Cavitation erosion and pitting corrosion behaviour of laser.docx

Cavitation erosion and pitting corrosion behaviour of laser.docx

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Cavitationerosionandpittingcorrosionbehaviouroflaser

Cavitationerosionandpittingcorrosionbehaviouroflaser

surface-meltedmartensiticstainlesssteelUNSS42000

C.T.Kwoka,H.C.Man

bandF.T.Chenga

aDepartmentofAppliedPhysics,TheHongKongPolytechnicUniversity,HungHom,Kowloon,HongKong,China

bDepartmentofManufacturingEngineering,TheHongKongPolytechnicUniversity,HungHom,Kowloon,HongKong,China

Received26October1999;accepted17January2000.Availableonline19April2000.

Abstract:

SurfacemodificationofmartensiticstainlesssteelUNSS42000wasachievedby

laser

surface-meltingusinga3.5-kWcontinuouswaveCO2

laser.

Thecavitationerosionandcorrosioncharacteristicsof

laser

surface-meltedspecimensin3.5%NaClsolutionat23°Cwerestudiedbymeansofa20-kHzultrasonicvibratoratapeak-to-peakamplitudeof30μmandapotentiostat,respectively.Inaseriesof4-hcavitationtests,thecavitationerosionresistanceof

laser

-meltedspecimensfabricatedunderapowerof1.7kWandascanningspeedof25mm/swas70timesthatoftheas-received（annealed）S42000and1.8timesthatofconventionallyheat-treatedspecimens.Theexcellentcavitationerosionresistancewasduetothecombinedeffectofahighvolumefractionofretained

austenite

（89%）andamoderatehardness（450Hv）.Byusingdifferentprocessingparameters,itwasfoundthatthecavitationerosionresistanceofthe

laser

-meltedspecimensincreasedwiththeincreaseinvolumefractionofretained

austenite,

aresultattributabletothehighmartensitictransformabilityofthe

austenite

inS42000.Ontheotherhand,cavitationerosionresistanceincreasedwiththeincreaseinhardnesstoamaximumvalueandthendroppedwithfurtherincreasesinhardness.Thisindicatedthatmartensitictransformabilityplayedamoreimportantrolethanhardnessincavitationerosion.Thepittingpotentialsofall

laser

-meltedspecimensshiftedinthenobledirection,andthepitsformedinthe

laser

-meltedspecimenswereshallowerascomparedwiththoseformedinas-receivedandhardenedS42000specimens.Theimprovementinpittingcorrosionresistanceresultedfromthedissolutionorrefinementofcarbideparticlesandthepresenceofretained

austenite,

asevidencedbythefactthatthepittingpotentialincreasedlinearlywiththeamountofretained

austenite.

Thepresentstudythusshows,thatwithproperprocessingparameters,boththecavitationerosionandpittingresistancemightbesimultaneouslyimprovedby

laser

-surfacemelting,atleastformartensiticstainlesssteels.

AuthorKeywords:

Laser

surface-melting;Cavitationerosion;Pittingcorrosion;Martensiticstainlesssteel;Retained

austenite

1.Introduction

Cavitationerosionisacommonprobleminengineeringpartsincontactwithaliquidinwhichthelocalpressurefluctuates,andcavitationdamagewillleadtoshorteningofservicelifeormaintenancecycle.Therearenumerousmeansofcombatingcavitationerosion[1and2],andamongthesetechniques,modificationofthecomponentsurfaceisespeciallyeffectiveandattractivesinceitdoesnotaffectthebulkpropertiesofthesubstrate.

Laser

surfacemodification,arelativelynewtechniqueinsurfaceengineering,isparticularlyattractiveasitcanproducesurfacelayerswithmicrostructuresandcompositionsnotattainablebyconventionalmethods.

Laser

surface-melting（LSM）forimprovingthecavitationerosionresistancebymeansofacontinuousNd-YAG

laser

hasbeenattemptedonausteniticstainlesssteelsUNSS31603andUNSS30400,andsuperduplexstainlesssteelUNSS32760inourpreviousstudy[3].Thecavitationerosionresistanceofthe

laser

surface-meltedstainlesssteelswasfoundtobehighlydependentonthemicrostructuralchangeandtheresidualstressinthe

laser

-meltedlayer.OnlythecavitationerosionresistanceofUNSS31603wasimprovedby22%afterLSM.ThelowcarboncontentpresentinthesestainlesssteelslimitstheirhardenabilityandhencelimitsthebeneficialeffectofLSM.

