ATIG welding of ferritic stainless steels.docx

ATIG welding of ferritic stainless steels.docx

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ATIGweldingofferriticstainlesssteels

A-TIGweldingofferriticstainlesssteels

A-TIGWELDINGOFFERRITICSTAINLESSSTEELS

12T.Sándor,J.Dobránszky

1ESABKft.,Hungary,2ResearchGroupforMetalsTechnologyoftheHungarianAcademyofSciences

Abstract

Asaconsequenceoftheenormouschangingofthealloysurchargesofausteniticcorrosionresistantsteelsinthelastyearsmoreandmoreattentionwasfocusedonthecheaperferriticstainlesssteels.Knowingtheweldabilityproblemsoftheferriticstainlesssteels（sensitivityofgraingrowthduringheatinput,lowductility,etc.）itseemedtheapplicationofactivatedTIG（ATIG）weldingwithitslowerheatinputandfocusedarcmaywillprovideadvantageousresults.Thispapersumsuptheexperimentsthatweredoneonferriticsteelswithdifferentthicknesses,differentweldingspeedsandheatinputs.TostrengthenourobservationsmicroscopicexaminationsalsoweredonetocomparetheconventionalweldingmethodsandtheATIGwelding.

Introduction

Inthelastsomeyearsanenormousmarketdemandappearedconcerningtovagy:

inconnectionwiththestainlesssteels.Hereandusualwhenwesay“stainless”wemean“austeniticstainlesssteel”asthe80-90%ofthetotalstainlesssteelproductionandconsumptionisaustenitic.Theadvantagesofausteniticstainlesstypesareverywellknown:

easyforming,verygoodweldability,goodcorrosionresistance,decorativeoutlook,andsoon.Anadditionalbenefitistheverywideliteratureofthepreviouslymentionedproperties,whichhelpstheusertofindsolutionsforanyproblemsthatmayoccurduringproduction.

Inoppositewithaustenitictypestheferriticandmartensitictypeshaveseveralproblemswhichdidnothelpthemtospreadintheindustry,butontheotherhandinsomespecificareasthesesteelsmaybetheuniquesolutions.

Asthewellsellableausteniticstainlesssteelscontains8,12,13,etcpercentnickel,parallelwiththeincreasingdemandofausteniticstainlesssteelsthedemandofnickelalsoincreased.Thisleadedtothesubstantialincreasingofalloysurchargesandnickelprices.

Thereactionofmarketdidnotwaittoolongtimeandsoonbeganthedevelopingofthelownickelcontentstainlesssteels.Amongtheduplexsteelsthesetypesarecalled“leanduplex”.Parallelwiththesedevelopmentsthefocusofthemarket’sattentionturnedtothedirectionof

elderandnewferriticstainlesssteels（FSSs）.ThisworkwouldliketorepresenttheresultsofweldingofthegraincoarseningandintergranularcorrosionsensitiveFSSsbytheActivatedTungstenInertGas（ATIG）welding.

TheATIGwelding

TheATIGweldingisthehighproductivityvariationoftheconventionalTIGwelding.Whenapplyingthishardlyknownweldingprocess,theweldingmaybeexecutedwithsubstantiallylowerweldingcurrentandhigherweldingspeedwhilethepenetrationis2-3timesdeeperasitiswithconventionalTIGwelding.WhenATIGweldingisappliedforweldingofstainlesssteelsthefollowingrulesmustbekept:

ATIGweldingisapplicablewithoutbevelling.Thisdecreasesthecostandtimeof

productionandlettheend-usereasierfitting;

Gapisnotrecommendedwhenfittingthepieces,becauseitincreasesthepossibilityof

porosity;

Onesizebiggertungstanelectrodeshouldbeusedtoresistthehigherreflectedheat;,Electrodesharpeningshouldbearound45ºforlongerlifeexpectancy;

Consistentactivefluxportioningisindispensable;

Weldingconsumableisnotaddedtotheweldpool.

Generallyaboutferriticstainlesssteels

InspiteoftheadvantageouspropertiesofFSSs（similarmechanicalpropertiestoaustenitictypes,verygoodcorrosionresistancetoCl-containingmedias,highstresscorrosioncrackingresistance,lowthermalcoefficientandconsequentlylowthermalfatiguetendency,stronglyadhereoxidelayerwhichisreallyprosperousatelevatedtemperaturesandquitestableandlowprice）unfortunatelytheusershouldalsobeawareofsomehandicaps:

IncaseofpresenceofcarbonintheFSStheγ-loopopenswhichleadstoformationof

austeniteandmartensite（withappropriatecoolingrate）（Figure1）;

ThepresenceofevenaverysmallcarboncontentinFSSsalsotendtoformcarbidesin

reallywidevariety（themostregularformsareMCandMCwhereMstandsfor23673

Fe+CrinFSSs）whichfinallyresultsthehighriskofintergranularcorrosion（Thisisthe

socalledsensitisation-effect）;

Aptitudeto475ºCembrittlement（especiallywhenCr-contentisabove18%）;,σ-phaseformationinthetemperaturerange500…800ºC（aptitudeisincreasingwithCr

content）;

InthecaseofNborTistabilizationknife-edgecorrosionmayoccurintheheataffected

zone（HAZ）;

GraincoarseninginHAZ;

Inpureferritictypesbecauseofthelackofγ?

αtransformationheattreatmentisnot

possibletorepairthegrain-coarsenedstructure.

