WeldingMetallurgy

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Welding Metallurgy istortionResidual Stress _

Post Weld HeatTreatment

cture 10

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ecture Sc ope

Typical patt rns distortion residual stress inwelded assemblies Effects residual stress Methods for prevention and correction distortion Heat treatmemt steel weldments

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Residual Stress efinitions Residual stresses are the stresses that exist in a structurein the absence external loads Residual stresses can be produced metal structures bymany processes in luding casting heat treatmenfforming and bending and thermal cutting Welding produces residual stress and distortion as aresult localised heating and cooling the work material

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ecture 10 P 3


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auses esidual stress During welding the weld and HAZ are heated totemperatures w f ~ above those the surrounding material The weld and HAZ deform plastically because theirthermal expansion is resisted by the surrounding material As the weld cools and contracts tensile stresses developelastically Welds contain tensile stresses that approach the yieldstress.

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Weld hermal trainsi

3 The contraction is againresisted by the surroundingmaterial, leaving the weldand HAZ in a state otension

l On heatin , thethennal strains areresisted by thesurrounding material,causing plasticdeformation

rt iWeld

2 On cooling, the weld Itand HAZ tend to contract

cture 10 p


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xial Residual Stress attern

The tensile stress in theweld is balanced bycompressive stresses olower magnitude in thesurrounding plate

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t Axial Stress


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Transverse Stresses utt ointx

y ytension compression

xAxial stress Distribution o y stresscomponent along x x

Lecture 1

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Pipe irth Weld isplacements _ _ _ ~

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Tensile hoopstress

Displaced sh p due tocircumferential shrinkagestrains exaggerated

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Typical Pipe irth Weld StressesLongitudinal Stresses Circumferential St[esses

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20 e uter Surfacee nner Surface e ueerSurface e nnerSurface-204

lO 4 6 8 1 4 6 8 1Distmce from weld CL in Distance from weld CL in

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iestraint tresses

When the workpiece is free to expand orcontract residual stresses are confined to the -region of t ~ weld When the workpiece is restrained e.g.between rigid anchors long range reactionstresses develop

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ontroll esidual Stress Use minimum required weld size and heat input U prE parations give smaller weld areas

Minimise constraint during welding Stress re lief

Heat treatment Mechanical e.g. vibratory

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stort on Distortion in welded fabrications is caused by

movements to accommodate thermal stresses Distortion in welded fabrications consists of:

- transverse shrinkage-longitudinal shrinkage- angular rotations

Distortion is problem mostly with thinnermaterials

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owing

ypical distortion pattern e x a g g e r a ~ ~ d < ~ ~ ~ ~ k a g e _

0.1 of length _

Transverse t hrinkage

1 5 3 mm

1..---=_ _=-= :_= _ J 1ngularRotation1_3

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r eaking tiffened anel

Examples: bridge deck ship hull

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cture 10 pH


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ontrol istortion Minimise heat input- Maximise melting efficiency, i.e. welding processes with high energy density

2 Use minimum required weld sizes,- e.g. intermittent rather than continuous fillet welds, or U preparations3 Balance welds about neutral axis

- e.g. double V preparation, simultaneously weld top and bottom plate girder4 Use clamps, jigs fixtures5 Preset the workpiece to compensate for anticipatedchanges, for instance:

FilletWelds

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ontrol distortion Avoid using welding for precision assembly achine to final dimensions after welding

__ ....ll ~ i l l t welds

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orrection distortion lame Straightening

welded [b =====:J]Area or areas h ~ ~

gas torch to b ~ ~ O C ~ = = = = = = = = = = _ ~ _

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lCorrection distortion Mechanical straightening

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at Treatment Welds eat treatment is costly nd should bevoided unless necessary for s tisf ctory performance ay e required y pplic le codes andst nd rds

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Weld H E ~ a t Treatments reheating- Heating prior to welding usually to t m p r t u r ~ lessthan 200C- Applied in welding C-Mn steels to decrease coolingrates and reduce HAZ hardness-


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ost eld I eat re tment efinitions

- Stress Relief Heating to a tl3mperature usually around 600C atwhich the yield strength is reduced such that residualstresses are relieved- Normalizing Heating to above the temperature for transformation toaustenite A3 and slow cooling for to refine andhomogenize the grain structure

Quenching Heating as in normalizing and rapid cooling in water,

brine or air to develop high strength.

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~ ~ost ~ l d eat Treatment

Definitions cont d)- Tempering .Reheating after quenching to below the transformationtemperature to reduce hardness and improve .ductility- Solution Treating

Heating to take into solution elements which will beprecipitated later in a controlled manner to producethe desired properties geing

Reheslting after solution treating to allow formation ofpreciplitates which strengthen the material.

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cture 10 p


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PW T ethods urnace eating

- Gas or electric most common- Restricted to parts that can be contained in a furnace- Optimum efficielncy and control Local e a t i n ~- Gas burners or electric resistance heaters most common- Mostly limited to relatively simple treatments typicallystress relief o 8teel weldments

- Main advantagl3 is capability to treat large or immovablestructures t low cost.

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PW T ffects on Steels Stress re efreduces residual stress and tempershardened heat affected zones. In most grades of steelbut notall it improves ductility and toughness.

orm lizing is used to refine the grain structure r ~ s u l t nfrom welding when optimum properties are required, e.g.electrosla1 welded pressure vessels Quench Temper omesteels require Q T treatmentsto develop high strength. Sometimes such steels can bewelded asquenched followed by a combinedtemperlstr1ass relief treatment.

- The metallurgical aspects of welding Q T steels must becarefully assessed.

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