Lecture 42

NPTELPhaseII:IITKharagpur:Prof.R.N.Ghosh,DeptofMetallurgicalandMaterialsEngineering|||||

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Module 42

Physical metallurgy of metal joining

Lecture 42

Physical metallurgy of metal joining


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Keywords:Commonmethodsofmetaljoining,bondingmechanism,wetability,effectoflocalheating,lawsgoverningheatflow,temperatureprofilearoundstationeryandmovingheatsource,weldpoolgeometry,weldsolidification,structureofweldmetal,heataffectedzone,

hardnessprofileIntroductionInthismoduleweshalllearntaboutthestructuralchangesthatmightoccurwhilejoiningtwosimilarordissimilarmetals.Thereareseveralwaystwopartscanbejoined.Someoftheseareasfollows:

Rivets/nuts&bolt:friction Adhesive:syntheticpolymer Soldering:lowmeltingmetals Brazing:Tmhigherthansolder Welding:similarmetals

Rivetsornuts&boltsholdtwopartstogetherprimarilyduetofriction.Youmayneedtodrillholes inthepartstobe joined. If it isdonewitha littlecareyoudonotexpectanystructuralchangewithinthemetal.Adhesiveusedtojointwopartsmakesuseofsyntheticpolymerthatsetsunderpressureorunderalittlethermalexposure.Oncetheadhesivesetsitholdsthetwoparts because of friction, Van der Waal force or chemical bonds. Even if a little thermalexposureisneededitisneverhighenoughtoinduceanystructuralchangeswithinthemetalsbeing joined. Soldering is donewith the help of lowmelting alloys.Often these are binaryeutectic.Thebondbetweenthesolderandthemetal isprimarilyduetovanderWaalforcesalthoughdiffusionmayplaysomerole.Themainpurposeistoprovideagoodcontactbetweenthepartsbeingjoined.Itplaysamajorroleinprovidinggoodelectricalandmechanicalcontactsbetweendifferentcomponentsinanelectroniccircuit.Theprocessofbrazingissimilartothatof soldering.However themeltingpointof thebrazingalloy ismuchhigher than thatof thesolder.Unlikesolderingorbrazinginthecaseofweldingthefillermetalissimilartothatofthepartsthatarebeing joined.Thereforeapartofthetwocomponentsbeing joinedwouldmeltandgetmixedwiththefillermetalandsolidifyresultinginamuchstrongerbond.Thisiswhereyouexpectmaximumchange inthestructureofthemetalsbeing joined.Thereforethemainfocusinthislecturewillbeonwelding.Intheprocessweshalllearntabout:

Bondingmechanisms Structuralchanges Effectofprocessingparameters Heataffectedzones Propertiesofjoints


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EffectofserviceexposureWhereeverpossibleweshallillustratethesewithspecificexamples.Bondingmechanisms:Thebondbetweenthepartsbeingjoinedbysoldering,brazingorweldingstartdevelopingrightfromthestagethemoltenfillermetalcomes incontactwiththe interfaces.Thereforehow itflows on the surface of themetal is of considerable interest. Slide 1 giveswhat is likely tohappenifadropofmoltenmetalrestsonasolidsurface.Therecanbetwodistinctsituationsdepending on whether the liquid metal wets the surface or not. This depends on themagnitudesoftheforcesactingalongthethreedifferentsurfaces.Notethatdenotessurfacetension. It isameasureof theenergyof the surfaceor the interfacebetween twodifferentphases. The suffix denotes a pair of interfaces. For example sv denotes the energy of thesurfaceofthesolidandvapor(atmosphereorenvironment).Lookatthetwosketchesinslide.Theangledenotesthecontactanglebetweentheliquidandthesolid.Theforcesactingatthepointofcontact(seeslide1)mustbalance.Thisgivesadefiniteexpression(seeslide1)forintermsofthesurfacetensionsofthethree interfaces.Thesketches inslide1showthat lowmeansgoodwetability.Thiswouldensurealargeareaofcontact.Theexpressionforgiveninslide1clearlysuggeststhat shouldbeas lowaspossibleforgoodwetability.Additionoffluxmayalsohelpimprovewetability.Thestrengthofthebondwouldcertainlydependonthequalityof the contactbetween the fillermetal and the interfaces.Thereforewetability is amajorcriterionforgoodbonding.


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Role of solid liquid interface

svlv

ls

cos

cos

sv sl lv

sv sl

lv

Liquid wets solid.

svlv

slNon wetting liquid

For good joints: wet-ability, smooth & clean surface, interface temp =/> Tm of filler (to avoid dry joint)

Apartfromgoodwetabilityyoualsoneedtohavecleanandsmoothsurfacesothatthemoltenfillermetalcouldfloweasily.Thetemperatureoftheinterfaceisalsoofconsiderableinterest.Itshouldbecloseorgreaterthanthemeltingpointofthefilleralloy.Thisisnecessarytoensurethat the filler alloy does not solidify before it spreads over the surface. A low interfacetemperaturemay lead toadry joint.This suggests thatpreheatingof thecomponentsbeingjoinedmaybenecessaryevenduring solderingandbrazing. In the caseofwelding theheatinputissointensionthatapartofthemetalmelts.Oncethemoltenfilleralloysolidifiesitactsabridgebetweenthetwopartsthatwerejoined.Thisisillustratedwiththehelpofasketchinslide2. In thecaseofwelding thecompositionof the fillermetal issameas thatof the twopartsbeing joined.However inthecaseofbrazingandsolderingthecompositionofthe fillermetal isdifferent.The strengthof fillermetaldependson itsmeltingpoint. Since the fillermetalused inweldinghas thehighestmeltingpoint, the strengthofwelded joint too is thehighest.Thisalsoexplainswhyasolderistheweakestofthethree.

