Lesson 5 - Distortion

7/24/2019 Lesson 5 - Distortion

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Distortion

What causes distortion?Because welding involves highly localised heating of joint edges to fuse the material, non-uniformstresses 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 poolis formed due to the thermal expansion of the hot metal (heat affected zone adjacent to the weldpool. !owever, tensile stresses occur on cooling when the contraction of the weld metal and theimmediate heat affected zone is resisted by the bul" of the cold parent metal.

#he magnitude of thermal stresses induced into the material can be seen by the volume change inthe weld area on solidification and subse$uent cooling to room temperature. %or example, whenwelding &'n steel, the molten weld metal volume will be reduced by approximately ) onsolidification and the volume of the solidified weld metal*heat affected zone (!+ will be reduced bya further ) 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 parentmetal, localised plastic deformation of the metal occurs. lastic deformation causes a permanentreduction in the component dimensions and distorts the structure.

Main types of distortion/istortion occurs in six main forms0

1ongitudinal shrin"age

#ransverse shrin"age

+ngular distortion

Bowing and dishing

Buc"ling

#wisting

#he principal features of the more common forms of distortion for butt and fillet welds are shown.

&ontraction of the weld area on cooling, results in both transverse 2 longitudinal shrin"age. 3on-uniform contraction (through thic"ness produces angular distortion in addition to above saidshrin"age.

%or example, in a single 4 butt weld, the first weld run produces longitudinal and transverseshrin"age and rotation. #he second run causes the plates to rotate using the first weld deposit as a


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fulcrum. !ence, balanced welding in a double side 4 butt joint can be used to produce uniformcontraction and prevent angular distortion.

5imilarly, in a single side fillet weld, non-uniform contraction produces angular distortion of theupstanding leg. /ouble side fillet welds can therefore be used to control distortion in the upstandingfillet but because the weld is only deposited on one side of the base plate, angular distortion will now

be produced in the plate.

1ongitudinal bowing in welded plates happens when the weld center is not coincident with theneutral axis of the section so that longitudinal shrin"age in the welds bends the section into a curvedshape. &lad plate tends to bow in two directions due to longitudinal and transverse shrin"age of thecladding6 this produces a dished shape. /ishing is also produced in stiffened plating. lates usuallydish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds

In plating, long-range compressive stresses can cause elastic buc"ling in thin plates, resulting indishing, bowing or rippling.

/istortion due to elastic buc"ling is unstable0 if you attempt to flatten a buc"led plate, it will probably7snap7 through and dish out in the opposite direction.

#wisting in a box section is caused by shear deformation at the corner joints #his is caused byune$ual longitudinal thermal expansion of the abutting edges. Increasing the number of tac" weldsto prevent shear deformation often reduces the amount of twisting.

Allowance for weld shrinkageIt is almost impossible to predict accurately the amount of shrin"ing. 3evertheless, a 7rule of thumb7has been composed based on the size of the weld deposit. 8hen welding steel, the followingallowances should be made to cover shrin"age at the assembly stage.

Transverse ShrinkageFillet Welds9.:mm per weld where the leg length does not exceed *; plate thic"nessButt weld99 4 joint, depending on number of runs

Longitudinal ShrinkageFillet Welds9.:mm per m of weldButt Weldsmm per m of weldIncreasing the leg length of fillet welds, in particular, increases shrin"age.

Factors affecting distortionIf a metal is uniformly heated and cooled there would be almost no distortion. !owever, because thematerial is locally heated and restrained by the surrounding cold metal, stresses are generatedhigher than the material yield stress causing permanent distortion. #he principal factors affecting the

type and degree of distortion, are0

arent material properties

+mount of restraint

?oint design

art fit-up

8elding procedure


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arent !aterial propertiesarent material properties, which influence distortion are coefficient of thermal expansion 2 specificheat per unit volume. +s distortion is determined by expansion and contraction of the material, thecoefficient of thermal expansion of the material plays a significant role in determining the stressesgenerated during welding and, hence, the degree of distortion. %or example, as stainless steel has ahigher coefficient of expansion than plain carbon steel, it is more li"ely to suffer from distortion.

