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1HardeningandTemperingofPlainCarbonSteelPlaincarbonsteelhasbeenvaluedfromearlytimebecauseofcertainproperties.thissoftsilvergraymetalcouldbeconvertedintoasuperhardsubstancethatwouldcutglassandmanyothersubstances，includingitselfwhensoft.Furthermore，itshardnesscouldbecontrolled.Thisconvertingofcarbonsteelintoasteelofusefulhardnessisdonewithdifferentheattreatments，twoofthemostimportantofwhicharehardeningandtemperingdrawing，whatstheplaincarbonsteelandwhichyouwillinvestigateinthislesson.PlainCarbonSteelInsimpleterms，aplaincarbonsteelmaybesaidtobeanalloyofironandcarboncontaininglessthan1.7carbon.Inpractice，however，thesesteelsrarelycontainmorethanabout1.4carbonandotherelementsarealsopresent，eitherasdeliberateadditionse.g.manganeseorasimpuritiese.g.sulphurandphosphorus.Plaincarbonsteelsmaybeclassifiedintothreemaingroups1.Lowcarbonsteelmildsteelcontaininglessthan0.3carbon.2.Mediumcarbonsteelcontaining0.3to0.6carbon.3.Highcarbonsteelcontaining0.6to1.4carbon.LowcarbonsteelLowcarbonsteelsmaybesubdividedinto1.Deadmildsteelcontaining0.05to0.10carbonInthemanufactureofthismaterial，completedeoxidationisnotcarriedoutanditismadeasrimmingsteel.Itisproducedashotandcoldworkedsheet，strip，rod，wireandtube，andisavailableinthehotworkedorprocessannealedconditionforthedeepdrawingofsheet，butthestrengthislow.Thistypeofsteelisusedforthemanufactureofmotorcarbodies，refrigerators，washingmachines，officefurniture，pressings，rivetsandnails.2.Carburisingsteelscontaining0.1to0.2carbonThestrengthandhardnessofthesesteelsislow，butahardsurfacecanbeobtainedbycarburizinginordertocombineawearresistantsurfacewithtoughcoreproperties.TheductilitymakesrapidmachiningdifficultunlesstheMnandScontentisincreasedby2usingashlphurcontentof0.2to0.3withabout1manganese.3.Constructionalmildsteelscontaining0.2to0.3carbonAsthecarboncontentisraisedabove0.2，thestrengthincreasesintotherangerequiredforconstructionalpurposes，buttheductilitydecreases.Thefabricationqualitiesworkingandweldingcharacteristicsofthismaterialareverygood.Thesteelishotrolledintoplateformakingboilers，shipsandvehicles，andalsointosectionse.g.girders，beams，joistsforuseinbridgesandbuildings.Thesteelisoftenusedinthehotrolledcondition，butthesmallersizedmaterialmaybenormalized.Ithasbeenestimatedthatabout90byweightofallsteelusedisofthislowcarbonclassification.MediumCarbonSteelsMediumcarbonsteelsareoftenquenchedhardenedandfullytemperedtogivegoodstrengthwithmaximumtoughness,whicharethepropertiesrequiredofcomponentssuchasshafts,gearsandconnectingrods.Steelscontaining0.4to0.5carbonmayalsobesurfacehardenedbylocalizedheatingandquenching.HighCarbonSteelHighcarbonsteelarequenchhardenedandlightlytemperedtogivehighhardnesswithonlylimitedtoughness.Materialinthisclassificationismainlyusedformakingfairlysmall,relativelyinexpensivecuttingtoolssothatthegroupasawholeisknownascarbontoolsteel.Steelscontaining0.6to0.8carbonareusedformakingdies,springs,wireropesandrailwaytypes.Manydifferentkindsoftoolsaremadeformhighercarbonsteelsasindicatedbelow.CarbonTypeoftool0.81.0Coldchisels,shearblades,punches,hammers1.01.2Files,axes,saws,knives1.21.4Razors,drills,woodcuttingandmetalcuttingtoolsLimitationsofPlainCarbonSteelsPlaincarbonsteelshavemanylimitationsincludingthefollowing1Ifreasonabletoughnessandductilityarerequired,themaximumtensilestrengthobtainableisabout700N/2mm.32Largesectionscannotbeeffectivelyhardened,thusrestrictingtheirusetorelativelythinsections.3Waterquenchingisnecessaryforfullhardeningwithconsequentriskofdistortionandcracking.4Rapidsofteningaboveabout300ºClimitstheiruseforhighspeedmetalcutting.5Poorresistancetocorrosionandtooxidationatelevatedtemperatures.Toovercometheselimitations,additionalelementsareaddedtothesteeltogivealloysteelswithspecificproperties.Themainalloyingelementsincludemanganese,nickel,chromium,molybdenum,tungsten,vanadium,cobaltandsilicon.HardeningofPlainCarbonSteelAssteelisheatedabovethelowedcriticaltemperatureof1330F721ºC，thecarbonthatwasinthefromoflayersofironcarbideinpearlitebeginstodissolveintheironandfromasolidsolutioncalledaustenite.Whenthissolutionofironandcarbonissuddenlycooledorquenched，anewmicrostructureisformed.Thisiscalledmartensite.Martensiteisveryhardandbrittle，havingamuchhighertensilestrengththanthesteelwithapearlitemicrostructure.Itisquiteunstable，however，andmustbetempereddrawntorelieveinternalstressesinordertohavetheductilityandtoughnessneededtobeuseful.AISIC1095，commonlyknownaswaterhardeningtoolW1steel，willbegintoshowhardnesswhenquenchedfromatemperaturejustover1330F721ºCbutwillnothardenatallifquenchedfromatemperaturelowerthan1330F721ºC.Thissteelwillbecomeashardasitcangetwhenheatedto1450788ºCandquenchedinwater.Thisquenchingtemperaturechangesasthecarboncontentchanges.Itshouldbe50F28ºCabovetheuppercriticaltemperatureforcarbonsteelscontaininglessthan0.83percentcarbon.Thereasoncarbonsteel，lessthaneutectoid，shouldbeheatedabovetheuppercriticaltemperatureisthattheferriteisnotalltransformedintoaustenitebelowthispoint，andwhenquenched，isretainedinthemartensiticstructure.Theretainedferritecausesbrittlenessevenaftertempering.LowcarbonsteelssuchasAISI1020willnot，forallpracticalpurposes，hardenwhentheyareheatedandquenched.Oilandairhardeningsteelshaveahigherhardenabilityanddonothavetobequenchedasrapidlyasplaincarbonsteels.Consequently，theyaredeeperhardeningthanwaterhardeningtypes，whichmustbecooledto200F93ºC4within1or2seconds.Plaincarbonsteelscontaining0.83percentcarboncangetashardRC67asanyplaincarbonsteelcontainingmorecarbon.TemperingofPlainCarbonSteelTempering，ordrawing，isaprocessofreheatingasteelpartthathasbeenpreviouslytotransformsomeofthehardmartensiteintosofterstructures.Thehigherthetemperingtemperatureused，themoremartensiteistransformed，andthesofterandtougherlessbrittlethepiecebecomes.Therefore，temperingtemperaturesarespecifiedaccordingtothestrengthandductilitydesired.Mechanicalpropertiescharts，whichmaybefoundinsteelmanufacturershandbooksandcatalogs，givethesedataforeachtypeofalloysteel.temperatureandholdingitthereforalengthoftime，thencoolingitinairorwater.ApartcanbetemperedinafurnaceorovenbybringingittotherequiredSometoolsteelsshouldbecooledrapidlyaftertemperingtoavoidtemperbrittleness.Temperingshouldbedoneassoonaspossibleafterhardening.Thepartshouldnotbeallowedtocoolcompletely，sinceuntempereditcontainsveryhighinternalstressesandtendstosplitorcrack.Temperingwillrelievetheinternalstresses.Ahardenedpartleftovernightwithouttemperingmaydevelopcracksbyitself.ClassificationofHeatTreatingProcessesInsomeinstances，heattreatmentproceduresareclearcutintermsoftechniqueandapplication．whereasinotherinstances，descriptionsorsimpleexplanationsareinsufficientbecausethesametechniquefrequentlymaybeusedtoobtaindifferentobjectives．Forexample，stressrelievingandtemperingareoftenaccomplishedwiththesameequipmentandbyuseofidenticaltimeandtemperaturecycles．Theobjectives，however，aredifferentforthetwoprocesses．ThefollowingdescriptionsoftheprincipalheattreatingprocessesaregenerallyarrangedNormalizingusuallyisusedasaconditioningtreatment，notablyforrefiningthegrainsofsteelsthathavebeensubjectedtohightemperaturesforforgingorotherhotworkingOperations.Thenormalizingprocessusuallyissucceededbyanotherheattreatingoperationsuchasaustenitizingforhardening，annealing，ortempering．AnnealingISagenerictermdenotingaheattreatmentthatconsisisofheatingtoandholdingatasuitabletemperaturefollowedbycoolingatasuitablerate．Itisused
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