Slide1


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Bonding mechanism

The continuous metallic bridge between the two parts provides the bond between the two. Its nature is

similar to that in metals

welding > brazing > solder

Heat flow during welding

Weld variables

Heat input (q)

Speed of heat source (v)

Thermal conductivity ()Plate thickness (d)

2 2 2

2 2 2

1T T T Tx y z t

c

= thermal diffusivity = densityc = specific heat

x

Effectofprocessingparameters:Thethreejoiningmethodscanbevisualizedasatwostepprocessconsistingoflocalheatingfora short duration followed by rapid cooling. The structural changes that may occur wouldthereforedependonthepeaktemperatureandthecoolingrate.Withoutgoingintodetailsletustrytohavearoughideaaboutthenatureofthetemperatureprofilearoundamovingheatsource.

Slide2

Slide3


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Slide3givesaschematicrepresentationofhowthetemperature(T)contoursaroundamovingpointsourcemaylooklike.Italsogivestheequationthatgovernsheatflowinacontinuous3Dmedium.The locationofapoint isgivenbytheCartesiancoordinatesx,y,z.Smalltdenotestimeandtheconstantisknownasthethermaldiffusivityofthemedium.Itdependsonthedensity,thermalconductivityandspecificheatofthemedium.Forsimplicityletusassumethatthese are independent of temperature and composition.Apart from these the temperaturedistributionatanyinstantwouldalsodependonthestrengthoftheheatsourceorheatinputperunittime,thespeed&geometryoftheheatsourceandthesize(thickness)ofthemedium.Ifthesource isstationeryweexpecttheheattoflowuniformlyalongalldirectionsinaplane.Thereforetheshapeofthetemperaturecontoursinaplaneshouldbecircular.Notethatalineor a curve joining points having identical temperatures is a contour. Heat flow direction isalwaysperpendiculartothedirectionofthecontour. Inthecaseofamovingsource itgetsalittledistortedor stretchedalong thedirectionofmotion.The contours inaplanearemoreclosely spaced near the heat source. These arewide apart at a distance far away from thesource.Thespacingisanindicatoroftemperaturegradient.

Temp profile : stationary heat sourcePoint source: q0 Line source: q1 Planar source: q2

Spherical surface: same temp

Cylindrical surface: same temp

planar surface: same temp

T = F(,r,t,q)T

r

T

t

q =F(power,t) ~ vit: efficiency: arc welding ~ 0.7

t = constant r = constant

Slide4 shows theeffectof thegeometryofaheat sourceon the shapeof the temperaturedistribution profile around it. From a point source heat can flow at the same rate along allpossibledirections.Thereforeatanyinstantthetemperatureatallpointsatidenticaldistancefromthesourceshouldbethesame.Thetemperaturecontoursaroundapointsourceshouldconsistofa setof concentric spheres.The same logic canbeextended to the caseofa linesourcetoshowthatthecontours(surfacehavingidenticaltemperature)wouldconsistofaset

Slide4


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of cylindrical surfaceswith the heat source being the common axis. Note that there is notemperaturegradientalongtheheatsourcethereforeheatcannotflowalongthisdirection.Inthe case of a planar heat source the temperature at all points on the plane are identical.Thereforeheatcanflowonly inadirectionperpendicularto it.The isothermalsurfaceswouldconsistof a setofparallelplanes. Slide4 also includes aplot representing the variationoftemperatureatagiven timeasa functionofdistancealongadirectionperpendicular to theisothermalsurfaces.Notethesymmetryoftheplotonthetwosidesofthesource.Slide4alsohasaplotdenotinghow the temperatureatapoint (r=constant) is likely tovarywith timeoncethesourceisswitchedoffaftersometime.Itkeepsincreasingwithtimeuntilitreachesapeakandthereafteritdecreasescontinuously.Thetemperatureapointnearaheatsourceofaspecificgeometryisthereforeafunctionofr(location),t(time),(thermophysicalproperties),and q (heat input or the power of the heat source). Duringwelding the heat inputwoulddependon theVoltage (V),current (i)and time (t)anarcspendsatapoint. In thecaseofacontinuouslymovingsourcethetimespentatapoint is inverselyproportionalto itsspeed. Inadditiontheexpressionshouldhaveatermrepresentingtheefficiency()ofheatabsorption.Itisaround0.7inthecaseofarcwelding.

Welding: moving heat source & finite geometry

Moving source: v = velocity

Stationary source2 2 2

2 2 2

T T T v Tx y z x

x

Quasi stationary state

Slide5 shows theeffectofvelocity (v)on temperaturedistributionatagiven timearoundapoint source. If it stationery the contours consistofa setof circles. In the caseofamovingsourcetheshapegetsstretchedalongthedirectionofmotion.Theequationthatdescribestheheatflowinthecaseofamovingisgiveninslide5.Notethattime(t)hasbeenreplacedbyx,

Slide5


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thedirectionofthemovementofthesource.This isobtainedbythefollowingsubstitution intheexpressiongiveninslide3.

