Weld Quality

WELDDISCONTINUITIES

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Lecture 12 p

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Lecture Scope

• Nature & causes of discontinuities in steelfusion welds>

• Significance for weld fitness-for-service

• Preventive measures

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Lecture 12

Weld Discontinuities

• Weld discontinuities are interruptions of thetypical structure of a weldment such as a lackof continuity in its physical, metallurgical ormechanical characteristics

• Discontinuities may be found in the weldmetal, the heat affected zone and the basemetal.

• Discontinuities that exceed establishedacceptance standards are termed "defects"

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Causes of Weld Discontinuity I

• Discontinuities can be thought of asoriginating in two sets of causes:1. Techno/au/cal: related to metallurgy and welding

process2. Execution: related to welding operator skill in

implementing procedures etc.

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Types of Weld Discontinuity• Discontinuities are usually classified according to their

physical characteristics

Lecture 12

Type Location Fig Ref

Cracks W,HAZ 12Laminations, BM 8,9,10seams and lapsLamellar tears BM 11

Porosity/Cavities W 1

Inclusions W 2

Lack of fusion or W 3,4penetration

Imperfect shape:undercut, BM, 5,overfill, W, 6,overlap W 7

W=weldHAZ=heat affected zoneBM=base metal

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Weld Defe!cts:Groove Weld

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Published by The Welding Institute,Abington Hall, Abington,Cambridge CB1 6AL, UK

C 1975, 1979, 1986

First published 1972Second edition 1986

ISBN 0 85300207 X

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This set of photographs illustrates the main faults found infusion welds in constructional steels. The notes in this bookie',provide a brief identification and description of the faultsillustrated; they do not attempt to provide a comprehensivetext.

Terms relating to weld imperfections are defined in BritishStandard BS 499: Part 1: 1983. The examples have beenchosen to illustrate the defects, and do not indicateacceptance levels, for which see the appropriate productstandards.

Each section has notes on causes, and where appropriate, onrp.ctifir.~tio!"'! ~!"'!d o!"'! pre'../e~tk~n.

Most of the photographs also appear in The Wp,lding Institutewallchart, 'Faults in fusion welds in constructional steels' (Ref.C1-86).

CAUSESCommon causes of faults in welds made by the conventionalarc welding processes are briefly described.

RECTIFICATIONMeasures to remove the defect, if it exceeds the applicableacceptance criteria. are outlined.

PREVENTIONPreventive measures, where they do not consist simply of anobvious treatment of the cause, are outlined.

Some joints call for considerable welder skill to obtain anacceptable weld; in these cases, preventive measures mayinvolve selection, training, and approval testing of welders.

..-.

SUDE SET 2The photographs are also available as a slide set, numbered0240 to 0279. Each slide carries its number in the picture area,repeated on the slide mount and indicating correct orientation.

DETECTIONSurface defects are normally detected by visual inspection,penetrant dye or magnetic particle inspection: in some cases itis feasible to use eddy current or potential drop testing.

To detect embedded defects.. volumetric methods, principallyradiography and ultrasonics, are needed: they also indicatesurface defects. The performance of either method dependscritically on orientation of the defect to the beam: this must betaken into account when selecting a test procedure.

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INDEXCRACKS0241,0242 Solidification cracking0243 Hydrogen-induced HAZ cracking0244, 0245, 0246 Hydrogen-induced weld metal cracking0247,0248 Lamellar tearing0249, 0250 Reheat cracking,..""ITI~~"",., ... I I ...'"

0251, 0252, 0253 Wormholes0254, 0255 Uniform porosity0256 Restart porosity0257 Surface porosity0258 Crater pipes0259 Microporosity

INCLUSIONS0260 Linear inclusions0261, 0262 Isolated inclusions

LACK OF FUSION AND PENETRATION0263 Lack of side fusion0264 Lack of inter-run fusion0265 Lack of root fusion0266, 0267 Incomplete root penetration

IMPERFECT SHAPE0268 Linear misalignment0269 Excess weld metal0270 Overlap0271 Undercut0272, 0273 Excess penetration bead0274, 0275 Root concavity

MISCELLANEOUS FAULTS0276 Stray arcing0277 Spatter0278 Copper pickup0279 Surface pocking

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CRACKSSOLIDIFICATION CRACKING

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CAUSESExcessively deep or wide weld bead.High current and/or walding speed.Large root gap.Carbon, sulphur or phosphorus pickup from parent steal.The risk of cracking is greatest with submerged-arc welding,and reduced with gas-shielded metal-arc, self-shielded metalare, manual metal are, to least with gas-shielded tungsten-arcHigh strength parent metal increases the risk.

