Effect of processing conditions on the corrosion performance of lasersurfacemelted AISI 440C.docx

Effect of processing conditions on the corrosion performance of lasersurfacemelted AISI 440C.docx

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EffectofprocessingconditionsonthecorrosionperformanceoflasersurfacemeltedAISI440C

Effectofprocessingconditionsonthecorrosionperformanceoflaser

surface-meltedAISI440Cmartensiticstainlesssteel

C.T.Kwoka,b,K.H.Lob,F.T.Cheng

bandH.C.Manc

aDepartmentofElectromechanicalEngineering,FacultyofScienceandTechnology,UniversityofMacau,Taipa,Macau,PRChina

bDepartmentofAppliedPhysics,TheHongKongPolytechnicUniversity,HungHom,Kowloon,HongKong,PRChina

cDepartmentofIndustrialandSystemsEngineering,TheHongKongPolytechnicUniversity,HungHom,Kowloon,HongKong,PRChina

Received26July2002; accepted27October2002. ;Availableonline21December2002.

Abstract:

LasersurfacemeltingofAISI440Cmartensiticstainlesssteelwasachievedusinga2.5-kWcontinuouswaveNd:

YAGlaser.Thepittingcorrosionbehaviorof

laser

surface-meltedspecimensprocessedunderdifferentprocessingconditionsin3.5%NaClsolutionat23°Cwasstudiedbypotentiodynamicpolarizationtechnique.Thecorrosionresistanceofall

laser

surface-meltedspecimenswassignificantlyimproved,asevidencedbyashiftfromactivecorrosiontopassivity,awidepassiverangeandalowpassivecurrentdensity.Thepittingpotentialofthe

laser

surface-meltedspecimensP08-440C-25（

laser

POWER=0.8kW,scanningSPEED=25mm/s）andP12-440C-25（

laser

POWER=1.2kW,scanningSPEED=25mm/s）wasincreasedto260and200mV（SCE）,respectively,andwasmuchhigherthanthatoftheconventionallyheat-treatedAISI440C.Thepittingcorrosioncharacteristicsofthe

laser

surface-meltedspecimenswerestronglydependentontheprocessingconditionswhichresultedindifferentmicrostructures.Theenhancedcorrosionresistancewasattributedtothedissolutionorrefinementofcarbideparticlesandthepresenceofretained

austenite.

Theamountofcarbidesinthemeltlayer,whichindirectlydeterminetheCrcontentinsolidsolutionandhence,thecorrosionresistance,wasrelatedtotheamountofCremaininginsolidsolutionandtodecarburization.Thepitmorphologyofthe

laser

surface-meltedspecimenwasalsostudied.

AuthorKeywords:

Lasersurfacemelting;Pittingcorrosion;Martensiticstainlesssteel;Nd:

YAGlaser

1.Introduction

Inmostpracticalapplicationsofengineeringcomponents,materialssufferfromdeteriorationbymechanicaland/orchemicaleffectspresentintheiroperatingenvironments.Martensiticstainlesssteelsarewidelyusedinengineeringapplicationssuchassteamandwatervalves,pumps,turbines,compressorcomponents,shafting,cutlery,surgicaltools,bearingsandplasticsmoulds,etc.whichdemandhighstrengthandhighresistancetowearandcorrosion.Amongthemartensiticstainlesssteels,AISI440Chasgoodmechanicalproperties（Table1）,ahighchromiumcontent（17wt.%）andahighcarboncontent（1.1wt.%）.However,thecorrosionresistanceofAISI440Cisthelowestamongthestainlessgroupsbecauseofitshighcarboncontent,whichresultsintheprecipitationofcarbidephases,althoughitschromiumcontentisclosetothatofAISI304austeniticstainlesssteel（18wt.%Cr）.

Table1.Typicalmechanicalpropertiesofselectedstainlesssteels

Theapplicationof

laser

surfacemodificationtoprolongtheservicelifeofengineeringcomponentsexposedtoaggressiveenvironmentshasgainedincreasingacceptanceinrecentyears.

Laser

surfacemelting（LSM）hasbeenproventobeapromisingmethodforimprovingcorrosion,wearandfatigueresistancesbyrefining,homogenizingortransformingthemicrostructureofawiderangeofengineeringalloys[1].Thesuperficiallayerofthecomponentsismodifiedwhilethebulkpropertiesofthesubstratearepreserved.AgreatdealofworkhasbeendonetoinvestigatetheeffectofLSMontheelectrochemicalcorrosionpropertiesofAISI304[2,3,4and5],316L[4]and310[5]austeniticstainlesssteels,Zeron100（superduplexstainlesssteel,UNSS32760）[4],AISI430ferriticstainlesssteel[5]andAISI420martensiticstainlesssteel[5,6and7]insodiumchloridesolution.Thepittingcorrosionresistanceof

laser

surface-meltedAISI304[2,3and5]and316L[4]wasimprovedbecauseoftheremovalorredistributionofmanganesesulfideinclusions.For

laser

surface-meltedAISI430andZeron100,theircorrosionbehaviorwasstronglydependentonthephasesformed[4and5].For

laser

surface-meltedAISI430,ahighercorrosionresistancewasobtainedwithamicrostructurecontainingasingleferriticorausteniticphase[5].

