外文翻译--金属热处理.doc

沈阳理工大学学士学位论文-1-英文原文HEATTREATMENTOFMETALAnnealingThewordannealhasbeenusedbeforetodescribeheat-treatingprocessesforsofteningandregainingductilityinconnectionwithcoldworkingofmaterial.Ithasasimilarmeaningwhenusedinconnectionwiththeheattreatingofallotropicmaterials.Thepurposeoffullannealingisodecreasehardness,increaseductility,andsometimesimprovemachinabilityofhighcarbonsteelsthatmightotherwisebedifficulttocut.Thetreatmentisalsousedtorelievestresses,refinegrainsize,andpromoteuniformityofstructurethroughoutthematerial.Machinabilityisnotalwaysimprovedbyannealing.Thewordmachinabilityisusedtodescribeseveralinterrelatedfactors,includingtheabilityofamaterialtobecutwithagoodsurfacefinish.Plainlowcarbonsteels,whenfullyannealed,aresoftandrelativelyweak,offeringlittleresistancetocutting,butuduallyhavingsufficientductilityandtoughnessthatacutchiptendstopullandtearthesurfacefromwhichitisremoved,leavingacomparativelypoorqualitysurface,whichresultsinapoormachinabilityrating.1Forsuchsteelsannealingmaynotbethemostsuitabletreatment.Themachinabilityofmanyofthehigherplaincarbonandmostofthealloysteelscanusuallybegreatlyimproyedbyannealing,astheyareoftentoohardandstrongtobeeasilycutatanybuttheirsoftestcondition.2Theprocedureforannealinghypoeutectoidsteelistoheatslowlytoapproximately60abovetheAc3line,3tosoakforalongenoughperiodthatthetemperatureequalizesthroughoutthematerialandhomogeneousausteniteisformed,andthentoallowthesteeltocoolveryslowlybycoolingitinthefumaceorburyingitinlimeotsomeotherinsulatingmaterial.Theslowcoolingiseasentialtotheprecipitationofthemaximumferriteandthecoarsestpearlitetoplacethesteelinitssoftest,mostductile,andleaststrainedcondition.NormalizingThepurposeofnormalizingissomewhatsimilartothatofannealingwiththeexceptionsthatthesteelisnotreducedtoitssoftestconditionandthepearliteisleftratherfineinsteadofcoarse.Refinementofgrainsize,reliefofinternalstresses,andimprovementof沈阳理工大学学士学位论文-2-structuraluniformitytogetherwithrecoveryofsomeductilityprovidehightoughnessqualitiesinnormalizedsteel.Theprocessisfrequentlyusedforimprovementofmachinabilityandforstressrelieftoreducedistortionthatmightoccurwithpartialmachiningoraging.Theprocedurefornormalizingistoaustenitizebyslowlyheatingtoapproximately80abovetheAo3orAccm3temperatureforhypoeutectoidorhyereutectoidsreels,respectively.Providingsoakingtimefortheformationofaustenite；andcoolingslowlyinstillair,NotethatthesteelswithmorecarbonthantheeutectoidcompositionareheatedaboutheAccminsteadoftheAc13usedforannealing.Thepurposeofnormalizingistoattempttodissolveallthecementiteduringaustenitizationtoeliminate,asfaraspossible,thesettlingofhard,brittleironcarbideinthegrainboundaries.Thedesireddecompositionproductsaresmallgrained,finepearlitewithaminimumoffreeferriteandfreecementite1SpheroidizingMinimumhardnessandmaximumductilityofsteelcanbeproducedbyaprocesscalledspheroidizing,whichcausestheironcarbidetoforminsmallspheresornodulesinaferritematrix.Inordertostartwithsmallgrainsthatspheroidizemorereadily,theprocessisusuallyperformedonnormalizedsteel.Severalvariationsofprocessingareused,butallrequiretheholdingofthesteelneartheA1temperatureusuallyslightlybelowforanumberofhourstoallow,theironcarbidetoformonitsmorestableandlowerenergystateofsmall,roundedglobules.Themainneedfortheprocessistoimprovethemachinabilityqualityofhighcarbonsteelandtopretreathardenedsteeltohelpproducegreaterstructuraluniformityafterquenching.Becauseofthelengthytreatmenttimeandthereforeratherhighcost,spheroidizingisnotperformednearlyasmuchasannealingornormalizing.HardeningofSteelMostoftheheattreatmenthardeningprocessesforsteelatebasedontheproductionofhighpercebtagesofmartensite.Thefirststep,therefore,isthatusedformostoftheotherheat-treatingprocessestreatmenttoproduceaustenite.Hypoeutectoidsteelsateheatedtoapproximately60abovetheAc3temperatureandallowedtosoaktoobtaintemperature沈阳理工大学学士学位论文-3-uniformityandaustenitehomogeneity.Hypereutectoidsteelsatesoakedatabout60abovetheAc1temperature,whichleavessomeironcarbidepresentinthematerial.ThesecondstepinvolvescoolingrapidlyinanattempttoavoidpearlitetransformationbymissingthenoseoftheITcurve.Thecoolingrateisdeterminedbythetemperatureandabilityofthequenchingmediatocarryheatawayfromthesurfaceofthematerialbeingquenchedandbytheconductionofheatthroughthematerialitself.Table111showssomeofthecommonlyusedmediaandthemethodofapplicationtoremoveheat,arrangedinorderofdecreasingcoolingability.Hightemperaturegradientscontributetohighstresserthatcausedistortionandcracking,sothequenchshouldonlyasextremeasisnecessarytoproducethedesiredstructure.Caremustbeexercisedinquenchingthatheatisremoveduniformlytominimizethermalstresses.Forexample,alongslenderbarshouldbeend-quenched,thatis,insertedintothequdenchingmediumverticallysothattheentiresectionissubjectedtotemperaturechangeatonetime.Ifashapeofthiskindweretobequenchedinawaythatcausedonesidetodropintempeiaturebeforetheother,changeofdimensionswouldlikelycausehighstressesproducingplasticflowandpermanentdistortion.Seyeralspecialtypesofquenchareconductedtominimizequenchingstressesanddecreasethetendencyfordistortionandcracking.Oneoftheseiscalledmartemoeringandconsistsofquenchinganaustenitizedsteelinasaltatatemperatureabovethatneededforthestartofmartensiteformation(Ms).Thesteelbeingquenchedisheldinthisbathuntilitisofuniformtemperaturebutisremovedbeforethereistimeforformationofbainitetostart.Completionofthecoolinginairthencausesthesamehardmartensitethatwouldhaveformedwithquenchingfromthehightemperature,butthehighthermalor“quench”stressesthataretheprimarysourceofcracksandwarpingwillhavebeeneliminated.Asimilarprocessperformedataslightlyhighertemperatureiscalledaustempering.Inthiscasethesteelisheldatthebathtemperatureforalongerperiod,andtheresultoftheformationofbainite.Thebainitestructureisnotashardasthemartensitethatcouldbeformedfromthesamecomposition,butinadditiontoreducingthethermalshocktowhichthesteelwouldbesubjectedundernormalhardeningprocedures,itisunnecessarytoperformanyfurthertreatmenttodevelopgoodimpactresistanceinthehighhardnessrange.4沈阳理工大学学士学位论文-4-TemperingAthirdstepusuallyrequiredtoconditionahardenedsteelforsweviceistempering,orasitissometimesreferredto,drawing.Withtheexceptionofaustemperedsteel,whichisfrequentlyusedintheashardenedcondition,moststeelarenotserviceable“asquenched”.Thedrasticcoolingtoproducemartensitecausesthesteeltobeveryhardandtocontainbothmacroscopicinternalstresseswiththeresultthatthematerialthislittleductilityandextremebrittleness.Reductionpgthesefaultsisaccomplishedbyreheatingthesteeltosometimesreferredto,drawing.Withtheexceptionofaustemperedsteel,whichisfrequentlyusedintheas-hardenedcognition,moststeelsarenotserviceable“asquenched”,Thedrasticcoolingtoproducemartensitecausesthesteeltobeveryhardandtocontainbothmacroscopicandmicroscopicinternalstresseswiththeresultthatthematerialhaslittleductilityandextremebrittleness.ReductionofthesefaultsisaccomplishedbyreheatingthesteeltosomepointbelowtheA1(lowertransformation)temperature.Thestructuralchangescausedbytemperingofhardenedsteelarefunctionsofbothtimeandtemperature,withtemperaturebeingthemostimportant.Itshouldbeemphasizedthattemperingisnotahardeningprocess,butis,instead,thereverse.Atemperedsteelisonethathasbeenhardenedbyheattreatmentandthenstressrelieved,softened,andprovidedwithincreasedductilitybyreheatinginthetemperingordrawingprocedure.Themagnitudeofthestructuralchangesandthechangeofpropertiescause