RECTIFICATION("1-1+ nlt+ ,.."""4,. : 1'" I L. _1 .. - ~ --' •• •• • .____ "",,1'1' OWIU n;;JIlldl.lI J.IIU;) ""11" ut1yuna VISIDle enaocrack and reweld.

PREVENTIONPlan welding parameters to reduce thermally induced strains.Adjust weld parameters to obtain weld width between 0.5 al'l!0.8 of depth. .'Avoid parent steels containing more than 0.6% total of sulphland phosphorus.Take care if submerged-arc welding steel with more than 0.2".carbon.Clean off all traces of cutting oils.Control joint fit-up to reduce gaps.Reduce weld metal dilution by reducing welding current, bywelding with electrode negative, or in the submerged-arcprocess with metal powder additions.

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CAUSESHardened HAZ - heat affected zone of the parent metal ­coupled with presence of hydrogen diffused from weld metal,caused by damp electrodes or flux, or contaminated wire.Susceptibility increases with increasing thickness of section,especially in steels with high carbon equivalent.Excessively fast cooling, allowing insufficient time forhydrogen to diffuse out at higher temperatures - crackingdoes not occur above about 200·C.High restraint. excessive root gaps, stress concentrations.Risk greatest with manual metal arc welding with cellulosic orrutile electrodes; least with gas-shielded tungsten-arc, solidwire gas-shielded metal-arc and manual metal arc withaustenitic or nickel-base or adequately dried and controlledbasic electrodes. NOTE: the cracks are fine and difficult todetect; they may not be found until after postweld heattreatment. They grow slowly, so inspection should be delayed.

8

RECTIFICATIONShort isolated lengths of crack, where accessible, can be cutout and rewelded. Extensive or inaccessible cracking may notbe possible to rectify. Austenitic or nickel-base electrodes maybe helpful if preheat must be limited. Note that any repair weldshould use the correct procedures as detailed below.

PREVENTIONUse the appropriate procedures, which may include:Hydrogen-controlled welding consumables;Consumables dried and stored in accordance with them~nllf~,...tllrcr'C! inC!t,.ll"ti "'t:'>·_.. -._--_. _. - '''-'' ... -~, " ... ,Preheat and minimum interpass temperatures reaching thespecified value in the whole joint;Heat input as specified, even for tack welds;Parent metal composition within limits specified for theprocedure in use.Avoid use of excessively strong weld metal.

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May be transverse (0244), at 90" or 45° to surface (0245).or longitudinal (0246).

CAUSESWeld metal alloyed as much as, or more than, parent steel,usually in thick sections with insufficient consumable drying orpreheat.High restraint.Excessive gap.Sharp notch in root run.

PREVENTIONAs for HAZ cracking.

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1011

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LAMEL .A TEARING

Fracture surface

12

CAUSESPoor ductility in through-thickness direction because ofnon-metallic inclusions.Only found in rolled plate.Occurs mainly where weld metal is deposited on plate surface,such as T, corner, and cruciform joints, and where restraint ishigh.

RECTIFICATIONGouge out cracked area. Butter affected plate with low~trp.nnth wplrl mpt~1 tn c:.nr,:J~rl rpc:irlll~l <:trQc:c:ac n\lor ::I brno_ , - . ----- --~ ---_ . ..,-area.

PREVENTIONDesign joints to avoid through-thickness tensile stress.Specify plate with sufficient through-thickness (shorttransverse) ductility, that is with a low sulphur and oxygencontent.Use forgings or castings instead of plate.

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treatment, particularly if they contain chromium, molybdenumand vanadium.