"estraintIf a component is welded without any external restraint, it distorts to relieve the welding stresses. 5o,methods of restraint, such as 7strong-bac"s7 in butt welds, can prevent movement and reducedistortion. +s restraint produces higher levels of residual stress in the material, there is a greater ris"of crac"ing in weld metal and !+ especially in crac"-sensitive materials.

#oint designBoth butt and fillet joints are prone to distortion. It can be minimised in butt joints by adopting a jointtype, which balances the thermal stresses through the plate thic"ness.( %or eg, a double-sided inpreference to a single-sided weld . /ouble-sided fillet welds should eliminate angular distortion ofthe upstanding member, especially if the two welds are deposited at the same time.

art fit$up%it-up should be uniform to produce predictable 2 consistent shrin"age. @xcessive joint gap can alsoincrease the degree of distortion by increasing the amount of weld metal needed to fill the joint. #he

joints should be ade$uately tac"ed to prevent relative movement between the parts during welding.

Welding procedure#his influences the degree of distortion mainly through its effect on the heat input. +s weldingprocedure is usually selected for reasons of $uality and productivity, the welder has limited scope forreducing distortion. +s a general rule, weld volume should be "ept to a minimum. +lso, the weldingse$uence and techni$ue should aim to balance the thermally induced stresses around the neutralaxis of the component.

Distortion $ prevention %y design

Aeneral guidelines are given below as 7best practice7 for limiting distortion when considering thedesign of arc welded structures.

Design principles+t the design stage, welding distortion can often be prevented, or at least restricted, by considering0

elimination of welding

weld placement

reducing the volume of weld metal

reducing the number of runs

use of balanced welding

&li!ination of welding+s distortion and shrin"age are an inevitable result of welding, good design re$uires that not only theamount of welding is "ept to a minimum, but also the smallest amount of weld metal is deposited.8elding can often be eliminated at the design stage by forming the plate or using a standard rolledsection, as shown in %ig


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Fig' ( &li!ination of welds %y) a* for!ing the plate+ %* useof rolled or e,truded section

If possible, the design should use intermittent welds ratherthan a continuous run, to reduce the amount of welding. %orexample, in attaching stiffening plates, a substantial reductionin the amount of welding can often be achieved whilst

maintaining ade$uate strength.Weld place!entlacing and balancing of welds are important in designing forminimum distortion. #he closer a weld is positioned to the

neutral axis of a fabrication, the lower the leverage effect of the shrin"age forces and the finaldistortion. @xamples of poor and good designs are shown in %ig .

Fig' - Distortion !ay %e reduced %y placing the weldsaround the neutral a,is

+s most welds are deposited away from the neutral axis,distortion can be minimised by designing the fabrication sothe shrin"age forces of an individual weld are balanced by

placing another weld on the opposite side of the neutral axis.8henever possible, welding should be carried outalternately on opposite sides, instead of completing one sidefirst. In large structures, if distortion is occurringpreferentially on one side, it may be possible to ta"e

corrective actions. for eg, by increasing welding on the other side to control the overall distortion.

"educing the volu!e of weld !etal#o minimise distortion, as well as for economic reasons, the volume of weld metal should be limitedto the design re$uirements. %or a single-sided joint, the cross-section of the weld should be "ept assmall as possible to reduce the level of angular distortion, as illustrated in %ig .

Fig' . "educing the a!ount of angular distortion and lateralshrinkage %y) a* reducing the volu!e of weld !etal+ %* usingsingle pass weld

?oint preparation angle and root gap should be minimisedproviding the weld can be made satisfactorily. #o facilitate access,it may be possible to specify a larger root gap and smallerpreparation angle. By cutting down the difference in the amountof weld metal at the root and the face of the weld, the degree ofangular distortion will be correspondingly reduced. Butt jointsmade in a single pass using deep penetration have little angular

distortion, especially if a closed butt joint can be welded (%ig . %or example, thin section material

can be welded using plasma and laser welding processes and thic" section can be welded, in thevertical position, using electrogas and electroslag processes. +lthough angular distortion can beeliminated, there will still be longitudinal and transverse shrin"age.

