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外文翻译--奥氏体的分解.doc

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外文翻译--奥氏体的分解.doc

DecompositionofAusteniteTheaustenitetopearlitetransformationisessentiallythedecompositionofausteniteintoalmostpureferriteandcementite.Attheequilibriumtemperature,thetransformationisimpossible,sincethefreeenergyoftheoriginalausteniteisequaltothatofthefinalproduct,pearlite.Thetransformationcanonlystartatacertainundercoolingwhenthefreeenergyoftheferritecarbidemixturepearliteislowerthanthatofaustenite.Thelowerthetransformationtemperature,thehigherthedegreeofundercoolingandthegreaterthedifferenceinfreeenergiesandthetransformationproceedsatahigherrat.Inthepearlitetransformation,thenewphasessharplydifferintheircompositionfromtheinitialphasetheyareferritewhichisalmostfreeofcarbon,andcementitewhichcontains6.67percentcarbon.Forthatreasontheaustenitetopearlitetransformationisaccompaniedwiththediffusion,redistributionofcarbon.Therateofdiffusionsharplydiminisheswithdecreasingtemperature,therefore,thetransformationshouldberetardedatagreaterundercooling.Thus,wehavecometoanimportantconclusionthatundercoolingloweringthetransformationtemperaturemayhavetwooppositeeffectsontherateoftransformation.Ononehand,alowertemperaturegreaterundercoolinggivesagreaterdifferenceinfreeenergiesofausteniteandpearlite,thusacceleratingthetransformationontheotherhand,itdiminishestherateofcarbondiffusion,andthusslowsdownthetransformation.Thecombinedeffectisthattherateoftransformationfirstincreasesasundercoolingisincreasedtoacertainmaximumandthendecreaseswithfurtherundercooling.At727℃A1andbelow200℃,therateoftransformationiszero,sinceat727℃thefreeenergydifferenceiszeroandbelow200℃therateofcarbondiffusioniszeromorestrictly,toolowforthetransformationtoproceed.AshasbeenfirstindicatedbyI.L.Mirkinin1939andthendevelopedbyR.F.Mehlin1941,theformationofpearliteistheprocessofnucleationofpearliteandgrowthofpearlitecrystals.Therefore,thedifferentrateofthepearlitetransformationatvariousdegreesofundercoolingisduetothefactthatundercoolingdifferentlyaffectstherateofnucleationNandtherateofcrystalgrowthG.AtemperatureA1andbelow200℃,bothparametersofcrystallizationNandGareequaltozeroandhaveamaximumatanundercoolingof150~200℃.Itfollowsfromtheforegoingthatassoonastheconditionsarefavorable,I.e.austeniteisundercooledbelowA1,thediffusionofcarbonisnotzero,centersofcrystallizationappear,whichgiverisetocrystals.thisprocessoccurswithtimeandcanberepresentedintheformofsocalledkineticcurveoftransformation,whichshowsthequantityofpearlitethathasformedduringthetimeelapsedfromthebeginningofthetransformation.Theinitialstageischaracterizedbyaverylowrateoftransformationthisiswhatiscalledtheincubationperiod.Therateoftransformationincreaseswiththeprogressinthetransformation.Itsmaximumapproximatelycorrespondstothemomentwhenrougly50percentofaustenitehastransformedintopearlite.Therateoftransformationthendiminishesandfinallystops.Therateoftransformationdependsonundercoolingthetransformationproceedsslowly,sinceNorGarelowintheformercase,owingtoalowdifferenceinfreeenergies,andinthekineticcurveshavesharppeaks,andthetransformationisfinishedinashorttimeinterval.Atahightemperatureslightundercooling,thetransformationproceedsslowlyandthetimeoftheincubationperiodandthetimeofthetransformationproperarelong.Atalowertemperatureofthetransformation,i.e.adeeperundercooling,therateoftransformationisgreater,andthetimeoftheincubationperiodandofthetransformationisshorter.Havingdeterminedthetimeofthebeginningofaustenitetopearlitetransformationincubationperiodandthetimeoftheendoftransformationatvariousdegreesofundercooling,wecanconstructadiagraminwhichthelefthandcurvedeterminesthetimeofthebeginningofthetransformation,i.e.thetimeduringwhichaustenitestillexistsintheundercooledstate,andthesectionfromtheaxisofordinatestothecurveisthemeasureofitsstability.Thissectionisshortestatatemperatureof500~600℃,i.e.thetransformationbeginsinshortesttimeatthattemperature.Therighthandcurveshowsthetimeneededtocompletethetransformationatagivendegreeofundercooling.Thistimeistheshortestatthesametemperature500~600℃.Notethattheabscissaofthediagramislogarithmic.Thisisdoneformoreconvenience,sincetheratesofformationofpearliteappreciablydifferthousandsofsecondsnearthecriticalpointA1andonlyoneortwosecondsatthebendofthecurve.Thehorizontallinebelowthecurvesinthediagramdeterminesthetemperatureofthediffusionlessmartensitetransformation.Themartensitetransformationoccursbyadifferentmechanismandwillbediscussedlater.DiagramsofthetypewediscussedareusuallycalledTTTdiagramstimetemperaturetransformation,orCcurves,owingtothespecificshapeofthecurves.Thestructureandpropertiesoftheproductsofaustenitedecompositiondependonthetemperatureatwhichthetransformationhastakenplaces.Athightemperature,i.e.lowdegreesofundercooling,acoarsegrainedmixtureofferriteandcementiteisformedwhichiseasilydistinguishedinthemicroscope.Thisstructureiscalledpearlite.Atlowertemperatures,andtherefore,greaterdegreesofundercooling,moredisperseandharderproductsareformed.Thepearlitestructureofthisfinertypeiscalledsorbite.AtstilllowertemperatureneartheendoftheCcurve,thetransformationproductsareevenmoredisperse,sothatthelamellarstructureoftheferritecementitemixtureisonlydistinguishableintheelectronmicroscope.Thisstructureiscalledtroostite.Thus,pearlite,sorbiteandtroostitearethestructuresofthesamenatureferritecementitebutadifferentdispersityofferriteandcementite.Pearlitestructuremaybeoftwotypesgranularinwhichcementiteispresentintheformofgrainsorlamellarwithcementiteplatelets.Homogeneousaustenitealwaystransformsintolamellarpearlite.Therefore,heatingtohightemperaturesetsupfavorableconditionsfortheformationofamorehomogeneousstructureandthuspromotestheappearanceoflamellarstructures.Inhomogeneousausteniteproducesgranularpearliteatalldegreesofundercooling,therefore,heatingtoalowtemperaturebelowAC3forhypereutectoidsteelsresultsintheformationofgranularpearliteoncooling.Theformationofgranularcementiteisprobablypromotedbythepresenceofundissolvedparticlesinaustenite,whichserveasadditionalcrystallizationnuclei.奥氏体的分解奥氏体向珠光体的转变本质上是奥氏体分解成纯净的铁素体和渗碳体。在平衡温度时,转变是不可能进行的,因为最初的奥氏体的自由能和最后产品,珠光体的自由能是相等的。当铁素体渗碳体混合物(珠光体)的自由能比奥氏体的低时,转变才能在一个特定的过冷下开始。较低的转变温度,较高程度的过冷度,并且较大的在较高的速度时的自由能和转变效益中的差异。在珠光体转变过程中,新形成相的成分明显不同于初相新相为含碳量几乎没有的铁素体和含碳量为6.67的渗碳体。因此对珠光体转变的奥氏体是伴随着碳原子的扩散和再分布进行的。随着温度的增加,扩散的速度明显地降低,因此,这个转变应该在较大的过冷下缓慢进行。因此,我们可以得到一个重要结论即过冷(降低转变温度)也许对转变速度上有两个迥然不同的效果。一方面,较低的温度(较大的过冷)给奥氏体和珠光体的自由能提供一种较大的差异,因而加速了转变另一方面,它减少了谈原子扩散的速度,因而减慢了转变。这种组合的效果是转变的速度首先增加,当过冷增加到某一个最大值然后减少与进一步过冷。温度在727℃(A1)和200℃以下时,转变的速度为零,因为哉727℃时自由能的差异为零和在200℃以下时碳原子的扩散速度为零(更严格地说,太低为了转变能进行)。在1939年首先由I.L.Mirkin提出的,然后在1941年由R.F.Mehl开发,

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