Influence of cyclic temperature changes on the microstructure of AISI 4140 after lasersurface.docx
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Influence of cyclic temperature changes on the microstructure of AISI 4140 after lasersurface.docx
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InfluenceofcyclictemperaturechangesonthemicrostructureofAISI4140afterlasersurface
InfluenceofcyclictemperaturechangesonthemicrostructureofAISI4140afterlaser
surfacehardening
T.Miokovića,V.Schulze
a,
O.VöhringeraandD.Löhea
aInstituteofMaterialsScienceandEngineeringI,UniversityofKarlsruhe,76131Karlsruhe,Germany
Received22February2006; revised14July2006; accepted22August2006. Availableonline15November2006.
Abstract
Inrecentyears
laser
surfacehardeningusingpulsed
laser
sourceshasbecomeanincreasinglyestablishedtechnologyinengineeringindustryandhasopenedupwiderpossibilitiesfortheapplicationofselectivesurfacehardening.However,thechoiceoftheprocessparametersisgenerallybasedonexperienceratherthanontheirempiricalinfluenceontheresultingmicrostructure,andforhardeningprocesseswithcyclictemperaturechanges,almostnocorrelationsbetweenprocessparametersandhardeningresultsareknown.Therefore,someproblemsregardingthechoiceoftheprocessparametersandtheirinfluenceontheresultingmicrostructurestillremain.Inparticular,thereisalackofdataconcerningtheeffectofcyclictemperaturechangesonhardening.Tofacilitateprocessoptimization,thispaperdealswithadetailedcharacterizationofthemicrostructurescreatedinquenchedandtemperedAISI4140(Germangrade42CrMo4)steelfollowingatemperature-dependent
laser
surfacehardeningtreatment.Thestructurepropertieswereobtainedfrommicrohardnessmeasurements,scanningelectronmicroscopyinvestigationsandX-raydiffractionanalysisofretained
austenite.
Keywords:
Austenite
formation;Martensitictransformation;
Laser
hardening;Cyclictemperaturechanges;Microstructuralcharacterization
ArticleOutline
1.Introduction
2.Experimental
3.Microstructuralcharacterization
3.1.Lasersurfacehardeningwithsingleheatingandcooling
3.2.Lasersurfacehardeningwithcyclictime–temperaturechanges
4.Discussion
5.Summary
Acknowledgements
References
1.Introduction
During
laser
surfacehardeningrapidthermalchangesareimposedonthematerial.Comparedwithconventionaltechniquesforselectivehardening,themainadvantageof
laser
surfacehardeningistheintenseenergyfluxwhichpromotesextremelyhighheatingrates.Thisleadstocomplexchangesintheprocessedmaterialwithlocallydifferentaustenizingandhardeningconditionsinthe
laser
-affectedzonecausedbythetransformationbehaviourofthesteel[1],[2],[3]and[4].Thepowerdensityandinteractiontimeatthesurfacedeterminethetemperatureprofileandthedepthofhardening.Byapplyingproperprocessparameters,asurfacelayercanbemodifiedtogiveawidevarietyofproperties[1],[2],[3],[4],[5],[6],[7],[8],[9]and[10].Whilethe
laser
-affectedzoneshavegenerallybeencharacterizedonlybythedistributionofhardnessandmacro-residualstresses,systematicinvestigationsontheeffectofhighratesofheatingandcoolingonthechangesinmicrostructurearescarcebecauseitisdifficulttocontroltheexperimentalconditions.Inthecaseofcyclictemperaturechangesduetotheuseofpulsed
laser
sources,veryfewpapershavedealtwiththeinfluenceof,forexample,pulsefrequencyontheresultingmicrostructureandmechanicalproperties[11]and[12].Thereforethegreatpotentialofcyclictemperaturesin
laser
hardeninghasnotyetberealised.Thisholdsespeciallyforlow-frequencycycling,whichprovidessignificantquenchingandreaustenizingeffectsinthesurfacelayerandcanbeperformedusingtemperature-controlled,high-powerdiode
lasers.
Inordertobeabletooptimizeprocessesbyapplyingtheproperparameters,adetailedknowledgeoftheeffectofhighheatingandcoolingratesandtheinfluenceofcyclictemperaturechangesonsurfacehardeningarenecessary.Therefore,wepresenthereasystematicstudyon
laser
surfacehardeningwithlargecyclictemperaturechangesusingthelow-alloyedsteelAISI4140(Germangrade42CrMo4)asamodelmaterial.Byaccuratelydocumentingthechangesinmicrostructureandtheexistingcarbides,theinfluenceofheatingrate,coolingrate,numberofcyclesandminimumtemperatureonthedistributionofmicrohardnessandretainedausteniteareevaluatedanddiscussed.
2.Experimental
Thematerialinvestigatedisthelow-alloyedsteelAISI4140(Germangrade42CrMo4).Itwasusedinaquenchedandtemperedstate(Ttemp = 570 °C),whichhasaferriticmicrostructurewithfinelydispersedcarbides.The
laser
surfacehardeningwascarriedoutusingahigh-powerdiode
laser
(6 kWmaximumbeampower)incontinuous-wavemode.Thistechniqueyieldedcyclictriangulartemperature–timecoursesatthesurfaceduetopyrometrictemperaturecontrolwithsetpointactualizationateachmillisecond.Thesetemperature–timecourseswerecarriedoutatheatingratesbetween1000and10,000 K s−1,coolingratesof1000and3000 K s−1,amaximumsurfacetemperatureof1150 °Candminimumtemperaturesof20and400 °Cforupto27cycles.
