機(jī)電專(zhuān)業(yè)英語(yǔ)Part1-Material-and課件

Part1MaterialandHeatTreatmentPart1

Unit1Metals

Unit2HeatTreatment

Unit3Iron-CarbonEquilibriumDiagramReadingTargetUnit1MetalsUnit2HeatTreaUnit1Metals

ReadingTarget1．Introducethecategoriesofmetalsandhowtodistinguishthem.2．Makeyouknowwhatmechanicalpropertiesmetalshaveandthedefinitionabouteachproperty.Unit1Metals

ReadingTargetText

Aswehaveseen,ferrousmetalsarealloysofironwithcarbon,thesealloysmaycontainalsosomeotherelementssuchassilicon(Si),phosphorus(P),etc.,butcarbonisthemostimportantofallelementspresentinferrousalloys[1].Text

Aswehaveseen,ferrousFerrousmetalsareusedinindustryintwogeneralforms:castironandsteel.Thesetwoferrousalloysareusuallyproducedfrompigiron,andtheyhavedifferentcarboncontent.Steelisironcontainingfrom0.0218to2.11percentcarbon,whilecastironisanalloyofironandcarbonwiththecarboncontentmorethan2.11percent[2].FerrousmetalsareusedinindPureironisnotusedinindustrybecauseitistoosoft.Steelisanalloyofironandcarbonwithotherelementsaddedtoproducespecificproperties[3].PureironisnotusedinindusThevarioustypesofsteelcanbegroupedundertwomajorheadings:（1）Carbonsteel.Steelinwhichthemainalloyingelementiscarbon.Carbonsteelsarefurtherdividedintothreegroups.Thevarioustypesofsteelcana．Lowcarbonsteel.Thissteelhasacarboncontentoflessthan0.30percent.Itisthemostcommontypeandisoftencalledmildsteel.Itisrelativelyinexpensive,ductile,soft,andiseasilymachinedandforged.Mildsteelcannotbeheat-treated(hardened).Lowcarbonsteelisageneralpurposesteel.a．Lowcarbonsteel.Thissteelb．Mediumcarbonsteel.Thissteelhasacarboncontentbetween0.30percentand0.80percent.Harderandstrongerthanmildsteel,itcanbehardenedbyheattreating.Mediumcarbonsteelismostcommonlyusedforforgings,castings,andmachinedpartsforautomobiles,agriculturalequipment,machines,andaircraft.b．Mediumcarbonsteel.Thisstc．Highcarbonsteel.Thistypeofsteeliseasilyheat-treatedtoproduceastrong,toughpart.Thematerialhasacarboncontentabove0.80percent.Itfindswideuseinhandtools,cuttingtools,springs,andpianowire.c．Highcarbonsteel.Thistype（2）Highalloysteel.Thesesteelscontainsignificantamountsofotherelementsinadditiontocarbon.Thecommonhighalloysteelsare:a．Stainlesssteelwhichisproducedbyusingchromiumasasignificantalloyingelementalongwithnickelandothermetals.Theresultisatough,hard,corrosion-resistantmetal.（2）Highalloysteel.Thesesteb．Toolsteelwhichisaspecialgroupofhighcarbonsteelsproducedinsmallquantitiestohighqualityspecifications.Toolsteelsareusedforawiderangeofcuttingtoolsandformingdies.b．Toolsteelwhichisaspeciac．Manganesesteelwhichisanalloycontaining12percentmanganeseandonepercentcarbon.Thismetalisusedinmining,railroad,andconstructionequipmentbecauseofitshightensilestrength.c．ManganesesteelwhichisanMechanicalpropertiesarethecharacteristicresponsesofamaterialtoappliedforces.Thesepropertiesfallintofivebroadcategories:strength,hardness,elasticity,ductility,andtoughness.Mechanicalpropertiesarethe（1）Strengthistheabilityofamaterialtoresistappliedforces.Bridgegirders,elevatorcables,andbuildingbeamsallmusthavethisproperty.