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1HardeningandTemperingofPlainCarbonSteelPlaincarbonsteelhasbeenvaluedfromearlytimebecauseofcertainproperties.thissoftsilver-graymetalcouldbeconvertedintoasuperhardsubstancethatwouldcutglassandmanyothersubstances,includingitselfwhensoft.Furthermore,itshardnesscouldbecontrolled.Thisconvertingofcarbonsteelintoasteelofusefulhardnessisdonewithdifferentheattreatments,twoofthemostimportantofwhicharehardeningandtempering(drawing),whatstheplaincarbonsteelandwhichyouwillinvestigateinthislesson.PlainCarbonSteelInsimpleterms,aplaincarbonsteelmaybesaidtobeanalloyofironandcarboncontaininglessthan1.7%carbon.Inpractice,however,thesesteelsrarelycontainmorethanabout1.4%carbonandotherelementsarealsopresent,eitherasdeliberateadditions(e.g.manganese)orasimpurities(e.g.sulphurandphosphorus).Plaincarbonsteelsmaybeclassifiedintothreemaingroups:1.Lowcarbonsteel(mildsteel)containinglessthan0.3%carbon.2.Mediumcarbonsteelcontaining0.3%to0.6%carbon.3.Highcarbonsteelcontaining0.6%to1.4%carbon.LowcarbonsteelLowcarbonsteelsmaybesub-dividedinto:1.Deadmildsteelcontaining0.05%to0.10%carbonInthemanufactureofthismaterial,completedeoxidationisnotcarriedoutanditismadeasrimmingsteel.Itisproducedashotandcoldworkedsheet,strip,rod,wireandtube,andisavailableinthehot-workedorprocessannealedconditionforthedeepdrawingofsheet,butthestrengthislow.Thistypeofsteelisusedforthemanufactureofmotorcarbodies,refrigerators,washingmachines,officefurniture,pressings,rivetsandnails.2.Carburisingsteelscontaining0.1%to0.2%carbonThestrengthandhardnessofthesesteelsislow,butahardsurfacecanbeobtainedbycarburizinginordertocombineawear-resistantsurfacewithtoughcoreproperties.TheductilitymakesrapidmachiningdifficultunlesstheMnandScontentisincreasedby2usingashlphurcontentof0.2%to0.3%withabout1%manganese.3.Constructionalmildsteelscontaining0.2%to0.3%carbonAsthecarboncontentisraisedabove0.2%,thestrengthincreasesintotherangerequiredforconstructionalpurposes,buttheductilitydecreases.Thefabricationqualities(workingandweldingcharacteristics)ofthismaterialareverygood.Thesteelishot-rolledintoplateformakingboilers,shipsandvehicles,andalsointosections(e.g.girders,beams,joists)foruseinbridgesandbuildings.Thesteelisoftenusedinthehot-rolledcondition,butthesmallersizedmaterialmaybenormalized.Ithasbeenestimatedthatabout90%(byweight)ofallsteelusedisofthislowcarbonclassification.MediumCarbonSteelsMediumcarbonsteelsareoftenquenched-hardenedandfullytemperedtogivegoodstrengthwithmaximumtoughness,whicharethepropertiesrequiredofcomponentssuchasshafts,gearsandconnectingrods.Steelscontaining0.4%to0.5%carbonmayalsobesurfacehardenedbylocalizedheatingandquenching.HighCarbonSteelHighcarbonsteelarequenchhardenedandlightlytemperedtogivehighhardnesswithonlylimitedtoughness.Materialinthisclassificationismainlyusedformakingfairlysmall,relativelyinexpensivecuttingtoolssothatthegroupasawholeisknownascarbontoolsteel.Steelscontaining0.6%to0.8%carbonareusedformakingdies,springs,wireropesandrailwaytypes.Manydifferentkindsoftoolsaremadeformhighercarbonsteelsasindicatedbelow.Carbon%Typeoftool0.81.0Coldchisels,shearblades,punches,hammers1.01.2Files,axes,saws,knives1.21.4Razors,drills,wood-cuttingandmetal-cuttingtoolsLimitationsofPlainCarbonSteelsPlaincarbonsteelshavemanylimitationsincludingthefollowing:(1)Ifreasonabletoughnessandductilityarerequired,themaximumtensilestrengthobtainableisabout700N/2mm.3(2)Largesectionscannotbeeffectivelyhardened,thusrestrictingtheirusetorelativelythinsections.