Figure1.Fe-Crbinarysystemwith<0,01%Ccontent（leftfigure）;Fe-Cr-Cternarysystemwith0,05%Ccontent（middlefigure）;Fe-Cr-Cternarysystemwith0,1%Ccontent（rightfigure）.[1],[6]

AftersolidificationtheFSSskeepstheirbodycenteredcubiclattice（bcc）untiltheroomtemperature.ThisexplainswhytheregeneratingheattreatmentisnotpossibleinFSSs,soconsequentlyitmaybestatedthatthebiggestdisadvantageofFSSsisthegraincoarseningofHAZ.Thisphenomenonsubstantiallydecreasesthemechanicalandcorrosionresistanceproperties.MoreoverifCispresentintheFSStheformationofcarbidesisalmostunavoidablewhichfinallyleadstoworsemechanicalpropertiesanddecreasedcorrosionresistance.InthefollowingthispaperwouldliketorepresenttheeffectofATIGweldingtograinstructureofFSSscomparedtoTIGwelding.[6],[8],[3]

Experiments

TheATIGweldingexperimentsweredoneinflatbuttweld（PA）positionwithoutbevellingandfittedwithnogap.Bothshieldingandbackinggaswerepureargon（T4.5）.Thearclength（thegapbetweentungstenelectrodeandtheplate）waskeptat2mm.TheconsistentarclengthandweldingspeedwasensuredwithmechanisedTIGtorchmovingtable.Heretheconstantarclengthparallelwithanaloguesettingofweldingspeedwithapotentiometerwasalsopossible.Thebasematerialwas430（X6Cr17）type（W.Nr.1.4016;UNSS43000）withtheplatethicknessof8mm（Table1）.

Table1.Chemicalcompositionofinvestigated430typeferriticstainlesssteelaccordingtoinspectioncertificate3.1.

CMnSiCrPS

0,0460,670,4616,360,020,003

Thecutedgesweregrindedmanuallyandcleanedwithalcoholtoremovethepossiblegreaseoroilresidualsfromtheproximatesurroundingofthewelding.

TheweldingparameterswereoptimisedforATIGweldingtoobtainabsolutelyperfectrootpenetration.ThesameparameterswereappliedforTIGweldingafterwardtoensurethesameheatinput.ThusthecomparisonofTIGandATIGweldingwaspossiblefromthepointofviewofheatinput.Themeasuredaveragegrainsizeofbasematerialwasintherangeof30…80μm.

Afterparameteroptimisationthefollowingweldingparameterswereapplied（Table2）.

Table2.WeldingparametersofTIGandATIGweldingof8mmthick430typeferriticstainlesssteel.

ArcWeldingWeldingHeatinputVoltage（V）Power（kW）efficiencyspeedcurrent（A）（kJ/mm）（%）（mm/min）

24020,85,007570ATIGwelding3,214

24020,85,007570TIGwelding3,214

ResultsofTIGwelding

AstheweldingparametersofTIGweldingweresettoATIGnaturallyperfectpenetrationwasnotexpected.Thusonlysimplebeadonplateweldswereexamined（Figure2）.

Figure2.CrosssectionofbeadonplateweldwithTIGprocess.

Asintheexamined430typeFSSthecarboncontentwas0,046%accordingtoFigure1.（middlefigure）austeniteandoptionallymartensiteformationwasexpectable.Consequentlythemostinterestingquestionswerehowlargemutationofgrainsizeoccurredduetothecounted3,214kJ/mmheatinputintheHAZandhowmuchaustenite/martensiteformedintheweldedjointandintheHAZ.

Naturallythegraincoarseningwasbefallen（Figure3）intheHAZ.Theaveragegrainsizeincreasedtotherangeof60…120μmintheHAZ.

Figure3.Macro-（A）andmicrostructure（B）oftheHAZofTIG-weldedjointandmicrostructureofthebasematerial（C）.

Thetremendousgrainsizeincreasingintheweldmetalmayhavebeenimprovedwithlowerheatinputbutitwasnottheaimofthisexperiment.[7]

ResultsofATIGwelding

ThejointsmadewithATIGweldingwiththeparametersstatedinTable2resultedtotalrootpenetration（Figure4）.

Figure4.CrosssectionofATIGweldedjoint;platethicknessis8mm.

Thecrosssectionofthejointsshowsthattheweldingcurrentcouldhavebeenlowerslightly.Thelittlebithighercurrentwasnecessarytoevadetherootpenetrationfaultsthatmayoriginfromthenotabsolutelyperfectfitting.ThemostinterestingobservationwasbetweenHAZandweldedjointofTIGandATIGwasthatincaseofATIGweldingthegraincoarseningwasalittlebitmoderateandthemartensiteformationwasbigger.ThisoriginsfromthattheweldpoolofTIGismoreshallow（thusitsvolumeisless）sothearcenergyheatsuptohighertemperaturetheweldpoolthusgrainshavemoretimetobecomelargerintheHAZastheyareinthecriticaltemperaturerangeforlongertime.IncaseofATIGweldingtheweldpoolisdeeperandthevolumeofmoltenmetalisbiggerwhichflowsfaster.Bythisthearcenergydoesnotheatsuptheweldpooloverly.SotheHAZ（withtheweldpool）cancooldownfasterthroughthecriticaltemperatureinterval,whichresultsfinergrainstructurethere[9].Ifthepreviousstatementisappropriatethefastercoolingratemustbeobservedintheweldpool