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Analysis of Laser Surface Hardened Layers of Automobile Engine Cylinder Liner

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Analysis of Laser Surface Hardened Layers of Automobile Engine Cylinder Liner

    Analysis of Laser Surface Hardened Layers

    of Automobile Engine Cylinder Liner Availableonlineatwww.sciencedirect.com

    :scienceDirect

    JOURNALOFIRONANDSTEELRESEARCH,INTERNATIONAL.2007,14(1):42-46

    AnalysisofLaserSurfaceHardenedLayersof

    AutomobileEngineCylinderLiner

    LIUXiubo.YUGang,GUOJian,SHANGQuanyi.,ZHANGZhenguo,GUYi

    jie.

    (1.LaboratoryforLaserIntelligentManufacturing,InstituteofMechanics,ChineseAcademyofSciences,Beijing

    100080,China;2.SchoolofMaterialsandChemicalEngineering,ZhongyuanInstituteofTechnology,Zhengzhou

    450007,Henan,China;3.CollegeofMaterialsScienceandEngineering,ShandongUniversityofScienceand

    Technology,Qingdao2665i0,Shandong,China)

    Abstract:GraycastironthatisusedforautomobileenginecylinderlinerswaslasersurfacehardenedusingNd;YAGqua

    si-continuousandCO2continuouswavelaser,respectively.Themacromorphologyandmicrostructureofthelaser

    surfacehardenedlayerswereinvestigatedusinganopticalmicroscope.Geometricdimensionsincludingdepthand

    widthandmicrohardnessdistributionofthehardenedlayerswerealsoexaminedinordertoevaluatethequalityofthe

    hardenedlayers.

    Keywords:graycastiron;lasersurfacehardening;microstructure;hardness Thenewlydevelopedlaserprocessingtechnolo

gyhasgeneratedconsiderableinterestsbothinre

    searchandindustrialapplicationfields,especiallyas advancedandgreenmanufacturingtoolsforhardfa

    cingorsurfacerepairinginmodernautomobilein

    dustry[?.However,todate,themainfoCUSinlaser surfacehardeningisonCO2laser,whichexhibitsa lowcouplinginteractionwithmetalmaterialsdueto therelativelylongerlaseroutputwavelength.Thus, paintingisnecessarytoincreasetheabsorbingrate beforelaserradiation.Thisnotonlyneedsadditional procedureintheproductlinebutalsobringsabout pollutionandhazardouseffectstotheenvironment. Onthecontrary.Nd:YAGlaserisemergingasa competitivetoolinsurfacemodificationduetothe shortwavelengthandhighabsorbingrateofthe metalmaterialsC.

    Theaimofthisstudyistoexaminethesurface hardenedlayers.whichwereprocessedusingNd: YAGquasicontinuousandCO2continuolwavela

    ser,respectively.Themacromorphologyandmicro

    structureofthelasersurfacehardenedlayerswere investigatedusingopticalmicroscope.Theresults andthequalityofthehardeninglayersprocessedby thetwokindsoflaserswerecompared.Certain methodsareproposedandtheirprospectsarealso reviewed.

    1Experimental

    Thematerialusedinthisstudywaspearlite graycastiron.Itissuitableforlasersurfaceharden

    ingandismainlyusedinenginecylindersandother

    tribologicalmovingcomponentsintheautomobile industry.Thelaserprocessingexperimentswere carriedoutusingaNd:YAGquasicontinuousanda

    CO2continuouswavelaser,respectively.Thelaser beamhadacircularcross-sectionandthepowerdis

    tributionwithinthemfollowedtheTEM01mode. Thelaserprocessingconditionsandsurfaceprepara

    tionusedintheexperimentsareshowninTable1. Specimensformetallographicexaminationbyoptical microscopy(mode:Nephot一?)werecutperpendic

    ulartothebeamdirection.Allthespecimenswere groundandpolished.Etchingwasperformedusing 4Nitalreagents.Microhardnesswasmeasuredby aIIMF3typemicrohardmeter,whichisexpressed asafunctionofdepthandwidthforallthelaserpro

    cessedspecimensusingaloadof0.1kg.

    Biography:UUXiu-bo(1968

    ),Male,AssociateProfessor;E-mail:liubobo0828@yahoo.com.cn;RevisedDale:July2

    0,2005

No.1AnalysisofLaserSurfaceHardenedLayersofAutomobileEngineCylinderLiner?43?

    Table1Processingconditionsandsurfacepreparationusedinexperimentsforgraycastiron

    Note:Specimen30wasprocessedusingaC02continuouslaser,whilealltheotherthreespeci

    menswereprocessed

    usinganNd:YAGquasicontinuouslaser.

