Prediction of phase transformations during lasersurface hardening of AISI 4140 including the.docx
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Prediction of phase transformations during lasersurface hardening of AISI 4140 including the.docx
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PredictionofphasetransformationsduringlasersurfacehardeningofAISI4140includingthe
Predictionofphasetransformationsduringlaser
surfacehardeningofAISI4140includingtheeffectsofinhomogeneous
austenite
formation
T.Miokovića,V.Schulze
a,
O.VöhringeraandD.Löhea
aInstitutfürWerkstoffkundeI,UniversitätKarlsruhe(TH),Germany
Received17March2006; revised12July2006; accepted13July2006. Availableonline11September2006.
Abstract
Modellingof
laser
surfacehardeningofAISI4140steelincludingtheeffectofinhomogeneous
austenite
formationduetolocallydifferentaustenizingandquenchingconditionsiscarriedouttogainabetterunderstandingofthematerialreactionduringshort-timehardeningprocesses.Dilatometricstudiesathighheatingandcoolingratesleadtobaseinformationsformateriallawsreflectingtheinfluenceoftheinhomogeneityof
austenite
onthequenchingprocess.Theywereimplementedinthefinite-element-programABAQUSasuserdefinedmateriallawsandallowacoupledcalculationoftemperatureandphasedevelopmentduringheatingandcooling.Theinfluenceofheatingrateandcoolingrateonthetime-dependenttemperaturefieldsandphasetransformationswithintheaffectedzonewasinvestigated.Agoodcorrespondencebetweentheresultsofsimulationandexperimentaldatawasobtainedinrespectoftheresultinghardnessprofilesandthedegreeofhomogeneityofthehardenedstructure.
Keywords:
Laser
surfacehardening;Phasetransformations;
Austenite
formation;FEM-simulation
ArticleOutline
1.Introduction
2.Effectsofheatingandcoolingratesonphasetransformations
3.Simulationsetup
4.Determinationofhardnessprofiles
5.Resultsanddiscussion
6.Summary
Acknowledgements
References
1.Introduction
During
laser
surfacehardeningthe
laser
beaminducesathermalcycleinthesurfaceregionofthematerial.Thegiventemperature–time-courseatthetopsurfaceofthespecimendeterminesthethermalcyclebelowthesurface.Directlyatthesurfaceofthespecimenthematrixisgenerallycompletelyaustenizedduringheatingandhomogeneousausteniteformationisexpectedresultinginhomogeneousmartensiticstructuresaftersubsequentquenching.Contrarilybelowthesurfacelocallydifferentaustenizingandquenchingconditionsoccurduetosteeptemperaturegradients.Thedegreeofaustenitehomogenisationisthendependentontheamountofdiffusionthatoccurs.Duetoshorttimesinducedbyrapidheatingandcoolingausteniteformationandcarbondiffusionreactionsarelimited[1]and[2].Thebasematerialtransformsintoaustenitewithoutreachingtheequilibriumweightfractionofaustenite.Thisresultsinundissolvedcarbidesandanausteniticmatrixwithreducedcarboncontent.Thelowercarboncontentoftheausteniticmatrixleadstoanincreaseinthetransformationtemperaturesformartensiteformationduringthesubsequentquenchingprocess[3]and[4].Thishasagreatimpactonthefinalmartensitemorphologyandthemechanicalpropertiesofthehardenedstructure[4].Consideringmainlythemartensitetransformationduringcoolinganestimationofthedevelopmentofthecarboncontentinthemartensiticmatrixformedinthe
laser
affectedzoneisnecessaryforanadequatepredictionofmechanicalpropertieslikethehardnessorthestrengthofthematerial.
Themathematicaldescriptionoftransformationhardeningisofgreatsignificanceforsurfacehardeningtreatmentssuchas
laser
surfacehardeningduetolocallydifferentaustenizingandquenchingconditionsintheheataffectedzone.Themodellingofthe
laser
hardeningprocessallowsthedeterminationoftime-dependenttemperaturefieldsandphasetransformationsduetotheheatimpactandsupportstheunderstandingofthehardeningprocess.Theinitialstateofausteniteisofgreatimportanceforthedevelopmentofthefinalmicrostructureanditsmechanicalproperties.Whiletherearemanystudiesonthesimulationofsurfacehardeningofsteelsconsideringtransformationhardeningwithhomogeneousausteniteformation[5],[6],[7],[8],[9],[10]and[11]themodellingoftheeffectofthestateofausteniteinvolvinginhomogeneousausteniteformationonthekineticsofphasetransformationduringcoolinghasbeentakenuponlybyafewauthors[10]and[11].