In thic" section material, as the cross sectional area of a double-4 joint preparation is often only halfthat of a single-4 preparation, the volume of weld metal to be deposited can be substantiallyreduced. #he double-4 joint preparation also permits balanced welding about the middle of the jointto eliminate angular distortion.


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+s weld shrin"age is proportional to the amount of weld metal, both poor joint fit-up and over-welding will increase the amount of distortion. +ngular distortion in fillet welds is particularly affectedby over-welding. +s design strength is based on throat thic"ness, over-welding to produce a convexweld bead does not increase the allowable design strength but it will increase the shrin"age anddistortion.

"educing the nu!%er of runs#here are conflicting opinions on whether it is better to deposit a given volume of weld metal using asmall number of large weld passes or a large number of small passes. @xperience shows that for asingle-sided butt joint, or a single-side fillet weld, a large single weld deposit gives less angulardistortion than if the weld is made with a number of small runs. Aenerally, in an unrestrained joint,the degree of angular distortion is approximately proportional to the number of passes.

&ompleting the joint with a small number of large weld deposits results in more longitudinal andtransverse shrin"age than a weld completed in a larger number of small passes. In a multi-passweld, previously deposited weld metal provides restraint, so the angular distortion per passdecreases as the weld is built up. 1arge deposits also increase the ris" of elastic buc"lingparticularly in thin section plate.

/se of %alanced weldingBalanced welding is an effective means of controlling angular distortion in a multi-pass butt weld byarranging the welding se$uence to ensure that angular distortion is continually being corrected andnot allowed to accumulate during welding. &omparative amounts of angular distortion from balancedwelding and welding one side of the joint first are shown schematically in %ig ;. #he balancedwelding techni$ue can also be applied to fillet joints.

Fig' 0 Balanced welding to reduce the a!ount ofangular distortion

If welding alternately on either side of the joint is not

possible, or if one side has to be completed first, anasymmetrical joint preparation may be used with more weldmetal being deposited on the second side. #he greatercontraction resulting from depositing the weld metal on thesecond side will help counteract the distortion on the firstside.

Best practice#he following design principles can control distortion0

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-4 joint in preference to a

single-4 joint

+dopting best practice principles can have surprising cost benefits. %or example, for a design filletleg length of >mm, depositing an :mm leg length will result in the deposition of =) additional weldmetal. Besides the extra cost of depositing weld metal and the increase ris" of distortion, it is costly


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to remove this extra weld metal later. !owever, designing for distortion control may incur additionalfabrication costs. %or example, the use of a double-4 joint preparation is an excellent way to reduceweld volume and control distortion, but extra costs may be incurred in production throughmanipulation of the wor"piece for the welder to access the reverse side

Distortion $ revention %y pre$setting1 pre$%ending or use of restraint

/istortion may be prevented by one of the following methods pre-setting of parts

pre-bending of parts

use of restraint

#he techni$ue chosen will be influenced by the size andcomplexity of the component or assembly, the cost of anyrestraining e$uipment and the need to limit residual stresses.

Fig' ( re$setting of parts to produce correct align!entafter welding

a*re-setting of fillet joint to prevent angular distortion%*re-setting of butt joint to prevent angular distortion

c*#apered gap to prevent closure

re$setting of parts#he parts are pre-set and left free to move during welding (see Fig 1. In practice, the parts are pre-set by a pre-determined amount so that distortion occurring during welding is used to achieve overallalignment and dimensional control.

#he main advantages compared with the use of restraint are that there is no expensive e$uipmentneeded and there will be lower residual stress in the structure.

Cnfortunately, as it is difficult to predict the amount of pre-setting needed to accommodateshrin"age, a number of trial welds will be re$uired. %or example, when ''+ or 'IA welding butt

joints, the joint gap will normally close ahead of welding6 when submerged arc welding6 the joint mayopen up during welding. 8hen carrying out trial welds, it is also essential that the test structure isreasonably representative of the full size structure in order to generate the level of distortion li"ely tooccur in practice. %or these reasons, pre-setting is a techni$ue more suitable for simple componentsor assemblies.

