Fracture-Toughness-of-AISI-M2-and-AISI-M7-High-Spe

FractureToughnessofAiSiM2andAISiM7High-SpeedSteelsA.R.JOHNSONAstudyhasbeenmadeofthefracturetoughnessofAISIM2andM7high-speedsteels.Fracturetoughnessofthesesteelswasfoundtodependprincipallyonaustenitizingtem-peratureandhardnesslevel.Theresultsarehighlyreproducible.AtusualworkinghardnesslevelsthefracturetoughnessofM7andM2wereapproximatelyequal.Instru-mentedunnotchedCharpytestsshowedM2tohavehigherimpactstrengththanM7.Amodelforfracturethatassumedpreexistingflawswastestedwiththedataobtained.ThemodelisconsistentwiththeresultsobtainedforM7butnotforM2.TheassumptionofpreexistingflawsinM2high-speedsteeldoesnotseemwarranted.Possiblemecha-nismsforcrackinitiationinM2arediscussedbriefly.THEtoughnessofhigh-speedsteelisapropertyofconsiderablepracticalimportanceandgenerallyimpliessomemeasureoftheabilityofthesteeltoabsorbimpactloadswithoutsignificantmacroscopicplasticdeformationorcatastrophicfailure.Acuttingtoolwhichisrequiredtomaintainprecisedimen-sionaltolerances,frequentlyunderconditionsofintermittentcuttinginvolvingrepeatedimpactload-ing,cannotbepermittedanylargeamountofirrever-sible(nonelastic)deformation.Forthisreason,ductilityaloneisofquestionablesignificanceasacriterionofadequatetoolperformance.Theliteratureontoughnesstestsforhigh-speedsteelismodest.Probablythemostcomprehensivestudiesoftoughnessofhigh-speedsteelshavebeenreportedbyGrobeandRoberts.l,2Theirdefini-tionofthetoughnessconceptis:1)Theabilitytodeformbeforebreaking(plasticdeflectionfromthebendtest).2)Theabilitytoresistpermanentdeformation(yieldstrengthfromthebendtest).AnalternativedefinitionhasbeenadvancedbyWeigand3whodefinestoughnessastheamountofplasticdeformationenergymeasuredinastaticbendortorsiontest.Weigand'sfindingsdemonstratethatthereisnotnecessarilyapositivecorrelationbetweenthetoughness,so-defined,andtoollifeinintermittentcutting.Inadditionhefindsnocorrela-tionbetweentoughness,so-defined,andgrainsizeoverarangeofinterceptgrainsizesfrom5to20.Dulls4findsthatatgrainsizesfinerthan20,duc-tilityasmeasuredinthebendtestincreasesandthereisacoincidentimprovementinintermittentcuttingperformance.Atthepresenttime,itisgenerallyagreedthattoughnessofhigh-speedsteelsisanimportantproperty.Anadequatedefinitionoftoughness,how-ever,isstilllacking.Theinfluenceofsuchstruc-turalparametersasgrainsizeaswellasthemor-phologyanddistributionofcarbidesontoughnessisstillcontroversial.A.R.JOHNSONisStaffMetallurgist,ClimaxMolybdenumCom-panyofMichigan,AnnArbor,MI48105.ManuscriptsubmittedAugust]1,1976.Recentyearshavewitnessedthedevelopmentofanewwayofviewingtheconceptoftoughnessasem-bodiedintheartandscienceoffracturemechanics.Fracturemechanicshasbeenparticularlysuccessfulinevaluatingthepropensityofamaterialtoundergobrittlefailurewhensubjectedtostress.Throughtheapplicationofappropriatetestingtechniquesitispossibletoobtainameasureoffracturetough-ness(Klc)whichisaninherentpropertyofthematerialinthesamesensethattensilestrengthisamaterialproperty.Thefracturetoughness,Kic,isnotdependentuponspecimengeometryinthewaythattheimpactenergyisinconventionalimpacttesting.Fracturetoughness,KIc,isameasureoftheabilityofamaterialtoresistpropagationofaninfinitelysharpcrackunderstress.Theliteratureontheapplicationoffracturetoughnesstestingtohigh-speedsteelsisrelativelymeager,s'6andthepracticalim-plicationsaresomewhatvague.Thepresentinvestigationshowshowthestudyoffracturetoughnessmayshednewlightontheme-chanismoffractureofhigh-speedsteels.Italsopointstopotentialavenuesforfurtherresearch.Thetechniquesemployedaretheevaluationofstaticfracturetoughness,Kic,bytestingcompacttensilespecimens,andthemeasurementofimpactproper-tiesbymeansofaninstrumentedCharpyimpacttest.EXPERIMENTALPROCEDURESThechemicalcompositionsofthematerialsusedinthisinvestigationareshowninTable].Thema-terialswereobtainedfromcommercialsuppliers.SinceAISIM7high-speedsteelwasnotavailableinthedesiredsize,a78mmdiam(3-1/16in.)roundbarwasreforgedto16x54mm(5/8�2-1/8in.)rectangularbarstock.Thereforgedmaterialwasburiedinaninsulatingmediumimmediatelyafterforgingandallowedtocooltoroomtemperature.Thebarswerethenannealedbyholdingfor1hat1600~(870~andfurnacecoolingataratelessthan50~(28~perhourtoabout1200~(650~whereuponthefurnacewasshutoffandthebarsal-lowedtocoolnaturallywiththefurnace.Compacttensionspecimenswereusedtodeter-METALLURGICALTRANSACTIONSAVOLUME8A,JUNE1977-891TableI.ChemicalCompositionsofExperimentalSteelsChemicalComposition,PctGradeSizeDesignationCSiMnSPWCrMoVAISIM2AISIM716X54mm16046(sAX21Ain.)78mmroundX51724(3'126in.)0.820.180.250.0070.020i.000.340.230.0070.0226.204.154.861.921.763.708.451.900.005IN.(0.3mm)~\L=2.12ItLF(54.0mm)W=1.62IN.Il:(4121n'~T)~h=0.50IN.It'~a~,~(12.7ram)lII~1~iJ110.25IN.(6.4mm)0.38IN.(9.5mm)DIA.Ig=0.50I{,!,,,I,I,',,(~2.7~)Fig.1--Compacttensionspecimen.minefracturetoughness(Kic),andstandardunnotchedCharpyspecimenswereusedintheinstrumentedim-pacttest.ThedimensionsofthecompacttensionspecimensareshowninFig.1.Thecompacttensionspecimenswererough-machinedtosizeleavingthethickness(B)atleast2.5mm(0.100in.)oversizeforsubsequentremovalofanydecarb