(1)

Whatisthebestapproximationforthegeometryofaheatsource?Thiswoulddependonthesizeofthe jobbeingwelded.Ifyouaretoweldathickplateheatmustflowalongthe length,width and the thickness. It is a case of 3D flow. Therefore point source is a betterapproximation.Inthecaseofthinsheetasinglepassweldingisgoodenough.Heatflowalongthethicknessmaynotbeimportant.Thereforealinesourceisagoodapproximation.

Moving source: solution: HAZ

Point source ~ multi-pass welding of thick plate Line source ~ single pass welding of thin sheet

2

0

2

0

/ exp2

/ exp44

q v rT T thick platet t

q v rT T thin platetd ct

T0:initial temp. Equations are valid outside the fusion zone.

Slide6


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Weld solidification

Volume of weld pool very small

Turbulent melt help distribute impurities

Composition of mold & metal similar

High temperature gradient

Dilution

Weld solidification: a dynamic process

Thick plate

Thin plate

Physical metallurgy helps us understand evolution of structures in various zones.

Molten pool Molten pool

Slide6givesapproximate solutions to themovingheat sourceproblemdescribed in slide5.Theseequationsarevalidjustoutsidetheheataffectedzone.Thiscanbeofhelpinpredictingtheevolutionofstructureintheheataffectedzonesofaweldjoint.Weld pool solidification: Welding represents solidification of a small pool of molten metalwithinamold.Heatflowsfromthehotpoolofmoltenmetalintothemoldsurroundingit.Thisisillustratedwiththehelpoftwodiagramsinslide7.Iftheplateisthickheatflowsinallthe3directions.Inthecaseofathinsheetheatflowsalongthewidthoftheplateasshown.Afewmajorpointsthatareworthconsideringare listed inslide7.Thesizeofthemoldthatactsasheatsinkisverylarge.Thetemperaturegradientwithinthemoldisveryhigh.Theheatmaygetdissipated through theplateor the sheet very fast.Theelectric fieldmay create turbulencewithinthemoltenpool.Thismayhelpdistributeimpurities.Thecompositionofthemoldorthemetalthat isbeing joinedandthefillermetal isusuallysimilar.Howeverthetwomaynotbeexactlythesame.Apartofthemetalbeing joinedmeltsandgetsmixedwiththefillermetal.The turbulence within the pool helps mixing. In the absence of turbulence there will besegregation.Thecentrewillberichinfillermetalwhereasthesidewillhavecompositionclosertothatoftheparentmetal.Thecompositionoftheweldmetal isgenerallyanaverageofthetwo.This is known asdilution.Theextentofdilutionwoulddependon the amountof fillermetalusedandtheamountofparentmetalthatmelts.Itmaydependontheweldgeometry.Theweldsolidification is thereforeacomplexdynamicprocess.Letusseehowwecouldusewhat has been covered in this course on physical metallurgy to explain the evolution ofstructureswithindifferentzonesinaweldedcomponent.

Slide7


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Weld pool geometryqivd

R = v cos

R: crystal growth speed

Rv

High v, , d

Low v, d &

Weld metal: cast structure. R is max = 0. Higher

speed higher segregation. Poor toughness.

Liquidus

High dT/dxT max

Weldpoolgeometry:Slide8showswiththehelpofasetofdiagramstheeffectofvariableslikevelocity(v),size(d)andthermalconductivity()ontheweldpoolgeometry.Theshapeofthemoltenpoolislikeadistortedellipse.Thesketchatthebottomleftcornerofslide8showstheshapeof thecontourcorresponding to the temperatureof the liquidus.The frontendof thedistorted ellipse denotes the locationwheremelting is about to begin. The tail end of thiscontour denotes the region where solidification is over. The direction of crystal growth isperpendicular to the liquidus.Theanglebetween thedirectionofgrowthand thevelocityoftheweldpool(v) is.TherateofcrystalgrowthR isequaltovcos. It isthehighestat=0.Thiscorresponds to the tailend.R iszerowherev is tangent to thedistortedellipse (or themoltenpool).Thisisthepointonthemovingweldpoolbeyondwhichthereisnonucleationorcrystalgrowth.Solidificationbeginsfromtheedgeofthesolidsurfacesoonaftertheweldpoolmovesaway from thepointwherewas0a littleearlier. Severalnucleimay formbutonlythosethatarefavorablyorientedwouldgrow.Dependingonthethermalgradientandthetypeof the alloy columnar or dendritic growth takes place. Usually axis is the favorabledirectionofgrowth.Thedirectionofgrowthisalwaysperpendiculartotheweldpoolboundary.Thealignmentofthegrainsdependsonthevelocity,thermalconductivityandthesizeofthecomponent.Thisisillustratedwiththehelpofsketchesinslide8.Amajorproblemduringthisstageisthesegregationofimpurities.Duringsolidificationsoluteatomsarepushedtowardsthemolten metal present along the centerline. This is the region that solidifies at the end. Itconsistsof inclusionsandprecipitates.Thecenterline segregation is responsible for thepoortoughnessofweldjoints.Higherthespeedhigheristhesegregation.