Fig' - re$%ending1 using strong%acks and wedges1 to acco!!odate angular distortion in

thin plates

re$%ending of partsre-bending, or pre-springing the parts before welding is atechni$ue used to pre-stress the assembly to counteract

shrin"age during welding. +s shown inFig 2,pre-bending bymeans of strongbac"s and wedges can be used to pre-set a

seam before welding to compensate for angular distortion. Deleasing the wedges after welding willallow the parts to move bac" into alignment. #he main photograph shows the diagonal bracings and


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centre jac" used to pre-bend the fixture, not the component. #his counteracts the distortionintroduced though out-of-balance welding.

/se of restraintBecause of the difficulty in applying pre-setting 2 pre-bending, restraint is the more widely practisedtechni$ue. #he basic principle is that the parts are placed in position and held under restraint to

minimise any movement during welding. 8hen removing the component from the restraininge$uipment, a relatively small amount of movement will occur due to loc"ed-in stresses. @itherapplying a small amount of pre-set or stress relieving before removing the restraint can cure this.

8hen welding assemblies,all component parts should be held in the correct position until completionof welding and a suitably balanced fabrication se$uence used to minimise distortion.8elding withrestraint will generate additional residual stresses in the weld, which may cause crac"ing. 8henwelding susceptible materials, a suitable welding se$uence 2 the use of preheating will reduce this.Destraint is relatively simple to apply using clamps, jigs 2 fixtures to hold the parts during welding.

Welding 2igs and fi,tures?igs and fixtures are used to locate the parts and to ensure that dimensional accuracy is maintained

whilst welding. #hey can be of a relatively simple construction, as shown in Fig 3a,but the weldingengineer will need to ensure that the finished fabrication can be removed easily after welding.

Fle,i%le cla!ps

+ flexible clamp (Fig 3b can be effective not only in applying restraint but also in setting up andmaintaining the joint gap (it can also be used to close a gap that is too wide. + disadvantage is thatas 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 $uite high.

Fig' . "estraint techni3ues to prevent distortion

a* Welding 2ig %* Fle,i%le cla!ps

c* Strong%acks with wedges d* Fully welded strong%acks

Strong%acks 4and wedges*


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5trongbac"s are a popular means of applying restraint especially for site wor". 8edgedstrongbac"s, Fig.3c,will prevent angular distortion in plate and help to prevent pea"ing in weldingcylindrical shells. +s these types of strongbac" will allow transverse shrin"age, the ris" of crac"ingwill be greatly reduced compared with fully welded strongbac"s.%ully welded strongbac"s (welded on both sides of the joint Fig 3d,will minimise both angulardistortion and transverse shrin"age. +s significant stresses can be generated across the weld, which

will increase any tendency for crac"ing, care should be ta"en in the use of this type of strongbac".

Best practice+dopting the following assembly techni$ues will help to control distortion0

re-set parts so that welding distortion will achieve overall alignment and dimensional control

with the minimum of residual stress re-bend joint edges to counteract distortion and achieve alignment and dimensional control

with minimum residual stress. +pply restraint during welding by using jigs and fixtures, flexible clamps, strongbac"s and

tac" welding but consider the ris" of crac"ing which can be $uite significant, especially forfully welded strongbac"s.

Cse an approved procedure for welding and removal of welds for restraint techni$ues which

may need preheat to avoid forming imperfections in the component surface.

Distortion $ revention %y fa%rication techni3ues

Asse!%ly techni3uesIn general, the welder has little influence on the choice of welding procedure but assemblytechni$ues can often be crucial in minimising distortion. #he principal assembly techni$ues are0

tac" welding

bac"-to-bac" assembly

stiffening

Tack welding#ac" welds are ideal for setting and maintaining the joint gap but can also be used to resisttransverse shrin"age. #o be effective, thought should be given to the number of tac" welds, theirlength and the distance between them. 8ith too few, there is the ris" of the joint progressivelyclosing up as welding proceeds. In a long seam, using ''+ or 'IA, the joint edges may evenoverlap. It should be noted that when using the submerged arc process, the joint might open up ifnot ade$uately tac"ed.#he tac" welding se$uence is important to maintain a uniform root gap along the length of the joint.#hree alternative tac" welding se$uences are shown in %ig


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tac"ing the rest of the jointc tac" weld the centre, then complete the tac" welding by the bac"-step techni$ue

/irectional tac"ing is a useful techni$ue for controlling the joint gap, for example closing a joint gap,which is (or has become too wide. 8hen tac" welding, it is important that tac"s which are to be

fused into the main weld, are produced to an approved procedure using appropriately $ualifiedwelders. #he procedure may re$uire preheat and an approved consumable as specified for the mainweld. Demoval of the tac"s also needs careful control to avoid causing defects in the componentsurface.

