and the residual stress left by the repair.

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Can the repair be inspected and tested?

For onerous service, radiography and/or ultrasonic examination are often
desirable, but problems are likely if stainless steel or nickel alloy filler is
used. Such repairs cannot be assessed by MPI as it is very important to
carry out the procedural tests very critically, to ensure there is no risk of
cracking nor likelihood of serious welder-induced defects.

For all repair welds it is vital to ensure that welders are properly motivated
and carefully supervised.

As-welded repairs
Repair without PWHT is, normal where the original weld was not heat
treated but some alloy steels and many thick-sectioned components require
PWHT to maintain a reasonable level of toughness, corrosion resistance,
etc. However, PWHT of components in-service is not always easy or even
possible and local PWHT may cause more problems than it solves except in
simple structures.

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 Section 19

Residual Stresses and Distortions

 Rev 2 April2013
Residual Stresses and Distortions
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19 Residual Stresses and Distortions

19.1 Development of residual stresses
Because welding involves highly localised heating of joint edges to fuse the
material, non-uniform stresses are set up in the components being joined,
generated because of expansion and contraction of the heated material.
Initially, compressive stresses are created in the surrounding cold parent
metal when the weld pool is formed due to the thermal expansion of the hot
metal HAZ adjacent to the weld pool. Tensile stresses occur on cooling
when contraction of the weld metal and the immediate HAZ is resisted by
the bulk of the cold parent metal.

As long as these stresses are above the yield point of the metal at the
prevailing temperature, they continue to produce permanent deformation,
but in so doing are relieved and fall to yield-stress level so cease to cause
further distortion. But, if at this point we could release the weld from the
plate by cutting along the joint line, it would shrink further because, even
when distortion has stopped, the weld contains an elastic strain equivalent
to the yield stress. Visualise the completed joint as weld metal being
stretched elastically between two plates.

Shrinkage of a weld metal element during cooling:

a) The entire length of weld is hot and starts to cool'
b) If unfused to the parent metal, the weld will shrink, reducing its dimensions on
all three directions;
c) Since the weld is fused to the parent metal under combined cooling residual
stresses will occur.

The stresses left in the joint after welding are referred to as residual
stresses. From the above it can be seen there will be both longitudinal and
transverse stresses (in the case of a very thick plate there is a through-
thickness component of residual stress as well).

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19.1.1 Distribution of residual stresses

The magnitude of thermal stresses induced into the material can be seen by
the volume change in the weld area on solidification and subsequent cooling
to room temperature. For example, when welc,ting C-Mn steel the molten
weld metal volume will be reduced by approximately 3% on solidification
and the volume of the solidified weld metal/ HAZ will be reduced by a further
7% as its temperature falls from the melting point of steel to room
temperature.

Perpendicular to the weld, in the transverse direction, the stresses in the

weld are more dependent on the clamping condition of the parts. Transverse
residual stresses are often relatively small although transverse distortion is
substantial. The distribution of transverse residual stresses in a butt joint is
shown below. Tensile stress of a relatively low magnitude is produced in the
middle section while compressive stress is generated at both ends of the
joint. It must be noted that the longer the weld, the higher the tensile
residual stress until yield stress is reached.

Tension

Compression

The pattern of residual stresses on transverse direction.

In longitudinal stresses the weld and some of the plate which has been
heated are at or near yield-stress level. Moving out into the plate from the
HAZ, the stresses first fall to zero (the tensile stress region extends beyond
the weld and HAZ into the parent plate} and beyond this there is a region of
compressive stress. The width of the band where tensile residual stresses
are present depends on the heat input during welding, the higher the heat
input the wider the band where these tensile residual stresses occur.

19-2
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Tension- -compression

The pattern of residual stresses in the longitudinal direction in the as~welded

conditions (the maximum level of tensile residual stress is equal to the value of the
yield strength of the weld metal at room temperature).

All fusion welds which have not been subjected to postweld treatments, the
vast majority of welded joints, contain residual stresses. Procedures
developed to minimise distortion may alter the distribution of the residual
stresses but do not eliminate them or even reduce their peak level.

One method of reducing the level of residual stress in a welded joint is to

perform PWHT to relieve these stresses. So once the temperature is raised
the yield stress starts to drop. As a result of this the peak residual stresses
which were up to the value of yield stress at room temperature, cannot be
carried over any more by the material and are relaxed by plastic deformation
(usually, this plastic deformation is confined at the level of grains but in
some situations can lead to significant deformation of the welded
component). This process ends when the material has reached the soaking
temperature. So the level of residual stresses will be reduced by the
difference between the value of the yield stress 'at room temperature (at the
beginning of PWHT) at the soaking temperature. As can be seen from the
following figure, this can reduce the level of tensile residual stresses up to a
quarter of its initial level but compressive residual stresses are left
unchanged. It must be pointed out that PWHT does not completely eliminate
these residual stresses.
Tension ----Compression

Maximum residual Maximum residual

stress before PWHT stress after PWHT
Pattern of residual stresses on longitudinal direction after PWHT.

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19.1.2 Effect of residual stresses

Since formation of residual stresses cannot be avoided, it is appropriate to
ask if their presence is a concern. As with many engineering situations the
answer is not a simple yes or no. There are numerous applications where
the existence of residual stresses would have little or no influence on the
service behaviour of the joint, storage tanks, building frames, low pressure
pipework and domestic equipment examples where the joints can be used in
the as-welded condition without detriment.