    2ResultsandDiscussion

    2.IGeometricfeaturesoflasersurfacehardened layers

    Fig.1showsthetypicalgeometricdimensionsof thelasersurfacehardenedlayers.Specimen26and24

wereprocessedusingaNd:YAGquasi-continuousIa

    ser.resultinginthedepthof0.15nlHl.0.20lainand widthof2.3nlHl,2.35nlHlofthehardenedlayers.re

    spectively.However,Specimen30processedusinga continuous0[)2laserhasalayerwithdepthof0.26n1Hl andwidthof3.6nM'n.Thereasonfortheabove-men

    tionedmeasuredresultscanbeexplainedasfollows: SincetheNd:YAGisaquas._continuouslaser,the beamsizeandthepulsedfrequencyarelowerthan thosefortheCO2continuouslaser,andifthespeci

    menswereprocessedwithsimilarparameters,the energyabsorbedbythespecimenwaslow,leading tolowerhardenedlayerdepthandwidthcompared tothatprocessedwithC02continuouslaser.Asfor Specimens26and24,thedifferencebetweenthem istheprocessingparameters.Thescanningspeed forSpecimen26was2200mm/min,whichwas morethanthatforSpecimen24(1800mm/min). Thefasterthescanningspeed,thelessertheinter

    actiontimebetweenthelaserbeamandthemateri

    als.thusthelesstheheatinputtothespecimenand thelessthetransformationzoneofthespecimen. Therefore,thespecimenwouldhaveashallower hardenedlayer.

    2.2Microstructureoflasersurfacehardenedlayers Ingeneral.themicrostructureofthelasersur

    facehardenedlayersisaresultofthefollowing process.First,thespecimenisquicklyheatedand mostofthestartingpearliteistransferredintoaus

    tenite.Theaustenitethenchangesintomartensite

    duringsubsequentrapidcooling,includingcertain unchangedaustenitethatisretained.Moreover.if thereexistscertainphosphorouseutecticinthelaser impactarea,thenitisdissolvedandtransferredinto ledeburiteorcoarseneedle-shapedmartensitedueto theirlowmeltingpoint.Sheetgraphitecanbepart

    lydissolvedanddiffusedasaresultoftheboundary reactionduetoaveryhightemperature.Thus,the typicalmicrostructureofthelasersurfacehardened layeriscomposedofmartensiteorledeburite.re

    tainedaustenite,andgraphitesheets.

    AscanbeseenfromFig.2(a)and(b),themi

    crostructureofthetypicalhardenedlayercanbe roughlydividedintotworegions,namely,thephase transformationzoneandthetransitionzone.The topsurfaceareaofthehardenedlayerobservedby theOM,showsauniformandbrightwhitecolorto bethemartensite,andausteniteisretaineddueto therapidcoolingofthehightemperatureaustenite. Theareaadjacenttothewhiteareawasidentifiedas thetransformedcoarseneedle-shapedmartensiteand somedissolvedgraphite[-Fig.2(b)].Thedissolved graphiteinthisareaisevidentlythinandshortcom

    paredtothestartingcoarsesheetgraphitemateria1.The (a)Specimen26,d=0?15rlllTI,W=2.3liana;(b)Specimen24,d=0.2rlllTI,W

    2.35mlTl;(c)Specimen30,d=0.26mm.W=3.6mm Fig.1Cross-sectionalima窘酷

    ofspecimensshowinggeometricdimensionsofhardenedlayers(d:depth,W:width)

?

    44?JournalofIronandSteelResearch,InternationalVo1.14 (a)TypicalmicrostructureforSpecimen22;(b)TypicalmicrostructureforSpecimen30;(c)

    Magnificationofmicrostructureof

    Specimen24inmiddleofhardenedlayeroftransitionzone,showingcoarsesheetgraphiteand

    pearlite-likethinmartensite

    Fig?2Opticalmicroscopemicrographsofgraycastironafterlasersurfacehardeningprocessi

    ng

    reasonforthisphenomenonwasduetotherather shortinteractiontime.ThegraphiteSdissolutionin theareawasincompleteandmaybebroughttothe upperareaandbrokenintopiecesbecauseofthe buoyancyanddynamicforces.

    Animportantphenomenonthatoccurredinthe hardenedlayerwasthatthemorphologicalcharac

    teristicofthemicrostructurewassimilartothatof thelamellarpearlite,butthehardnesstesting showedthatitsaveragehardnesswas680700HV.

    whichindicatesthatmartensitephasetransformation hadoccurred.Thiswasduetothehinderingand segregationeffectsofthesheetcarbide(cementite). Thehightemperatureausteniteandthesubsequent coolingtransformationproductscouldonlybefor

    mulatedwithintheverynarrowsheetcementite, leadingtotheformationoftheextremelythinmar

    tensite[Fig.2(c)][.Anotherimportantcharacter

    isticofphasetransformationofthegraycastiron wasitseutectic-typemicrostructure(Fig.3).Since severalgraphitesheetsexistedingraycastiron, therealsoexistedseveralgraphiteferritearea.Fer

ritetransferredintoausteniteduringtherapidheat

    F3Eutecticcharacteristicsdudngphasetransformation processofSpecimen27

    ingprocess.Thus,slightmeltingphenomenonOC

    curredinthegraphiteausteniteinterphasearea.Al

    thoughtheheatingtemperaturewasbelowth6melt

    ingpointofthebulkcastiron,theactualheating temperaturemightexceedthemeltingpointslightly,

    austenite leadingtoaminormeltingofthegraphite

    initially.Astherimofthegraphitesheetdissolved, thecarbonconcentrationinthesurroundingareain

    creasedtoalargeextent.Thustheeutectic-typemi

N0.1AnalysisofLaserSurfaceHardenedLayersofAutomobileEngineCylinderLiner?45?