Back$to$%ack asse!%ly

By tac" welding or clamping two identical components bac"-to-bac", welding of both componentscan be balanced around the neutral axis of the combined assembly (%ig a. It is recommended thatthe assembly is stress relieved before separating the components. If stress relieving is not done, it

may be necessary to insert wedges between thecomponents (%ig b so when the wedges are removed, the

parts will move bac" to the correct shape or alignment.

Fig' - Back$to$%ack asse!%ly to control distortion whenwelding two identical co!ponentsa* asse!%lies tacked together %efore welding%* use of wedges for co!ponents that distorton separation after welding

Stiffening

Fig' . Longitudinal stiffeners prevent%owing in %utt welded thin plate

2oints1ongitudinal shrin"age in butt welded seamsoften results in bowing, especially whenfabricating thin plate structures. 1ongitudinalstiffeners in the form of flats or angles, weldedalong each side of the seam (%ig areeffective in preventing longitudinal bowing.5tiffener location is important0 they must beplaced at a sufficient distance from the joint sothey do not interfere with welding, unless

located on the reverse side of a joint weldedfrom one side.

Welding procedure+ suitable welding procedure is usually determined by productivity and $uality re$uirements ratherthan the need to control distortion. 3evertheless, the welding process, techni$ue and se$uence doinfluence the distortion level.

Welding process


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Aeneral rules for selecting a welding process to prevent angular distortion are0


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Best practice#he following fabrication techni$ues are used to control distortion0

using tac" welds to set up and maintain the joint gap

identical components welded bac" to bac" 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 choice of welding procedure, process and techni$ue should aim to deposit theweld metal as $uic"ly as possible6 'IA in preference to ''+ or gas welding andmechanised rather than manual welding

in long runs, the whole weld should not be completed in one direction6 bac"-step or s"ip

welding techni$ues should be used

Distortion $ corrective techni3ues

Local heating of the flange edges to producecurved %ea!s for a %ridge structure

@very effort should be made to avoid distortion at the designstage and by using suitable fabrication procedures. +s it isnot always possible to avoid distortion during fabrication,several well-established corrective techni$ues can beemployed. !owever, rewor"ing to correct distortion shouldnot be underta"en lightly as it is costly 2 needs considerables"ill to avoid damaging the component. In this issue, generalguidelines are provided on 7best practice7 for correctingdistortion using mechanical or thermal techni$ues.

Mechanical techni3ues#he principal mechanical techni$ues are hammering andpressing. !ammering may cause surface damage and wor"hardening.In cases of bowing or angular distortion, the completecomponent can often be straightened on a press without thedisadvantages of hammering. ac"ing pieces are insertedbetween the component and the platens of the press. It isimportant to impose sufficient deformation to give over-correction so that the normal elastic spring-bac" will allow thecomponent to assume its correct shape.

Fig' ( /se of press to correct %owing in T %utt 2oint

ressing to correct bowing in a flanged plate is illustrated in %ig.


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#he following should be adopted when using pressing techni$ues to remove distortion0 Cse pac"ing pieces which will over correct the distortion so that spring-bac" will return the

component to the correct shape &hec" that the component is ade$uately supported during pressing to prevent buc"ling

Cse a former (or rolling to achieve a straight component or produce a curvature

+s unsecured pac"ing pieces may fly out from the press, the following safe practice must be

adopted0- bolt the pac"ing pieces to the platen- place a metal plate of ade$uate thic"ness to intercept the 7missile7- clear personnel from the hazard area

Ther!al techni3ues#he basic principle behind thermal techni$ues is to create sufficiently high local stresses so that, oncooling, the component is pulled bac" into shape.

