1SinteringbehaviourandmicrostructuredevelopmentofT42powdermetallurgyhighspeedsteelunderdifferentprocessingconditionsHighspeedsteelpowders(T42grade)havebeenuniaxiallycold-pressedandsubsequentlydensifiedthroughdifferentsinteringroutesincluding:supersolidusliquidphasesintering(SLPS)undervacuumanddifferentnitrogenpressures(0.2,0.9,and8bar)andthroughsolidstatesintering(SSS)byhotisostaticpressing(HIP).HIPtemperaturesaslowas850Cledtonearfulldensificationofthematerial(98%theoreticaldensity)withaveragesizeofM6CandMCcarbideslowerthan1mandgrainsize3m.Pressurelesssinteringunderdifferentnitrogenpressures(upto0.39wt.%Nabsorption)ledtoasignificantreductionoftheoptimumsinteringtemperature(OST)andapronouncedincreaseinthesinteringwindow(SW)ascomparedtovacuumsintering.Pressurelesssinteringunder8barN2ledtoafurtherreductioninOSTtogetherwiththeprecipitationofmassiveeutecticstructures.Therefore,theSWwasjudgedtobenegligible.Theresponseoftheas-sinteredmaterialstotheheattreatmentisbasicallydeterminedbytheamountofCavailableinthematrixpriortoquenchingandthegrainsize.Thehighesthardnessachievableforthesinteringconditionsevaluatedranges7001100HV2afteraustenitizingat1100C,oilquenchingandmultitemperingat500550C.Toolsteelsservealargerangeofapplicationsincludinghotandcoldworkingofmetalsandinjectionmouldingofplasticsorlightalloys.Highspeedsteels(HSS)aremorespecificallyusedascuttingtoolsandwearparts.Morerecently,thesematerialshavealsobeenusedforstructuralapplications.Thehighperformanceexhaustvalveseatinsertsforpassengervehiclesconstitutethemostnotableexample1.Ingeneralterms,forthesestructuralapplications,acombinationofhighstrength,wearresistanceandhardnesstogetherwithanappreciabletoughness(comparedwithothermaterialsusedastools)andfatigueresistanceisrequired.Fromamicrostructuralpointofview,HSScanbedescribedasmetallicmatrixcompositesformedbyaferrousmatrixwithadispersionofhard,wearresistantcarbides.Thetype,size,morphology,distributionandvolumefractionofcarbidesaswellasthecharacteristicsoftheferrousmatrixdependonboththecompositionofthematerialandthemanufacturingprocess2and3.Thebasicalloyingelementsofhighspeedsteelsareapproximately1530wt.%ofcarbideformers(Cr,Mo,W,V),sometimesCoand2sufficientcarbontopromotetheformationofcarbides.TungstenandmolybdenummainlycontributetotheformationoftheprimaryM6CandM2CcarbidesandvanadiumisthemainconstituentoftheMCtype.Conventionalmanufacturingprocessesfortheproductionofcomponentswiththesematerialsincludewroughtmetallurgyandpowdermetallurgy(directsinteringandhotisostaticpressing；HIP).Themainmanufacturingstepsforwroughtprocessingaremelting,casting,hotworking,machiningandheattreating.Normally,extensivehotworking(areareductions90%)isnecessarytodispersethecarbidenetworksformedduringthesolidificationoftheas-castingots.Thishotworkingprocessleadstothealignmentofcarbideinstrings,whichisresponsibleforanisotropicproperties2.Powdermetallurgy(PM)techniqueswereinitiallydevelopedtoovercometheseproblems.Thestartingrawmaterialsarepre-alloyedgasorwateratomisedpowders.Gasatomisedpowdersarecleanerthanwateratomisedpowdersandbothofthemarefreeofsegregationsduetothehighcoolingratesinvolved.GasatomisedpowdersareusedforHIP4andpowderinjectionmoulding(PIM)5