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  • 7/27/2019 03 Welding Imperfections 29-11-03

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 1 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Welding Imperfections:

    What are welding imperfections?

    Welding imperfections are material discontinuities caused by, or during, the process of

    welding.

    All things contain imperfections, but it is only when they fall outside of a level ofacceptance that they should be termed defects, as they may render the productdefective, or unfit for its purpose.

    As welds can be considered as castings they may contain all kinds of imperfectionsassociated with the casting of metals, plus any other particular imperfections associated

    with the specific welding process being used.

    Welding imperfections can be classified as follows:

    1) Cracks 2) Gas pores and cavities3) Solid inclusions 4) Lack of fusion5) Surface and profile 6) Mechanical/Surface damage7) Misalignment

    1) Cracks:Cracks sometimes occur in welded materials, and may be caused by a great number of

    factors. Generally, we can say that for any crack like imperfection to occur in a material,

    there are 3 criteria that must be present:

    a) A force b) Restraint c) A weakened structureTypical types of hot and cold cracks that will be discussed later in the course are:

    1) H2 Cracks 2) Solidification Cracks 3) Lamellar Tears

    A Materials likelihood to crack during welding can be evaluated under the term

    Weldability. This may be defined as:

    The ease with which materials may be welded by the common welding processes

    All cracks have sharp edges, which produce high stress concentrations. This generally

    results in rapid progression, however this also depends on the properties of the metal.

    Cracks are classed as planar imperfections as they generally have only 2 visible, ormeasurable dimensions i.e. length and depth. Most fall into the defects category, though

    some standards will allow a degree of so calledcrater, or star cracking.

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 2 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    2) Gas pores, porosity and cavities:Gas pores:These are defined as singular gas filled cavities < 1.6mm diameter, which are createdduring final solidification by expulsion of gases from solution in solidifying weld metal.

    Porosity:Porosity is a term used to describe a collection, or area of gas pores. These areas may be

    classified by their number, size and grouping of the pores within the area. (i.e. Fine, or

    coarse cluster porosity) Porosity is mainly produced when welding improperly cleaned

    plate, or when using damp welding consumables. Gases may also be formed by the

    breakdown of paints, oil based products, corrosion or anti corrosion products that have

    been left on the plates to be welded. A singular gas filled cavity = or > 1.6mm diameter

    is termed a blow hole

    Porosity can be frequently formed during the MIG orTIG process by the temporary lossof gas shield, and ingress of air into the arc column. This may be caused by movement ofthe surrounding atmosphere, or wind. Porosity may also be caused by improper settings

    of shielding gas flow rate. Gas pores may also be break the welds surface where they are

    collectively known as surface porosities. Porosity may also found in deep Sub Arc weldsdue to the distance that trapped gases formed in the root area need to travel to escape

    from the surface, and may also occur when using damp MMA welding electrodes, ordamp Sub Arc Fluxes. Porosity may be prevented by correct cleaning of materials,correct setting and shielding when using the TIG or MIG welding processes, and using

    dry welding consumables. Porosity may generally be identified on a radiograph as a

    spherical imperfection that has varying density shades, from highest in the centre,

    decreasing to its outer edges i.e.

    Shrinkage cavities:These are internal voids, or cavities that are formed during the solidification of single

    welds of high depth to width ratio (d:w) as with SAW or MIG. They may be defined asa hot plastic tears caused by opposing contractional strains. Shrinkage cavities producehigh stress concentrations at their sharp edges, and are thus generally treated as cracks.

    Surface breaking cluster porosity

    Fine cluster porosityBlow hole > 1.6 mm

    Hollow root bead

    Coarse cluster porosityShrinkage cavity

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 3 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    3) Solid inclusions:Solid inclusions include metallic andnon-metallic inclusions that may be trapped in the

    weld during the process of welding. The type of solid inclusion that may be expected isreally dependant on the welding process being used. In welding processes that use fluxes

    to form all the required functions of shielding and chemical cleaning, such as MMA and

    Submerged Arc welding, slag inclusions may occur. Other welding processes such as

    MIG and TIG use silicon, aluminium and other elements to de-oxidise the weld. These

    may form silica, or alumina inclusions. Any of these non-metallic compounds may be

    trapped inside a weld during welding. This often happens after slag traps, such as

    undercut have been formed. Slag traps are mostly caused by incorrect welding technique.