Fig' - Localised heating to correct distortion

#his is achieved by locally heating the material to a temperature

where plastic deformation will occur as the hot, low yield strengthmaterial tries to expand against the surrounding cold, higher yieldstrength metal. &ooling to room temperature the heated area willattempt to shrin" to a smaller size than before heating. 5tressesgenerated thereby pull the component into re$uired shape

1ocal heating is, therefore, a relatively simple but effective means of correcting welding distortion.5ize, number, location and temperature of the heated zones determine shrin"age level. #hic"nessand plate size determines the area of the heated zone. 3umber and placement of heating zones arelargely a $uestion of experience. %or new jobs, tests will often be needed to $uantify the level ofshrin"age.Spot1 line1or wedge$shapedheating techni$ues can all be used in thermal correction of distortion.

Spot heatingFig' . Spot heating for correcting %uckling

5pot heating (%ig. , is used to remove buc"ling, forexample when a relatively thin sheet has been welded toa stiff frame. /istortion is corrected by spot heating onthe convex side. If the buc"ling is regular, the spots canbe arranged symmetrically, starting at the centre of thebuc"le and wor"ing outwards.

Line heating

Fig' 0 Line heating to correct angular distortion in a fillet weld

!eating in straight lines is often used to correct angular distortion, forexample, in fillet welds (%ig. ;. #he component is heated along the line ofthe welded joint but on the opposite side to the weld so the inducedstresses will pull the flange flat.


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Wedge$shaped heating#o correct distortion in larger complex fabrications it may be necessary to heat whole areas inaddition to employing line heating. #he pattern aims at shrin"ing one part of the fabrication to pullthe material bac" into shape.

Fig' 5 /se of wedge shaped heating to straighten plate

+part from spot heating of thin panels, a wedge-shapedheating zone should be used, (%ig. = from base to apex andthe temperature profile should be uniform through the platethic"ness. %or thic"er section material, it may be necessary touse two torches, one on each side of the plate.

+s a general guideline, to straighten a curved plate (%ig. =wedge dimensions should be0

0

a. Buc"le at edge of plate as an alternative to rolling (%i.g >b. Box section fabrication which is distorted out of plane (%ig. >c

Fig' 6 Wedge shaped heating to correct distortion

a* standard rolledsteel section

%* %uckled edge of plate c* %o, fa%rication

7eneral precautions#he dangers of using thermal straightening techni$ues are the ris" of over-shrin"ing too largean area or causing metallurgical changes by heating to too high a temperature. +s a generalrule, when correcting distortion in steels the temperature of the area should be restricted toapproximately to >9F - >=9F& - dull red heat.If the heating is interrupted or heat lost, the operator must allow the metal to cool 2 thenbegin again.

Best practice for distortion correction %y ther!al heating#he following should be adopted when using thermal techni$ues to remove distortion0


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use spot heating to remove buc"ling in thin sheet structures

other than in spot heating of thin panels, use a wedge-shaped heating techni$ue

use line heating to correct angular distortion in plate

restrict the area of heating to avoid over-shrin"ing the component

limit the temperature to >9F to >=9F& (dull red heat in steels to prevent metallurgical

damage

in wedge heating, heat from the base to the apex of the wedge, penetrate evenlythrough the plate thic"ness and maintain an even temperature

7eo!etric shape i!perfections $ types 8 causes

9ntroductionIn the job "nowledge series welding imperfections such as crac"s, lac" of fusion, penetration andporosity have been discussed. #his article loo"s at those imperfections related to poor geometricshape and will concentrate on the following0

@xcess weld metal

Cndercut

Gverlap


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1inear misalignment

Incompletely filled groove

5uch imperfections might be considered as anomalies in the joint and they will always be present tosome degree so that it becomes necessary to separate the acceptable from the unacceptable. /==99 5pecification for unfired fusion welded pressure vessels.B5 >;9 5pecification for &lass II oxy-acetylene welding of carbon steel pipewor" for carrying fluids

B5 @3 =:


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8hen arc and gas welding, undercut isprobably the most common shape imperfection.8ith single-sided pipe welds it may also befound at the bore surface. It may also be seenon the vertical face of fillet welds made in thehorizontal vertical position.