Asfarasstainlesssteelsareconcerned,themartensiticstainlesssteelspossessthehighestcavitationerosionresistancefollowedbyausteniticstainlesssteelswhiletheferriticsteelshavethelowestcavitationerosionresistance[4].Heathcocketal.[4]studiedthecavitationerosionoftwomartensiticstainlesssteelsBS431S29（Fe-15.6%Cr-2.34%Ni-1%Mn-0.8%Si-0.11%C）andDIN4112（Fe-18.5%Cr-1.15%Mo-1%Mn-1%Si-0.91%C）,andfoundthatthecavitationerosionresistanceofthehardenedandtemperedmartensiticsteelsincreasedwiththeincreaseinhardness.DIN4112exhibitedahighercavitationerosionresistancethanBS431S29forsimilarheattreatmentconditionsbecauseofthedifferenceinthecarboncontentandtoalesserdegree,thechromiumcontent.Carbonhasbothpositiveandnegativeeffectsoncavitationerosionresistance.ThehighcarboncontentinDIN4112resultedinahigherhardnessinitsmartensiticmatrixontheonehand,andalsoledtotheformationofthecoarsechromiumcarbideswhichprovidedsitesfortheinitiationoferosiondamageontheother.Inaddition,Yebisuyaetal.[5]reportedthatcastmartensiticstainlesssteel（Fe-11–14%Cr-0.5–8%Ni-2–9%Mn-1%Si-0.1%C）hashighcavitationerosionresistanceandissuitableforuseasturbineelementsinwaterplants.Theapplicationofmartensiticstainlesssteelsashigh-speedcomponentsinmarineenvironmentsagainstcavitationdamageisfeasiblebecauseofitshighhardenabilitybysuitableheattreatment.LSMnaturallyoffersaconvenientandeffectivesolutiontosuchaproblemsinceonlythesuperficiallayerismodifiedandthebulkpropertiesofthesubstrateareretained.

Similartoothertypesofsteels,LSMofmartensiticstainlesssteelsleadstothedissolutionoflargecarbides,refinementofthemicrostructure,homogenisationofchemicalcomposition[6],resultinginimprovementofhardness,toughness[7],wearresistance[8],andcorrosionresistance[9]ofthesesteels.Ontheotherhand,unlikeothertypesofstainlesssteels,retainedausteniteisingeneralpresentinthe

laser

-meltedmartensiticstainlesssteels[10,11,12and13].Recently,ColacoandVilar[14]reportedanincreaseoftheproportionofretainedaustenitewithdecreasingpowerdensityandincreasingscanningspeedfor

laser

surface-meltedS42000.Thepresenceandtheamountofretainedausteniteasamicrostructuralcomponenthasaprofoundeffectonthepropertiesandperformanceofmartensiticstainlesssteels.Dependingonthepropertiesconcernedandtheamountpresent,retainedaustenitecanbebeneficialordetrimentaltoanalloy[15].Forexample,retainedausteniteupto10%wasfoundtoincreasethewearresistanceofsteelT10[16],andretainedaustenitecontrolledat20–25%couldincreasetherollingfatiguelifeofbearingsteelsby11.3times[17].TongandZhong[18]investigatedtheeffectofretainedausteniteinhighchromiumcastironsubjecttoimpactabrasivewear,andtheyconcludedthatunderhigh-energyimpact,retainedaustenitewasharmfulwhileunderlow-energyimpact,itwasbeneficial.Theeffectofretainedausteniteinmartensiticstainlesssteelsonthecavitationerosionresistance,however,hasnotbeenreportedintheliterature,norhasstudyontheLSMofmartensiticsteelstoincreasethecavitationerosionresistance.

ThepresentworkaimstoimprovethecavitationerosionresistanceofmartensiticstainlesssteelUNSS42000byLSM,andtoinvestigatetheeffectoftheamountofretainedaustenite.Thecorrosionbehaviourofthe

laser

surface-meltedS42000in3.5%NaClsolutionwillalsobestudied.

2.Experimentalprocedures

2.1.Materialandspecimenpreparation

Theas-receivedS42000,whichhadbeenheat-treatedto780°C,cooledinafurnaceat10°C/hto650°C,andthenair-cooled,wasintheformofaroundbarwithdiameterofhalfaninchandhadahardnessof220Hv.Thenominalchemicalcompositioninwt.%was:

Fe-13.6%Cr-0.5%Mn-0.9%Si-0.38%C-0.3%V.HardeningofS42000wasachievedbyheattreatmentinafurnaceforcomparisonwiththe

laser

-meltedspecimens.SpecimensHT-420-1andHT-420-2werepre-heatedto,andkeptat,850°Cfor45minandthenheat-treatedthroughtheaustenitisingtemperature（1020°C）for15min,followedbyquenchinginliquidnitrogentoroomtemperature.Stressrelieving（tempering）wasdonebykeepingspecimensHT-420-1andHT-420-2at650and250°C,respectively,for150min,followedbyair-cooling.

Theroundbarmaterialwasmachinedtoflat-endstudswithathreadedsectionforattachmenttothevibrationhornforthecavitationtest.Thespecimensforpolarisationstudieswereintheformofadiscof16mmindiameterand3.2mmthickness.ThesurfaceofthespecimensforLSMwassand-blastedinordertoreducethereflectivitytothe

laser

beam.Priortocavitationandpolarisationtests,thesurfaceofallspecimenswasmechanicallypolishedto1μmdiamondfinishinordertokeepthesurfaceroughnessconsistent.Subsequently,thespecimenswerecleaned,degreased,driedandweighedbeforeandaftereachcavitationandpittingcorrosiontest.

2.2.

Laser

surface-melting

LSMwascarriedoutusinga3.5-kWCWCO2laser.ThelaserbeamwasfocusedontothespecimenbyaZnSelenswithfocallengthof170mm.Laser

powerof1100W（156W/mm2）and1700W（240W/mm2）atworkpiecewithabeamsizeof3mmindiameterandbeamscanningspeeds5,15and25mm/swereused.Argonflowingat15l/minwasusedastheshieldinggas.ThedetailsofthelaserparametersforeffectingthemelttracksareshowninTable1.Themeltsurfacewasachievedbyoverlappingofsuccessivemel