Welding-metallurgy-of-stainless-steels-during-resi

WeldingmetallurgyofstainlesssteelsduringresistancespotweldingPartI:fusionzoneM.Pouranvari*1,M.Alizadeh-Sh2andS.P.H.Marashi3Weldabilityisoneofthekeyrequirementsforautomotivematerials.Thistwo-partpaperaimsatunderstandingthemetallurgicalphenomenaduringresistancespotweldingofstainlesssteels,asinterestingcandidatesforautomotivebodyinwhite.PartIaddressesthephasetransformationsinthefusionzoneofthreetypesofstainlesssteelsincludingaustenitic,ferriticandduplextypes.Thesolidificationandsolidstatephenomenaincludingcolumnartoequiaxedtransition,ferrite–aus-tenitepost-solidificationtransformation,martensitictransformationandcarbideprecipitationarediscussed.Particularattentionisgiventotheeffectofhighcoolingrateofresistancespotweldingprocessontheferrite–austenitetransformation.Keyfactorscontrollingthehardnessofthefusionzonearehighlighted.Keywords:Resistancespotwelding,Weldingmetallurgy,Stainlesssteel,Phasetransformations,FusionzoneIntroductionMaterialsselectionisakeystageforthevehicledesign.Increasingglobaldemandsforenergyconservation,environmentalprotectionandimprovedcrashworthinesshavepromptedautomotivemanufacturestodeveloplightweightautomobiles.1,2Vehicleweightreductioncanbeachievedbyreplacingheavysteelcomponentswithmaterialssuchasadvancedhighstrengthsteel(AHSS),aluminium,magnesiumorglassfibrereinforcedpolymercomposites.3Thepossibilityofusingstainlesssteelsinstructuralautomotivewasstudiedbyanextgenerationvehicle(NGV)programme.4,5Nextgenerationvehicledemon-stratedthatusingstainlesssteelsreducesweightand,atthesametime,increasessafety,sustainabilityandenvironmentalresistanceofthestructuralautomotivesystems.StainlesssteelcanbeusedindoorpillarsincludingAandBpillarreinforcementparts,bumperbeams,rolloverbars,crashboxes,suspension/wheelhousingsandsubframes.4–6Underimpact,highstrengthausteniticandduplexstainlesssteels(DSSs)offerexcellentenergyabsorptioninrelationtostrainrate.Therefore,theyareidealfortherevolutionary‘spaceframe’carbodystructureconcept.Moreover,nowa-days,usingausteniticstainlesssteels(ASSs)andferriticstainlesssteels(FSSs)instructuralframeworksandbodypanellingofbusesandcoachesiswidelyaccepted.5Resistancespotwelding(RSW)isacriticaljoiningprocessinvehicleproduction.Vehiclecrashworthiness,whichisdefinedasthecapabilityofacarstructuretoprovideadequateprotectiontoitspassengersagainstinjuriesintheeventofacrash,largelydependsontheintegrityandthemechanicalperformanceofthespotwelds.7–9Therefore,studyofweldingmetallurgyofstainlesssteelsheetsisaprerequisitefortheiruseinautomotivestructuralparts.Mechanicalperformanceofresistancespotweldsissignificantlyaffectedby:7,10–15(i)weldphysicalattributes,particularlythefusionzone(FZ)size:theweldnuggetsizeiscontrolledbyheatgenerationandheatdissipation,whichinturnaregovernedbyelectricalresistivityandthermalconductivityofthemetalsheetsbeingweldedrespectively.7Table116showsasum-maryofphysicalpropertiesoftheASS,FSS,martensiticstainlesssteel(MSS)andDSScom-paredwithlowcarbonsteel.Thehigherelectricalresistivityandlowerthermalconductivitycoupledwiththelowermeltingtemperaturesofstainlesssteelscontributetothelowercurrentsandshorterweldingtimes,whicharenecessaryforthedevelopmentofanacceptableweldnugget.Thehighcoefficientofthermalexpansionofstainlesssteelscomparedtothelowcarbonsteelsmakethemmoresusceptibletosolidifica-tioncrackingduringwelding(ii)metallurgicalfactors:mechanicalpropertiesandfailurebehaviourofthespotweldsdependontheweldmetallurgicalcharacteristicsincludingmicrostructuralandhardnesscharacteristicsoftheweldment.Itisshownthatthehardnesscharacteristic,whichinturnisgovernedbyweldphasetransformations,isthekeycontrollingfactorininterfacialtopulloutfailuremodetransitionofresistancespotwelds.7Therefore,studyingthephasetransformationsinFZandheataffectedzoneiscriticalforunderstandingthefailuremodebehaviourandmechanicalpropertiesofthejoints.1MaterialsandMetallurgicalEngineeringDepartment,DezfulBranch,IslamicAzadUniversity,Dezful,Iran2YoungResearchersandEliteClub,SouthTehranBranch,IslamicAzadUniversity,Tehran,Iran3MiningandMetallurgicalEngineeringDepartment,AmirkabirUniversityofTechnology,Tehran,Iran*Correspondingauthor,[e-mail]
2015InstituteofMaterials,MineralsandMiningReceived15December2014;accepted04February2015DOI10.1179/1362171815Y.0000000015ScienceandTechnologyofWeldingandJoining2015VOL00NO01ThephasetransformationsinFZofstainlesssteelweldmentsincludingsolidificationtransformations,fer-rite–austenitetransformationandcarbideprecipitationarecontrolledbytheratioofchromiumequivalenttonickelequivalent(Creq/Nieq)andthecoolingrate.17–20ThecoolingrateofRSWissignificantlyhigherthanthatoftheconventionalarcweldingprocesses.Gouldetal.21proposedasimpleanalyticalmodelpredictingcoolingratesofresistancespotwelds,asfollowsLTLt¼2ap24t2S
TTP
TP2T1þ2pkEkStStEcosp2tSx2435ð1Þwhereaisthermaldiffusivityofthesteelsheets,TPisthemaximumtemperatureexperiencedinFZduringweld-ingprocess,tsisthesheetthickness,tEistheelectrodefacethicknesses(i.e.thedistanceofwatercooledholetotheelectrodesurface),kSandkEaret