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沈阳理工大学学士学位论文1英文原文HEATTREATMENTOFMETALAnnealingThewordannealhasbeenusedbeforetodescribeheattreatingprocessesforsofteningandregainingductilityinconnectionwithcoldworkingofmaterial.Ithasasimilarmeaningwhenusedinconnectionwiththeheattreatingofallotropicmaterials.Thepurposeoffullannealingisodecreasehardness,increaseductility,andsometimesimprovemachinabilityofhighcarbonsteelsthatmightotherwisebedifficulttocut.Thetreatmentisalsousedtorelievestresses,refinegrainsize,andpromoteuniformityofstructurethroughoutthematerial.Machinabilityisnotalwaysimprovedbyannealing.Thewordmachinabilityisusedtodescribeseveralinterrelatedfactors,includingtheabilityofamaterialtobecutwithagoodsurfacefinish.Plainlowcarbonsteels,whenfullyannealed,aresoftandrelativelyweak,offeringlittleresistancetocutting,butuduallyhavingsufficientductilityandtoughnessthatacutchiptendstopullandtearthesurfacefromwhichitisremoved,leavingacomparativelypoorqualitysurface,whichresultsinapoormachinabilityrating.1Forsuchsteelsannealingmaynotbethemostsuitabletreatment.Themachinabilityofmanyofthehigherplaincarbonandmostofthealloysteelscanusuallybegreatlyimproyedbyannealing,astheyareoftentoohardandstrongtobeeasilycutatanybuttheirsoftestcondition.2Theprocedureforannealinghypoeutectoidsteelistoheatslowlytoapproximately60abovetheAc3line,3°°tosoakforalongenoughperiodthatthetemperatureequalizesthroughoutthematerialandhomogeneousausteniteisformed,andthentoallowthesteeltocoolveryslowlybycoolingitinthefumaceorburyingitinlimeotsomeotherinsulatingmaterial.Theslowcoolingiseasentialtotheprecipitationofthemaximumferriteandthecoarsestpearlitetoplacethesteelinitssoftest,mostductile,andleaststrainedcondition.NormalizingThepurposeofnormalizingissomewhatsimilartothatofannealingwiththeexceptionsthatthesteelisnotreducedtoitssoftestconditionandthepearliteisleftratherfineinsteadofcoarse.Refinementofgrainsize,reliefofinternalstresses,andimprovementof沈阳理工大学学士学位论文2structuraluniformitytogetherwithrecoveryofsomeductilityprovidehightoughnessqualitiesinnormalizedsteel.Theprocessisfrequentlyusedforimprovementofmachinabilityandforstressrelieftoreducedistortionthatmightoccurwithpartialmachiningoraging.Theprocedurefornormalizingistoaustenitizebyslowlyheatingtoapproximately80°abovetheAo3orAccm3temperatureforhypoeutectoidorhyereutectoidsreels,respectively.Providingsoakingtimefortheformationofausteniteandcoolingslowlyinstillair,NotethatthesteelswithmorecarbonthantheeutectoidcompositionareheatedaboutheAccminsteadoftheAc13usedforannealing.Thepurposeofnormalizingistoattempttodissolveallthecementiteduringaustenitizationtoeliminate,asfaraspossible,thesettlingofhard,brittleironcarbideinthegrainboundaries.Thedesireddecompositionproductsaresmallgrained,finepearlitewithaminimumoffreeferriteandfreecementite1SpheroidizingMinimumhardnessandmaximumductilityofsteelcanbeproducedbyaprocesscalledspheroidizing,whichcausestheironcarbidetoforminsmallspheresornodulesinaferritematrix.Inordertostartwithsmallgrainsthatspheroidizemorereadily,theprocessisusuallyperformedonnormalizedsteel.Severalvariationsofprocessingareused,butallrequiretheholdingofthesteelneartheA1temperature{usuallyslightlybelow}foranumberofhourstoallow,theironcarbidetoformonitsmorestableandlowerenergystateofsmall,roundedglobules.Themainneedfortheprocessistoimprovethemachinabilityqualityofhighcarbonsteelandtopretreathardenedsteeltohelpproducegreaterstructuraluniformityafterquenching.Becauseofthelengthytreatmenttimeandthereforeratherhighcost,spheroidizingisnotperformednearlyasmuchasannealingornormalizing.HardeningofSteelMostoftheheattreatmenthardeningprocessesforsteelatebasedontheproductionofhighpercebtagesofmartensite.Thefirststep,therefore,isthatusedformostoftheotherheattreatingprocessestreatmenttoproduceaustenite.Hypoeutectoidsteelsateheatedtoapproximately60°abovetheAc3temperatureandallowedtosoaktoobtaintemperature沈阳理工大学学士学位论文3uniformityandaustenitehomogeneity.Hypereutectoidsteelsatesoakedatabout60°abovetheAc1temperature,whichleavessomeironcarbidepresentinthematerial.ThesecondstepinvolvescoolingrapidlyinanattempttoavoidpearlitetransformationbymissingthenoseoftheITcurve.Thecoolingrateisdeterminedbythetemperatureandabilityofthequenchingmediatocarryheatawayfromthesurfaceofthematerialbeingquenchedandbytheconductionofheatthroughthematerialitself.Table111showssomeofthecommonlyusedmediaandthemethodofapplicationtoremoveheat,arrangedinorderofdecreasingcoolingability.Hightemperaturegradientscontributetohighstresserthatcausedistortionandcracking,sothequenchshouldonlyasextremeasisnecessarytoproducethedesiredstructure.Caremustbeexercisedinquenchingthatheatisremoveduniformlytominimizethermalstresses.Forexample,alongslenderbarshouldbeendquenched,thatis,insertedintothequdenchingmediumverticallysothattheentiresectionissubjectedtotemperaturechangeatonetime.Ifashapeofthiskindweretobequenchedinawaythatcausedonesidetodropintempeiaturebeforetheother,changeofdimensionswouldlikelycausehighstressesproducingplasticflowandpermanentdistortion.Seyeralspecialtypesofquenchareconductedtominimizequenchingstressesanddecreasethetendencyfordistortionandcracking.OneoftheseiscalledmartemoeringandconsistsofquenchinganaustenitizedsteelinasaltatatemperatureabovethatneededforthestartofmartensiteformationMs.Thesteelbeingquenchedisheldinthisbathuntilitisofuniformtemperaturebutisremovedbeforethereistimeforformationofbainitetostart.Completionofthecoolinginairthencausesthesamehardmartensitethatwouldhaveformedwithquenchingfromthehightemperature,butthehighthermalorquenchstressesthataretheprimarysourceofcracksandwarpingwillhavebeeneliminated.Asimilarprocessperformedataslightlyhighertemperatureiscalledaustempering.Inthiscasethesteelisheldatthebathtemperatureforalongerperiod,andtheresultoftheformationofbainite.Thebainitestructureisnotashardasthemartensitethatcouldbeformedfromthesamecomposition,butinadditiontoreducingthethermalshocktowhichthesteelwouldbesubjectedundernormalhardeningprocedures,itisunnecessarytoperformanyfurthertreatmenttodevelopgoodimpactresistanceinthehighhardnessrange.4沈阳理工大学学士学位论文4TemperingAthirdstepusuallyrequiredtoconditionahardenedsteelforsweviceistempering,orasitissometimesreferredto,drawing.Withtheexceptionofaustemperedsteel,whichisfrequentlyusedintheashardenedcondition,moststeelarenotserviceableasquenched.Thedrasticcoolingtoproducemartensitecausesthesteeltobeveryhardandtocontainbothmacroscopicinternalstresseswiththeresultthatthematerialthislittleductilityandextremebrittleness.Reductionpgthesefaultsisaccomplishedbyreheatingthesteeltosometimesreferredto,drawing.Withtheexceptionofaustemperedsteel,whichisfrequentlyusedintheashardenedcognition,moststeelsarenotserviceableasquenched,Thedrasticcoolingtoproducemartensitecausesthesteeltobeveryhardandtocontainbothmacroscopicandmicroscopicinternalstresseswiththeresultthatthematerialhaslittleductilityandextremebrittleness.ReductionofthesefaultsisaccomplishedbyreheatingthesteeltosomepointbelowtheA1lowertransformationtemperature.Thestructuralchangescausedbytemperingofhardenedsteelarefunctionsofbothtimeandtemperature,withtemperaturebeingthemostimportant.Itshouldbeemphasizedthattemperingisnotahardeningprocess,butis,instead,thereverse.Atemperedsteelisonethathasbeenhardenedbyheattreatmentandthenstressrelieved,softened,andprovidedwithincreasedductilitybyreheatinginthetemperingordrawingprocedure.Themagnitudeofthestructuralchangesandthechangeofpropertiescausedbytemperingdependuponthetemperaturetowhichthesteelisreheated.Thehigherthetemperature,thegreatertheeffect,sothechoiceoftemperaturewillgenerallydependonwillingnesstosacrificehardenssandstrengthtogainductilityandtoughness.Reheatingtobelow100°haslittlenoticeableeffectonhardenedplaincarbonsteel.Between100°and200°,thereisevidenceofsomestructuralchanges.Above200°markedchangesinstructureandpropertiesappear.ProlongedheatingatjustundertheA1temperaturewillresultinaspheroidizedstructuresimilartothatproducedbythespheroidizingprocess.Incommercialtemperingthetemperaturerangeof250425°Cisusuallyavoidedbecauseofanunexplainedembrittlement,orlossofductility,thatoftenoccurswithsteelstemperedinthisrangeof425600°C,particularlywhencooledslowlyfromorthroughthisrangeoftemperature.whenhightemperatureremperaturetemperingisnecessaryforthese
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