（1）StrengthistheabilityofAmaterialcanbesubjectedtoanumberofdifferenttypesofforces.Theymaybetension,shear,torsion,compression,oracombinationoftheseforces.Eachpossibleforcecausesamaterialtorespondinadifferentway.Amaterialhasseveraldifferentmechanicalstrengths.Thestrengthdependsontheforceapplied.AmaterialcanbesubjectedtoThemostcommonmechanicalstrengthsare:a．Tensilestrength:themaximumtensionloadsamaterialcanwithstandbeforefracturing.Tensilestrengthistheeasieststrengthtomeasureand,therefore,iswidelyused.Themostcommonmechanicalstrb．Compressionstrength:theabilitytoresistforceswhichtendtosqueezethematerialintoanewshape.Itisbasicallytheoppositeoftensilestrength.Excessivecompressionforcewillcausethematerialtorupture(bucklingandsplitting).b．Compressionstrength:theabc．Shearstrength:theabilitytoresistfractureundershearforces.Theshearforceiscausedbyoffsetforcesappliedinoppositedirections.Theseforcescausethegrainsormoleculesofthematerialtoslidebyoneanotherandeventuallyfracture.c．Shearstrength:theabilityd．Torsionstrength:theabilitytoresisttwistingforces.Forceswhichexceedthetorsionstrength(modulusofrupture)willcausethematerialtorupture.d．Torsionstrength:theabilit（2）Hardnessistheresistanceofamaterialtopenetrationorscratching,itaccountsforabrasionresistanceaswellasresistancetodenting.Hardnessisalsodirectlyrelatedtostrength.Theharderamaterialthestrongeritis.（2）HardnessistheresistanceMetallicmaterialsarealmostalwaysharderandstrongerthanpolymericmaterials.Anumberofdifferenttestershavebeendesignedtotestthehardnessofavarietyofmaterials.Cuttingtools,files,anddrillsmustresistabrasion,orwear.Armorplate,crushingmachinery,andmetalrollsforsteelmillsallmustresistpenetration.Metallicmaterialsarealmost（3）Elasticityistheabilitytospringbacktooriginalshape.Autobumpersandallspringsshouldhavethisquality.Asstressisapplied,thematerialfirstresistspermanentdeforming.Thisareaisinthemateri-al'selasticrange.Thisisarangeinwhichthematerialwillreturntoitsoriginallengthwhentheforceisreleased.（3）ElasticityistheabilitytApplyingadditionalstress(force)willbringthematerialtoitsyieldpoint.Atthispoint,additionalstrain(elongation)occurswithoutadditionalforce(stress)beingapplied.Strainabovethispointisproducedwithsmalleramountsofforce[4].Applyingadditionalstress(foTheforcealsoproducespermanentchangesinthelengthofthematerial.Thiselongationwhichisabovethematerial'selasticlimit(pointatwhichthematerialwillnotreturntoitsoriginallength)iscalledplasticdeformation.Theforcealsoproducesperman（4）Ductilityistheplasticflowcharacteristicofamaterialundernormaltemperature.Thehighertheductilityofamaterialthegreaterisitsabilitytobeformedwithoutfracturing.Highlyductilematerialscanbeeasilybent,drawnintowire,orextruded.Modern,deep-formedautobodiesandfenders,andotherstampedandformedproductsmusthavethisproperty.（4）Ductilityistheplasticfl（5）Toughnessistheabilitytoabsorbmechanicallyappliedenergy.Strengthandductilitydetermineamaterial'stoughness.Toughnessisneededinrailroadcars,automobileaxles,hammers,rails,andsimilarproducts.（5）ToughnessistheabilitytoNotes［1］butcarbonisthemostimportantofallelementspresentinferrousalloys.

但黑色金屬里存在的各種元素中，要數(shù)碳最為重要?！皃resent”是形容詞作后置定語(yǔ)，修飾“allelements”，宜譯作“在黑色金屬所存在的各種元素中”。Notes［1］butcarbonisthemost［2］Steelisironcontainingfrom0.0218to2.11percentcarbon,whilecastironisanalloyofironandcarbonwiththecarboncontentmorethan2.11percent.

