热处理外文翻译.docx
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热处理外文翻译.docx
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热处理外文翻译
附录一
外文翻译
原文:
HeatTreatment
Theunderstandingofheattreatmentisembracedbythebroaderstudyofmetallurgy.Metallurgyisthephysics,chemistry,andengineeringrelatedtometalsfromoreextractiontothefinalproduct.
Heattreatmentistheoperationofheatingandcoolingametalinitssolidstatetochangeitsphysicalproperties.Accordingtotheprocedureused,steelcanbehardenedtoresistcuttingactionandabrasion,oritcanbesoftenedtopermitmachining.
Withtheproperheattreatmentinternalstressesmayberemoved,grainsizereduced,toughnessincreased,orahardsurfaceproducedonaductileinterior.Theanalysisofthesteelmustbeknownbecausesmallpercentagesofcertainelements,notablycarbon,greatlyaffectthephysicalproperties.
Alloysteelowetheirpropertiestothepresenceofoneormoreelementsotherthancarbon,namelynickel,chromium,manganese,molybdenum,tungsten,silicon,vanadium,andcopper.Becauseoftheirimprovedphysicalpropertiestheyareusedcommerciallyinmanywaysnotpossiblewithcarbonsteels.
Thefollowingdiscussionappliesprincipallytotheheattreatmentofordinarycommercialsteelsknownasplaincarbonsteels.Withthisprocesstherateofcoolingisthecontrollingfactor,rapidcoolingfromabovethecriticalrangeresultsinhardstructure,whereasveryslowcoolingproducestheoppositeeffect.
Ifwefocusonlyonthematerialsnormallyknownassteels,asimplifieddiagramisoftenused.
Thoseportionsoftheiron-carbondiagramnearthedeltaregionandthoseabove2%carboncontentareoflittleimportancetotheengineerandaredeleted.Asimplifieddiagram,suchastheoneinFig.2.1,focusesontheeutectoidregionandisquiteusefulinunderstandingthepropertiesandprocessingofsteel.
Thekeytransitiondescribedinthisdiagramisthedecompositionofsingle-phaseaustenite(γ)tothetwo-phaseferritepluscarbidestructureastemperaturedrops.
Controlofthisreaction,whicharisesduetothedrasticallydifferentcarbonsolubilityofausteniteandferrite,enablesawiderangeofpropertiestobeachievedthroughheattreatment.
Tobegintounderstandtheseprocesses,considerasteeloftheeutectoidcomposition,0.77%carbon,beingslowcooledalonglinex-x’inFig.2.1.Attheuppertemperatures,onlyausteniteispresent,the0.77%carbonbeingdissolvedinsolidsolutionwiththeiron.Whenthesteelcoolsto727℃(1341℉),severalchangesoccursimultaneously.
TheironwantstochangefromtheFCCaustenitestructuretotheBCCferritestructure,buttheferritecanonlycontain0.02%carboninsolidsolution.
Therejectedcarbonformsthecarbon-richcementiteintermetallicwithcompositionFe3C.Inessence,thenetreactionattheeutectoidisaustenite0.77%C→ferrite0.02%C+cementite6.67%C.
Sincethischemicalseparationofthecarboncomponentoccursentirelyinthesolidstate,theresultingstructureisafinemechanicalmixtureofferriteandcementite.Specimenspreparedbypolishingandetchinginaweaksolutionofnitricacidandalcoholrevealthelamellarstructureofalternatingplatesthatformsonslowcooling.
Thisstructureiscomposedoftwodistinctphases,buthasitsownsetofcharacteristicpropertiesandgoesbythenamepearlite,becauseofitsresemblancetomother-of-pearlatlowmagnification.
Steelshavinglessthantheeutectoidamountofcarbon(lessthan0.77%)areknownashypo-eutectoidsteels.Considernowthetransformationofsuchamaterialrepresentedbycoolingalongliney-y’inFig.2.1.
Athightemperatures,thematerialisentirelyaustenite,butuponcoolingentersaregionwherethestablephasesareferriteandaustenite.Tie-lineandlevel-lawcalculationsshowthatlow-carbonferritenucleatesandgrows,leavingtheremainingaustenitericherincarbon.
At727℃(1341℉),theausteniteisofeutectoidcomposition(0.77%carbon)andfurthercoolingtransformstheremainingaustenitetopearlite.Theresultingstructureisamixtureofprimaryorpro-eutectoidferrite(ferritethatformedabovetheeutectoidreaction)andregionsofpearlite.
Hypereutectoidsteelsaresteelsthatcontaingreaterthantheeutectoidamountofcarbon.Whensuchsteelcools,asshowninz-z’ofFig.2.1theprocessissimilartothehypo-eutectoidcase,exceptthattheprimaryorpro-eutectoidphaseisnowcementiteinsteadofferrite.
Asthecarbon-richphaseforms,theremainingaustenitedecreasesincarboncontent,reachingtheeutectoidcompositionat727℃(1341℉).Asbefore,anyremainingaustenitetransformstopearliteuponslowcoolingthroughthistemperature.
Itshouldberememberedthatthetransitionsthathavebeendescribedbythephasediagramsareforequilibriumconditions,whichcanbeapproximatedbyslowcooling.Withslowheating,thesetransitionsoccurinthereversemanner.