Residual stresses are considered detrimental because they:

Lead to distortion and an out~of-shape welded structure is not fit-for-

purpose.
Affect dimensional stability of the welded assembly. When machining
welded components, removing layers of metal near the joint may disturb
the balance between the tensile and compressive residual stresses and
further defonnation or warping can occur. This can make it difficult to
hold critical machining tolerances and it may be desirable to stress-
relieve to achieve dimensional stability.
Enhance the risk of brittle fracture. When residual stresses are present in
a welded component, a small extra stress added may initiate a brittle
fracture, providing other conditions are met {ie low temperature).
Can facilitate certain types of corrosion. Some metals in certain
environments corrode rapidly in the presence of tensile stress, ie stress
corrosion cracking. In these cases, a joint in the as-welded condition
containing residual stresses suffers excessive attack; retarded if the joint
is stress-relieved.

If the service requirements indicate that residual stresses are undesirable,

the designer must take them into account when selecting materials and
deciding upon a safe working stress. This can be seen in the design of
ships, where the combination of low temperatures and residual stress could
lead to brittle fracture. The designer selects a material not susceptible to this
mode of failure even at the low temperatures which may be experienced
during the working life of the ship; the presence of residual stresses is then
not important. Similarly, in many structures subjected to loads which
fluctuate during service, for example, bridges, earthmoving equipment and
cranes, the designer recognises the existence of residual stresses by
choosing a working stress range which takes account of the role these
stresses play in the formation and propagation of fatigue cracks. There are
some specific applications where it is essential to reduce the level of
residual stresses in the welded joint: With pressure vessels because of the
risk of a catastrophic failure by brittle fracture, stress-relieving is often a
statutory or insurance requirement.

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19.1.3 Factors affecting residual stresses

Can be grouped into five categories:

8 Material properties.
9 Amount of restraint.
10 Joint design.
11 Fit-up.
12 Welding sequence.

19.2 Causes of distortion

Welding involves highly localised heating of joint edges to fuse the material
and non-uniform stresses are set up in the component because of
expansion and contraction of the heated material.

Initially, compressive stresses are created in the surrounding cold parent

metal when the weld pool is formed due to the thermal ex.pansion of the hot
metal (HAZ) adjacent to the weld pool. Tensile stresses occur on cooling
when the contraction of the weld metal and immediate HAZ is resisted by
the bulk of the cold parent metal.

The magnitude of thermal stresses induced into the material can be seen by
the volume change in the weld area on solidification and subsequent cooling
to room temperature. For example, when welding C-Mn steel, the molten
weld metal volume will be reduced by appro~imately 3% on solidification
and the volume of the solidified weld metal/HAl will be reduced by a further
7% as its temperature falls from the melting point of steel to room
temperature.

If the stresses generated from thermal expansion/contraction exceed the

yield strength of the parent metal, localised plastic deformation of the metal
occurs, causing a permanent reduction in the component dimensions and
distorts the structure.

19.3 The main types of distortion

Longitudinal shrinkage.
Transverse shrinkage.
Angular distortion.
Bowing and dishing.
Buckling.

Contraction of the weld area on cooling results in both transverse and

longitudinal shrinkage.

Non-uniform contraction (through-thickness) produces angular distortion as

well as longitudinal and transverse shrinking.

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For example, in a single V butt weld, the first weld run produces longitudinal
and transverse shrinkage and rotation. The second run causes the plates to
rotate using the first weld deposit as a fulcrum so balanced welding in a
double-sided V butt joint can produce uniform contraction and prevent
angular distortion.

In a single-sided fillet weld, non-uniform contraction will produce angular

distortion of the upstanding leg so double-sided fillet welds can be used to
control distortion in the upstanding fillet but because the weld is only
deposited on one side of the base plate, angular distortion will now be
produced in the plate.

Longitudinal bowing in welded plates happens when the weld centre is not
coincident with the neutral axis of the section so that longitudinal shrinkage
in the welds bends the section into a curved shape. Clad plate tends to bow
in two directions due to longitudinal and transverse shrinkage of the
cladding, producing a dished shape.

Dishing is also produced in stiffened plating. Plates usually dish inwards

between the stiffeners, because of angular distortion at the stiffener
attachment welds.

In plating, long range compressive stresses can cause elastic buckling in

thin plates, resulting in dishing, bowing or rippling, Figure 19.1 .

..
- ... ...... -;:

Figure 19.1 Examples of distortion.

Increasing the leg length of fillet welds, in particular, increases shrinkage.

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19.4 Factors affecting distortion

If a metal is uniformly heated and cooled there would be almost no distortion
but because the material is locally heated and restrained by the surrounding
cold metal, stresses are generated higher than the material yield stress
causing permanent distortion. The principal factors affecting the type and
degree of distortion are:

Parent material properties.

Amount of restraint.
Joint design.
Part fit-up.
Welding procedure.