Slide8


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Weld metal structure

% C

TsTm TsT

HAZCellular Dendritic

No super-coolingLow constitutional

super coolingHigh constitutional

super cooling

xx

x

constitutional super cooling

TmTm

Ts

S S SL L L

Weldpoolstructure:The temperatureof thesolid interfacemaygobeyond itsmeltingpointwhenitcomesincontactwiththemoltenpoolofweldmetal.Athinregionoftheparentmetalmaythereforemeltandmixwiththeliquid.Thisisknownasdilution.Ithelpsdevelopagoodcontactbetweenbasemetalandtheweldthroughtheformationofafusionzone.Theregionthat melts and resolidifies is known as the fusion zone. The subsequent solidification isgovernedbynucleationandgrowth.Theorientationofthegrainsthatnucleateisexactlysameas those at the solidliquid interface. Inwelding literature it is known epitaxial growth. Thesubsequentgrowthdependsontheshapeofthesolidification front.This is influencedbythesolutecontentinthemoltenweldmetal,thethermalgradient(G)intheweldpoolandtherate(R)atwhich the frontmoves forward.Slide9shows thebasic featuresof the threedifferenttypesofgrainstructurethatcanbeseeninweldmetal.Itfollowsfromequation1that:

(2)

Note thatTmdenotes themeltingpointof themetalandxl is thedistancebetween theheatsourceandthetailendofthemoltenweldpool (see fig1. ItshowsthatR=v). Themeltingpointofafilleralloy isknown. It isfixed.Asv increasesxltoowould increase.ThereforehighvelocityoftheweldpoolmeanslowerthermalgradientorG.

Slide9


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Thesurfaceoftheparentmetalmustmeltandmixwiththefilleralloy.Itisknownasthefusionzone. Itplays amajor role in thedevelopmentof a goodbondbetween the two. Figure2explainswhytheresolidificationofthefusionzonemusttakeplacethroughepitaxialgrowth.Thethinfilmsubsequentlygrowsintocolumnargrainsordendritesdependingonthethermalgradient (G)andR (the rateatwhich the solidification frontmoves).Colorhasbeenused torepresentthedifference inorientationofthegrains.Theextendedpartshownbydashed linedenotesthefusionzone.

Apart from G & R the concentration of solute too has a pronounced effect on themicrostructure that develops in theweldmetal.A set of concentration versus distance andtemperature versus distance plots in slide 9 describe three different situations. If thermalgradientatthesolidliquidinterface isgreaterthanacriticalvaluetheinterface isplanar.Thispromotes columnar grains. The critical gradient depends on the solute buildup and thecorrespondingphasediagram.Ifitisalittlelower,thebuildupofsoluteattheinterfaceleadstotheformationofacellularstructure.Lookatequation2.ItgivesarelationbetweenG/Randthemeltingpoint.Thevelocityoftheweldpool (v)andxl (thedistancebetweentheheatsourceand the tail of the weld pool) are constant for a given welding setup. Therefore the onlyvariable(liquidustemperature)whichisafunctionofcompositionisTm.

Fig 2: Explains the concept of epitaxialcrystal growth. (a) A part of the parentmetal that comes in contact with theliquidmelts.Thenewgrain thatnucleatemust have identical orientation. 0 (b) = 180. The other twosurface tensions should be equal andopposite. The shape of the new grain islikeathinfilm.

(a) Liquid

FusionzoneMold

NewGrain

0, M L

S(b)

R

v

xl

Heatsource

MeltingstartsSolidification

ends

R&v

Rv

Fig 1: The shape of themoltenweld pooldependson thevelocityatwhich theheatsourcemoves. Ifthevelocity(v) ishighthelengthofthepool is longer.At the farendwheresolidificationends the rateatwhichthesolidificationfrontmoves(R)isequaltothevelocityoftheheatsourcebecausetheanglebetweenthetwoiszero(=0).


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Figure3givesaschematicdiagramshowingtheeffectofconcentrationofsoluteandtheratioG/R on the nature of structure that evolves within the weld metal. Figure 4 gives mainstructuralfeaturesofexpectedmicrostructure.

Heataffected (HAZ):Wehave so far talkedabout theevolutionofmicrostructurewithin theweldmetal.Thisconsistsofthesolidifiedfilleralloyandthefusionzone.Apartfromthesethereareregionswherethetemperaturecangoupsignificantly.Thereforethemicrostructurewithintheseislikelytochange.Thesizeofsuchregionsisafunctionofthefollowingfactors:

Peaktemperature(Tp) Phasediagram Transformationkinetics

Planargrowth

Cellulargrowth

Cellulardendritic

Columnardendritic

Equiaxeddendritic

Fig4:Aschematicrepresentationofdifferenttypesofstructurelikelytobeseenintheweldsolidifiedzone.

%C

G/R

Planargrowth

Equiaxeddendriticgrowth

Cellulargrowth

Cellulardendriticgrowth

Columnardendriticgrowth

Fig 3: A schematic diagram showing theeffectof% concentration andG/Ron thenatureofthestructurethatdevelopsintheweldmetal.