鋼是含碳量為0.0218%到2.11%的鐵；而鑄鐵是含碳量超過(guò)2.11%的鐵碳合金。［2］Steelisironcontainingfr“containingfrom0.0218to2.11percentcarbon”為分詞短語(yǔ)作定語(yǔ)，修飾“iron”?！皐hile”此處表示同時(shí)存在的兩種事物的對(duì)比，引出并列的分句，譯成“而”?！癱ontainingfrom0.0218［3］Steelisanalloyofironandcarbonwithotherelementsaddedtoproducespecificproperties.

鋼是一種包含鐵碳以及其他元素的鐵碳合金，加入其他元素可產(chǎn)生某種特殊性能。［3］Steelisanalloyofirona“addedtoproducespecificproperties”作定語(yǔ)，修飾“otherelements”。而“with…”同樣又是定語(yǔ)，修飾“analloy”。“addedtoproducespeci［4］Atthispoint,additionalstrain(elongation)occurswithoutadditionalforce(stress)beingapplied.Strainabovethispointisproducedwithsmalleramountsofforce.［4］Atthispoint,additionals

在屈服點(diǎn)上，即使沒(méi)有施加附加的應(yīng)力，附加應(yīng)變還是會(huì)出現(xiàn)。在這一點(diǎn)上存在很小的應(yīng)力這種應(yīng)變都會(huì)發(fā)生?！啊瓀ithout…”、“…with…”此處的用法一致，均是作狀語(yǔ)，分別修飾“…occurs…”、“…isproduced…”。在屈服點(diǎn)上，即使沒(méi)有施加附加的應(yīng)力，附加應(yīng)變Unit2HeatTreatmentReadingTarget1．Illustratethefunctionoftheheattreatmentandwhatwillhappenaftertheheattreatment.2．Makeyouknowcommonformsoftheheattreatmentandthedifferentrolesofthem.Unit2HeatTreatmentReadingTaTextHeattreatmentisthermalcyclinginvolvingoneormorereheatingandcoolingoperationsafterforgingforthepurposeofobtainingdesiredmicrostructuresandmechanicalpropertiesinaforging[1].TextHeattreatmentisthermalFewforgingsofthetypesareproducedwithoutsomeformofhealtreatment.Untreatedforgingsareusuallyrelativelylow-carbonsteelpartsfornoncriticalapplicationsorarepartsintendedforfurtherhotmechanicalworkandsubsequentheattreatment.FewforgingsofthetypesareThechemicalcompositionsofthesteel,thesizeandshapeoftheproduct,andthepropertiesdesiredareimportantfactorsindeterminingwhichofthefollowingproductioncyclestouse.ThechemicalcompositionsoftTheobjectofheattreatingmetalsistoimpartcertaindesiredphysicalpropertiestothemetalortoeliminateundesirablestructuralconditionswhichmayoccurintheprocessingorfabricationofthematerial[2].TheobjectofheattreatingmeIntheapplicationofanyheattreatmentitisdesirablethatthe"previoushistory"orstructuralconditionofthematerialbeknownsothatamethodoftreatmentcanbeprescribedtoproducethedesiredresult.Intheabsenceofinformationastothepreviousprocessing,amicroscopicstudyofthestructureisdesirabletodeterminethecorrectproceduretobefollowed.IntheapplicationofanyheatThecommercialheattreatmentsincommonuseareannealing,normalizing,hardeningandtempering.Theyinvolvetheheatingofthematerialtocertainpredeterminedtemperatures,"soaking"orholdingatthetemperature,andcoolingataprescribedrateinair,liquids,orretardingmedia[3].Theabovetreatmentsmaybebrieflydefinedasfollows:ThecommercialheattreatmentsAnnealing:Annealingconsistsofheatingsteelslightlyaboveitscriticalrangeandcoolingveryslowly.Annealingrelievesinternalstressesandstraincausedbypreviousheattreatment,machining,orothercold-workingprocesses.Annealing:AnnealingconsistsThetypeofsteelgovernsthetemperaturetowhichthesteelisheatedfortheannealingprocess.Thepurposeforwhichannealingisbeingdonealsogovernstheannealingtemperature.ThetypeofsteelgovernstheTherearethreedifferenttypesofannealingprocessesusedinindustry:(1)fullannealing,(2)processannealing,(3)spheroidizing.TherearethreedifferenttypeFullannealingisusedtoproducemaximumsoftnessinsteel.Machinabilityisimproved.Internalstressesarerelieved.Processannealingisalsocalledstressrelieving.Itisusedforrelievinginternalstressesthathaveoccurredduringcold-