19.4.1 Parent material properties

Parent material properties which influence distortion are coefficient of
thermal expansion, thermal conductivity and to a lesser extent, yield stress
and Young's modulus. As distortion is determined by expansion and
contraction of the material so the coefficient of thermal expansion of the
material plays a significant role in determining the stresses generated during
welding and the degree of distortion. For example, as stainless steel has a
higher coefficient of expansion and lesser thermal conductivity than plain
carbon steel, it generally has significantly more distortion.

19.4.2 Restraint
If a component is welded without any external restraint, it distorts to relieve
the welding stresses. Methods of restraint such as strongbacks in butt
welds., can prevent movement and reduce distortion. Restraint produces
higher levels of residual stress in the material, so there is a greater risk of
cracking in weld metal and HAZ especially in crack-sensitive materials.

19.4.3 Joint design

Both butt and fillet joints are prone to distortion. It can be minimised in butt
joints by adopting a joint type which balances the thermal stresses through
the plate thickness, eg double-sided in preference to a single-sided weld.
Double-sided fillet welds should eliminate angular distortion of the
upstanding member especially if the two welds are deposited at the same
time.

19.4.4 Part fit-up

Fit-up should be uniform to produce predictable and consistent shrinkage.
Excessive joint gap can increase the degree of distortion by increasing the
amount of weld metal needed to fill the joint. The joints should be
adequately tacked to prevent relative movement between the parts during
welding.

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19.4.5 Welding procedure

This influences the degree of distortion mainly through its effect on heat
input. As welding procedures are usually selected for reasons of quality and
productivity, the welder has limited scope for reducing distortion. As a
general rule weld volume should be kept to a minimum. Also, the welding
sequence and technique should balance the thermally induced stresses
around the neutral axis of the component.

19.5 Prevention by pre-setting, pre-bending or use of restraint

Distortion can often be prevented at the design stage, eg by placing the
welds about the neutral axis, reducing the amount of welding and depositing
the weld metal using a balanced welding technique. Where this is not
possible, distortion may be prevented by one of the following:

Pre-setting of parts.
Pre-bending of parts.
Use of restraint.

The technique chosen will be influenced by the size and complexity of the
component or assembly, the cost of any restraining equipment and the need
to limit residual stresses.

l~~
b)~~

Figure 19.2 Pre-setting of parts to produce correct alignment after welding:

d Fillet joint to prevent angular distortion.
e Butt joint to prevent angular distortion.

19.5.1 Pre-setting
The parts are pre-set and left free to move during welding, see Figure 19.2.
The parts are pre-set by a pre-determined amount so that distortion
occurring during welding is used to achieve overall alignment and
dimensional control.

The main advantages compared with restraint are there is no expensive

equipment needed and it gives lower residual stress in the structure.

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As it is difficult to predict the amount of pre-setting needed to accommodate

shrinkage, a number of trial welds will be required. For example when MMA
or MIG/MAG welding butt joints the joint gap will normally close ahead of
welding; whereas with SAW the joint may open up during welding. When
carrying out trial welds it is essential that the test structure is reasonably
representative of the full size structure to generate the level of distortion
likely to occur. So pre-setting is more suitable for simple components or
assemblies.

19.5.2 Pre-bending
Pre-bending or pre-spnngmg parts before welding pre-stresses the
assembly to counteract shrinkage during welding. As shown in Figure 19.3,
pre-bending using strongbacks and wedges can pre-set a seam before
welding to compensate for angular distortion. Releasing the wedges after
welding will allows the parts to move back into alignment.

The figure below shows the diagonal bracings and centre jack used to pre-
bend the fixture, not the component, counteracting the distortion introduced
through out-of-balance welding.

Figure 19.3 Pre-bending using strongbacks and wedges to accommodate angular

distortion in thin plates.

19.5.3 Use of restraint

Because of the difficulty in applying pre-setting and pre-bending restraint is
the more widely used technique. The basic principle is that the parts are
placed in position and held under restraint to minimise any movement during
welding. When removing the component from the restraining equipment a
relatively small amount of movement will occur due to locked-in stresses
which can be cured by applying a small amount of pre-set or stress-relieving
before removing the restraint.

When welding assemblies all the component parts should be held in the
correct position until completion of welding and a suitably balanced
fabrication sequence used to minimise distortion.

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Welding with restraint will generate additional residual stresses in the weld
which may cause cracking. When welding susceptible materials a suitable
welding sequence and the use of preheating will reduce this risk. Restraint
is relatively simple to apply using clamps, jigs and fixtures.

Welding jigs and fixtures

Used to locate the parts and ensure that dimensional accuracy is
maintained whilst welding, can be of a relatively simple construction as
shown in Figure 19.4a but the welding engineer will need to ensure that the
finished fabrication can be removed easily after welding.

Flexible clamps
Can be effective in applying restraint but also in setting up and maintaining
the joint gap (can also be used to close a gap that is too wide), Figure
19.4b.

A disadvantage is that as the restraining forces in the clamp will be

transferred into the joint when the clamps are removed, the level of residual
stress across the joint can be quite high.

a) b)

c) d)

Figure 19.4 Restraint techniques to prevent distortion:

a Welding jig;
b Flexible clamp;
c Strongbacks with wedges;
d Fully wedged strongbacks.

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Strongbacks and wedges

A popular way of applying restraint especially for site work, wedged
strongbacks (Figure 19.4c), will prevent angular distortion in plate and help
prevent peaking in welding cylindrical shells. As they allow transverse
shrinkage the cracking risk will be greatly reduced compared with fully
welded strongbacks.

