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  • Knife Steel Basics

    Selection of Knife Steels Theutilityofaknifedependsonoptimizingallthefactorsaffectingitsperformance.Usually,conditionsofuse(appliedloads,abrasiveenvironments,impacts,andotherfactors)determinehowwelltheknifeholdsup.Mostknifefailuresarerelatedtosuchmechanicalcauses.However,withavarietyofsteelsavailableformanufacturingknives,itisoftenpossibletochooseaknifesteelwithafavorablecombinationofpropertiesforparticularuses.Bycomparingthelevelsofmetallurgicalpropertiesofferedbydifferentsteels,knifemakerscandeterminewhichsteelsarebestsuitedforfixingorresistingperformanceproblems,orforenhancingknifeperformance.Knifesteelscanbecategorizedandcomparedbythosepropertieswhichhaveadirectinfluenceonperformance:hardness,toughness(impactresistance),wearresistance,andcorrosionresistance.

    Knife Steels

    GeneralPurpose ImprovedWearResistanceO1,A2,D2 M4,10V,S30V,S90V

    ImprovedToughness CorrosionResistance3V 440C,154CM,CPM154,S30V,S35VN,S90V

    Historically,customknifesteelshaveincludedO1,A2andD2,withoccasionaluseofcorrosionandwearresistantgradessuchas440Cand154CM.O1,A2andD2arefamiliartomostcustomknifemakersandusers.Theycombinegoodallaroundperformancepropertieswithlowcost,wideavailability,andrelativelyeasyfabrication.However,theysometimesdonotprovidethelevelofperformanceneededforchallengingapplications.Specifically,whereabusiveuseandinfrequentresharpeningaredesired,otherhigheralloysteelsmightbeused.Traditionally,otherproperties,suchasimpactresistance,maybesacrificedinordertogainthehigherwearproperties.Conversely,steelschosenfortheirresistancetoimpactorbreakagemaynotbecapableofhighwearresistance.Anunderstandingoftheseknifesteelpropertiesandrelatedissuespermitsselectionoftheoptimumsteelformostapplications.Knifeuserscanexamineheavilyusedbladestodeterminewhichpropertymayhavebeenlacking,orwhichpropertiesshouldbeimproved,andwhatotherpropertiesmustbeconsideredinalternatematerialswiththerequiredimprovedproperties.

    Properties of Knife Steel Hardness isameasureofasteelsresistancetodeformation.HardnessinknifesteelsismostcommonlymeasuredusingtheRockwellCtest.Hardenedknifesteelsaregenerallyabout58/62HRC(hardnessRockwellC),dependingonthegrade.Mostaretypicallyabout58/60HRC,althoughsomeareoccasionallyuseduptoabout62HRC.Knifeedgeswhichplasticallydeforminservicepossessinsufficienthardness.Permanentbendingofthebladeorpermanentdeflectionofthecuttingedgeindicatesinsufficienthardness.Becauseasteelsresistancetopermanentdeflectionisdirectlyrelatedtothehardness,notthegrade,correctiveactionsfordeformationmayincludeincreasinghardness,ordecreasingoperatingloadsbyincreasingbladethickness.Changinggradeswillnothelpadeformationproblem,unlessthenewgradeiscapableofhigherhardness. Toughness,asconsideredforhighhardnessknifesteels,istherelativeresistanceofamaterialtobreakage,chipping,orcrackingunderimpactorstress.Toughnessmaybethoughtofastheoppositeofbrittleness.Toughnesstestingisnotasstandardizedashardnesstesting.Itmaybedifficulttocorrelatetheresultsofdifferenttestmethods.Commontoughnesstestsincludevariousimpacttestsandbendfracturetests.Inservice,wearfailuresareusuallypreferabletotoughnessfailures(breakage).Breakagefailurescanbeunpredictable,catastrophic,andevenasafetyconcern.Conversely,wearfailuresareusuallygradual,andcanbeanticipatedandplannedfor.Toughnessfailuresmaybetheresultofinadequatematerialtoughness,oranumberofotherfactors,includingheattreatment,fabrication(grindingabuse),oramultitudeofusageissues.Toughnessdataisusefultopredictwhichsteelsmaybemoreorlesspronetochippingorbreakagethanothersteels,buttoughnessdatacannotalonepredicttheperformancelifeofaknife. Wear resistance istheabilityofmaterialtoresistbeingabradedorerodedbycontactwithworkmaterial,oroutsideinfluences(dirt,grit,bone,etc.)Wearresistanceisprovidedbyboththehardnesslevelandthechemistryoftheknifeblade.Weartestsarequitespecifictothecircumstancescreatingthewearandtheapplicationoftheknife.Mostweartestsinvolve