workingormachiningprocesses.FullannealingisusedtoprodSpheroidizingisusedtoproduceaspecialkindofgrainstructurethatisrelativelysoftandmachinable.Thisprocessesgenerallyusedtoimprovethemachinabilityinhigh-carbonsteelsandinwire-drawingprocesses.SpheroidizingisusedtoproduNormalizing:Normalizingisaprocessusedtorelievetheinternalstressesduetohot-working,cold-working,andmachining.Theprocessconsistsofheatingsteelslightlyabovetheuppercriticalrange30℃to50℃andcoolingtoroomtemperatureafterholdingforawhile.Normalizing:NormalizingisaThisprocessisusuallyusedwithlowandmedium-carbonaswellasalloysteels.Normalizingremovesallpreviouseffectsduetoheattreatment.ThisprocessisusuallyusedwThemorerapidcoolinginair，usedinnormalizing,causestheaustenitetodecomposeatlowertemperatures.Thisincreasesthedispersityoftheferrite-cementitestructureandtheamountofpearliteor,moreexactly,thequasi-eutectoid(ofthesorbiteortroostitetype).Themorerapidcoolinginair，Thisraisesthestrengthandhardnessofnormalizedmedium—andhigh—carbonsteelsby10percentto15percentascomparedtoannealedsteel.ThisraisesthestrengthandhThenormalizingofhot-rolledsteelincreasesitsresistancetobrittlefailurebyloweringthecold-shortnessthresholdandincreasingtheworkrequiredforcrackpropagation.Thenormalizingofhot-rolledThepurposeofnormalizingmaydifferinaccordancewiththecompositionofthesteel.Normalizingisappliedforlow-carbonsteelsinplaceofannealing.Byslightlyincreasingthehardness,normalizingenablesabettersurfacefinishtobeobtainedinmachiningandraisestheproductioncapacity.ThepurposeofnormalizingmayNormalizingaloneornormalizingwithhigh-temperaturetemperingisusedforcastingsofmedium-carbonsteelinsteadofhardeningandhigh-temperaturetempering.Theresultingmechanicalpropertiesarelower,buttheworkiswarpedlessthaninhardeningandthepossibilityofcrackformationispracticallyexcluded.NormalizingaloneornormaliziNormalizingfollowedbyhigh-temperaturetempering(at600℃to650℃)isoftenappliedinsteadoffullannealingtocorrectthestructureofalloysteelbecausetheproductivityofthetwooperationsisgreaterthanthatofasingleannealingoperation.Normalizingfollowedbyhigh-tHardening:Inanyheat-treatingoperationtherateofheatingisimportant.Heatflowsfromtheexteriortotheinteriorofsteelatadefinitemaximumrate.Ifsteelisheatedtoofast,theoutsideofthepartbecomeshotterthantheinterior.Auniformstructureishardtoobtain.Hardening:Inanyheat-treatinThehardnessthatcanbeobtainedfromagiventreatmentdependsuponthefollowingthreefactors:1．Quenchingrate.2．Carboncontent.3．Work-piecesue.ThehardnessthatcanbeobtaiRapidquenchingisneededtohardenlowcarbonandmediumplaincarbonsteels.Waterisgenerallyusedasaquenchforthesesteels.Forhigh-carbonoralloysteel,oilisused.Itsactionisnotassevereasthatofwater.Whereextremecoolingisdesired,brineisused.RapidquenchingisneededtohThemaximumdegreeofhardnessobtainableinsteelbydirecthardeningisdeterminedlargelybythecarboncontent[4].Steelwithalowcarboncontentwillnotrespondgreatlytothehardeningprocess.Carbonsteelsaregenerallyconsideredasshallowhardeningsteels.Thehardeningtemperaturevariesfordifferentsteels.Thetemperaturedependsuponthecarboncontent.ThemaximumdegreeofhardnessThetemperatureatwhichsteelisusuallyquenchedforhardeningisknownasthehardeningtemperature.Itisusually10℃to38℃abovetheuppercriticaltemperatureatwhichstructuralchangetakesplace.ThetemperatureatwhichsteelTempering:Hardeningmakeshigh-carbonsteelsandtoolsteelsextremelyhardandbrittleandnotsuitableformostuses.Bytemperingor"drawing"internalstressesdevelopedbythehardeningprocessarerelieved.Temperingincreasedthetoughnessofthehardenedpiece.Italsoseemstomakethemmoreplastic,orductile.Tempering:HardeningmakeshigThetemperingtemperaturehastheprincipaleffectonthepropertiesofthesteel.Distinctionismadebetweenthreetemperingprocedures.ThetemperingtemperaturehasLow-temperaturetemperingisperformedbyheatingto250℃.Thisreducestheinternalstresses,themartensiteproducedbyquenchingistransformedintotemperedmartensite,thestrengthisincreasedandthetoughnessisimprovedslightlywithoutanyappreciablelossinhardness.Low-temperaturetemperingispHardenedsteel(containingfrom0.6%Cto1.3%C)retainsahardnessof58to63RCfollowingsuchtemperingand,consequently,highwearresistance.Thetemperedcomponents,however,cannotwithstandconsiderableimpactloads(unlesstheyhaveatoughcore).Hardenedsteel(containingfroForthesereasons,low-temperaturetemperingisappliedinthemanufactureofcuttingandmeasuringtoolsofcarbonandlow-alloysteels,aswellascomponentsthatundergosurfacehardening,carburizing,cyanidingorcarbonitriding.Theholdingtimeusuallyrangesfrom1to2.5hours,butalongertimeisspecifiedforworkoflargecrosssectionormeasuringtools.Forthesereasons,low-temperaMedium-temperaturetemperingat350℃to500℃isemployedchieflyforcoilandlaminatedsprings,aswellasfordies.Thisoperationprovidesahighelasticlimit,endurancelimitandresistancetorelaxation.Medium-temperaturetemperingaThetemperedsteelhasastructureconsistingoftempertroostiteandtroosto-martensite,andahardnessof40to50RC.Thetemperingtemperatureshouldbeselectedsoastoavoidirreversibletemperbrittleness.ThetemperedsteelhasastrucAftertemperingat400℃to450℃,theworkshouldbecooledinwater.Thispromotesthedevelopmentofcompressiveresidualstressesatthesurface,whichraisetheendurancelimitofsprings.Aftertemperingat400℃to450High-temperaturetemperingisperformedintherangefrom500℃to680℃.Thetemperedsteelhasastructureconsistingoftempersorbite.Thisoperationprovidesforthebestcombinationofstrengthandtoughnessofthesteel.High-temperaturetemperingisIncomparisontothenormalizedoras-annealedcondition,hardeningfollowedbyhigh-temperaturetemperingsimultaneouslyraisesthetensilestrengthandyieldpoint,reductioninareaand,especially,theimpactstrength.Forthisreason,aheat-treatmentconsistingofhardeningfollowedbyhigh-temperaturetemperingiscalledstructuralimprovement.IncomparisontothenormalizeStructuralimprovementisappliedtomedium-carbonstructuralsteels(0.3%Cto0.5%C)whichmustmeethighrequirementswithrespecttotheiryieldpoint,endurancelimitandimpactstrength.Owingtothereducedhardness,however,thewearresistanceofstructurallyimprovedsteelisnothigh.StructuralimprovementisapplThisoperationsubstantiallyraisesthestructuralstrengthofsteelbyreducingitssensitivitytostressraisers,byincreasingtheworkofplasticdeformationin,crackpropagation(workrequitedincrackdevelopment),andbyreducingthetemperaturesoftheupperandlowercold-shortnessthresholds.ThisoperationsubstantiallyrTemperingintherangefrom550℃to600℃for1to2hoursalmostcompletelyrelievesresidualstressesdevelopedinhardening.Theholdingtimeforahigh-temperaturetemperingoperationmoreoftenrangesfrom1to6hours,dependingupontheoverallsizeofthework.Sometimestheholdingtimeisincreasedtoseveraldozenhourstoreducethedangerofflakesbeingformedinthesteel.Temperingintherangefrom55Notes

