外文翻译材料的热处理.docx

外文翻译材料的热处理.docx

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外文翻译材料的热处理

外文资料

HEATTREATMENTOFMETALS

Theunderstandingofheattreatmentisembracebythebroaderstudyofmetallurgy.Metallurgyisthephysics,chemistry,andengineeringrelatedtometalsfromoreextractiontothefinalproduct.Heattreatmentistheoperationdoheatingandcoolingametalinitssolidstatetochangeitsphysicalproperties.Accordingtotheprocedureused,steelcanbehardenedtoresistcuttingactionandabrasion,oritcanbesoftenedtopermitmachining.Withtheproperheattreatmentinternalductileinterior.Theanalysisofthesteelmustbeknownbecausesmallpercentagesofcertainelements,notablycarbon,greatlyaffectthephysicalproperties.

Alloysteelsowetheirpropertiestothepresenceofoneormoreelementsotherthancarbon,namelynickel,chromium,manganese,molybdenum,tungsten,silicon,vanadium,andcopper.Becauseoftheirimprovedphysicalpropertiestheyareusedcommerciallyinmanywaysnotpossiblewithcarbonsteels.

Thefollowingdiscussionappliesprincipallytotheheattreatmentofordinarycommercialsteelknownasplain-carbonsteels.Withthisprovestherateofcoolingisthecontrollingfactor,producestheoppositeeffect.

ASIMPLIFIEDIRON-CARBONDAGRAM

Ifwefocusonlyonthematerialsnormallyknownassteels,asimplifieddiagramisoftenused.Thoseportionsoftheiron-carbondiagramnearthedeltaregionandthoseabove2%carboncontentareoflittleimportancetotheengineerandaredeleted.Asimplifieddiagram,suchastheoneinFig.2.1focusesontheeutectoidregionandisquiteusefulinunderstandingthepropertiesandprocessingofsteel.

Thekeytransitiondescribedinthisdiagramisthedecompositionofsingle-phaseaustenite（）tothetwo-phaseferritepluscarbidestructureastemperaturedrop.Controlofthisreaction,whicharisesduetothedrasticallydifferentcarbonsolubilitiesofausteniteandferrite,enablesawiderangeofpropertiestobeachievedthroughheattreatment.

Tobegintounderstandtheseprocesses,considerssteeloftheeutectoidcomposition,0.77%carbon,beingslowcooledalonglineinFig.2.1Attheuppertemperatures,onlyausteniteispresent,the0.77%carbonbeingdissolvedinsolidsolutionwiththeiron.Whenthesteelcoolsto727,severalchangesoccursimultaneously.TheironwantstochangefromthebccaustenitestructuretothebccferriteStructure,buttheferritesanonlycontain0.02%carboninsolidsolution.Therejectedcarbonformsthecarbon-richcementiteintermetallicwithcomposition.Inessence,thenetreactionattheeutectoidis:

Austeniteferrite+cementite

Sincethischemicalseparationofthecarboncomponentoccursentirelyinthesolidstate,theresultingstructureisafinemechanicalmixtureofferriteandcementite.Specimentspreparedbyplolishingandetchinginaweaksolutionlfnitricacidandalcoholrevealthelamellarstructurelfalternatingplatesthatformsonslowcooling.Thisstructureiscomposedoftwodistinctphases,buthasitsownsetofcharacteristicpropertiesandgoesbythenamepearlite,becauseofitsresemblancetomother-of-pearlatlowmagnification.

Steelshavinglessthantheeutectoidamountofcarbon（lessthan0.77%）areknownashypoeutectoidsteels.Considernowthetransformationofsuchamaterialrepresentedbycoolingalongliney-y′inFig.2.1.Athightemperatures,thematerialisentrirelyaustenite,butuponcoolingentersaregionwherethestablephasesareferriteandaustenite.Tie-lineandlever-lawcalculationsshowthatlow-carbonferritenucleatesandgrows,leavingtheremainingaustenitericherincarbon.At727°C（1341°F）,theausteniteisofeutectoidcompositon（0.77%carbon）andfurthercoolingtransformstheremainingaustenitetopearlite.Theresultingstructureisamixturelfprimaryorproeutectoidferrite（ferritethatformedabovetheeutectoidreaction）andregionsofpearlite.

Hypereutectoidsteelsaresteelsthatcontaingreaterthantheeutectoidamountofcarbon.Whensuchasteelcools,asinz-z′ofFig.2.1theprocessissimilartothehypoeutectoidcase,exceptthattheprimaryorproeutectoidphaseisnowcementiteinsteadlfferrite.Asthecarbon-richphaseforms,theremainingaustenitedecreasesincarboncontent,reachingtheeutectoidcompositionat727°C（1341°F）.Asbefore,anyremainingaustenitetransformstopearliteuponslowcoolingthroughthistemperature.

Itshouldberememberedthatthetransitionsthathavebeendescribedbythephasediagramsareforequilibriumconditions,whichcanbeapproximatedbyslowcooling,Withslowheating,thesetransitionsoccurintherevertsemanner.However,whenalloysarecooledrapidly,entirelydifferentresultsmaybeobtained,becausesufficienttimeisnotprovidedforthenormalphasereactionstooccur,Insuchcases,thephasediagramisnolongerausefultoolfor