Therefore,inthepresentworksystematicexperimentalinvestigationsonphasetransformationprocessesduringheatingandcoolingwithhighestratesincludingtheeffectofinhomogeneousausteniteformationontheformationofmartensiteofAISI4140(EN42CrMo4)inastatequenchedandtemperedat450 °Cwillbeusedtodeducemodelsforthedescriptionoftheseeffects.Additionallythesemodelswereimplementedinthecommercialfinite-element-systemABAQUSandappliedtosurfacehardeningusinga3 kWhighpowerdiode
laser
whichallowstocontrolsurfacetemperatureseverymillisecond.Bymodellingthedevelopmentofthemartensitestartandmartensitefinishtemperaturefordifferentsurfacehardeningconditionstheeffectofheatingandcoolingrateonthemicrostructureinthesurfacelayerandaustenitehomogenisationwasanalyzed.BasedontheresultsobtainedfordifferentT,t-coursesatthesurfaceandbelowwithlocallyvaryingheatingandcoolingratescalculationsoftheresultinghardness-depth-profileswereperformed.
2.Effectsofheatingandcoolingratesonphasetransformations
Inordertostudytheshort-timeaustenizingandquenchingbehaviourofAISI4140dilatometricexperimentsvaryingheatingratesvheat,coolingratesvcoolandmaximumtemperaturesTA,maxwereperformedusingthedevicepresentedin[12].Asalreadyexplainedin[12],hollowspecimenswithanouterdiameterof8 mmandawallthicknessof1 mmwereheatedusingelectriccurrentandcooledinternallyusingamixtureofwaterandpressurizedair.Thisallowedtogetalmosthomogeneousheattreatmentsinlargespecimensatheatingratesupto10,000 K/sandcoolingratesupto3000 K/s.ThestrainsmeasuredduringheatingareexpemplarilyshowninFig.1aandallowedtodeterminethethermalexpansioncoefficientsoftheferritic–carbidicinitialmicrostructureandofaustenite,thetransformationstrainforthetransformationofferriteintoausteniteandthekineticsofthistransformation.Fig.1ashowsthatthetransformationisshiftedtohighertemperatureswithincreasingheatingrateandthatthereductionofstrainduringthetransformationisreducedduetothehigherthermalexpansioncoefficientofaustenitecomparedtoferrite.UsinganAvrami-approach[13]aspresentedin[12]and[14]forthetransformationandtheparametersmentionedbeforethedatameasuredcouldbedescribedverywell(Fig.1b).TheparametersoftheAvrami-approachcorrespondtotheideallyisothermalTTT-diagramusedinthesimulationhere[12].
DisplayFullSizeversionofthisimage(32K)
Fig.1. Thermalexpansion
(T)vs.temperature(a)andvolumefractionofaustenitefAtemperature(b)fordifferentheatingratesandtheirdescriptionusingtheAvrami-approach.
Dilatometricexperimentswithrapidcoolingaftershort-timeaustenizationwereperformedusingthesamedevices[15].Theyshowedthataustenizationcontinuesduringthefirstpartofcoolingifitwasnotcompletedatmaximumtemperature.ThisallowedthedeterminationoftheeffectsofheatingandcoolingratesandmaximumtemperatureonthemaximumaustenitecontentasexemplarilyshowninFig.2.Theanalysisofthedilatometricstrainsallowedthedeterminationofthethermalexpansioncoefficientofmartensiteand–consideringtheausteniteformationduringcooling–thedeterminationofmartensitecontentversustemperatureasdepictedinFig.3fordifferentheatingandcoolingratesaswellasmaximumtemperatures.IfthesecoursesarefittedusingtheKoistinen–Marburger-approach[16]forthetransformationofausteniteintomartensitestarttemperatures
andmartensitefinishtemperatures
canbedeterminedforthedifferentheattreatmentconditions.SomeofthesevaluesaregiveninFig.4andshowthatthemartensitestartandfinishtemperatureincreasewithincreasingheatingandcoolingrateduetotheinhomogeneityoftheausteniteformedandtheincreasingamountsofdissolvedcarbidesleadingtogrowingmeancarboncontentssolutedintheaustenite[15].Therefore,themartensitestartandmartensitefinishtemperaturedependontheaustenizingandquenchingconditionslikeheatingrate,coolingrateandthemaximumaustenizingtemperatureandhavetobedescribedmathematically[15].Thisisdonebyapplying
(2.1)
DisplayFullSizeversionofthisimage(20K)
Fig.2. Developmentofthemaximumvolumefractionsofaustenite
undfA,max(vheat)forthepureheatingprocessandduringheatingandsubsequentcoolingwithcoolingratesof1000and3000 K/sforTA,max = 850 °C.
DisplayFullSizeversionofthisimage(41K)
Fig.3. ExperimentallydeterminedandcalculatedvolumefractionofmartensiteforTA,max = 850and1150 °Canddifferentheatingratesatcoolingratesof1000 K/s(a)and3000 K/s(b).
DisplayFullSizeversionofthisimage(26K)
Fig.4. Experimentallydeterminedandcalculateddevelopmentof
(TA,max)(a)and
(TA,max)(b)fordifferentheatingratesatacoolingrateof1000 K/s.
TheparameterA1,hom = 730 °Cistheequilibriumtemperatureofinitiationofausteniteformationand
andthe
arethemartensitestartandmartensitefinishtemperatureformartensiteformationafterhomogeneousaustenizationrespectively,correspondingtothefullcarboncontent
ofthesteelAISI4140.Additionally,kistheBoltzmannconstant,v0=1 K/sisastandardheatingrate
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