机械专业英文翻译Word文档下载推荐.docx
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机械专业英文翻译Word文档下载推荐.docx
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Therefore,themainfactorsactinginheattreatmentaretemperatureandtime,sothatanyprocessofheattreatmentcanberepresentedintemperature—time(
)coordinates.
Heattreatmentconditionsarecharacterizedbythefollowingparameters:
heatingtemperature
i.e.themaximumtemperaturetowhichanalloymetalisheated;
timeofholdingattheheatingtemperature
;
heatingrate
andcoolingrate
.
Ifheating(orcooling)ismadeataconstantrate,thetemperature-timerelationshipwillbedescribedbyastraightlinewitharespectiveangleofincline.
Withavaryingheating(orcooling)rate,theactualrateshouldbeattributedtothegiventemperature,morestrictly,toaninfinitechangeoftemperatureandtime:
thatisthefirstderivativeoftemperatureintime:
Heattreatmentmaybeacomplexprocess,includingmultipleheatingstages,interruptedorstepwiseheating(cooling),coolingtosubzerotemperature,etc.Anyprocessofheattreatmentcanbedescribedbyadiagramintemperature-timecoordinates.
2、FormationofAustenite
ThetransformationofpearliteintoaustenitecanonlytakeplaceattheequilibriumcriticalpointonaveryslowheatingasfollowsfromtheFe-Cconstitutionaldiagram.Undercommonconditions,thetransformationisretardedandresultsinoverheating,i.e.occursattemperaturesslightlyhigherthanthoseindicatedintheFe-Cdiagram.
Whenoverheatedabovethecriticalpoint,pearlitetransformsintoaustenite,therateoftransformationbeingdependentonthedegreeofoverheating.
Thetimeoftransformationatvarioustemperatures(dependingonthedegreeofoverheating)showsthatthetransformationtakesplacefaster(inashortertime)atahighertemperatureandoccursatahighertemperatureonaquickerheating.
Forinstance,onquickheatingandholdingat780℃,thepearlitetoaustenitetransformationiscompletedin2minutesandonholdingat740℃,in8minutes.
Theendofthetransformationischaracterizedbytheformationofausteniteandthedisappearanceofpearlite(ferrite+cementite).Thisausteniteishoweverinhomogeneouseveninthevolumeofasinglegrain.Inplacesearlieroccupiedbylamellae(orgrains)ofapearlitecementite,thecontentofcarbonisgreaterthaninplacesofferriticlamellae.Thisiswhytheaustenitejustformedisinhomogeneous.
Inordertoobtainhomogeneousaustenite,itisessentialonheatingnotonlytopassthroughthepointoftheendofpearlitetoaustenitetransformation,butalsotooverheatthesteelabovethatpointandtoallowaholdingtimetocompletethediffusionprocessesinausteniticgrains.
Therateofhomogenizationofausteniteappreciablydependsontheoriginalstructureofthesteel,inparticularonthedispersionandparticleshapeofcementite.Thetransformationsdescribedoccurmorequicklywhencementiteparticlesarefineand,therefore,havealargetotalsurfacearea.
3、CoarseningofAusteniteGrains
Atthebeginningofpearlitetoaustenitetransformation,thegrainsofausteniteformattheboundariesbetweentheferriteandcementite——thetwostructuralconstituentsofpearlite.Sincetheseboundariesareverydeveloped,thetransformationstartsfromformationofamultitudeoffinegrains.Thereforeattheendofthetransformationtheaustenitewillbecomposedofagreatmultitudeoffinegrainswhosesizecharacterizewhatiscalledtheoriginalausteniticgrainssize.
Furtherheating(orholding)uponthetransformationwillcausecoarseningofausteniticgrains.Theprocessofgraincoarseningisspontaneous,sincethetotalsurfaceareaofgrainsdiminishes(thesurfaceenergydecreases)andahightemperaturecanonlyacceleratetherateofthisprocess.
Inthatconnection,twotypesofsteelsaredistinguished:
inherentfinegrainedandinherentcoarsegrained,theformerbeinglessliabletograincoarseningthanthelatter.Thesizeofgrainsformedinasteelbyheattreatmentiscalledtheactualgrainsize.
Thus,adistinctionshouldbemadebetween:
(1)originalgrain,i.e.thesizeofausteniticgrainsimmediatelyafterthepearlitetoaustenitetransformation;
(2)inherent(natural)grain,i.e.theliabilityofaustenitetograincoarsening;
and(3)actualgrain,i.e.thesizeofausteniticgrainsundergivenparticularconditions.
Thesizeofpearliticgrainsatthesametemperatureoftheaustenitetopearlitetransformationdependsonthatoftheausteniticgrainsfromwhichtheyhaveformed.Austeniticgrainsgrowonlyduringheating(butarenotrefinedinsubsequentcooling),becauseofwhichthehighesttemperatureasteelisheatedtointheausteniticstateandtheinherentgrainsizeofthatsteeldeterminethefinalgrainsize.