Fully welded (welded on both sides of the joint) strongbacks (Figure 19.4d)
will minimise both angular distortion and transverse shrinkage. As significant
stresses can be generated across the weld which will increase any tendency
for cracking, care should be taken in their use.

19.5.4 Best practice

Adopting the following assembly techniques will help control distortion:

Pre-set parts so that welding distortion will achieve overall alignment and
dimensional control with the minimum of residual stress.
Prebend joint edges to counteract distortion and achieve alignment and
dimensional control with minimal residual stress.
Apply restraint during welding using jigs and fixtures, flexible clamps,
strongbacks and tack welding but consider the cracking risk which can
be quite significant, especially for fully welded strongbacks.
Use an approved procedure for welding and removal of welds for
restraint techniques which may need preheat to avoid inperfections
forming in the component surface.

19.6 Prevention by design

Design principles
At the design stage welding distortion can often be prevented or restricted
by considering:

Elimination of welding .
Weld placement.
Reducing the volume of weld metal.
Reducing the number of runs.
Use of balanced welding.

19.6.1 Elimination of welding

As distortion and shrinkage are an inevitable result of welding, good design
requires that welding is kept to a minimum and the smallest amount of weld
metal is deposited. Welding can often be eliminated at the design stage by
forming the plate or using a standard rolled section, as shown in Figure
19.5.

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Figure 19.5 Elimination of welds by:

a) Forming the plate;
b) Use of rolled or extruded section.

If possible the design should use intermittent welds rather than a continuous
run to reduce the amount of welding . For example in attaching stiffening
plates, a substantial reduction in the amount of welding can often be
achieved whilst maintaining adequate strength.

19.6.2 Weld placement

Placing and balancing of welds are important in designing for minimum
distortion. The closer a weld is to the neutral axis of a fabrication, the lower
the leverage effect of the shrinkage forces and the final distortion. Examples
of poor and good designs are shown in Figure 19.6.

Neutral
/axis

Poor Good

Figure 19.6 Distortion may be reduced by placing the welds around the neutral
axis.

As most welds are deposited away from the neutral axis, distortion can be
minimised by designing the fabrication so the shrinkage forces of an
individual weld are balanced by placing another weld on the opposite side
of the neutral axis. Where possible welding should be carried out alternately
on opposite sides instead of completing one side first. In large structures if
distortion is occurring preferentially on one side it may be possible to take
corrective action, for example, by increasing welding on the other side to
control the overall distortion.

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19.6.3 Reducing the volume of weld metal

To minimise distortion as well as for economic reasons the volume of weld
metal should be limited to the design requirements. For a single~sided joint,
the cross-section of the weld should be kept as small as possible to reduce
the level of angular distortion, as illustrated in Figure 19.7.

Figure 19.7 Reducing the amount of angular distortion and lateral shrinkage.

Ways of reducing angular distortion and lateral shrinkage:

Reducing the volume of weld metaL

Using single pass welds.
Ensure fillet welds are not oversize.

Joint preparation angle and root gap should be minimised providing the weld
can be made satisfactorily. To facilitate access it may be possible to specify
a larger root gap and smaller preparation angle. By reducing the difference
in the amount of weld metal at the root and face of the weld the degree of
angular distortion will be correspondingly reduced. Butt joints made in a
single pass using deep penetration have little angular distortion, especially if
a closed butt joint can be welded (Figure 19.7). For example thin section
material can be welded using plasma and laser welding processes and thick
section can be welded in the vertical posftion using electrogas and
electroslag processes. Although angular distortion can be eliminated there
will still be longitudinal and transverse shrinkage.

In thick section material, as the cross-sectional area of a double V joint

preparation is often only half that of a single V, the volume of weld metal to
be deposited can be substantially reduced. The double V joint preparation
also permits balanced welding about the middle of the joint to eliminate
angular distortion.

As weld shrinkage is proportional to the amount of weld metal both poor

joint fit-up and over-welding will increase the amount of distortion. Angular
distortion in fillet welds is particularly affected by over-welding. As design
strength is based on throat thickness, over-welding to produce a convex
weld bead does not increase the allowable design strength but will increase
the shrinkage and distortion.

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19.6.4 Reducing the number of runs

There are conflicting opinions on whether it is better to deposit a given
volume of weld metal using a small number of large weld passes or a large
number of small passes. Experience shows that for a single-sided butt joint,
or fillet weld, a large single weld deposit gives less angular distortion than a
weld made with a number of small runs. Generally in an unrestrained joint
the degree of angular distortion is approximately proportional to the number
of passes.

Completing the joint with a small number of large weld deposits results in
more longitudinal and transverse shrinkage than using in a larger number of
small passes. In a multi-pass weld, previously deposited weld metal
provides restraint, so the angular distortion per pass decreases as the weld
is built up. Large deposits also increase the risk of elastic buckling
particularly in thin section plate.

19.6.5 Use of balanced welding

Balanced welding is an effective means of controlling angular distortion in a
multi-pass butt weld by arranging the welding sequence to ensure that
angular distortion is continually being corrected and not allowed to
accumulate during welding. Comparative amounts of angular distortion from
balanced welding and welding one side of the joint first are shown in Figure
19.8, Balanced welding can also be applied to fillet joints.