However,whenalloysarecooledrapidly,entirelydifferentresultsmaybeobtained,becausesufficienttimeisnotprovidedforthenormalphasereactionstooccur,insuchcases,thephasediagramisnolongerausefultoolforengineeringanalysis.
Hardening
Hardeningistheprocessofheatingapieceofsteeltoatemperaturewithinoraboveitscriticalrangeandthencoolingitrapidly.
Ifthecarboncontentofthesteelisknown,thepropertemperaturetowhichthesteelshouldbeheatedmaybeobtainedbyreferencetotheiron-ironcarbidephasediagram.However,ifthecompositionofthesteelisunknown,alittlepreliminaryexperimentationmaybenecessarytodeterminetherange.
Agoodproceduretofollowistoheat-quenchanumberofsmallspecimensofthesteelatvarioustemperaturesandobservetheresult,eitherbyhardnesstestingorbymicroscopicexamination.Whenthecorrecttemperatureisobtained,therewillbeamarkedchangeinhardnessandotherproperties.
Inanyheat-treatingoperationtherateofheatingisimportant.Heatflowsfromtheexteriortotheinteriorofsteelatadefiniterate.Ifthesteelisheatedtoofast,theoutsidebecomeshotterthantheinterioranduniformstructurecannotbeobtained.
Ifapieceisirregularinshape,aslowrateisallthemoreessentialtoeliminatewarpingandcracking.Theheavierthesection,thelongermustbetheheatingtimetoachieveuniformresults.
Evenafterthecorrecttemperaturehasbeenreached,thepieceshouldbeheldatthattemperatureforasufficientperiodoftimetopermititsthickestsectiontoattainauniformtemperature.
Thehardnessobtainedfromagiventreatmentdependsonthequenchingrate,thecarboncontent,andtheworksize.Inalloysteelsthekindandamountofalloyingelementinfluencesonlythehardenability(theabilityoftheworkpiecetobehardenedtodepths)ofthesteelanddoesnotaffectthehardnessexceptinunhardenedorpartiallyhardenedsteels.
Steelwithlowcarboncontentwillnotrespondappreciablytohardeningtreatment.Asthecarboncontentinsteelincreasesuptoaround0.60%,thepossiblehardnessobtainablealsoincreases.
Abovethispointthehardnesscanbeincreasedonlyslightly,becausesteelsabovetheeutectoidpointaremadeupentirelyofpearliteandcementiteintheannealedstate.Pearliterespondsbesttoheat-treatingoperations;andsteelcomposedmostlyofpearlitecanbetransformedintoahardsteel.
Asthesizeofpartstobehardenedincreases,thesurfacehardnessdecreasessomewhateventhoughallotherconditionshaveremainedthesame.Thereisalimittotherateofheatflowthroughsteel.
Nomatterhowcoolthequenchingmediummaybe,iftheheatinsidealargepiececannotescapefasterthanacertaincriticalrate,thereisadefinitelimittotheinsidehardness.However,brineorwaterquenchingiscapableofrapidlybringingthesurfaceofthequenchedparttoitsowntemperatureandmaintainingitatorclosetothistemperature.
Underthesecircumstancestherewouldalwaysbesomefinitedepthofsurfacehardeningregardlessofsize.Thisisnottrueinoilquenching,whenthesurfacetemperaturemaybehighduringthecriticalstagesofquenching.
Tempering
Steelthathasbeenhardenedbyrapidquenchingisbrittleandnotsuitableformostuses.Bytemperingordrawing,thehardnessandbrittlenessmaybereducedtothedesiredpointforserviceconditions.
Asthesepropertiesarereducedthereisalsoadecreaseintensilestrengthandanincreaseintheductilityandtoughnessofthesteel.Theoperationconsistsofreheatingquench-hardenedsteeltosometemperaturebelowthecriticalrangefollowedbyanyrateofcooling.
Temperingispossiblebecauseoftheinstabilityofthemartensite,theprincipalconstituentofhardenedsteel.Low-temperaturedraws,from300℉to400℉(150℃~205℃),donotcausemuchdecreaseinhardnessandareusedprincipallytorelieveinternalstrains.
Althoughthisprocesssoftenssteel,itdiffersconsiderablyfromannealinginthattheprocesslendsitselftoclosecontrolofthephysicalpropertiesandinmostcasesdoesnotsoftenthesteeltotheextentthatannealingwould.Thefinalstructureobtainedfromtemperingafullyhardenedsteeliscalledtemperedmartensite.
Asthetemperingtemperaturesareincreased,thebreakdownofthemartensitetakesplaceatafasterrate,andatabout600℉(315℃)thechangetoastructurecalledtemperedmartensiteisveryrapid.Thetemperingoperationmaybedescribedasoneofprecipitationandagglomerationorcoalescenceofcementite.
Asubstantialprecipitationofcementitebeginsat600℉(315℃),whichproducesadecreaseinhardness.Increasingthetemperaturecausescoalescenceofthecarbideswithcontinueddecreaseinhardness.
Intheprocessoftempering,someconsiderationshouldbegiventotimeaswellastotemperature.Althoughmostofthesofteningac
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