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Duration(t)Thepeaktemperatureinthefusionzoneduringweldingisalwayshigherthanthemeltingpointof theparentmetal.Theheat affected zoneextends from the fusion zone to apoint in theparentmetalwherethetemperaturedoesnotgobeyondtherecrystallizationtemperatureofparentmetal.

Figure5showshowthetemperatureatapointintheparentmetalislikelytoincreaseinitiallyreachapeakand thereafterdecrease.Thepeak temperaturemaygoashighas themeltingpoint in the fusion zone. Evolution of microstructure in the fusion zone has already beendiscussed. Inthissectionourfocus isonthestructuralchangesoccurring intheheataffectedzone(HAZ).Thepeaktemperature(Tp)andtheduration(t)orthetimeittakestocoolthroughthecriticalrangeoftemperature(800500C)determinethetypeofstructurethatisexpectedintheheataffectedzone.

Time(t)

T

TsPeaktemperature

(Tp)

Solidustemperature

t

Fig5:Showsasetofschematictemperatureversustimeplotsatthree locationsnearthefusion zone of a welded structure. Tp, thepeak temperature is a function of distancefrom the fusion zone. The range oftemperatureisshownbyapairofhorizontallines.Thepeaktemperaturemaygobeyondorliewithinthisrangeoveraregionneartheweld.Thisisknownastheheataffectedzonebecauseofthestructuralchangesthatoccurshere.

Transformationtemperature

range


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Heat affected zone

1. Weld metal 2. Partially melted zone 3. Excessive grain growth 4. Re-austenitized 5. Partial austenitized

6. Tempered 7. Unaltered

%Coriginal

T T

1 2 765

+ P + cm

L +

LL +

Slide10givesTpversusdistanceplotandFeFe3Cphasediagramsidebysideso that ithelpsappreciatetheeffectofweldingonthestructurethatevolveswithintheHAZ.Onthebasisofthethermalprofiletheweldjointmaybedividedintosevendistinctzonesasindicatedinslide10.Thenomenclatureitselfgivesanideaaboutthemainstructuralfeatureswithinthesezones.Table 1 gives important features of these zones. It also gives the type of failure onemightencounter duringwelding. Solidification cracks are usually found in theweldmetal. This isassociatedwiththesegregationof lowmeltingconstituentswithindendriticchannelsandthethermalstressthatdevelopsduetocontraction.Thepartiallymeltedzoneispronetoliquationcracking. This too is associatedwith segregation. However itmay occur during subsequentheating (forexampleduringmultipasswelding).The regionwhere the temperature is lowerthanthemeltingrangebuthigherthanthenormalaustenitizationtemperatureissusceptibletoexcessive grain growth. Coarse grain austenite may give rise to a structure consisting of anetworkofferritearoundmartensiteparticularlyinsteelshavinghighcarbonequivalent.ThepresenceofNb/V/Timakesmicroalloyedsteelextremelyresistanttoexcessivegraingrowth.ThecarbidesofNb,VandTiareextremelystableevenatveryhigh temperatures (~1200C).Theirpresenceataustenitegrainboundary inhibitsexcessivegraingrowth.Theregionwherethetemperaturegoesbeyondtheuppercriticaltemperaturewouldtransformintoaustenite.Inthepresenceof strong carbide formers theaustenitegrainsare relatively fine.Thereforeonsubsequentcoolingitislikelytogiveastructureconsistingofferriteandpearlite.Thiszonemaybewiderinmicroalloyedsteelthanthatinnormalplaincarbonsteel.TheaustenitethatformsintheregionwherepeaktemperatureisbetweenA1andA3hasrelativelyhighcarboncontent.Therefore depending on the cooling rate itmay transform into platemartensite. The zone

Slide10


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wherethetemperaturedoesnotgobeyondthelowercriticaltemperaturewoulddependaloton its initialstructure.Theheatingtothisrangeoftemperatureamountstotempering inthecaseofairhardeningsteel.Thiswouldpromotesofteningduetocoarseningofcarbides.Inthecase of low carbon steel only ferrite grains may recrystallize. The pearlite may remainunaltered.Thetemperatureintheregionbeyondthisistoolowtohaveavisiblechangeinitsmicrostructure.The temperaturemay stillbehighenough toallow themovementofcarbonatomsdissolvedinferritetopinthedislocations.Thismayleadtostrainageing.Barringthisthestructurewithinthisremainsunaltered.

Table1:Mainstructuralfeaturesofvariouszonesinweldedsteel Zone Structuralfeature1 Weldmetal Caststructure:solidificationcracking2 Partialmelting Coarsegrain+caststructure:liquationcracking3 Excessivegraingrowth ProeutectoidferritenetworkandmartensiteifCequivalentishigh4 Reaustenitiztion Finegrain+Pstructure:thezoneisrelativelywideinNb/Vsteel5 Partialaustenitization Dependingont85+Pearlite/Bainite/platemartensite6 Tempering Spheroidisedcarbide(700750C)7 Noapparentchange PronetostainageinginpresenceofresidualstressesThe structural change in theHAZhas a pronounced effect on the hardness andmechanicalproperties of theweld joint. Slide 11 showswith the help of a set of schematic plots thevariation inhardness andprior austenitic grain size as a functionofdistance from theweldparentmetal interface. The coarse austenite grains aremore likely to give hardmartensiticstructure.This is the zonehaving thehighesthardness.Grain refinementoccurs in the zonewhere the temperature goes beyond A3 but remains lower than the temperature atwhichexcessivegraingrowthtakesplace.Asaresultgrainswithinthiszonemaybefinerthanthatoftheparentmetal.