    Metallic inclusions include tungsten inclusions that may be produced during TIG

    welding by a poor welding technique, an incorrect tungsten vertex angle, or too high

    amperage for the diameter of tungsten being used. Copper inclusions may be caused

    during MIG/MAG welding by a lack of welding skill, or incorrect settings in

    mechanised, or automated MIG welding. (Mainly welding Aluminium alloys)

    Other welding phenomena such arc blow or the deviation of the electric arc by

    magnetic forces, can cause solid inclusions to be trapped in welds. The locations of these

    inclusions may be within the centre of a deposited weld, or between welds where the

    result causes Lack of inter-run fusion, or at the sidewall of the weld preparation

    causing Lack of side wall fusion Generally solid internal inclusions may be caused by:

    1) Lack of welder skill. (Incorrect welding technique)2) Poor manipulation of the welding process, or electrode.3) Incorrect parameter settings, i.e. voltage, amperage, speed of travel.4) Magnetic arc blow.5) Incorrect positional use of the process, or consumable.6) Incorrect inter-run cleaning.

    Internal solid

    Internal solid inclusion causinga lack of sidewall fusion

    Solid inclusions from base metalundercut in the root run, or hotpass (Slag traps)

    Surface breaking solid inclusionInternal solid inclusioncausing a lack of inter-runfusion

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 4 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    4) Lack of fusion:Lack of fusion imperfections, are defined as a lack of union between two adjacent areas

    of material. This may be accompanied, or caused by other imperfections as explained in

    the last section. Lack of fusion can be considered a serious imperfection, as like cracks,

    they produce areas of high stress concentration. Lack of fusion, or overlap (a form of

    lack of fusion) may occur in the weld face area during positional welding caused by the

    action of gravity and incorrect use of the process.

    Arc blow is a prime cause of lack of fusion imperfections, particularly when using highcurrent processes, such as Sub Arc using high direct electric currents. (DC+ or DC -)

    Lack of fusion may also be formed in the root area of the weld where it may be found on

    one, or both plate edges. It may also be accompanied by incomplete root penetration.

    Lack of fusion is also a common imperfection in Dip transfer MIG welding of metals

    over 3mm thickness, especially when welding vertically down. This is caused by the

    inherent coldness of this form of metal transfer, and the action of gravity.

    Like solid inclusions, lack of fusion imperfections may be caused by:

    1) Lack of welder skill. (Incorrect welding technique)2) Poor manipulation of the welding process, or electrode.3) Incorrect parameter settings, i.e. voltage, amperage, speed of travel.4) Magnetic arc blow.5) Incorrect positional use of the process, or consumable.6) Incorrect inter-run cleaning.7) Incorrect or non-featheredtack welds. (Lightly ground prior to welding)

    Lack of root fusion

    Overlap (Causing a Cold laps)

    Lack of inter-run

    Lack of sidewall fusion

    Lack of sidewall fusion(Also causing an Incompletely filled groove)

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 5 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    5) Surface and profile:Surface and profile imperfections are generally caused by poor welding techniques. This

    includes the use of incorrect welding parameters, electrode/blowpipe sizes and/or

    manipulation and joint set up. This category may be split into two further groups of weld

    face and weld root. Surface and profile imperfections are shown pictorially in A & Bbelow:

    A:

    Spatter is not a major factor in lowering the weldments strength, though it may maskother imperfections, and should therefore be cleaned off before inspection.

    Spatter may also hinder NDT and be detrimental to coatings. It can also cause micro

    cracking or hard spots in some materials due to the localised heating/quenching effect.

    An incompletely filled groove may bring the weld below its DTT. It is a major stressconcentration when accompanied by lack of sidewall fusion.

    Lack of root fusion causes a serious stress concentration to occur in the root.It may also render the root area more susceptible to corrosion in service

    An Incompletely filled groove

    Lack of root fusion

    Spatter

    A

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 6 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    B:

    A bulbous contour is an imperfection as it causes sharp stress concentrations at the toes

    of individual passes and may also contribute to overall poor toe blend

    Arc strikes, Stray-arcing, or Stray flash may cause many problems including severaltypes of cracks to occur. They can also cause depressions in the plate bringing it below

    its DTT. Arc strikes would normally be NDT inspected and then repaired.