.HIPisdevotedforaprimequalityproductduetothecleannessoftherawmaterialandtothefactthatdensificationtakesplacebyasolidstatesintering(SSS)process.Consequently,afineandhomogeneousdistributionofcarbidesembeddedinapore-freeferrousmatrixisobtainedleadingtoexceptionalproperties.PIMisbestsuitedforsmallcomponentswithcomplexgeometriesanddensificationtakesplacebydirectsintering(i.e.,pressurelesssintering)throughasupersolidusliquidphasesintering(SLPS)mechanism5and6.Wateratomisedpowdersarenormallyprocessedbythedirectsinteringroute.Partialdensificationisachievedbycold-pressingthepowderswithasuitablecompactionlubricant.Subsequently,sinteringtofulldensitytakesplacebyaSLPSmechanism.Thedirectsinteringroutehasinherentadvantagesintermsofachievablepropertiesversusprocessingcostsandenvironmentalconsiderationsrelatedtothehighlyefficientmaterialuse.Duringthelast20years,ahighresearchefforthasbeenmainlyaddressedattheunderstandingofthephysicalandchemicalmechanismsinvolvedinthedensificationviaSLPS7andSSS4.Additionally,researchhasalsobeenfocusedonthemicrostructuralchangesoccurringduringheattreatmentsleadingtothedesiredmechanicalproperties(i.e.,strength,toughness,wear,fatigue)dependentontheserviceapplications8.3Previousstudies9,10,11and12havebeenfocusedontheoptimisationofthedirectsinteringrouteofHighVanadiumHSSgrades(mainlyM35MHV)byunderstandingtheeffectofthesinteringatmosphere.Thisapproachhasledtoasignificantdecreaseofthesinteringtemperaturesneededforfulldensificationinnitrogenrichatmospherescomparedtovacuumsintering.Theoptimumsinteringtemperature(OST)forM35MHVdecreasedfrom1220Cforvacuumsinteringto1140Cwhensinteredundernitrogen-richatmosphere3and10.Theeffectofboththesinteringatmosphereandalloymodifications(i.e.,carbonadditions)hasbeenunderstoodbytheuseofcomputationalthermodynamics(calculationofmulticomponentphasediagrams)10and11.Moreover,thecorrespondencebetweencomputationalthermodynamicsandexperimentaldataconstitutedapreliminarystepforalloydesignofnewHSScompositions12and13.ThepresentstudyisaimedatthedescriptionofthesinteringbehaviourofthePMT42gradeunderdifferentnitrogenpressures.ThismaterialhasbeenchosenfortheinvestigationsinceitisacommercialgradecontaininganadequateamountofVformicrostructuraldesignthroughtheadditionofN.Additionally,theexcellenthothardnessofT42duetothe10.58wt.%Co,makesthismaterialveryinterestingforbothstructuralandtribologicalapplications.ThesatisfactoryresultsofearlierinvestigationsonthesinteringbehaviourofT42inN-richatmospheres9invitedforamoredetailedinvestigationstrivingforacommercialimpactoftheresearch.ThemainobjectivewasthemicrostructuraldesignofPMHSSthroughthedensificationrouteandtheheattreatmentsequenceselected.Awidesetofdifferentmicrostructureshasbeenobtainedbyusingthedifferentsinteringconditions.Theeffectoftheas-sinteredmicrostructure(mainlyabsorbedNcontentandgrainsize)ontheheattreatment(austenitizing+quenching+multitempering)isalsodiscussed.References1H.Kawata,K.Hayashi,K.Ishii,K.Maki,A.EhiraandM.Toriumi,SAETrans.107(5)(1998),pp.194200.2G.Hoyle,HighSpeedSteels,Butterworths,BoroughGreen,SevenoaksKent(1988).3S.GimenezandI.Iturriza,J.Mater.Process.Technol.143/144(2003),pp.555560.44E.Arzt,M.F.AshbyandK.E.Easterling,Metall.T