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Hardness profile in HAZ

1500

300

200

400

700

VH

N

ASTM

gra

in s

ize

0

10 Fine

CoarseGrain size

Hardness

TempWeld metal

Base metal

Slide11showsthatprioraustenitegrainsizemayvaryfromaround2attheweldinterfacetoaround 8 within the reaustenitized zone. CCT diagrams help us predict the evolution ofstructure in steel.Thisdependsonaustenitegrain size.Therefore topredict the structureofHAZ it isnecessary tohaveaseriesofCCTdiagrams forarangeofprioraustenitegrainsize.Slide12givesaschematic(nottoscale)CCTdiagramofacoarsegrainsteel.Thissuggeststhatthediffusioncontrolledtransformation insteeloccurswithin800500C.Thetime ittakestocoolthroughtherangeoftemperatureafteritgetsheatedduringweldingisanindicatorofthestructure that would develop within a HAZ. High t800500 means soft and coarse structurewhereas low t800500 means fine and hard structure. The structure of HAZ is thereforedeterminedbybothTpandt800500.TheformergivesanideaabouttheaustenitegrainsizeandsuggestwhichCCTdiagramto look forandthe latterhelpspredictthemicrostructureonthebasisoftherelevantCCTdiagram.

Slide11


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Weld CCT diagram

A3

T500

800 P

B

+Mtime

Higher austenitising temp 1250-1400: excessive grain growth zone: CCT = F( Grain size). t800-500

is an indicator of microstructure & hardness

Hardness vs. t8-5

VHN

t800-500

%C

6 5 15eqMn Cr Mo V Cu NiC C Weldable:

Ceq < 0.4

High hardness: Poor ductility &

toughness

Slide 13 gives a set of hardness (expressed as VHN) versus t800500 plots for differentconcentrationsofcarbon(%C).Thelowerthemagnitudeoft800500higheristhehardnessforagiven%C.Thehardness increaseswith%C.Apartfrom%Cthepresenceofalloyadditionalsoaffects theCCTdiagramsandhence thehardenabilityof steel.Thecombinedeffectofall isoften expressed in terms of carbon equivalent (Ceq).A commonly used expression forCeq isgiveninslide13.Ifcarbonequivalentislessthan0.4asteelisconsideredtobeweldable.Ifitis

Slide12

Slide13


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higher itmayneedpreheatingandpostweldheat treatment.Weldmetalmaypickupgasesduringwelding.Hydrogenbeing the smallestdiffuses fast into theHAZ.Thismakes the steelparticularly thosewithhighcarbonequivalentprone tocoldcracking.Useofproperlybakedlowhydrogenelectrodesandpostweldheattreatmentmayhelpovercomecoldcracking.HAZinagehardenablealloy:Heretoothetemperaturecouldgoashighasthatofthesolidusduringwelding.Slide14presentsthermalprofile(peaktemperatureversusdistanceplot)andthephasediagramofanagehardenablealloysidebyside.Thetemperatureoftheweldmetalandthefusionzoneishigherthanthatoftheliquidus.Thestructureofthiszoneislikelytobethesameasthatofacastmetalthatsolidifiesinametallicmold.Wearealreadyfamiliarwithitsmainfeatures.Besidesthistheweldjointshouldconsistof(i)apartiallymeltedzonewherethetemperaturegoesbeyondthesolidusbutdoesnotexceedthe liquidus,(ii)asinglephaseregion where the preexisting precipitates dissolve into the matrix (iii) a two phase regionconsistingofsoftmatrixandprecipitatesthatmayhavecoarsenedduringweldingand(iv)theparent(base)metalwherethetemperaturewasnothighenoughtocauseanyvisiblechangeinitsstructure.Incomparisontothatofsteelinterpretationofstructurehereismuchsimpler.