Figure 19.8 Balanced welding to reduce the amount of angular distortion.

If welding alternately on either side of the joint is not possible or if one side
has to be completed first, an asymmetrical joint preparation may be used
with more weld metal being deposited on the second side. The greater
contraction resulting from depositing the weld metal on the second side will
help counteract the distortion on the first side.

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19.6.6 Best practice

The following design principles can control distortion:

Eliminate welding by forming the plate and using rolled or extruded

sections.
Minimise the amount of weld metal.
Do not over-weld.
Use intermittent welding in preference to a continuous weld pass.
Place welds about the neutral axis.
Balance the welding about the middle of the joint by using a double
rather than a single V joint.

Adopting best practice principles can have cost benefits. For example, for a
design fillet leg length of 6mm, depositing an 8mm leg length will result in
the deposition of 57% additional weld metal. Besides the extra cost of
depositing weld metal and the increased risk of distortion, it is costly to
remove this extra weld metal later. Designing for distortion control may incur
additional fabrication costs, for example, the use of a double V joint
preparation is an excellent way to reduce weld volume and control distortion
but extra costs may be incurred in production through manipulation of the
workpiece for the welder to access the reverse side.

19.7 Prevention by fabrication techniques

19.7.1 Assembly techniques
In general, the welder has little influence on the choice of welding procedure
but assembly techniques can often be crucial in minimising distortion. The
principal assembly techniques are:

Tack welding.
Back-to-back assembly.
Stiffening.

Tack welding
Ideal for setting and maintaining the joint gap but can also be used to resist
transverse shrinkage. To be effective, thought should be given to the
number of tack welds, their length and the distance between them. Too few
risks the joint progressively closing up as welding proceeds. In a long seam
using MMA or MIG/MAG the joint edges may even overlap. When using the
submerged arc process the joint might open up if not adequately tacked .

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The tack welding sequence is important to maintain a uniform root gap

along the length of the joint. Three alternative tack welding sequences are
shown:

Straight through to the end of the joint (Figure 19.9a). It is necessary to

clamp the plates or use wedges to maintain the joint gap during tacking.
One end then use a back stepping technique for tacking the rest of the
joint (Figure 19.9b).
Centre and complete the tack welding by back stepping (Figure19.9c).

Figure 19.9 Alternative procedures used for tack welding to prevent transverse
shrinkage.

Directional tacking is useful for controlling the joint gap, for example closing
a joint gap which is or has become too wide.

When tack welding it is important that tacks to be fused into the main weld
are produced to an approved procedure using appropriately qualified
welders. The procedure may require preheat and an approved consumable
as specified for the main weld. Removal of the tacks also needs careful
control to avoid causing defects in the component surface.

Back-to-back assembly
By tack welding or clamping two identical components back-to-back,
welding of both components can be balanced around the neutral axis of the
combined assembly (see Figure 19.10a). It is recommended that the
assembly is stress-relieved before separating the components or it may be
necessary to insert wedges between the components (Figure 19.10b) so
when the wedges are removed the parts will move back to the correct shape
or alignment.

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a) 1-~ ~ ~ ~ m]- I

"~j j ~ ] pV>kctles

Figure 19.10 Back-to-back assembly to control distortion when welding two

identical components:
a) Assemblies tacked together before welding;
b) Use of wedges for components that distort on separation after welding.

Stiffening

Figure 19.11 Longitudinal stiffeners prevent bowing in butt welded thin plate joints.

Longitudinal shrinkage in butt welded seams often results in bowing,

especially when fabricating thin plate structures. Longitudinal stiffeners in
the form of flats or angles, welded along each side of the seam (Figure
19.11) are effective in preventing longitudinal bowing. Stiffener location is
important unless located on the reverse side of a joint welded from one side
they must be placed at a sufficient distance from the joint so they do not
interfere with welding.

19.7.2 Welding procedure

A suitable procedure is usually determined by productivity and quality
requirements rather than the need to control distortion. The welding
process, technique and sequence do influence the distortion level.

Welding process
General rules for selecting a welding process to prevent angular distortion:

Deposit the weld metal as quickly as possible.

Use the least number of runs possible to till the joint.

Unfortunately, selecting a suitable welding process based on these rules

may increase longitudinal shrinkage resulting in bowing and buckling.

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In manual welding, MIG/MAG, a high deposition rate process, is preferred to

MMA. Weld metal should be deposited using the largest diameter electrode
(MMA), or the highest current level (MIG/MAG) without causing lack-of-
fusion imperfections. As heating is much slower and more diffuse, gas
welding normally produces more angular distortion than the arc processes.

Mechanised techniques combining high deposition rates and welding

speeds have the greatest potential for preventing distortion. As the distortion
is more consistent, simple techniques such as pre-setting are more effective
in controlling angular distortion.

Welding technique
General rules for preventing distortion are:

Keep the weld (fillet) to the minimum specified size.

Use balanced welding about the neutral axis.
Keep the time between runs to a minimum.

Figure 19.12 Angular distortion of the joint as determined by the number of runs in
the fillet weld.