Formartensiticstainlesssteels,LSMcausesthedissolutionoflargecarbides,refinementofthemicrostructureandhomogenizationofchemicalcomposition[8],resultinginimprovementofhardness,toughness[9],wearresistance[10]andcavitationerosionresistance[6]ofthesesteels.Thecorrosionbehaviorof

laser

surface-meltedAISI420inNaClsolution[5,6and7]andinH2SO4[11]wasreportedbyseveralworkers.Thecorrosionresistanceof

laser

surface-meltedmartensiticstainlesssteelswasfoundtobehighlydependentonthemicrostructuralchangewhichwasrelatedtoprocessingconditionssuchaspowerdensityandscanningspeedofthe

laser

beam[5,6and7].EscuderoandBello[7]reportedthatthebestcorrosionbehaviorof

laser

surface-meltedAISI420wasobservedinthecompletelymeltedregion.Ontheotherhand,itwasreportedthatLSMimprovedthepassiveperformanceofAISI420andthepittingcorrosionresistanceinH2SO4approachedthatofAISI304[11].

Retainedausteniteiscommonlypresentin

laser

surface-meltedmartensiticstainlesssteels[10,12,13and14].ColacoandVilar[13]reportedanincreaseintheproportionofretainedaustenitewithdecreasingpowerdensityandincreasingscanningspeedintheLSMofAISI420martensiticstainlesssteel.Theeffectofthepresenceandamountofretainedausteniteonthecorrosionbehaviorisstillacontroversialissue.Noharmfuleffectofretainedausteniteonthecorrosionresistanceofa13%-CrmartensiticstainlesssteelwasobservedbyKimuraetal.[15].Ontheotherhand,Kraposhin[16]reportedthattherewasanoptimumamountoftheretainedausteniteforbestresistancetoanodicdissolution.

StudiesrelatedtotheeffectofLSMonthecorrosionbehaviorAISI440Cmartensiticstainlesssteelarescarceintheliterature.Thepresentwork,thus,aimsatimprovingthepittingcorrosionresistanceofAISI440Cin3.5%NaClsolutionbyLSM,andatinvestigatingtheeffectofretainedausteniteandundissolvedcarbidespresent.Therelationshipbetweenthecorrosionparametersandthe

laser

processingconditionswillalsobestudied,aimingatfindingtheoptimumprocessingparameters.

2.Experimentaldetails

2.1.Materialandspecimenpreparation

Theas-receivedAISI440C（designatedasAR-440C）wasinannealedconditionandintheformofroundbarwithahardnessof260Hv.Thenominalchemicalcompositioninwt.%was:

17%Cr;0.75%Mo;1%Mn;1.1%C;1%Si;0.049%P;0.03%S;andbalanceFe.HardenedspecimenofAISI440C（designatedasHT-440C）wasachievedbyconventionalheattreatmentinafurnaceforcomparisonwiththe

laser

surface-meltedspecimens.Thespecimenswerepreheatedto,andkeptat,850°Cfor45minandthenheat-treatedthroughtheaustenitizingtemperature（1060°C）for30min,followedbyquenchinginliquidnitrogen.Temperingwasachievedbykeepingthespecimensat250°Cfor210min,followedbyaircooling.

Thespecimensforpolarizationstudiesweremachinedtodiscs12.7mmindiameterand3.2-mmthick.ThesurfaceofthespecimensforLSMwassandblastedinordertoreducethereflectivitytothe

laser

beam.Priortopolarizationtests,thesurfaceofall

laser

surface-meltedspecimenswasmechanicallypolishedwith1-μmdiamondpasteinordertokeepthesurfaceroughnessconsistent.Thespecimenswerecleaned,degreasedanddriedbeforethepolarizationtest.

2.2.

Laser

surfacemelting

LSMwascarriedoutusinga2.5-kWCWNd:

YAG

laser.Thelaser

beamwasfocusedontothespecimenwithaBK-7glasslensoffocallength100mm.Preliminarytrialsonthe

laser

processingparametersforfeasibletreatmentconditionswerecarriedout.

Laser

power（P）of1.2kW（4.2kW/cm2）and0.8kW（2.8kW/cm2）atworkpiecewitha

laser

spotsizeof6mmindiameterandbeamscanningspeeds（v）of25to85mm/swereused.Atscanningspeedslowerthan25mm/s,theenergyinputfromtheYAG

laser

wastoohighandthermaldistortionofthespecimenoccurred.Atscanningspeedhigherthan85mm/s,crackswerepresentinthemeltlayerduetoveryhighcoolingrate.Argonflowingat15l/minwasusedasshieldinggas.ThedesignationsofthesamplesareshowninTable2.Themeltsurfacewasachievedbyoverlappingofsuccessivemelttracksat50%trackwidthinterval.

Table2.

Laser

parametersfor

laser

surfacemeltingofAISI440C

2.3.Metallographicandmicrostructuralexamination

AfterLSM,thespecimensweresectioned,polishedandetchedinacidicferricchloridesolution（25gFeCl3,25mlHCland100mlH2O）.Themicrostructureandpitmorphologyofthe

laser

surface-meltedlayerswerestudiedbyopticalmicroscopy（OM）,scanningelectronmicroscopy（SEM）andenergy-dispersiveX-rayanalysis（EDX）.ThephasespresentwereidentifiedbyX-raydiffractometry（XRD）.TheradiationsourceusedwasCuKαwithnickelfilterandgeneratedat40kVand35mA.

2.4.Electrochemicalmeasurements

Polarizationstudieswereperformedin3.5%NaClsolutiontoinvestigatetheelectrochemicalcorrosionbehavioroftheas-received,theconventionallyheat-treatedandthe

laser

surface-meltedspecimens.Cyclicpoten