材料成形及控制工程专业英语阅读.docx

材料成形及控制工程专业英语阅读.docx

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材料成形及控制工程专业英语阅读

CHAPTERIMATERIALSANDTHEIRPROPERTIES

1.1MetalsandNon-metals

Amongnumerouspropertiespossessedbymaterials,theirmechanicalproperties,inthemajorityofcases,arethemostessentialandtherefore,theywillbegivenmuchconsiderationinthebook.Allcriticalpartsandelements,ofwhichahighreliabilityisrequired,aremadeofmetals,ratherthanofglass,plasticsorstone.AshasbeengiveninSec1-l,metalsarecharacterizedbythemetallicbond;wherepositiveionsoccupythesitesofthecrystallatticeandaresurroundedbyelectrongas.Allnon-metalshaveanionicoracovalentbond.Thesetypesofbondarerigidandareduetoelectrostaticattractionoftwoionsofunlikecharges.Becauseofthemetallicbond,metalsarecapableofplasticdeformationandself-strengtheninguponplasticdeformation.Therefore,ifthereisadefectinamaterialoriftheshapeofanelementissuchthattherearestressconcentrators,thestressesinthesepointsmayattainagreatvalueandevencausecracking.Butsincetheplasticityofthematerialishigh,themetalisdeformedplasticallyinthatpoint,say,atthetipofacrack,undergoesstrengthening,andtheprocessoffracturecomestoanarrest.Thisdoesnotoccurinnon-metals.Theyareuncapableofplasticdeformationandself-strengthening;therefore,fracturewilloccurassoonasthestressesatthetipofadefectexceedadefinitevalue.Thesefactsexplainwhymetalsarereliablestructuralmaterialsandcannotbeexcelledbynon-metallicmaterials.

WordsandTerms:

mechanicalproperty机械（力学）性能criticalpartandelement关键零部件covalentbond共价键metalicbondcrystallattice金属键晶格electrostaticattraction静电吸引

plasticdeformation塑性变形self-strengthening自强化stressoncentrator应力集中点thetipofacrack裂纹尖端

Questions:

1）Whatarethedifferencesinpropertiesbetweenmetalsandnon-metals?

2）Whyaremetalscapableofplasticdeformationandself-strengthening?

1.2FerrousAlloys

Morethan90%byweightofthemetallicmaterialsusedbyhumanbeingsareferrousalloys.Thisrepresentsanimmensefamilyofengineeringmaterialswithawiderangeofmicrostructuresandrelatedproperties.Themajorityofengineeringdesignsthatrequirestructuralloadsupportorpowertransmissioninvolveferrousalloys.Asapracticalmatter,thesealloysfallintotwobroadcategoriesbasedonthecarboninthealloycomposition.Steelgenerallycontainsbetween0.05and2.0wt%carbon.Thecastironsgenerallycontainbetween2.0and4.5wt%carbon.Withinthesteelcategory，weshalldistinguishwhetherornotasignificantamountofalloyingelementsotherthancarbonisused.Acompositionof5wt%totalnon-carbonadditionswillserveasanarbitraryboundarybetweenlowalloyandhighalloysteels.Thesealloyadditionsarechosencarefullybecausetheyinvariablybringwiththemsharplyincreasedmaterialscosts.Theyarejustifiedonlybyessentialimprovementsinpropertiessuchashigherstrengthorimprovedcorrosionresistance.

WordsandTerms:

ferrous铁的；含铁的corrosionresistance耐腐蚀；抗蚀力arbitrary特定的；武断的Questions:

l）Whatisthedifferenceincompositionbetweensteelandcastiron?

2）Howcanyoudistinguishlowalloysteelsfromhighalloysteels?

CHAPTER2HEATTREATMENTOFSTEEL

2.1PrincipleofHeatTreatmentofSteel

Theroleofheattreatmentinmodernmechanicalengineeringcannotbeoverestimated.Thechangesinthepropertiesofmetalsduetoheattreatmentareofextremelygreatsignificance.

2.1.1TemperatureandTime

Thepurposeofanyheattreatingprocessistoproducethedesiredchangesinthestructureofmetalbyheatingtoaspecifiedtemperatureandbysubsequentcooling.

Therefore,themainfactorsactinginheattreatmentaretemperatureandtime,sothatanyprocessofheattreatmentcanberepresentedintemperature-time（t-τ）coordinates.

Heattreatmentconditionsarecharacterizedbythefollowingparameters:

heatingtemperaturetmax,i.e.themaximumtemperaturetowhichanalloymetalisheated;timeofholdingattheheatingtemperatureτh;heatingrateνhandcoolingrateνc．

Ifheating（orcooling）ismadeataconstantrate,thetemperature-timerelationshipwillbedescribedbyastraightlinewitharespectiveangleofincline.