Without restraint angular distortion in both fillet and butt joints is due to joint
geometry, weld size and the number of runs for a given cross-section.
Angular distortion, measured in degrees as a function of tne number of runs
for a 1 Omm leg length fillet weld is shown.

If possible, balanced welding around the neutral axis should be done, for
example on double-sided fillet joints, by two people welding simultaneously.
In butt joints, the run order may be crucial as balanced welding can be used
to correct angular distortion as it develops.

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Welding sequence
The welding sequence or direction is important and should be towards the
free end of the joint. For long welds the whole of the weld is not completed
in one direction. Short runs, for example using the back-step or skip welding
technique, are very effective in distortion control (Figure 19.13).

Figure 19.13 Use of welding direction to control distortion.

a) Back-step welding.
b) Skip welding.

Back-step welding involves depositing short adjacent weld lengths - the

appropriate direction to the general progression, Figure19.13a.

Skip welding is laying short weld lengths in a predetermined, evenly spaced,

sequence along the seam (Figure 19.13b). Weld lengths and the spaces
between them are generally equal to the natural run-out length of one
electrode. The direction of deposit for each electrode is the same, but it is
not necessary for the welding direction to be opposite to the direction of
general progression.

19.7.3 Best practice

The following fabrication techniques are used to control distortion:

Tack welds to set-up and maintain the joint gap.

Identical components welded back-to-back so welding can be balanced
about the neutral axis.
Attachment of longitudinal stiffeners to prevent longitudinal bowing in
butt welds of thin plate structures.
Where there is a choice of welding procedure, process and technique
should aim to deposit the weld metal as quickly as possible; MIG/MAG in
preference to MMA or gas welding and mechanised rather than manual
welding.
In long runs, the whole weld should not be completed in one direction;
back-step or skip welding techniques should be used.

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19.8 Corrective techniques

Distortion should be avoided at the design stage and by using suitable
fabrication procedures. As it is not always possible to avoid distortion during
fabrication, several well-established corrective techniques can be used.
Reworking to correct distortion should not be undertaken lightly as it is
costly and needs considerable skill to avoid damaging the component.

General guidelines are provided on best practice for correcting distortion

using mechanical or thermal techniques.

19.8.1 Mechanical techniques

The principal techniques are hammering which may cause surface damage
and work hardening.

With bowing or angular distortion the complete component can often be

straightened on a press without the disadvantages of hammering. Packing
pieces are inserted between the component and the platens of the press. It
is important to impose sufficient deformation to give over-correction so that
the normal elastic spring-back will allow the component to assume its
correct shape.

Figure 19. 14 Use of press to co"ect bowing in a T butt joint.

Pressing to correct bowing in flanged plate in long components distortion is

removed progressively in a series of incremental pressings; each one acting
over a short length. With flanged plate, the load should act on the flange to
prevent local damage to the web at the load points. As incremental point
loading will only produce an approximately straight component, it is better to
use a former to achieve a straight component or to produce a smooth
curvature.

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Best practice for mechanical straightening

The following should be adopted when using pressing techniques to remove
distortion:

Use packing pieces which will over-correct the distortion so that the
spring-back will return the component to the correct shape.
Check that the component is adequately supported during pressing to
prevent buckling.
Use a former or rolling to achieve a straight component or produce a
curvature.
As unsecured packing pieces may fly out from the press, the following
safe practices must be adopted:
- Bolt the packing pieces to the platen.

19.8.2 Thermal techniques

The basic principle is to create sufficiently high local stresses so that on
cooling the component is pulled back into shape.

Achieved by locally heating the material to a temperature where plastic

deformation will occur as the hot, low yield strength material tries to expand
against the surrounding cold, higher yield strength metal. On cooling to
room temperature the heated area will attempt to shrink to a smaller size
than before heating. The stresses generated pull the component into the
required shape (Figure 19.15),

Figure 19.15 Localised heating to correct distortion.

Local heating is a relatively simple but an effective means of correcting

welding distortion. Shrinkage level is determined by size, number, location
and temperature of the heated zones. Thickness and plate size determine
the area of the heated zone, number and placement of heating zones are
largely a question of experience and for new jobs, tests will often be needed
to quantify the level of shrinkage.

Spot, line, or wedge~shaped heating techniques can be used in thermal

correction of distortion.

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Spot heating

Q
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Figure 19.16 Spot heating for correcting buckling.

Spot heating is used to remove buckling, for example when a relatively thin
sheet has been welded to a stiff frame. Distortion is corrected by spot
heating on the convex side. If the buckling is regular, the spots can be
arranged symmetrically, starting at the centre of the buckle and working
outwards.

line heating

Figure 19.17 Line heating to correct angular distortion in a fillet weld.

Heating in straight lines is often used to correct angular distortion, for

example, in fillet welds, Figure 19.17. The component is heated along the
line of the welded joint but on the opposite side to the weld so the induced
stresses will pull the flange flat.

Wedge-shaped heating
To correct distortion in larger complex fabrications it may be necessary to
heat whole areas in addition to using line heatihg. The pattern aims at
shrinking one part of the fabrication to pull the material back into shape.

Apart from spot heating of thin panels, a wedgeshaped heating zone should
be used; Figure 19.18 from base to apex and the temperature profile should
be uniform through the plate thickness. For thicker section material it may
be necessary to use two torches, one on each side of the plate.

