175-paper-Preparation of Ultrafine Grain Austeniti

[e-mail]eGrainAusteniticStainlessSteelbySPSLIUJing，ZHANGXin，LIGuang-qiang（SchoolofMaterialsandMetallurgy,WuhanUniversityofScienceandTechnology,Wuhan430081,China）Abstract:Themilled304austeniticstainlesssteelpowderwassinteredthroughtheSPS(SparkPlasmaSintering)process.Intheprocess,thesinteringtemperaturewas1173K,andsinteringstresswas30MPaand50MPa,respectively.ThephasecompositionandgrainsizeofsinteredsampleswereobservedbyXRD,SEMandTEM.Theresultsshowthatmatrixofthesinteredsamplesisaustenite,andtheaveragegrainsizeisbetween150nmand200nm.Thedensityofthesamplesisclosetosteel,whichshowsthathighdensificationhasbeenattained.TherigidityofthesamplesismuchhigherthanthatofthecommonausteniticStainlessSteel.Theresultsofelectrochemiscalcorrosiontestcombinedwithmetallographicobservationshowthatthemorecompactthesamplesis,thebettercorrosionresistanceofthesamples.Keyword:Sparkplasmasintering;Austeniticstainlesssteel;Corrosionresistance;Averagegrainsize;Ultrafinegrain1IntroductionPowdermetallurgytechniqueandgranularmaterialsprocessingtechniqueareabsolutelynecessaryinthepreparationandexploitationofhigh-techengineeringmaterialssuchasnewmetalmaterials,fineceramicsandcomplexmaterials.Sineringisaveryimportanttechnology.Generally,sinteringprocessisclassifiedtwokinds,oneispressurelesssintering,theotherispressure-assistedsintering.Thesinteringdrivingforceofthemetalpower，whichisknownassurfaceenergy,isverylow.Inordertomaketheperformanceofsinteredmaterialsbecompetitivetocastmaterials,improvingthedensificationofthesinteriedsamplesisneeded,[1].Amongsomanystrengthen-sinteringtechniquesusedatpresent.Sothesparkplasmasintering(SPS)isanadvancedprocess,SPSwasappliedtothesinteringofstainlesssteelpowderpreparedbymechanicalalloying[2].Sparkplasmasintering(SPS)isarelativelynewsinteringtechnologyinpowdermetallurgywhichiscapableofsinteringmetalandceramicpowdersquicklytofulldensityatafairlylowtemperatureduetoitsuniquefeatures.Fig.1showsthebasicconfigurationoftheSPSsystemand.Thegraphitediesetisfilledwiththerawmaterialpowder,andisplacedbetweenthelowerandupperelectrodes.Anexternalpowersourceprovidesapulsedplasmatoactivatethesurfaceofthesinteringparticles.Bycharginganddischargingthespacesbetweenpowderparticleswithelectricalenergy,ahightemperaturesparkdischarge,orplasmaiseffectivelygeneratedmomentarily.TheSPSfeaturesaveryhighthermalefficiencybecauseofthedirectheatingofthesinteringgraphitemoldandstackedpowdermaterialsbythelargesparkpulsecurrent.SPScanreadilyconsolidateahomogeneous,high-qualitycompactbecauseoftheuniformheating,surfacepurificationandactivationmadebydispersingthesparkpoints[3–8].Inthesinteringprocess,manyparametershaveeffectsonperformanceofsinteriredmaterials,suchasthesinteringtemperature,thesinteringpressureandtheheatingrateandsoon.Theeffectofdifferentsinteringpressureonperformanceofsinteriredsamplesisdiscussedinthispaper.Fig.1BasicconfigurationandtheprincipleofplasmagenerationinSPSprocess.2Experimentalprocedures2.1PreparationofsamplesSUS304stainlesssteelpowdercontaining17.16%Cr,8.71%Niinsizeof47μmwasusedasmainmaterial.Nitridedferromanganesepowdercontaining90%Mnand5.85%N,highnitrogenferrochromiumpowdercontaining62.6%Crand8.48%Nwereusedasnitrogenadditives.PureMoandTipowderswerealsoadded.ThechemistrycompositionofthemixpowderislistedinTable1.Thealloypowdersweremixedonplanetballandmilledwitharotatespeedof260r/minfor100hours,thenweresinteredbyasparkplasmasinteringmachine(SumitomoCoalMiningCo,ModelSPS-1050).Thesinteringtemperaturewas1173K,thesinteringpressurewas30MPaand50MPa,respectively.Thetimeforheatingupis600s,andthetemperaturewaskeepedat1173Kfor300s.Thenthetemperaturewasdecreasedfrom1173Kto473Kinthesinteringequipment,atfinallyreducedtoroomtemperaturebynature.Thesinteredsamplesarecylinderwithdiameterof32mmandhightof5mm-6mm.2.2ExperimentalTechniquesXRDpatternsofmilledpowderandsinteredsampleswasobtainedbyPhilipsX’Pertdiffractometer.ThesinteredsampleswereetchedbyHCl+HNO3andobservedbyFE-SEM.ThesamplesforTEMobservationwerepreparedbyArionetching.ThegrainsizewasmeasuredfromtheTEMimaginesandthesizedistributionwasanalyzedbytheImage-ProPlussoftware.AnothermethodwasalsousedtocalculateaveragegrainsizeusingScherrer’sformulathroughtheXRDpatterns.DensityofsinteredsampleswasmeasuredusingmethodofArchimedes.Formulaofrelativedensityisρr＝(ρ/ρ0)*100%.ρ0istheoreticaldensityofstainlesssteel.HardnessofsinteredsampleswasmeasuredbyVickershardnesstester.CorrosionresistanceofsinteredsampleswasmeasuredbyCHI600Agalvano-chemistryworkstationmadeinChenghuaofShanghai.Corrosionmediumisc(H2SO4)=0.05mol/Landc(Na2SO4)＝0.25mol/L.Table1.chemicalcompositionofmixedpowderElementsCrNiMnMoTiNFeMass%18.139.068.432.000.480.94balance3ResultsandDiscussion3.1analysisofphasetransformationFigure2showsXRDpatternofpowderbeforesintering.Itindicatesthatpowderiscomposedofmartensite;austeniteandalittleTiN;Cr2N.Figure3showsXRDpatternofsinteredstainlesssamplein50MPasinteredpressure.ItindicatethatsinteredsampleiscomposedofausteniticandalittleTiNandCr2N.Sowecanknowduringthesinteringp