  • creatingamovingcontactbetweenthesurfaceofasampleandsomedestructivemedium.Thereare2basictypesofweardamageinknives,abrasiveandadhesive.Wearinvolvingerosionorroundingofedgesiscalledabrasivewear.Abrasiveweardoesnotrequirehighpressures.Abrasiveweartestingmayinvolvesand,sandpaper,orvariousslurriesorpowders.Wearfromintimatecontactbetweentworelativelysmoothsurfaces,suchassteelonsteel,carbideonsteel,etc.,iscalledadhesivewear.Adhesivewearmayinvolveactualtearingofthematerialatpointsofhighpressurecontactduetofriction.Corrosion Resistance isameasureofaknifesteelsresistancetoattackinhighhumidity,damp,orsaltenvironments.Thisresistanceisestablishedbytheadditionofchromiumtothecomposition.Developingcorrosionresistanceinaheattreatable,wearresistantsteelisachallengethathasbeenmetwithnumerousspecialtyCPMalloys.Relativeresistanceisoftenmeasuredinsaltsprayandwatersprayenvironments.

    Heat Treatment of Tool Steels Knifesteelsareusuallysuppliedintheannealedconditiontofacilitatemanufacture.Thesesteelsmustbeheattreatedtodeveloptheircharacteristicproperties.Theheattreatingprocessaltersthealloydistributionandtransformsthesoftmatrixintoahardmatrixcapableofwithstandingthepressure,abrasionandimpactsinherentinknifeuse.Eachstepoftheheattreatingcycleisdesignedtoperformaspecificfunction,and,likelinksinachain,thefinalproductisonlyasgoodasitsweakestcomponent. Preheating,orslowheating,ofknifesteelsduringheattreatmentisnecessarytoavoidthermalshockanddistortion.Preheatingisdonetojustbelowacriticaltransformationtemperature.Thepartisthenheldlongenoughtoallowthefullcrosssectiontoreachauniformtemperature.Oncetheentirepartisequalized,furtherheatingtotheaustenitizingtemperaturewillallowthematerialtotransformmoreuniformly. Austenitizing atanelevatedtemperatureisnecessarytohardenknifesteels.Theactualtemperatureuseddependsonthechemicalcompositionofthesteel.Thetemperaturemaybevariedsomewhattotailortheresultingpropertiestospecificapplications.Highertemperaturespermitslightlyhigherhardnessorcompressivestrength.Lowertemperaturespermithighermaterialtoughnessataslightlylowertemperature. Quenching thesteelfromtheaustenitizingtemperaturecausesthesteeltofullyhardentomartensite,whichwillprovidethematerialsstrength.Howfastasteelmustbecooledtofullyhardendependsonthechemicalcomposition.Forhighlyalloyedtoolsteelswhichhardenfromover2000F,thequenchratefromabout1800Ftobelow1200Fiscriticalforoptimumheattreatresponseandmaterialtoughness.Nomatterhowknifesteelsarequenched,theresultingstructure,martensite,isextremelybrittle,andundergreatstress.Ifputintoserviceinthiscondition,mostknifesteelswouldshatter.Somesteelswillspontaneouslycrackinthisconditionevenifleftuntouchedatroomtemperature.Forthisreason,assoonasknifesteelshavebeenquenchedbyanymethodtohandwarm(about125/150F),theyshouldbeimmediatelytempered. Tempering isperformedtostressrelievethebrittlemartensitewhichwasformedduringthequench.Moststeelshaveafairlywiderangeofacceptabletemperingtemperatures.Ingeneral,usethehighesttemperingtemperaturewhichwillprovidethenecessaryhardnessfortheknife.Therateofheatingto,andcoolingfromthetemperingtemperatureisnotcritical.Suddendrastictemperatureswingsshouldbeavoided.Thematerialshouldbeallowedtocoolcompletelytoroomtemperature(50/75F)orbelowbetweenandaftertempers.Moststeelsmustbeheldattemperatureforaminimumoftwotofourhoursforeachtemper.Aruleofthumbistoallowonehourperinchofthickestsectionfortempering,butinnocaselessthantwohoursregardlessofsize. Size Change isunavoidableinheattreatedtoolsteelsbecauseoftheresultantchangesintheirmicrostructure.Mosttoolsteelsgrowbetweenabout0.0005and0.002inchperinchoforiginallengthduringheattreatment.

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