ThespeciallydesignedspecimensshowninFig.1wereusedforhardeningatthetopsurfacewithastationary
laser
beam.Thediameterof6 mmwasidenticaltothebeamdiameterandthereforeallowedforanalmostuniaxialheatconductionalongthespecimenaxis.Thepartofthespecimenwiththelargerdiametersupportedthis,andservedasaheatsink.Afterthehardeningprocess,differentzoneswithinthesurfaceareaoccur,andtheseareoutlinedinFig.1.Atthetopacompletelymartensiticstructuredevelops,whichisreferredtoasthehardenedzone(HZ).Thetransitionzone(TZ)consistsofpartlyaustenizedandeventuallyhardenedmicrostructure,andtherestisbasematerialwhichdidnottransformduringthe
laser
irradiation.Dependingonthestateofthebasematerial,aheat-affectedzonecanbeobserved,andunderneaththistheunaffectedbasematerial.Inthefollowing,thedepthofhardeningwasdeterminedasthedistancefromthesurfaceatwhichthehardnessvaluesfirstreachthehardnessofthebasematerial,i.e.
370HV0.1.
DisplayFullSizeversionofthisimage(23K)
Fig.1. Geometryofspecimensfor
laser
hardeningandschemeshowingtheregionsofthe
laser
-affectedzone.
Microscopyanalyseswerecarriedouttocharacterizemicrostructuraldetailssuchasthemartensiteandcarbidemorphologyobtainedby
laser
surfacehardening.Themicrostructuralobservationsweremadefrompolishedandetchedspecimensusingscanningelectronmicroscopy(SEM)methods.TheetchantNitalwasused.MicrohardnessmeasurementswereperformedonthepolishedspecimensurfacesusingacalibratedShimadzumicrohardnesstester.Additionally,theX-raydiffractionsix-linemethod[13]wasusedtodeterminetheretainedaustenitecontentinthesamples,withX-raysproducedfromaCuKαradiationsource.
3.Microstructuralcharacterization
3.1.
Laser
surfacehardeningwithsingleheatingandcooling
TheresultspresentedinFig.2giveanoverviewonthecourseofmicrohardnessvs.distancetothesurfaceafter
laser
surfacehardeningwithsingleheatingandcooling,forheatingratesbetween1000and10,000 K s−1andcoolingratesof1000and3000 K s−1.ThemicrohardnessprofilesofthetransformedregionspresentedinFig.2showthetypicalplateaucorrespondingtoafullytransformedmicrostructureandaregioninwhichthehardnesssteeplydecreasestothatofthebasematerial,representingmixedmicrostructuresoftransformedanduntransformedphasesandundissolvedcarbides.AsshowninFig.2,anincreaseinheatingandcoolingratelowersthesizeofcompletelyhardenedmaterialandthedepthofhardening,respectively.Duetoveryfastheating,austeniteformationandcarbondiffusionarelimited,andthisshiftsthetransformationtemperatures,Ac1andAc3,forthestartandfinishofausteniteformationtohighertemperatures.WithincreasingcoolingratethetimespentaboveAc1duringcontinuouscoolingisevenshorter.Thisresultsinashiftinthesizeofthehardenedandtransitionzonetolowerdepths,astheheatingandcoolingratesbecomehigher.Moreover,
laser
surfacehardeningwitharelativelysmallheatingandcoolingrateof1000 K s−1(Fig.2)resultsinaslightdecreaseinhardnessinthehardenedzonewithdecreasingdistancefromthesurface.Incontrast,higherratesofheatingandcoolingleadtoafairlyconstanthardness.AccordingtoRefs.[9]and[10],thisisduetothegrowthofaustenitegrains,whichleadstocoarsemartensite.Thematerialbecomessofterandthehardnessdecreases.Aheat-affectedzoneisnotfoundforallheattreatmentvariants.
DisplayFullSizeversionofthisimage(36K)
Fig.2. Microhardnessvs.distancetosurfaceafter
laser
surfacehardeningwithheatingratesbetweenνheat = 1000and10,000 K s−1andcoolingratesofνcool = 1000and3000 K s−1(error = ±20HV0.1).
Additionalmicroscopyinvestigationsoftheresultingmicrostructureswereperformedandanalyzed.Fig.3illustratesSEMmicrographsofthehardenedandtransitionzonesafter
laser
surfacehardeningwithheatingandcoolingratesof1000 K s−1andafterhardeningwithνheat = 10,000 K s−1andνcool = 3000 K s−1.Themicrostructureinthetransitionzoneatx = 0.9 mmafterheatingandsubsequentquenchingwith1000 K s−1(Fig.3a)consistsofprimarilyferriticbasematerialwithplate-like(I)carbidesandglobular(II)carbides.Theplate-likecarbidesareferrouscarbides(Fe3C)andtheglobularcarbidesaremixedcarbides((Fe, M)3CwithM = Cr,Mo,Mn).Ascanbeseenfromthemicrograph,thedistributionoftheplate-likecarbidesismor
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