+ wide spreading arc (high arc voltage with insufficient fill (low current or high travel speed is theusual cause. !owever, welder techni$ue, especially when weaving, and the way the welding torch isangled can both cause and be used to overcome undercutting ( ieangled to push the weld metal tofill the melted groove. !igh welding current will also cause undercut - this is generally associatedwith the need for a high travel speed to avoid overfilling of the joint.

Acceptance1argely because this imperfection is widespread, most standards permit some level of undercutalthough they do re$uire that a 7smooth transition is re$uired. #he limits in B5 @3 =:


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Linear !isalign!ent4Also known in the /SA as high$low*'

#his imperfection relates to deviations from the correct position*alignment of the joint.

:o!!on causes

#his is primarily a result of poor component fit-up before welding, which can be compoundedby variations in the shape and thic"ness ofcomponents (egout of roundness of pipe.#ac"s that brea" during welding may allow thecomponents to move relative to one another,again resulting in misalignment.

Acceptance#he acceptability of this defect is related to the design function of the structure or pipe line either interms of the ability to ta"e load across the misalignment or bHcoz such step impedes the flow of fluid.

+cceptance varies with the application.

B5 @3 =:


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has not been sufficiently filled. #he result is that the thic"ness of weldment is less than that specifiedin the design, which could lead to failure.

Acceptance'ost standards will not accept this type of imperfection, except perhaps over short lengths and eventhen a smooth transition is re$uired. #he designer expects the joint to be ade$uately filled, but not

too much so (see excess weld metal.Gften the presence of this imperfection is an indication of poor wor"manship and could suggest thatfurther training is re$uired.

A general review of the causes and acceptance of shape i!perfections $ art -#his second article on shape imperfections refers mostly to fillet welds but there are two additionalbutt weld imperfections that re$uire some comment.

&,cessive penetration 4&,cess penetration %ead*

@xcess weld metal protruding through the root of a fusion (butt weld made from one side only.8ith pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion

and*or corrosion problems.

:o!!on causesenetration becomes excessive when the jointgap is too large, the root faces are too small,the heat input to the joint is too high or acombination of these causes.

Acceptance#he criteria which sets the level of acceptablepenetration depends primarily on theapplication code or specification.

B5 J< re$uires that the 7penetration bead shall not exceed mm for pipes up to and includingmm for pipes over gives specific limits for smaller diameters pipes, eg for pipe size =-=9mm the maximumallowed bore penetration is .=mm.

+5'@ B


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AvoidanceIt is important to ensure that joint fit-up is as specified in the welding procedure. If welder techni$ueis the problem then re training is re$uired.

"oot concavity 4suck$%ack+ underwashing*

:o!!on causesDoot concavity is caused by shrin"age of the weld pool in the through-thic"ness direction of theweld. 'elting of the root pass by the secondpass can also produce root concavity.

#his imperfection is fre$uently associated with#IA welding with the most common causebeing poor preparation leaving the root gapeither too small or, in some cases, too large.@xcessively high welding speeds ma"e theformation of root concavity more li"ely.

Acceptance#he root concavity may be acceptable. #his will depend on the relevant standard being wor"ed to.%or example0B5 J< re$uires that0a there is complete root fusionb the thic"ness of the weld is not less than the pipe thic"ness.

+5'@ B


20/22

:o!!on causesoor techni$ue and the deposition of large volumes of 7cold7 weld metal.

Acceptance#he idealised design re$uirement of a 7mitre7 fillet weld is often difficult to achieve, particularly with

manual welding processes.B5 @3 =:


21/22

#he welding related causes are associated with highwelding speeds and low welding currents.

AcceptanceIt is normally assumed that fillet welds will be at least ofthe size specified. B5 @3 =:


22/22

#here are instances where asymmetry may be specified (egto place the toe stress concentration ina particular region.

B5 @3 =:

Date post:	23-Feb-2018
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Lesson 5 - Distortion

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