Thepropertiesofsteelareaffectedonlybytheactualgrainsizeandnotbytheinherentgrainsize.Iftwosteelsofthesamegrade(oneinherentcoarsegrained,theotherfinegrained)havethesameactualgrainsizeuponheattreatmentatdifferenttemperatures,theirpropertieswillalsobethesame;
ifotherwise,andmanypropertiesofthetwosteelswillalsobedifferent.
4、DecompositionofAustenite
Theaustenitetopearlitetransformationisessentiallythedecompositionofausteniteintoalmostpureferriteandcementite.
Attheequilibriumtemperature,thetransformationisimpossible,sincethefreeenergyoftheoriginalausteniteisequaltothatofthefinalproduct,pearlite.Thetransformationcanonlystartatacertainundercoolingwhenthefreeenergyoftheferritecarbidemixturehigherthedegreeofundercoolingandthegreaterthedifferenceinfreeenergiesandthetransformationproceedsatahigherrate.
Inthepearlitetransformation,thenewphasessharplydifferintheircompositionfromtheinitialphase;
theyareferritewhichisalmostfreeofcarbon,andcementitewhichcontains6.67percentcarbon.Forthisreasontheaustenitetopearlitetransformationisaccompaniedwiththetemperature,redistributionofcarbon.Therateofdiffusionsharplydiminisheswithdecreasingtemperature;
therefore,thetransformationshouldberetardedatagreaterundercooling.
Thus,wehavecometoanimportantconclusionthatundercooling(loweringthetransformationtemperature)mayhavetwooppositeeffectsontherateoftransformation.Ononeausteniteandpearlite,thusacceleratingthetransformation;
ontheotherhand,diminishedtherateofcarbondiffusion,andthusslowsdownthetransformation.Thecombinedeffectisthattherateoftransformationfirstincreasesasundercoolingisincreasedtoacertainmaximumandthendecreaseswithfurtherundercooling.
At270℃(A1)andbelow200℃,therateoftransformationiszero,sinceat727℃thefreeenergydifferenceiszeroandbelow200℃therateofcarbondiffusioniszero(morestrictly,toolowforthetransformationtoproceed).
AshasbeenfirstindicatedbyI.L.Mirkinin1939anddevelopedbyR.F.Mehlin1941,theformationofpearliteistheprocessofnucleationofpearliteandgrowthofpearliticcrystals.Therefore,thedifferentrateofthepearlitetransformationatvariousdegreesofundercoolingisduetothefactthatundercoolingdifferentlyaffectstheratenucleationNandtherateofcrystalsgrowthG.attemperatureA1andbelow200℃,bothparametersofcrystallizationNandGareequaltozeroandhaveamaximumatanundercoolingof150~200℃.
Itfollowsfromtheforegoingthatassoonastheconditionsarefavorable,i.e.austeniteisundercoolingbelowA1,thediffusionofcarbonisnotzero,andcentersofcrystallizationappearwhichgiverisetocrystal.Thisprocessoccurswithtimeandcanberepresentedintheformofsocalledkineticcurveoftransformation,whichshowsthequantityofpearlitethathasformedduringthetimeelapsedfromthebeginningofthetransformation.
Theinitialstageischaracterizedbyaverylowrateoftransformation;
thisiswhatiscalledtheincubationperiod.Therateoftransformationincreaseswiththeprogressinthetransformation.Itsmaximumapproximatelycorrespondstothemomentwhenroughly50percentofaustenitehastransformedintopearlite.Therateoftransformationthendiminishesandfinallystops.
Therateoftransformationdependsonundercooling.Atlowandhighdegreesofundercoolingthetransformationproceedsslowly,sinceNandGarelow;
intheformercase,owingtoalowdifferenceinfreeenergy,andinthelater,duetoalowdiffusionmobilityofatoms.Atthemaximumrateoftransformationthekineticcurveshavesharppeaks,andthetransformationisfinishedinashorttimeinterval.
Atahightemperature(slightlyundercooling),thetransformationproceedsslowlyandtheincubationperiodandthetimeofthetransformationproperarelong.Atalowertemperatureofthetransformation,i.e.adeeperundercooling,therateoftransformationisgreater,andthetimeoftheincubationperiodandofthetransformationisshorter.
5、TTTDiagramorC-Curve
Havingdeterminedthetimeofthebeginningofaustenitictopearlitetransformation(incubationperiod)andthetimeoftheendoftransformationatvariousdegreesofundercooling,wecanconstructadiagraminwhichthelefthandcurvedeterminesthetimeofthebeginningoftransformation,i.e.thetimeduringwhichaustenitestillexistsintheundercoolingstate,andthesectionfromtheaxisofordinatestothecurveismeasureofitsstab
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