CAUSESPoor creep ductility in HAZ coupled with thermal stress.Accentuated by severe notches such as pre-existing cracks, ortears at weld toes, or unfused root of partial penetration weld.

PREVENTIONHeat treatment may need to include low temperature soaking,followed by fairly rapid heating to final temperature.Grinding or peening of weld toes after welding can beh"n"fi"i,,1Can be avoided by 'two-layer' welding technique to refine thecoarse-grained heat affected zone, together with the use ofnon-susceptible weld metal.

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Fracture surface

Resulting from entrapment of gas between solidifyingdendrites of weld metal, often showing a 'herring-bone' array,as in radiograph 0252,

CAUSESThe gas may arise from contamination of surfaces to bewelded, or be prevented from escaping from beneath the weldby joint crevices,

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Gas may originate from dampness or grease on consumablesor workpiece. or by nitrogen contamination from theatmosphere. If the welding wire used contains insufficientdeoxidant it is also possible for carbon monoxide to causeporosity.

PREVENTIONRemove the sources of gas by avoiding or removingcontamination.Ensure correct shielding gas flow.Use consumables with higher levels of deoxidants.Where porosity is the result of an intentional surfacetreatment, such as priming paint, remove it locally.

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RESTAh. t>OROSITY

Radiograph

CAUSESUnstable arc conditions at weld start, where protection may beincomplete and temperature gradients have not had time toequilibrate, coupled with inadequate manipulative technique toallow for this instability.

The radiograph shows internal restart porosity.

PREVENTIONImprove the welder's restart technique.Start on a run-on tab if practicable.

20

SURFACE POROSITY

CAUSESExcessive contamination from grease, dampness, oratmospheric entrainment.Occasionally caused by excessive sulphur in consumables orparent metal.

PREVENTIONAs for uniform porosity.Reduce sulphur content where appropriate.

21

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Resulting from shrinkage at the end crater of a weld run.

CAUSESIncorrect manipulative technique or current decay to allow forcrater shrinkage.

PREVENTIONImprove welder's technique.Use a welding set with a current decay facility.Use a run-off tab.

22

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Resulting from shrinkage at, or just below, the surface of asubmerged-arc weld.

CAUSESHigh welding speed with deep weld pool.

23

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LINEAR INCLUSIONS

Radiogrsph

CAUSESIncomplete removal of slag in multipass welds, oftenassociated with the presence of undercut or irregular surfacesin underlying passes. The radiograph in the slide shows twoslag lines in a weld root zone.

25

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LACK OF FUSION ANUPENETRATIONCAUSESIncorrect welding conditions, such as too Iowa current, orincorrect torch/gun angle.Incorrect edge preparation, for example too large a root face.

LACK OF SIDE FUSION

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EXCESS WELD METALAlternative term - REINFORCEMENT

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Weld metal normally extends above the parent metal surface:only a fault if exceeding specification,

CAUSESDeposition of too much weld metal, often associated withunsuitable weld preparation, incorrect welding parameters, ortoo large an electrode,

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CAUSESPoor manipulative technique or insufficient heat input, withcurrent or voltage too low,Depositing too large a fillet weld run in the horizontal-vertical

position.

Results from washing away of the edge preparation whenmolten.

CAUSESPoor welding technique and/or unbalanced weldingconditions.

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EXCESS PENETRATION BEAD

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Weld metal normally extends below the parent metal surface:only a fault if exceeding specification.

CAUSESIncorrect edge preparation providing insufficient support at theroot, and/or incorrect welding parameters.

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MISCELLANEOUS FAULTSSTRAY ARCING

CAUSESAccidental contact of electrode or welding torch with platesurface remote from weld. These usually result in small hardspots just beneath the surface which may contain cracks, and

are thus to be avoided.

CAUSESIncorrect welding conditions, such as too high a weldingcurrent, and/or contaminated consumables or preparations,giving rise to explosions within the arc and weld pool.Globules of molten metal are thrown out, and adhere to theparent metal remote from the weld.

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CAUSESMelting of copper contact tube in gas-shielded metal-arcwelding because of incorrect welding conditions.

42

Surface depression on face of submerged-arc weld.

CAUSESFlux inadequately dried or containing insufficient deoxidants.

43

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