Withavaringheating（orcooling）rate,theactualrateshouldbeattributedtothegiventemperature,morestrictly,toaninfinitechangeoftemperatureandtime:

thatisthefirstderivativeoftemperatureintime:

νact=dt/dτ.

Heattreatmentmaybeacomplexprocess,includingmultipleheatingstages,interruptedorstepwiseheating（cooling）,coolingtosubzerotemperatures,etc.Anyprocessofheattreatmentcanbedescribedbyadiagramintemperature-timecoordinates.

WordsandTerms:

coordinates坐标系heatingrate加热速度straightline直线heatingtemperature加热温度coolingrate冷却速度firstderivative一阶导数

Questions:

1）Whatarethetwomainfactorsactinginheattreatment?

2）Howmanystagesmayusuallybeincludedintheheattreatmentofsteel?

2.1.2FormationofAustenite

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）ofapearliticcementite,thecontentofcarbonisgreaterthaninplacesofferriticlamellae.Thisiswhytheaustenitejustformedisinhomogeneous.

Inordertoobtainhomogeneousaustenite,itisessentialonheatingnotonlytopassthroughthepointoftheendofpearlitetoaustenitetransformation,butalsotooverheatthesteelabovethatpointandtoallowaholdingtimetocompletethediffusionprocessesinaustenitcgrains.

Therateofhomogenizationofausteniteappreciablydependsontheoriginalstructureofthesteel,inparticularonthedispersionandparticleshapeofcementite.Thetransformationsdescribedoccurmorequicklywhencementiteparticlesarefineand,ctherefore,havealargetotalsurfacearea.

WordsandTerms:

pearlite珠光体constitutionaldiagrm状态图inhomogeneous不均匀的lamellae层片criticalpoint临界温度overheat过热grain晶粒diffuse扩散

Questions:

1）Istherenodiffusionprocessinthetransformationfrompearlitetoaustenite?

2）Isittruethatthehigherthetemperature,thefasterthetransformationfrompearliteintoaustenite?

3）Howtoobtainhomogeneousaustenite?

CHAPTER3PRINCIPLESOFPLASTICFORMING

3.1PhysicalMetallurgyofHotWorking

Theprinciplesofthephysicalmetallurgyofhotworkingarenowwellrecognized.Duringthedeformationprocessitself,e.g.arollingpass,workhardeningtakesplacebutisbalancedbythedynamicsofteningprocessesofrecoveryandrecrystallization.Theseprocesses,whicharethermallyactivated,leadtoaflowstressthatdependsonstrainrateandtemperatureaswellasonstrain.Thestructuralchangestakingplacewithinthematerialresultinanincreaseindislocationdensitywithstrainuntilinausteniticsteelsandnickel-andcopper-basealloysacriticalstrain（εc）isreachedwhenthestoredenergyissufficientlyhightocausedynamicrecrystallization.Withfurtherstrain,dynamicrecrystallizationtakesplacerepeatedlyasthenewrecrystallizedgrainsarethemselveswork-hardenedtothecriticallevelofstoredenergy.Thesedynamicstructuralchangesleavethemetalinanunstablestateandprovidethedrivingforceforstaticrecoveryandstaticrecrystallizationtotakeplaceafterthedeformationpass.Staticrecrystallizationmaybefollowedbygraingrowthifthetemperatureissufficientlyhigh.Inordertobeabletoapplytheseprinciplestocommercialworkingprocesses,werequireanswerstotwomainquestions:

（a）howlongdoesrecrystallizationtakeplaceafteradeformationpass;and（b）whatgrainsizeisproducedbyrecrystallizationandgraingrowth?

Theanswersdeterminethestructureofthematerialenteringthenextandsubsequentpassesandhenceinfluencetheflowstressofthematerialandtheworkingforcesrequired.Eventuallytheydeterminethestructureandpropertiesofthehotworkedproducts.

WordsandTerms:

physicalmetallurgy物理冶金workhardening加工硬化

staticrecovery静态回复thermallyactivated热激活的hotworking热加工dynamicsoftening动态软化recrystallization再结晶dislocationdensity位错密度criticalstrain临界应变

Questions:

l）Whendoesdynamicrecrystallizationtakeplacewithinthematerialworkhardened?

2）Whatdotheanswerstothetwoquestionsdetermine?

3.1.1DynamicStructuralChange

Duringthedeformationofausteniteathot-workingtemperaturesandconstantstrainrate,thecharacteristicformofstress-straincurveobservedisillustratedinFig.3.1.Thesecurvesareforlow-alloysteels,testedintorsion,butaresimilartothoseobtainedforothersteelsintheausteniticconditiontested