    crostructurewasformedintheminorareainthe startinggraphite-ferriteinterphase.Asshownin Fig.3,theledeburitemicrostructurewhichisthe eutecticsofcementiteandpearlite,wasformedin thestartinggraphite-ferriteinterphasearea[.

    2.3MicrohardnessofIasersurfacehardenedlayer Fig.4(a)and(b)showthevariationofmicro

    hardnesswithdepthandwidthofthelasersurface hardenedSpecimen26.

    Thelasersurfacehardeningprocessingyieldsa unique.thinmicrostructure,andmostofthemis supersaturatedsolidsolution--veryhardmartensite andthereexistsalargeamountofretainedausten

    ite,eliminatingthesoftgraphiteatthetopsurface. Theretainedausteniteisstrengthenedtoalargeex

tentthroughdislocationandsolid-solutionstrength

    eningmechanism.So,itisnaturallyanticipatedthat thehardenedlayerscontainaveryhighanduniform hardnessdistributi0n.

    Fromthemicrohardnessexamination,itisap

    parentthatthemicrohardnesslevelinthehardened layer.whichaveragesabout700HV,issignificant

    lyhigherthanthatatthesubstrateregion,which averagesabout200HV.Itisimpotantthatthemi- crohardnessdistributioncorrespondstotheabove- mentionedmicrostructureanalysis.Itcanbeseen veryclearlyfromFig.4(a)and(b)thattheharden- edlayersdepthandwidthare0.15mmand2.3 mm,respectively.Sincethetopsurface(bright whitearea)ofthehardenedlayermainlyconsistsof retainedaustenite.thehardnessthereinisrelatively slightly1owerthanthatoftheadjacentarea,which Distancefromstartingpoinffmm

    F.4Variationofmicrohardnesswithdepth(a)and width(b)ofhardenedSpecimen26

    showsthehighesthardnessduetoitsveryhardmar

    tensitephase.Moreover,inthetransitionzone,the microhardnessstartstodecreaseasthemicrostruc

    tureismainlycomposedofunchangedpearliteand graphitephases.Fig.3(b)showsthatthehardness paralleltothesurface,whichfluctuatestoacertain extentduetothefactthatataparticulartime,the testedareachieflyisaveryhardmartensitephase andatanotherpointoftime,thetestedareamaybe asoftgraphitephasetoalargeextent.However,

    microhardnessdistributionofthisspecimenwithin thehardenedlayerisuniformandstatisticalconstant aboutameanleve1.

    Sincethelasersurfacehardenedlayersofthe graycastironexhibitfavorablemicrostructurechar

    acteristicandveryhighhardnessdistribution,the qualityofthehardenedlayersishigh.Itisanticipa

    tedthatthehardenedlayershaveaverygoodwear resistanceundertribologicalmovingservicecondi

    tions.Itshowsthatthelasersurfacehardeningisa verypromisingandeconclmictechniqueintheauto

    mobileindustry.

    (1)Thelasersurfacehardenedlayersofgray castironmainlyconsistoftwozones:hardenedzone andtransitionzone.Thedepthandwidthofthe hardenedlayerarerelatedtothelasermode.pro

    cessingparameters,andSOon.CO2continuouswave laseryieldsarelativelydeeperandwiderhardened layerthanthatoftheNd:YAGquasicontinuous

    laserunderthesameprocessingconditions.The depthofthehardenedlayerdecreasesiftheprocess

    ingscanningspeedincreasesinthesamelasermode. Butfromthelongtermpointofview.Nd:YAGla

    serwouldbecomethecompetitivetoolinsurface hardeningduetoitshighabsorbingrateandhigh productionefficiency.

    (2)Lasersurfacehardeningforgraycastiron producesapredominantmartensite,or/andlede- buriteeutectic,includingcertainretainedaustenite

andcertainchangedandunchangedgraphitemicro

    structuresThecorrespondingmicrohardnessdistri- butionshowsthesignificantlyenhancedhardnessof thehardenedlayersandconfirmstheresultsofthe microstructureanalysis.Itisexpectedthatthelaser surfacehardenedlayersforgraycastironshould havegoodwearresistanceundertribologicalservice conditions.andthereforelasersurfacehardeningisa promisingandeconomictechniqueintheautomobile Iu)IIIIIII2u

?

    46?JournalofIronandSteelResearch,InternationalV01.14 industry.

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