［1］Heattreatmentisthermalcyclinginvolvingoneormorereheatingandcoolingoperationsafterforgingforthepurposeofobtainingdesiredmicrostructuresandmechanicalpropertiesinaforging.Notes

［1］Heattreatmentisthe

熱處理是鍛后進(jìn)行一次或多次重新加熱和冷卻操作的熱循環(huán)過(guò)程，以便使鍛件獲得所需的顯微組織和機(jī)械性能。句中involvingoneormore…是分詞短語(yǔ)，作thermalcycling的后置定語(yǔ)，inaforging是動(dòng)名詞obtaining的狀語(yǔ)。熱處理是鍛后進(jìn)行一次或多次重新加熱和冷卻操作［2］Theobjectofheattreatingmetalsistoimpartcertaindesiredphysicalpropertiestothemetalortoeliminateundesirablestructuralconditionswhichmayoccurintheprocessingorfabricationofthematerial.［2］Theobjectofheattreating

金屬熱處理的目的是使金屬獲得所需的物理性能，或者消除那些在材料生產(chǎn)和加工中可能出現(xiàn)的不好的組織狀態(tài)。句中不定式toimpart…ortoeliminate…為并列結(jié)構(gòu)，作is的表語(yǔ)，which引導(dǎo)定語(yǔ)從句修飾conditions。金屬熱處理的目的是使金屬獲得所需的物理性能，［3］Theyinvolvetheheatingofthematerialtocertainpredeterminedtemperatures,"soaking"orholdingatthetemperature,andcoolingataprescribedrateinair,liquids,orretardingmedia.［3］Theyinvolvetheheatingof

熱處理包括把材料加熱到預(yù)定的溫度，在此溫度下，“均熱”即保溫，然后在空氣、液體或保溫介質(zhì)中按規(guī)定的速率冷卻。句中的theheating…，"soaking"…，andcooling是involve的賓語(yǔ);orholding是"soaking"的同位語(yǔ),or譯為“即”。熱處理包括把材料加熱到預(yù)定的溫度，在此溫度下［4］Themaximumdegreeofhardnessobtainableinsteelbydirecthardeningisdeterminedlargelybythecarboncontent.

直接冷卻后的鋼所能獲得的最大硬度在很大程度上是由碳的含量所決定。［4］Themaximumdegreeofhardn

句中的obtainableinsteelbydirecthardening作定語(yǔ)修飾hardness，主語(yǔ)為T(mén)hemaximumdegreeofhardness。句中的obtainableinsteelUnit3Iron-CarbonEquilibriumDiagram

