Heat-Treatment Process of S-130 Steel Produced by Selective Laser Melting br

Objective The Fe-Cr-Ni-Co-Mo system is a new type of maraging stainless steel (MSS) possessingultra-highstrengthandgoodtoughnesswhichisconsideredasapotentialstructuralmaterialforfabricatinglandinggearpartspowertrainpartsofvehiclesandliquid-oxygenkeroseneenginecomponentsItisdifficulttofabricatethesecomplex-structuredprecisionmetalcomponentsbytraditionalprocessingmethodsegcastingandforgingAsafast-growingadditivemanufacturingtechnologyselectivelasermeltingSLMcanbeusedtofabricatecomplex-structuredandhigh-performancepartswithashortprocessingperiodahighmaterialutilizationrateandlowproductioncostHoweverfewpublicreportsonthefabricationofFe-Cr-Ni-Co-MoMSSsbySLMareavailableBasedonthishereanS-130MSSisproducedbySLMwhosechemicalcompositionsmassfractionsareCr 105%--120%Ni60%--90%Co30%--70%Mo15%--35%andtherestisFeToobtaintheoptimalheat-treatmentregimetheeffectsoffive-stepandthree-stepheat-treatmentsonmicrostructuresandpropertiesarecomparedandanalyzedMethodsThesphericalgasatomizedS-130powderwithanaveragesizeof456 mu misthestartingmaterialTheSLMexperimentiscarriedoutonaself-developedmachineAseriesofS-130cuboidsamplesandtensiletestsamplesaredepositedusingtheoptimizedSLMparameterslaserpowerof360Wscanningspeedof800mmshatchspacingof01mmlayerthicknessof40 mu mandphaseangleof90 degrees Heat-treatmentsareconductedinamufflefurnaceataheatingrateof10 degrees CminTheSLMas-depositedS-130samplesareheat-treatedundertwokindsofheat-treatmentprocesses1homogenization1150 degrees Cx3haircooling+grainrefinement950 degrees Cx1haircooling+solution800 degrees Cx1haircooling+cryogeniccooling-73 degrees Cx2hwarminginair+aging500 degrees Cx3haircoolingfive-stepheattreatment2solution800 degrees Cx1haircooling+cryogeniccooling-73 degrees Cx2hwarminginair+aging500 degrees Cx3haircoolingthree-stepheattreatmentMicrostructureandphasecharacteristicsareobservedandidentifiedbythemeansofX-raydiffractometerXRDscanningelectronmicroscopeSEMandtransmissionelectronmicroscopyTEMMicrohardnessandtensilepropertiesarealsoevaluatedResultsandDiscussionsThephasecompositionsoftheSLMasdepositedS-130samplesconsistofthedominantmartensiteandfewretainedaustenitevolumefractionof612%Thecontentsvolumefractionsofausteniteareincreasedto852%and1131%afterthefive-stepandthree-stepheat-treatmentsrespectivelyFig3whichismainlyascribedtotheformationofrevertedausteniteTheSLMas-depositedS-130samplespresentcellulardendritestructuresFig4TheTEMresultsrevealalargenumberofmartensitelathswithahighdensityofdislocationsFig5AftertwokindsofheattreatmentsthelargequantityofnanosizedspheroidalprecipitatesaregeneratedanduniformlydispersedinthemartensitelathsandthelamellarretainedrevertedaustenitearedistributedamongthemartensitelathsFigs6and7Afterthefive-stepheattreatmentthemartensitewidthis82365 +/- 3005nmandtheprecipitatesizeis1522 +/- 283nmComparativelythemartensitewidthobtainedafterthethree-stepheattreatment56259 +/- 2006nmisfinerandtheprecipitatesize858 +/- 213nmissmallerFig8ThisismainlyduetothelargerresidualstressintheSLMsampleswithouthomogenization+grainrefinementtreatmentwhichprovidesagreaterdrivingforceformartensitictransformationandpromotestheformationoffinerlathmartensiteduringthesolution+cryogeniccoolingMeanwhileitprovidesagreaterdrivingforceforthe alpha-gamma phasetransformationandprecipitationofthesecondphaseparticlesduringthesubsequentagingthusobtainingmorefineprecipitatesTheaveragemicrohardnessyieldstrengthandultimatestrengthoftheSLMas-depositedS-130sampleare35151 +/- 1116HV8526 +/- 4086MPaand107289 +/- 296MParespectivelyAfterthefive-stepheattreatmenttheaveragemicrohardnessyieldstrengthandultimatestrengthincreaseto38308 +/- 1106HV114182 +/- 5609MPaand12337 +/- 956MParespectivelyAfterthethree-stepheattreatmenttheaveragemicrohardnessyieldstrengthandultimatestrengthincreaseto40494 +/- 1032HV123576 +/- 617MPaand126697 +/- 662MParespectivelyTheelongationforthetwokindsofheattreatments1769 +/- 023%and1747 +/- 015%respectivelyiscomparabletothatoftheSLMas-depositedsample1766 +/- 011%Fig10Comparedwiththatafterthefive-stepheattreatmentthesampleafterthree-stepheattreatmentpossesseshighmicrohardnessandtensilestrengthAccordingtotheHall-PetchformulathesmallerthemartensitelaththehighertheresistanceofthemartensitephaseinterfacetodislocationmovementandthehigherthemartensitehardnessandyieldstrengthBesidestheinteractionbetweenthedispersedfinerprecipitatesanddislocation si senhancedtheresistancetodislocationmovementisincreasedandthusthehardnessandyieldstrengtharefurtherimprovedInadditionthesamplehasahighaustenitecontentafterthree-stepheattreatmentwhichensuresahighelongationConclusionsInthispaperanS-130MSSissuccessfullyproducedbySLMForSLMas-depositedsamplesthephaseanalysisrevealsalargeamountofmartensiteandasmallamountofretainedausteniteandthemicrostructureischaracterizedbythecellulardendritestructuresandalargenumberofmartensitelathsAfterthetwokindsofheattreatmentstheformationofrevertedausteniteincreasesthecontentofausteniteandalargenumberofnanosizedprecipitatesareuniformlydispersedinthemartensitelathsMeanwhilethelamellarretainedrevertedausteniteisdistributedamongthemartensitelathsComparedwiththoseoftheSLMas-depositedsamplesthemicrohardnessandtensilestrengtharesignificantlyimprovedandtheelongationisnotreducedaftertwokindsofheattreatmentsMoreoverincomparisontothatafterfive-stepheattreatmentthesampleafterthree-stepheattreatmentpossessesahighcontentofausteniteandslenderermartensitelathsaswellasfinerprecipitatewhichcontributestohigh microhardnessandtensilestrengthundertheconditionofensuringelongation The optimized heat-treatment regime for the SLM as-deposited S-130 MSS is three-step heat treatment (solution+cryogenic cooling+aging)