    Incomplete root penetration may be caused by too small a root gap, insufficientamperage, or poor welding technique. It also causes high stress concentrations to occur.

    It also generally produces a weld with less throat thickness than the DTT of the joint.

    Poor toe blend

    Bulbous contour

    Arc Strikes

    Incomplete rootpenetration bead

    B

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 7 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    C:

    An irregular bead width is a surface imperfection, which is often referenced inapplication standards as. The weld bead should be regular along its linear length

    Undercut:

    Undercut can be defined as a depression at the toe of a weld in a previous deposited

    weld, or base metal, caused by welding. Undercut is generally caused by incorrect

    welding technique, including the use of too high a current for the electrode being used,

    and the welding position. It is often caused in the top toe of fillet welds when attempting

    to produce a large leg length fillet weld in one run. Undercut can also be considered a

    serious imperfection particularly if it is sharp, as again it causes high stress

    concentrations. It is gauged in severity by its length, depth and sharpness.

    Parent metal, top toe undercut

    Parent metal, surface undercut

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 8 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Shrinkage grooves:

    Shrinkage grooves may occur in the root area and are caused by contractional forces

    pulling on the hot plastic base metal in the root area. It is often mistakenly termed as rootundercut.

    Weld metal, surface undercut

    Shrinkage grooves

    Root Run or Hot Pass undercut

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 9 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Root concavity: (Suck back)

    This may be caused when using too high a gas backing pressure in purging. It may also

    be produced when welding with too large a root gap and depositing too thin a root bead,

    when the hot pass may pull back the root bead through contractional strains.

    Excess penetration:Often caused by using too high a welding current, and/or, slow travel speed, coupled with

    a large root gap, and/or a small root face for the current or process being used. It is often

    accompanied by burn through, which can be defined as a local collapse of the weld

    puddle causing a hole, or depression in the final weld root bead.

    Root oxidation:Root oxidation may take place when welding re-active metals such as stainless steels with

    contaminated, or inadequate purging gas flow.

    Incompletely fused Tack Welds:It is often a procedural requirement for tack welds to be feathered (Lightly ground and

    blended) prior to welding. This requirement is mainly dependent upon the class of work.

    Feathering should enable the tack welds to be more easily fused and thus more smoothly

    blended into the root/penetration bead during welding. Failure to achieve this correctly

    may result in a degree oflack of root fusion/penetration occurring in the weld root run.

    Root concavity

    An un-featheredroot tack weld

    Adjacent un-smootharea showing a lack ofroot fusion and/or rootpenetration

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 10 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Crater pipes:This often occurs during TIG welding in the crater at the end of a weld run during final

    solidification. It is caused by insufficient filler material to meet the solidification process

    and can be eliminated by applying adequate filler metal, or using a slope out control.

    Crater pipe

    Excess root penetration bead

    Root oxidation inStainless Steel

    This may lead to aburn throughA local collapse of the weld pool

    leaving a hole in the root area.

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 11 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    To summarize, we can list surface or profile welding imperfections as follows:

    1) Incompletely filled grove/lack of fusion.2) Spatter.3) Arc strikes. (Stray arcs)4) Incomplete root penetration.5) Lack of root fusion.6) Bulbous or irregular contour.7) Poor toe blend.8) Irregular bead width.9) Undercut. (Weld and Base metal)10) Root concavity. Root shrinkage grooves.11) Excess penetration. Burn through.12) Root oxidation.

    6) Mechanical/Surface damage:

    Mechanical/Surface damage:This can be defined as any surface material damage caused during the manufacturing

    process, or in-service conditions. This can include damage caused by:

    1) Grinding. 2) Chipping.3) Hammering. 4) Braking off welded attachments by hammering.5) Chiselling. 6) Using needle guns to compress weld capping runs.7) Corrosion.

    As with the stray arcing, the above imperfections can be detrimental as they reduce the

    through thickness dimension of the plate in that area. They can cause local stress

    concentrations and should be repaired prior to completing the job.

    Chisel Marks Pitting Corrosion Grinding Marks

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 12 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    7) Misalignment:

    There are 2 main forms of misalignment in plate materials, which are termed:

    1) Linear misalignment. 2) Angular misalignment.Linear misalignment: can be controlled during weld set up by the correct use/control ofthe weld set up technique i.e. tacking, bridging, clamping etc. Excess weld metal heightand the root penetration bead are always measured from the lowest plate to thehighest point of the weld metal, as shown below.

