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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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