ReadingTarget

1．Makeyouknowtherepresentationofiron-carbonequilibriumdiagram.2．Illustratethereactionofiron-carbonequilibriumdiagramandwhatphase-changereactionindifferentregions.Unit3Iron-CarbonEquilibriumTextAnequilibrium,phaseorconstitutionaldiagramisagraphicrepresentationoftheeffectsoftemperatureandcompositionuponthephasespresentinanalloy[1].TextAnequilibrium,phaseorcAnequilibriumdiagramisconstructedbyplottingtemperaturealongthey-axisandpercentagecompositionofthealloyalongthex-axis.Thisdiagramshowsrangesoftemperatureandcompositionswithinwhichthevariousphasechangesarestableandalsotheboundariesatwhichthephasechangesoccur.AnequilibriumdiagramisconsIron-carbonequilibriumdiagram(referFig1.3.1)indicatesthephasechangesthatoccurduringheatingandcoolingandthenatureandamountofthestructuralcomponentsthatexistatanytemperature[2].Iron-carbonequilibriumdiagraBesides,itestablishesacorrelationbetweenthemicrostructureandpropertiesofsteelandcastironsandprovidesabasisfortheunderstandingoftheprinciplesofheat-treatment.Besides,itestablishesacorrAniron-carbonequilibriumdiagramformsabasisfordifferentiatingamongiron(0.008%carbonorless),hypoeutectoidsteels(0.008%to0.77%carbon),hypereutectoidsteels(0.77%to2.11%carbon),hypoeutecticcastirons(2.11%to4.3%carbon)andhypereutecticcastirons(above4.3%carbon).Aniron-carbonequilibriumdiaTheironcarbonequilibriumdiagramhasaperitectic(pointJ)aeutectic(pointC)andaeutectoid(pointS).TheironcarbonequilibriumdiPeritecticreactionequationmaybewrittenasThehorizontallineat1495℃showstheperitecticreaction.PeritecticreactionequationmTheeutecticreactiontakesplaceat1148℃anditsequationmaybewrittenasTheeutecticreactiontakesplEutecticpointisat4.3%carbon.Eutecticmixtureisnotusuallyseeninthemicrostructure,becauseausteniteisnotstableatroomtemperatureandmustundergoanotherreactionduringcooling.Eutecticpointisat4.3%carbTheeutectoidreactionisrepresentedbythehorizontallineof727℃and(point)Smarkstheeutectoidpoint.TheeutectoidequationmaybewrittenasTheeutectoidreactionisreprFig1.3.1Iron-CarbonequilibriumdiagramFig1.3.1Iron-CarbonequilibTransformationswhichtakeplaceinthestructuresofsteelscontaining0.4%,0.77%and1.2%carbonrespectively(referFig1.3.1)whenheatedtoatemperaturehighenoughtomakethemausteniticandthenallowedtocoolslowly(underequilibriumconditions),havebeenexplainedbelow[4].Transformationswhichtakepla1．Steelcontaining0.4%carbonisahypoeutectoidsteelandiscompletelyausteniteaboveA3,i.e.,uppercriticaltemperatureline.AsitiscooledbelowA3linetheironbeginstochangefromF.C.C.toB.C.C.Asaresult,smallcrystalsofbodycenteredcubic(B.C.C.)ironbegintoseparateoutfromtheaustenite(F.C.C.).1．Steelcontaining0.4%carbonTheB.C.C.crystalsretainasmallamountofcarbon(lessthan0.03%)andarereferredascrystalsofferrite.Asthecoolingproceeds,ferritecrystalsgrowinsizeattheexpenseofaustenite.TheB.C.C.crystalsretainaBythetimethesteelhasreachedA1line,i.e.727℃(calledlowercriticaltemperature)itiscomposedofapproximatelyhalfferriteandhalfaustenite.Atthisstagetheaustenitecontains0.77%carbonandsinceaustenitecanholdnomorethan0.77%carboninsolidsolutionat727℃,thusatthistemperature,carbonbeginstoprecipitateascementite.BythetimethesteelhasreacThiscementiteandstillseparatingferriteformalternatelayersuntilalltheremainingausteniteisconsumed.Thelamellarstructure,i.e.,eutectoidofferriteandcementitecontains0.77%carbonandisknownasPearlite(referFig1.3.2).ThiscementiteandstillseparAllhypoeutectoidsteelswhencooledfromaustenitestatewilltransformintoferriteandpearliteinthesamewayasexplainedabove.Allhypoeutectoidsteelswhen2．Considerthetransformationofaeutectoidsteelcontaining0.77%carbon.ItwillremainausteniteuptothepointS.Thetransformationwillbeginandendatthesametemperature:i.e.,727℃.Sinceeutectoidsteelcontains0.77%carboninitially,itfollowsthatthefinaltransformedstructurewillbecompletelypearlite(seeFig1.3.2)[5].2．Considerthetransformation機(jī)電專(zhuān)業(yè)英語(yǔ)Part1-Material-and課件3．Considerthetransformationofahypereutectoidsteel(saycontaining1.2%carbon).AsthetemperaturedropsandsteelcrossesAcm(i.e.uppercriticaltemperature)lineatpointdandmovestowardse,theexcesscarbonabovetheamountrequiredtosaturateaustenite(i.e.0.77%)isprecipitatedascementiteprimarilyalongthegrainboundaries(Fig1.3.1).3．ConsiderthetransformationAsthetemperaturedropsbelow727℃,theaustenitehasbecomelessrichi