Welding of age hardenable alloy

T

L

original

T

1 2 54

1. Weld metal 2. Partial melting 3. Ppt dissolution 4. Ppt coarsening 5. Original metal

+ L + L

L

+ +

TA

% solutex

Slide14


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Hardness profile

Tp

time

VHN

Distance from fusion zone

aging

+ + ss

+ T

t

SS

ss

T versus t plot superimposed on CCT curve

Hardness profile of HAZ

The hardness of the HAZ is likely to be lower than that of the parent metal because ofcoarseninganddissolutionofprecipitates.Thecoolingrateafterwelding isusuallyveryhigh.Therefore nucleation of second phase gets suppressed. The regionwhere there is completedissolutionthehardnessmaybelowinitiallyhoweveritmayincreaselaterduetoageing.Slide15hastwoplots.Thediagramonthe leftshowshowthetemperatureatapointgoesupandthencomesdownduringwelding.During theheating theprecipitatesmaydissolveonce thepeak temperaturegoesbeyond the solvus curve in thephasediagram. If the rateof coolingbeyondthepeak ishigherthanacriticalratetheprecipitationofsecondphase iscompletelysuppressed.This is illustratedwith thehelpofaC curve shown in thisdiagram. In this casesomeamountofprecipitatemayform.Heretoothecriticalrangeoftemperature(T)andthetime(t)spentduringcoolingwithinthisrangeoftemperaturewoulddeterminethestructurethatevolves.The seconddiagram in slide15gives theexpectedhardnessprofileof theheataffectedzone.Residualstress:Stressescandevelopinafusionweldjointbecauseofpostsolidificationthermalcontractionoftheweldmetal.AsaroughapproximationthemagnitudeofstressisgivenbyETwhereEisthe elastic modulus, is the coefficient of linear expansion and T is the temperaturedifferencebetween theweldmetalandHAZ soonafter solidification. If the stress isgreaterthanthefracturestrengthoftheregionitleadstocracking.Ifitisgreaterthanitsyieldstrengthitwould leadtodistortion.Sincethestressthatdevelopsgetsrelaxedbyplasticdeformationthehighestresidualstressthatcouldexit inaweld joint istheyieldstrengthofthematerial.

Slide15


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Thestressfieldthatexists isalwaysunderequilibrium. Itmeansthat ifthestress istensile incertain locationof theweld joint theremustbesimilarcompressivestress ina regiona littleawayfrom it.Welddistortioncanbecontrolledbydecreasingtemperaturedifference(T)bypreheatingorbyadoptingappropriateweldprocedure(Lookatfig6).Thetotalheatinputtooaffectstheextentofdistortion. Italsodependsonweldingmethodthat isadopted.Excessiveheatinputasinthecaseofgasweldingcausesgreaterdamagetotheworkpieceresultinginaweakerjointandhigherdistortion.UseofintenseLASERorsharplyfocusedelectronbeamcanmeltthematerialinstantaneouslygivinglittletimefortheheattodissipatetotheneighboringregionbyconductiontherebyresultinginahighqualityjointwithminimumdistortion.Figure7givestotalheatinputasafunctionofpowerdensityofthreedifferentweldingtechniques.

(b)(a)

(c) (d)

Fig6:Showshowdistortioncanbeminimizedbyproperplacementofthetwopartsbeingjoined.Ifthetwopartsarekeptasshownin(a)duringweldingitresultsinjointhavingdistortion(b).Ifthesearealignedasin(c)duringwelding,subsequentcontractionmakestheplatesstraight.


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Coldcrackingsusceptibility:Thisisaproblemassociatedwiththepresenceofhydrogeninmoltenweldpoolfromwhereitcoulddiffuse intotheheataffectedzone(HAZ)ofthewelded joint.ThepresenceofdissolvedhydrogeninHAZmakesitsusceptibletodelayedcrackingafteritcoolsdown.Itisalsoknownasunderbeadcracking.Considerableeffortshavegone intofindoutthefactorsthatdeterminesusceptibilityof steel tocoldcracking.Both%Cand thepresenceofalloyingelementshavebeenfoundtobethetwomostimportantfactors.Theformerdeterminesthehardnessofsteelwhereasthelatterdeterminesthesizeoftheheataffectedzone(HAZ)havingpoorresistancetocrackgrowth.

Heat

input

Gaswelding

Powerdensity

Arcwelding

Highenergybeamwelding

Fig 7: Total heat input as a functionofpowerdensityof theheat sourceusedduringwelding.Intenseheatsource(focusedLASERorelectronbeam)althoughcomesatacostbutensureshigherweldspeed,increasedpenetration,lessdistortionandbetterweldquality.


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Susceptibility of steel to such cracking is best described byGraville diagram (Weldability ofHSLA:microalloyedsteel,Int.Conf.Proc.ASMNov1976).Figure8showshowthesusceptibilityofsteeltocoldcrackingcanbeclassifiedintothreezones.SteelwhosecompositionfallswithinzoneIisnotsusceptibletocoldcracking.SteellyingwithinzoneIIIismostdifficulttoweld(Seefig8).Itisextremelysusceptibletocoldcracking.Itneedslowhydrogenelectrode,preheatingandpostweldheattreatment.SteellyinginzoneIIismoderatelysusceptibletocoldcracking.Useoflowhydrogenelectrodeoftenmaybeenough.Incaseofthickerplatespreheatingmaybenecessary.Diagramalso includes the rangeof%C&CE for fourdifferentgradesof steel.TMCP steel having low carbon and fine grain structure is least susceptible to cold crackingwhereasQTsteelsaremostsusceptibletosuchcracking.SummaryIn thismodulewehave looked atdifferentmethodsof joining and themechanismofbondformation.However themain focuswason the structuralchanges thatoccurduringweldingwherethetemperatureishighenoughtomeltthesurfaceofthemetalbeingjoined.Thishelps

0.2 1.2

0.6

00.5 0.8

0.3%C

CE

LCQTHSLATMCP

ZoneIII

ZoneI

ZoneII

Fig8:AschematicrepresentationofGravillediagramshowing locationsoffourcommongradesofsteel ina%C CEspace.LCdenotes lowcarbonsteel,QTdenotesquenchedand temperedsteel,HSLAdenoteshighstrength lowalloysteel,andTMCPdenotessteelmadebythermomechanicalcontrolledprocessingroute.CE=%C+(%Mn+%Si)/6+(%Ni+%Cu)/15+(%Cr+%Mo+%V)/5