(3)Waterquenchingisnecessaryforfullhardeningwithconsequentriskofdistortionandcracking.(4)Rapidsofteningaboveabout300Climitstheiruseforhigh-speedmetalcutting.(5)Poorresistancetocorrosionandtooxidationatelevatedtemperatures.Toovercometheselimitations,additionalelementsareaddedtothesteeltogivealloysteelswithspecificproperties.Themainalloyingelementsincludemanganese,nickel,chromium,molybdenum,tungsten,vanadium,cobaltandsilicon.HardeningofPlainCarbonSteelAssteelisheatedabovethelowedcriticaltemperatureof1330F(721C),thecarbonthatwasinthefromoflayersofironcarbideinpearlitebeginstodissolveintheironandfromasolidsolutioncalledaustenite.Whenthissolutionofironandcarbonissuddenlycooledorquenched,anewmicrostructureisformed.Thisiscalledmartensite.Martensiteisveryhardandbrittle,havingamuchhighertensilestrengththanthesteelwithapearlitemicrostructure.Itisquiteunstable,however,andmustbetempered(drawn)torelieveinternalstressesinordertohavetheductilityandtoughnessneededtobeuseful.AISIC1095,commonlyknownaswater-hardeningtool(W1)steel,willbegintoshowhardnesswhenquenchedfromatemperaturejustover1330F(721C)butwillnothardenatallifquenchedfromatemperaturelowerthan1330F(721C).Thissteelwillbecomeashardasitcangetwhenheatedto1450(788C)andquenchedinwater.Thisquenchingtemperaturechangesasthecarboncontentchanges.Itshouldbe50F(28C)abovetheuppercriticaltemperatureforcarbonsteelscontaininglessthan0.83percentcarbon.Thereasoncarbonsteel,lessthaneutectoid,shouldbeheatedabovetheuppercriticaltemperatureisthattheferriteisnotalltransformedintoaustenitebelowthispoint,andwhenquenched,isretainedinthemartensiticstructure.Theretainedferritecausesbrittlenessevenaftertempering.LowcarbonsteelssuchasAISI1020willnot,forallpracticalpurposes,hardenwhentheyareheatedandquenched.Oil-andair-hardeningsteelshaveahigherhardenabilityanddonothavetobequenchedasrapidlyasplaincarbonsteels.Consequently,theyaredeeperhardeningthanwaterhardeningtypes,whichmustbecooledto200F(93C)4within1or2seconds.Plaincarbonsteelscontaining0.83percentcarboncangetashard(RC67)asanyplaincarbonsteelcontainingmorecarbon.TemperingofPlainCarbonSteelTempering,ordrawing,isaprocessofreheatingasteelpartthathasbeenpreviouslytotransformsomeofthehardmartensiteintosofterstructures.Thehigherthetemperingtemperatureused,themoremartensiteistransformed,andthesofterandtougher(lessbrittle)thepiecebecomes.Therefore,temperingtemperaturesarespecifiedaccordingtothestrengthandductilitydesired.Mechanicalpropertiescharts,whichmaybefoundinsteelmanufacturershandbooksandcatalogs,givethesedataforeachtypeofalloysteel.temperatureandholdingitthereforalengthoftime,thencoolingitinairorwater.ApartcanbetemperedinafurnaceorovenbybringingittotherequiredSometoolsteelsshouldbecooledrapidlyaftertemperingtoavoidtemperbrittleness.Temperingshouldbedoneassoonaspossibleafterhardening.Thepartshouldnotbeallowedtocoolcompletely,sinceuntempereditcontainsveryhighinternalstressesandtendstosplitorcrack.Temperingwillrelievetheinternalstresses.Ahardenedpartleftovernightwithouttemperingmaydevelopcracksbyitself.ClassificationofHeatTreatingProcessesInsomeinstances,heattreatmentproceduresareclearcutintermsoftechniqueandapplicationwhereasinotherinstances,descriptionsorsimpleexplanationsareinsufficientbecau
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