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1、DecompositionofAustenite粮禾瓞暴璞题槊缴邋弘病Theaustenitetopearlitetransformationisessentiallythedecompositionofausteniteintoalmostpureferriteandcementite.啖跛钦阮捞弓淫术恼摈巢Attheequilibriumtemperature,thetransformationisimpossible,sincethefreeenergyoftheoriginalausteniteisequal
2、tothatofthefinalproduct,pearlite.剩娲蜀纾瑭磉伎联粮撂啊Thetransformationcanonlystartatacertainundercoolingwhenthefreeenergyoftheferritecarbidemixture(pearlite)islowerthanthatofaustenite.钞锤圄绸崔裱鞴愀殪其朔Thelowerthetransformationtemperature,thehigherthedegreeofundercoolingandthe
3、greaterthedifferenceinfreeenergiesandthetransformationproceedsatahigherrat.喁枚濒群斯派圃弘亩议睡Inthepearlitetransformation,thenewphasessharplydifferintheircompositionfromtheinitialphase;theyareferritewhichisalmostfreeofcarbon,andcementitewhichcontains6.67percentcarbon.F
4、orthatreasontheaustenitetopearlitetransformationisaccompaniedwiththediffusion,redistributionofcarbon.Therateofdiffusionsharplydiminisheswithdecreasingtemperature,therefore,thetransformationshouldberetardedatagreaterundercooling.莶癸锶涸嗔帷犏纩揪躇甓Thus,wehavecometoanimp
5、ortantconclusionthatundercooling(loweringthetransformationtemperature)mayhavetwooppositeeffectsontherateoftransformation.熔孜凶桑鲁沛做季枚坏烀Ononehand,alowertemperature(greaterundercooling)givesagreaterdifferenceinfreeenergiesofausteniteandpearlite,thusacceleratingthetr
6、ansformation;ontheotherhand,itdiminishestherateofcarbondiffusion,andthusslowsdownthetransformation.Thecombinedeffectisthattherateoftransformationfirstincreasesasundercoolingisincreasedtoacertainmaximumandthendecreaseswithfurtherundercooling.冖恩糁媚盗茑咭眦莸区淳At727℃(A1
7、)andbelow200℃,therateoftransformationiszero,sinceat727℃thefreeenergydifferenceiszeroandbelow200℃therateofcarbondiffusioniszero(morestrictly,toolowforthetransformationtoproceed).南郯女堂莺薮瞠阒镅岐怩AshasbeenfirstindicatedbyI.L.Mirkinin1939andthendevelopedbyR.F.Mehlin1941
8、,theformationofpearliteistheprocessofnucleationofpearliteandgrowthofpearlitecrystals.看胎携糙铺涧鸵诸祥笏檗Therefore,thedifferentrateofthepearlitetransformationatvariousdegreesofunderc
