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外文翻译--材料选择与热处理.doc

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外文翻译--材料选择与热处理.doc

1MaterialSelectionandHeatTreatmentofMetalDuringrecentyearstheselectionofengineeringmaterialshasassumedgreatimportance.Moreover,theprocessshouldbeoneofmaybeadecreasingavailabilityofthem.Concernsregardingenvironmentalpollution,recyclingandworkerhealthandsafetyoftenimposenewconstraints.Thedesireformightreductionorenergysavingsforeigncompellation,increasedserviceabilityrequirements,andcustomerliabilityactions,oftentheresultofimpropermaterialuse,hashadamarkedimpact.Inaddition,theinterdependencebetweenmaterialsandtheirprocessinghasbecomebetterrecognized.Thedevelopmentofnewprocessesoftenforcesreevaluationofthematerialsbeingprocessed.Therefore,itisimperativethatdesignandmanufacturingengineersexerciseconsiderablecareinselecting,specifying,andutilizingmaterialsiftheyaretoachievesatisfactoryresultsatreasonablecostandstillassurequality.Thefirststepinthemanufactureofmyproductisdesign,whichusuallytakesplaceinseveraldistinctstagesaconceptualbfunctionalcproductiondesign.Duringtheconceptualdesignstage,thedesignerisconcernedprimarilywiththefunctiontheproductistofulfill,UsuallyseveralconceptsarevisualJailsandconsidered,andadecisionismadeeitherthattheideaisnotpracticalorthattheideaissoundandoneormoreoftheconceptualdesignsshouldbedevelopedfurther.Here,theonlyconcernformaterialsisthatmaterialsexistthatcanprovidethedesiredproperties,Ifnosuchmaterialsareavailable,considerationisgivenastowhetherthemisareasonableprospectthatnewonescouldbedevelopedwithincostandtimelimitations.Atthefunctionalorengineeringdesignstage,apractical,workabledeignisdeveloped.Fairlycompletedrawingsaremade,andmaterialsareselectedandspecifiedforthevariouscomponents.0ftenaprototypeorworkingmodelismadethatcanbetestedtopermitevaluationoftheproductastofunction,relativity,appearance,serviceability,andsoon.Althoughitisexpectedthatsuchtestingmightshowthatsomechangesmayhavetobemadeinmaterialsbeforetheproductisadvancedtotheproductiondesignstage,thisshouldnotbetakenasanexcusefornotdoingathoughjobofmaterialselections.Appearance,costandreliabilityfactionshouldbeconsideredindetail,togetherwiththefunctionalfactors.Thereismuchmerittothepracticeofoneverysuccessfulcompanywhichrequiresthatallprototypesbebuiltwiththesamematerialsthatwillbeusedinproductionand,insofaraspossible,with2thesamemanufacturingtechniques.Itisoflittlevaluetohaveaperfectlyfunctioningprototypethatcannotbemanufacturedeconomicallyintheexpectedsalesvolume,oronethatissubstantiallydifferentfromwhattheproductionunitswillbeinregardtoqualityandreliability.Also,itismuchbetterfordesignengineerstodoacompletejobofmaterialanalysis,selection,andspecificationatthedevelopmentstageofdesignratherthantoleaveittotheproductiondesignstage,wherechangesmaybemadebyother,possiblylessknowledgeableaboutallofthefunctionalaspectsoftheproduct.Attheproductiondesignstage,theprimarycornerrelativetomaterialsshouldbethattheyarespecifiedfully,thattheyarecompatiblewith,andcanbeprocessedeconomicallyby,existingequipment,andthattheyarereadilyavailableintheneededquantities.Asmanufacturingprogresses,itisinevitablethatsituationswillarisethatmayrequiremodificationsofthematerialsbeingused.Experiencemayrevealthatsubstitutionofcheapermaterialscanbemade.Inmostcases,however,changesaremuchmorecostlytomakeaftermanufacturingisinprogressthanbeforeitstarts.Goodselectionduringtheproductiondesignphasewilleliminatethenecessityformostofthistypeofchange.Themorecommontypeofchangetheoccursaftermanufacturingstartsistheresultoftheavailabilityofnewmaterials.These,ofcourse,presentpossibilitiesforcostreductionandimprovedperformance.However,newmaterialsmustbeevaluatedverycarefullytomakesurethatalltheircharacteristicsarewellestablished.Oneshouldalwaysrememberthatitisindeedrarethatasmuchisknownaboutthepropertiesandreliabilityofanewmaterialasaboutthoseofanexistingone.Alargeproportionofproductfailureandproductliabilitycaseshaveresultedfromnewmaterialsbeingsubstitutedbeforetheirlongtermpropertieswerereallyknown.Productliabilityactionshavemadeitimperativethatdesignersandcompaniesemploytheverybestproceduresinselectingmaterials.Thefivemostcommonfaultsinmaterialselectionhavebeenafailuretoknowandusethelatestandbestinformationavailableaboutthematerialsutilizedbfailuretoforesee,andtakeintoaccountthereasonableusesfortheproductwherepossible,thedesignerisfartheradvisedtoforeseeandaccountformisuseoftheproduct,asthemhavebeenmanyproductliabilitycasesinrecentyearswheretheclaimant,injuredduringmisuseoftheproduct,hassuedthemanufacturerandwonetheuseofmaterialsaboutwhich3therewasinsufficientoruncertaindata,particularlytoitslongtermpropertiesdinadequate,andunverified,qualitycontrolproceduresandematerialselectionmadebypeoplewhoarecompletelyunqualifiedtodoso.Anexaminationofthefaultsabovewillleadonetoconcludethattheeisnogoodreasonwhytheyshouldexist.Considerationofthemprovidesguidanceastohowtheycanbeeliminated.Whilefollowingtheverybestmethodsinmaterialselectionmaynoteliminateallproductliabilityclaims,theuseofproperproceduresbydesignersandindustriescangreatlyreducetheirnumbers.Fromthepreviousdiscussion,itisapparentthatthosewhoselectmaterialsshouldhaveabroad,basicunderstandingofthenatureandpropertiesofmaterialsandtheirprocessing.Thegenerallyaccepteddefinitionforheattreatingmetalsandmetalalloysisheatingandcoolingasolidmentaloralloyinawaysoastoobtainspecificconditionsand/orproperties.Heatingforthesolepurposeofhotworkingasinforgingoperationsisexcludedfromthisdefinition.Likewise,thetypesofheattreatmentthataresometimesusedforproductssuchasglassorplasticsarealsoexcludedfromcoveragebythisdefinition.TransformationCurvesThebasisisthetimetemperaturetransformationcurvesorTTTcurveswhere,inasinglediagramallthethreeparametersareplotted.Becauseoftheshapeofthecurves,theyarealsosometimescalledCcurvesorScurves.ToplotTITcurves,theparticularsteelisheldatagiventemperatureandthestructureisexaminedatpredeterminedintervalstorecordtheamountoftransformationtakenplace.ItisknownthattheeutectoidsteelC80underequilibriumconditionscontains,allausteniteabove723C.whereasbelow,itispearlier.Toformpearlier,thecarbonatomsshoulddiffusetoformcommentate.Thediffusionbeingarateprocess,wouldrequiresufficienttimeforcompletetransformationofaustenitetopearlier.Fromdifferentsamples,itispossibletonotetheamountofthetransformationtakingplaceatanytemperature.Thesepointsarethenplottedonagraphwithtimeandtemperatureastheaxes.Throughthesepoints,transformationcurvescanbeplottedasshowninFig.1foreutectoidsteel.Thecurveatexcretaleftrepresentsthetimerequiredforthetransformationofaustenitetopearliertostartatanygiventemperature.Similarly,thecurveatextremerightrepresentsthetimerequiredforcompletingthetransformation.Betweenthetwocurvesarethepointsrepresentingpartialtransformation.ThehorizontallinesSM4andfMrepresentthestartandfinishofmartenstictransformation.ClassificationofHeatTreatingProcessesInsomeinstances,heattreatmentproceduresareclearcutintermsoftechniqueandapplication,whereasinotherinstances,descriptionsorsimpleexplanationsareinsufficientbecausethesametechniquefrequentlymaybeusedtoobtaindifferentobjectives.Forexample,stressrelievingandtemperingareoftenaccomplishedwiththesameequipmentandbyuseofidenticaltimeandtemperaturecycles.Theobjectives,however,aredifferentforthetwoprocesses.Thefollowingdescriptionsoftheprincipalheattreatingprocessesamgenerallyarrangedaccordingtotheirinterrelationships.Fig.1IsothermalDecompositionDiagramofC80SteelNormalizingconsistsofheatingaferrousalloytoasuitabletemperatureusually50Fto100For28Cto56Caboveitsspecificuppertransformationtemperature.Thisisfollowedbycoolinginstillairtoatleastsometemperaturewellbelowitstransformationtemperaturerange.Forlowcarbonsteels,theresultingstructureandpropertiesarethesameasthoseachievedbyfullannealingformostferrousalloys,normalizingandannealingarenotsynonymous.Normalizingusuallyisusedasaconditioningtreatment,notablyforrefiningthegrainsofsteelsthathavebeensubjectedtohightemperaturesforforgingorotherhotworkingoperations.Thenormalizingprocessusuallyissucceededbyanotherheattreatingoperationsuchasausteniticforhardening,annealing,ortempering.Annealingisagenerictermdenotingaheattreatmentthatconsistsofheatingtoandholdingatasuitabletemperaturefollowedbycoolingatasuitablerate.Itisusedprimarilytosoftenmetallicmaterials,butalsotosimultaneouslyproducedesiredchangesinotherpropertiesorinmicrostructure.Thepurposeofsuchchangesmaybe.

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