    Angular misalignment: may be controlled by the correct application of distortioncontrol techniques, i.e. balanced welding, offsetting, or use of jigs, clamps, etc.

    Hi-Lo is a term that is generally used to describe the unevenness across the root facesbetween pipes found during setting up for welding. This unevenness is often caused by

    an un-matching and/or irregular wall thickness, or between pipes having any degree of

    ovality. It is not a term that should be used when describing misalignment in plates.

    Angular misalignment measured in degrees

    15

    Linear misalignment measured in mm

    3 mm

    Excess weld metal height

    Hi-Lo

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 13 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Summary of Welding Imperfections:

    Group Type Causes/Location

    Centreline Weld Metal

    H2 Weld Metal & HAZ

    1) Cracks

    Lamellar Tears Base metal

    Porosity

    Gas pore < 1.6mm

    Blow hole > 1.6mm

    Damp electrodes

    Un-cleaned plates/pipes

    Loss of gas shield2) Porosity/Cavities

    Shrinkage cavity Weld metal (high d:w)

    Slag MMA/SAW

    Silica TIG/MAG(Fe steels)

    Poor Inter-run cleaning

    Slag traps. Arc blow

    Tungsten TIG Dipping tungsten in pool3) Solid Inclusions

    Copper (MIG/MAG) Dipping contact tip in pool

    Lack of side wall fusion

    (Can be surface breaking)

    Arc Blow

    Incorrect welding techniqueLack of root fusion Non feathering of tack welds

    4) Lack of Fusion

    Cold lapping Positional welding technique

    Poor toe blend Incorrect welding technique

    Arc Strikes Poor welding technique

    Incomplete penetration < Root gap/Amps. > Root face

    Incompletely filled groove Incorrect welding technique

    Spatter Damp consumables

    Bulbous contour Incorrect welding technique

    Undercut:

    Surface and internal

    Too high an amperage

    Poor welding technique

    Shrinkage groove (Root) Contractional strainsRoot concavity Too high gas pressure

    Excess Penetration

    Burn through

    > Root gap/Amps < Root face

    5) Surface & Profile

    Crater Pipes (Mainly TIG) Incorrect current decay

    6) Mechanical damage Hammer/Grinding marks etc. Poor workmanship

    Angular Misalignment () Poor fit-up. Distortion

    Linear Misalignment (mm) Poor fit-up.7) Misalignment

    Hi-Lo (mm)Only in pipe Irregular pipe wall, or ovality

    Notes:

    The causes given in the above table should not be considered as the only possiblecauses of the imperfection given, but as an example of a probable cause.

    Good working practices and correct welder training will minimise the occurrence ofunacceptable welding imperfections, or welding defects.

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 14 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Identify and name the following Welding Imperfections:(As indicated within the ovals)

    A A

    A A

    A A

    B B

    A

    B

    A

    B

    A A

    A A

    1 2

    3 4

    5 6

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 16 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    A A

    A A

    A A

    B B

    B

    A

    AB

    A

    A

    AA

    13 14

    15 16

    17 18

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    THE WELDING INSTITUTE

    Welding Inspection of Steels WIS 5Section 03 Welding Imperfections

    Rev 09-09-03 Copyright 2003, TWI Ltd

    3. 17 WORLD CENTRE FORMATERIALS JOINING

    TECHNOLOGY

    Solutions:

    Page 3:14

    1. Weld metal undercut

    2. Lack of root fusion

    3. Root concavity

    4. Incomplete root penetration

    5a. Lack of sidewall fusion 5b Cold lap (Bulbous contour)

    6a. Slag inclusion 6a Base metal undercut

    Page 3:15

    7. Pitting Corrosion

    8. Lack of sidewall fusion with an incompletely filled groove

    9. Spatter

    10. Bulbous contour with a Poor toe blend

    11a. Arc strikes 11b. Lack of root fusion

    12a. Base metal undercut 12b. Weld metal undercut

    Page 3:16

    13. Chisel marks

    14. Incompletely filled groove with a bulbous contour

    15. Shrinkage grooves

    16. Burn through

    17a. Spatter 17b. Undercut (In the top toe)

    18a. Excess root penetration bead 18b. Burn through