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propermixingofthefillermetalandthefusionzone.Thisiswhythestrengthofweldedjointismuchhigherthanthatofthesolder.Weldingisdonewiththehelpofafilleralloyhavingnearlythesamecompositionasthatoftheparentmetal.However,thisisnotsointhecaseofbrazingor soldering. Themeltingpointof solder alloy ismuch lower than thatof thebrazing alloy.Therefore the strength of solder joint is lower than that of the brazing joint. Usually thestrengthofbrazingjointislowerthanthatofwelding.Someideaabouttheheattransferduringweldingisimportant.Thebasicconcepthasbeendescribed.Thishelpsusappreciatetheeffectofvariousweldparametersontheshapeoftheweldpoolandthesubsequentstructurethatdevelops.Apart from theweld pool the region surrounding it is also exposed to significantthermalexposure.Therefore itundergoes structural changes.Thebasic conceptsofphysicalmetallurgy couldbeapplied topredict the structureandpropertiesofdifferent regionsofaweldjoint.Thestructuralevolutioninsteeliscertainlymorecomplexthanthatinotheralloys.Why it isso isevidentfromthe illustrations included inthismodule.Themainfocushoweverhas been on fusion welding. This is where you are likely to have a wide variation in themicrostructure and properties of themetals being joined.Apart from this there are severalotherjoiningprocesses.Solidstatejoiningprocessessuchasfrictionorfrictionstirweldinghasrecentlygainedconsiderablepopularity.Bothuse theheat that isgeneratedat the interfaceduetofrictioneitherbetweenarotatingandastationarypart(frictionwelding)orbetweenarotating cylindrical tool and two rigidly clamped parts touching each other (friction stirwelding).Therotatingtoolhasaprobethatploughsthroughtheadjoiningregionsasitmovesalongtheinterface.Theheatthatisproducedisnothighenoughforfusionbutissufficienttojoin the two parts. Severe plastic deformation at the interface is responsible for thedevelopmentofagoodbondbetween the two.Theplasticdeformationand theheat that isgenerated at the interfacewould alter themicrostructure of thework piece. Inmost casestherewillbethreedistinctzones:thermallyaffectedzone,thermomechanicallyaffectedzoneanddynamicallyrecrystallizedzone. Inthethermallyaffectedzonethere isnochange inthegrainstructurealthoughitmaysoftenalittle.Inthethermomechanicallyzonesthegrainsgetseverelytwistedordeformedwhereasalltheoldgrainsinthedynamicallyrecrystallizedzonebynumeroustinygrains.Thiszoneiscommonlyknownasnugget.Thebasicphysicalconceptsgiveninthismodulewouldhelpyouinterpretsuchstructuralchanges.Exercise:

1. Why there isexcessive grain growth in theheat affected zoneof awelded structureevenifitismadeofmicroalloyedsteel?

2. MoststructuralsteelshavesomeamountMnprimarilytotakecareofthehotshortnessproblem because of residual sulfur in steel. Some amount of Si is also present. It isaddedtoremovedissolvedoxygeninsteel.ASTMA516Grade60isacommongradeof


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steelusedfortheconstructionofweldedvessels.Ithas021C,0.66Mn,0.45Si,S


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4. Totallydifferentkindsofproblemare facedduringweldingofaustenitic steel.One is

related to solidification shrinkage which is much more than in ferritic steel. This isovercome by suitable control of the composition of weld consumables so that onsolidification it has some amount of delta ferrite. Ferrite meters, a non destructivetesting tool todetect if there is somedelta ferrite.The secondproblem isassociatedwith the precipitation of Cr23C6 at grain boundaries in the heat affected zone. Thecarbideshaveveryhighamountofchromium.Wheneversuchprecipitatesareformedchromiumcontentofthesurroundingmatrixdropsbelowtheminimumamountneededtoformprotectiveoxidelayer.Theseregionsbecomepronetocorrosiveattack.Oftenitleads to inter granular failure. This phenomenon is known as sensitization. Stainlesscharacteristics of sensitized steel can be restored by giving post weld annealing byheating to 950C followed by rapid cooling to suppress carbide precipitation.Alternativelyselectspecial lowcarbonaustenticsteelsorthosehavingadditionalalloyelementslikeNb.Thisisastrongcarbideformingelement.ItdoesnotallowanycarbontobepresentinthematrixtoformCr23C6.

5. Temperatureatdistance r fromamovingpointheat source isgivenby the followingexpression:

whereT is temperature, t is time, is thermal

diffusivity,v isweldingspeed,isthermalconductivity,T0 is initialtemperatureoftheplate,q=iVwherei=current,V=voltage,=efficiency,=/(c)where=densityandc=specificheat.TofindouttimeatwhichTreachesmaximumatagivenpointrequate: 0

. Since

or

0

On substituting this in the expression for T:

On substituting numerical values one directly gets both tmax and Tmax.

. 7850 450 1.77 and

300 .... 777.

6. The expression for Tmax can be rewritten as /2 = 0.6 150 20 1.42 10/2 50 2.718 0.0025 1200 0.00316

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Lecture 42