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Figure 19.18 Use of wedge-shaped heating to straighten plate.

As a general guideline, to straighten a curved plate wedge dimensions

should be:
Length of wedge - two-thirds of the plate width.
Width of wedge (base), one sixth of its length (base to apex).

The degree of straightening will typically be 5mm in a 3m length of plate.

Wedge-shaped heating can be used to correct distortion in a variety of
situations, Figure 19.19:

a. b.

c.

Figure 19.19 Wedge-shaped heating to correct distortion:

a) Standard rolled section which needs correction in two planes.
b) Buckle at edge of plate as an alternative to rolling;
c) Box section fabrication distorted out of plane.

TWI 19-23

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General precautions
The dangers of using thermal straightening techniques are over-shrinking
too large an area or causing metallurgical changes by heating to too high a
temperature. When correcting distortion in steels the temperature of the
area should be restricted to approximately 600-650C, dull red heat. If the
heating is interrupted ,or the heat lost, the operator must allow the metal to
cool then begin again.

Best practice for distortion correction by thermal heating

The following should be adopted when using thermal techniques to remove
distortion:

Use spot heating to remove buckling in thin sheet structures.

Other than in spot heating of thin panels, u"se a wedge-shaped heating
technique.
Use line heating to correct angular distortion in plate.
Restrict the area of heating to avoid over~shrinking the component.
Limit the temperature to 600-650C (dull red heat) in steels to prevent
metallurgical damage.
In wedge-shaped heating, heat from the base to the apex of the wedge,
penetrate evenly through the plate thickness and maintain an even
temperature.

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 Section 20

Heat Treatment
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Heat Trealment
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20 Heat Treatment
20.1 Introduction
The heat treatment given to a particular grade of steel by the steelmaker/
supplier should be shown on the material test certificate and may be
referred to as the supply condition.

Welding inspectors may need to refer to material test certificates so must be

familiar with the terminology used and have some understanding of the
principles of some of the most commonly applied heat treatments.

Welded joints may need to be subjected to heat treatment after welding

(PWHT) and the tasks of monitoring the thermal cycle and checking the heat
treatment records are often delegated to welding inspectors.

20.2 Heat treatment of steel

The main supply conditions for weldable steels are:

As-rolled, hot roller and hot finished

Plate is hot rolled to finished size and allowed to air cool; the temperature at
which rolling finishes may differ from plate to plate and so strength and
toughness properties vary and are not optimised.

Applied to
Relatively thin, lower strength C-steel.

Thermomechanical controlled processing (TMCP), control-rolled,

thermomechanically rolled
Steel plate given precisely controlled thickness reductions during hot rolling
within carefully controlled temperature ranges; final rolling temperature is
also carefully controlled.

Applied to
Relatively thin, high strength low alloy (HSLA) steels and some steels with
good toughness at low temperatures, eg cryogenic steels.

Normalised
After working (rolling or forging) the steel to size, it is heated to -9oooc then
allowed to cool in air to ambient temperature; which optimises strength and
toughness and gives uniform properties from item to item for a particular
grade of steel (Figure 20.1).

Applied to
C-Mn steels and some low alloy steels.

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Normalising:
Rapid heating to soak temperature (100% austenite)
Short soak time at temperature
Cool in air to ambient temperature

Time
Figure 20.1 Typical normalising heat treatment applied to C-Mn and some /ow
alloy steels.

Quenched and tempered

After working the steel (rolling or forging) to size it is heated to then -9oooc
cooled as quickly as possible by quenching in water or oil. After quenching,
the steel must be tempered (softened) to improve the ductility of the as-
quenched steel (Figure 20.2).

Applied to
Some low alloy steels to give higher strength toughness or wear resistance.

Quenching and tempering:

Rapid heating to soak temperature (100% austenite).
Short soak time at temperature.
Rapid cooling by quenching in water or oil.
Reheat to tempering temperature, soak and air cool.

Quenching
cycle

Time
Figure 20.2 A typical quenching and tempering heat treatment applied to some low
alloy steels.

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Solution annealed
Hot or cold working to size, steel heated to -11 oooc after.

Slab heating temperature > - 1050C

INNNIM
Austenite Rolling period
(y}

- 900C
- -- -- ---
Austenite + ferrite
(y+<X)

As-rolled or
Ferrite + pearlite hot rolled
(a) iron carbide

Time
Figure 20.3 Comparison of the control-rolled (TMCP) and as-rolled (hot rolling)
conditions.

Solution heat treated

Rapidly cooled by quenching in water to prevent any carbides or other
phases forming {Figure 20.4).

Solution heat treatment:

Rapid heating to soak temperature (100% austenite}.
Short soak time at temperature.
Rapid cool cooling by quenching in water or oil.
o
0
> -1050"C
PI
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a.
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Time

Figure 20.4 Typical solution heat treatment (solution annealing) applied to

austenitic stainless steels.

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Applied to
Austenitic stainless steels such as 304 and 316 grades.

Annealed
After working the steel (pressing or forging, etc) to size, it is heated to
-900C then allowed to cool in the furnace to ambient temperature; this
reduces strength and toughness but improves ductility (Figure 20.5).

