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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

DECOMPOSITIONOFAUSTENITETHEAUSTENITETOPEARLITETRANSFORMATIONISESSENTIALLYTHEDECOMPOSITIONOFAUSTENITEINTOALMOSTPUREFERRITEANDCEMENTITEATTHEEQUILIBRIUMTEMPERATURE,THETRANSFORMATIONISIMPOSSIBLE,SINCETHEFREEENERGYOFTHEORIGINALAUSTENITEISEQUALTOTHATOFTHEFINALPRODUCT,PEARLITETHETRANSFORMATIONCANONLYSTARTATACERTAINUNDERCOOLINGWHENTHEFREEENERGYOFTHEFERRITECARBIDEMIXTUREPEARLITEISLOWERTHANTHATOFAUSTENITETHELOWERTHETRANSFORMATIONTEMPERATURE,THEHIGHERTHEDEGREEOFUNDERCOOLINGANDTHEGREATERTHEDIFFERENCEINFREEENERGIESANDTHETRANSFORMATIONPROCEEDSATAHIGHERRATINTHEPEARLITETRANSFORMATION,THENEWPHASESSHARPLYDIFFERINTHEIRCOMPOSITIONFROMTHEINITIALPHASE;THEYAREFERRITEWHICHISALMOSTFREEOFCARBON,ANDCEMENTITEWHICHCONTAINS667PERCENTCARBONFORTHATREASONTHEAUSTENITETOPEARLITETRANSFORMATIONISACCOMPANIEDWITHTHEDIFFUSION,REDISTRIBUTIONOFCARBONTHERATEOFDIFFUSIONSHARPLYDIMINISHESWITHDECREASINGTEMPERATURE,THEREFORE,THETRANSFORMATIONSHOULDBERETARDEDATAGREATERUNDERCOOLINGTHUS,WEHAVECOMETOANIMPORTANTCONCLUSIONTHATUNDERCOOLINGLOWERINGTHETRANSFORMATIONTEMPERATUREMAYHAVETWOOPPOSITEEFFECTSONTHERATEOFTRANSFORMATIONONONEHAND,ALOWERTEMPERATUREGREATERUNDERCOOLINGGIVESAGREATERDIFFERENCEINFREEENERGIESOFAUSTENITEANDPEARLITE,THUSACCELERATINGTHETRANSFORMATION;ONTHEOTHERHAND,ITDIMINISHESTHERATEOFCARBONDIFFUSION,ANDTHUSSLOWSDOWNTHETRANSFORMATIONTHECOMBINEDEFFECTISTHATTHERATEOFTRANSFORMATIONFIRSTINCREASESASUNDERCOOLINGISINCREASEDTOACERTAINMAXIMUMANDTHENDECREASESWITHFURTHERUNDERCOOLINGAT727℃A1ANDBELOW200℃,THERATEOFTRANSFORMATIONISZERO,SINCEAT727℃THEFREEENERGYDIFFERENCEISZEROANDBELOW200℃THERATEOFCARBONDIFFUSIONISZEROMORESTRICTLY,TOOLOWFORTHETRANSFORMATIONTOPROCEEDASHASBEENFIRSTINDICATEDBYILMIRKININ1939ANDTHENDEVELOPEDBYRFMEHLIN1941,THEFORMATIONOFPEARLITEISTHEPROCESSOFNUCLEATIONOFPEARLITEANDGROWTHOFPEARLITECRYSTALSTHEREFORE,THEDIFFERENTRATEOFTHEPEARLITETRANSFORMATIONATVARIOUSDEGREESOFUNDERCOOLINGISDUETOTHEFACTTHATUNDERCOOLINGDIFFERENTLYAFFECTSTHERATEOFNUCLEATIONNANDTHERATEOFCRYSTALGROWTHGATEMPERATUREA1ANDBELOW200℃,BOTHPARAMETERSOFCRYSTALLIZATIONNANDGAREEQUALTOZEROANDHAVEAMAXIMUMATANUNDERCOOLINGOF150~200℃ITFOLLOWSFROMTHEFOREGOINGTHATASSOONASTHECONDITIONSAREFAVORABLE,IEAUSTENITEISUNDERCOOLEDBELOWA1,THEDIFFUSIONOFCARBONISNOTZERO,CENTERSOFCRYSTALLIZATIONAPPEAR,WHICHGIVERISETOCRYSTALSTHISPROCESSOCCURSWITHTIMEANDCANBEREPRESENTEDINTHEFORMOFSOCALLEDKINETICCURVEOFTRANSFORMATION,WHICHSHOWSTHEQUANTITYOFPEARLITETHATHASFORMEDDURINGTHETIMEELAPSEDFROMTHEBEGINNINGOFTHETRANSFORMATIONTHEINITIALSTAGEISCHARACTERIZEDBYAVERYLOWRATEOFTRANSFORMATION;THISISWHATISCALLEDTHEINCUBATIONPERIODTHERATEOFTRANSFORMATIONINCREASESWITHTHEPROGRESSINTHETRANSFORMATIONITSMAXIMUMAPPROXIMATELYCORRESPONDSTOTHEMOMENTWHENROUGLY50PERCENTOFAUSTENITEHASTRANSFORMEDINTOPEARLITETHERATEOFTRANSFORMATIONTHENDIMINISHESANDFINALLYSTOPSTHERATEOFTRANSFORMATIONDEPENDSONUNDERCOOLINGTHETRANSFORMATIONPROCEEDSSLOWLY,SINCENORGARELOW;INTHEFORMERCASE,OWINGTOALOWDIFFERENCEINFREEENERGIES,ANDINTHEKINETICCURVESHAVESHARPPEAKS,ANDTHETRANSFORMATIONISFINISHEDINASHORTTIMEINTERVALATAHIGHTEMPERATURESLIGHTUNDERCOOLING,THETRANSFORMATIONPROCEEDSSLOWLYANDTHETIMEOFTHEINCUBATIONPERIODANDTHETIMEOFTHETRANSFORMATIONPROPERARELONGATALOWERTEMPERATUREOFTHETRANSFORMATION,IEADEEPERUNDERCOOLING,THERATEOFTRANSFORMATIONISGREATER,ANDTHETIMEOFTHEINCUBATIONPERIODANDOFTHETRANSFORMATIONISSHORTERHAVINGDETERMINEDTHETIMEOFTHEBEGINNINGOFAUSTENITETOPEARLITETRANSFORMATIONINCUBATIONPERIODANDTHETIMEOFTHEENDOFTRANSFORMATIONATVARIOUSDEGREESOFUNDERCOOLING,WECANCONSTRUCTADIAGRAMINWHICHTHELEFTHANDCURVEDETERMINESTHETIMEOFTHEBEGINNINGOFTHETRANSFORMATION,IETHETIMEDURINGWHICHAUSTENITESTILLEXISTSINTHEUNDERCOOLEDSTATE,ANDTHESECTIONFROMTHEAXISOFORDINATESTOTHECURVEISTHEMEASUREOFITSSTABILITYTHISSECTIONISSHORTESTATATEMPERATUREOF500~600℃,IETHETRANSFORMATIONBEGINSINSHORTESTTIMEATTHATTEMPERATURETHERIGHTHANDCURVESHOWSTHETIMENEEDEDTOCOMPLETETHETRANSFORMATIONATAGIVENDEGREEOFUNDERCOOLINGTHISTIMEISTHESHORTESTATTHESAMETEMPERATURE500~600℃NOTETHATTHEABSCISSAOFTHEDIAGRAMISLOGARITHMICTHISISDONEFORMORECONVENIENCE,SINCETHERATESOFFORMATIONOFPEARLITEAPPRECIABLYDIFFERTHOUSANDSOFSECONDSNEARTHECRITICALPOINTA1ANDONLYONEORTWOSECONDSATTHEBENDOFTHECURVETHEHORIZONTALLINEBELOWTHECURVESINTHEDIAGRAMDETERMINESTHETEMPERATUREOFTHEDIFFUSIONLESSMARTENSITETRANSFORMATIONTHEMARTENSITETRANSFORMATIONOCCURSBYADIFFERENTMECHANISMANDWILLBEDISCUSSEDLATERDIAGRAMSOFTHETYPEWEDISCUSSEDAREUSUALLYCALLEDTTTDIAGRAMSTIMETEMPERATURETRANSFORMATION,ORCCURVES,OWINGTOTHESPECIFICSHAPEOFTHECURVESTHESTRUCTUREANDPROPERTIESOFTHEPRODUCTSOFAUSTENITEDECOMPOSITIONDEPENDONTHETEMPERATUREATWHICHTHETRANSFORMATIONHASTAKENPLACESATHIGHTEMPERATURE,IELOWDEGREESOFUNDERCOOLING,ACOARSEGRAINEDMIXTUREOFFERRITEANDCEMENTITEISFORMEDWHICHISEASILYDISTINGUISHEDINTHEMICROSCOPETHISSTRUCTUREISCALLEDPEARLITEATLOWERTEMPERATURES,ANDTHEREFORE,GREATERDEGREESOFUNDERCOOLING,MOREDISPERSEANDHARDERPRODUCTSAREFORMEDTHEPEARLITESTRUCTUREOFTHISFINERTYPEISCALLEDSORBITEATSTILLLOWERTEMPERATURENEARTHEENDOFTHECCURVE,THETRANSFORMATIONPRODUCTSAREEVENMOREDISPERSE,SOTHATTHELAMELLARSTRUCTUREOFTHEFERRITECEMENTITEMIXTUREISONLYDISTINGUISHABLEINTHEELECTRONMICROSCOPETHISSTRUCTUREISCALLEDTROOSTITETHUS,PEARLITE,SORBITEANDTROOSTITEARETHESTRUCTURESOFTHESAMENATUREFERRITECEMENTITEBUTADIFFERENTDISPERSITYOFFERRITEANDCEMENTITEPEARLITESTRUCTUREMAYBEOFTWOTYPESGRANULARINWHICHCEMENTITEISPRESENTINTHEFORMOFGRAINSORLAMELLARWITHCEMENTITEPLATELETSHOMOGENEOUSAUSTENITEALWAYSTRANSFORMSINTOLAMELLARPEARLITETHEREFORE,HEATINGTOHIGHTEMPERATURESETSUPFAVORABLECONDITIONSFORTHEFORMATIONOFAMOREHOMOGENEOUSSTRUCTUREANDTHUSPROMOTESTHEAPPEARANCEOFLAMELLARSTRUCTURESINHOMOGENEOUSAUSTENITEPRODUCESGRANULARPEARLITEATALLDEGREESOFUNDERCOOLING,THEREFORE,HEATINGTOALOWTEMPERATUREBELOWAC3FORHYPEREUTECTOIDSTEELSRESULTSINTHEFORMATIONOFGRANULARPEARLITEONCOOLINGTHEFORMATIONOFGRANULARCEMENTITEISPROBABLYPROMOTEDBYTHEPRESENCEOFUNDISSOLVEDPARTICLESINAUSTENITE,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667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ILMIRKIN提出的,然后在1941年由RFMEHL开发,

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