Annealing:
Rapid heating to soak temperature (1 00% austenite).
Short soak time at temperature.
Slow cool in furnace to ambient temperature.

p - 900"C
~
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Time
Figure 20.5 Typical annealing heat treatment applied to C-Mn and some low alloy
steels.

Applied to
C-Mn steels and some low alloy steels.

Figures 20.1-20.5 show thermal cycles for the main supply conditions and
subsequent heat treatment that can be applied to steels.

20.3 Postweld heat treatment (PWHT)

Postweld heat treatment has to be applied to some welded steels to ensure
that the properties of the weldment is suitable for the intended applications.

The temperature at which PWHT is usually carried out well below the
temperature where phase changes can occur (see Note), but high enough
to allow residual stresses to be relieved quickly and to soften (temper) any
hard regions in the HAZ.

Note There are circumstances when a welded joint may need to be

normalised to restore HAZ toughness, these are relatively rare and it is
necessary to ensure that welding consumables are carefully selected
because normalising will significantly reduce weld metal strength.

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The major benefits of reducing residual stress and ensuring that the HAZ
hardness is not too high for steels for particular service applications are:

Improves the resistance of the joint to brittle fracture.

Improves the resistance of the joint to stress corrosion cracking.
Enables welded joints to be machined to accurate dimensional
tolerances.

Because the main reason for and benefit of PWHT is to reduce residual
stresses, PWHT is often called stress-relief.

20.4 PWHT thermal cycle

The Application Standard/Code will specify when PWHT is required to give
the first two benefits above and also 9ive guidance about the thermal cycle
that must be used.

To ensure that a PWHT cycle is carried out in accordance with a particular

Code, it is essential that a PWHT procedure is prepared and the following
parameters are specified:

Maximum heating rate.

Soak temperature range.
Minimum time at the soak temperature (soak time).
Maximum cooling rate.

20.4.1 Heating rate

Must be controlled to avoid large temperature differences, (large thermal
gradients) within the fabricated item. Which will produce large stresses and
may be high enough to cause distortion or even cracking .

Application Standards usually require control of the maximum heating rate

when the temperature of the item is above -300C because steels start to
show significant loss of strength above this temperature and are more
susceptible to distortion if there are large thermal gradients.

The temperature of the fabricated item must be monitored during the

thermal cycle by thermocouples attached to the surface at locations
representing the thickness range of the item.

By monitoring fumace and item temperatures the rate of heating can be

controlled to ensure compliance with Code requirements at all positions
within the item.

Maximum heating rates specified for C-Mn steel depend on the thickness of
the item but tend to be in the range -60 to -200"C/h.

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20.4.2 Soak temperature

The soak temperature specified by the Code depends on the type of steel
and thus the temperature range required reducing residual stresses to a low
level.

C and C-Mn steels require a soak temperature of -soooc whereas some

low alloy steels (such as Cr-Mo steels used for elevated temperature
service) require higher temperatures, typically in the range -700--760C.

Soak temperature is an essential variable for a WPQR, so it is very

important it is controlled within the specified limits otherwise it may be
necessary to carry out a new WPQ test to validate the properties of the item
and at worst it may not be fit-for-purpose.

20.4.3 Soak time

It is necessary to allow time for all the welded joints to experience the
specified temperature throughout the full joint thickness.

The temperature is monitored by surface contact thermocouples and it is

the thickest joint of the fabrication that governs the minimum time for
temperature equalisation.

Typical specified soak times are 1h per 25mm thickness.

20.4.4 Cooling rate

It is necessary to control the rate of cooling from the PWHT temperature for
the same reason that 1heating rate needs to be controlled, to avoid distortion
or cracking due to high stresses from thermal gradients.

Codes usually specify controlled cooling to -300C. Below this temperature

the item can be withdrawn from a furnace and allowed to cool in air because
steel is relatively strong and unlikely to suffer plastic strain by any
temperature gradients that may develop.

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PWHT (C-Mn steels):

Controlled heating rate from 300C to soak temperature.
Minimum soak time at temperature.
Controlled cooling to - 300C.

u
0

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{!!.
and cooling rates
-300C
I ......... e

Soak
time Air cool

Time
Figure 20.6 Typical PWHT applied to C-Mn steels.

20.5 Heat treatment furnaces

Oil- and gas-fired furnaces used for PWHT must not allow flame contact
with the fabrication as this may induce large thermal gradients.

It is also important to ensure that the fuel particularly for oil-fired furnaces
does not contain high levels of potentially harmful impurities, such as
sulphur.

20.5.1 Local PWHT

For a pipeline or pipe spool it is often necessary to apply PWHT to individual
welds by local application of heat.

For this, a PWHT procedure must specify the previously described

parameters for controlling the thermal cycle but it is also necessary to
specify the following:

Width of the heated band (must be within the soak temperature range).
Width of the temperature decay band (soak temperature to -300C).

Other considerations are:

Position of the thermocouples in the heated band width and the decay
band.
If the item needs support in a particular way to allow movement/
avoid distortion.

The commonest method of heating for local PWHT is by insulated electrical

elements (electrical mats) attached to the weld.

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Gas-fired, radiant heating elements can also be used.

Figure 20.7 shows typical control zones for localised PWHT of a pipe butt
weld.

Figure 20.7 Local PWHT of a pipe girth seam.

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