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,whichusuallytakesplaceinseveraldistinctstages:(a)conceptual；(b)functional；(c)productiondesign.Duringtheconceptual-designstage,thedesignerisconcernedprimarilywiththefunctiontheproductistofulfill,UsuallyseveralconceptsarevisualJailsandconsidered,andadecisionismadeeitherthattheideaisnotpracticalorthattheideaissoundandoneormoreoftheconceptualdesignsshouldbedevelopedfurther.Here,theonlyconcernformaterialsisthatmaterialsexistthatcanprovidethedesiredproperties,Ifnosuchmaterialsareavailable,considerationisgivenastowhetherthemisareasonableprospectthatnewonescouldbedevelopedwithincostandtimelimitations.Atthefunctional-orengineering-designstage,apractical,workabledeignisdeveloped.Fairlycompletedrawingsaremade,andmaterialsareselectedandspecifiedforthevariouscomponents.0ftenaprototypeorworkingmodelismadethatcanbetestedtopermitevaluationoftheproductastofunction,relativity,appearance,serviceability,andsoon.Althoughitisexpectedthatsuchtestingmightshowthatsomechangesmayhavetobemadeinmaterialsbeforetheproductisadvancedtotheproductiondesignstage,thisshouldnotbetakenasanexcusefornotdoingathoughjobofmaterialselections.Appearance,costandreliabilityfactionshouldbeconsideredindetail,togetherwiththefunctionalfactors.Thereismuchmerittothepracticeofoneverysuccessfulcompanywhichrequiresthatallprototypesbebuiltwiththesamematerialsthatwillbeusedinproductionand,insofaraspossible,with2thesamemanufacturingtechniques.Itisoflittlevaluetohaveaperfectlyfunctioningprototypethatcannotbemanufacturedeconomicallyintheexpectedsalesvolume,oronethatissubstantiallydifferentfromwhattheproductionunitswillbeinregardtoqualityandreliability.Also,itismuchbetterfordesignengineerstodoacompletejobofmaterialanalysis,selection,andspecificationatthedevelopmentstageofdesignratherthantoleaveittotheproduction-designstage,wherechangesmaybemadebyother,possiblylessknowledgeableaboutallofthefunctionalaspectsoftheproduct.Attheproduction-designstage,theprimarycornerrelativetomaterialsshouldbethattheyarespecifiedfully,thattheyarecompatiblewith,andcanbeprocessedeconomicallyby,existingequipment,andthattheyarereadilyavailableintheneededquantities.Asmanufacturingprogresses,itisinevitablethatsituationswillarisethatmayrequiremodificationsofthematerialsbeingused.Experiencemayrevealthatsubstitutionofcheapermaterialscanbemade.Inmostcases,however,changesaremuchmorecostlytomakeaftermanufacturingisinprogressthanbeforeitstarts.Goodselectionduringtheproduction-designphasewilleliminatethenecessityformostofthistypeofchange.Themorecommontypeofchangetheoccursaftermanufacturingstartsistheresultoftheavailabilityofnewmaterials.These,ofcourse,presentpossibilitiesforcostreductionandimprovedperformance.However,newmaterialsmustbeevaluatedverycarefullytomakesurethatalltheircharacteristicsarewellestablished.Oneshouldalwaysrememberthatitisindeedrarethatasmuchisknownaboutthepropertiesandreliabilityofanewmaterialasaboutthoseofanexistingone.Alargeproportionofproductfailureandproduct-liabilitycaseshaveresultedfromnewmaterialsbeingsubstitutedbeforetheirlong-termpropertieswerereallyknown.Productliabilityactionshavemadeitimperativethatdesignersandcompaniesemploytheverybestproceduresinselectingmaterials.Thefivemostcommonfaultsinmaterialselectionhavebeen:(a)failuretoknowandusethelatestandbestinformationavailableaboutthematerialsutilized；(b)failuretoforesee,andtakeintoaccountthereasonableusesfortheproduct(wherepossible,thedesignerisfartheradvisedtoforeseeandaccountformisuseoftheproduct,asthemhavebeenmanyproductliabilitycasesinrecentyearswheretheclaimant,injuredduringmisuseoftheproduct,hassuedthemanufacturerandwon)；(e)theuseofmaterialsaboutwhich3therewasinsufficientoruncertaindata,particularlytoitslong-termproperties；(d)inadequate,andunverified,qualitycontrolprocedures；and(e)materialselectionmadebypeoplewhoarecompletelyunqualifiedtodoso.Anexaminationofthefaultsabovewillleadonetoconcludethattheeisnogoodreasonwhytheyshouldexist.Considerationofthemprovidesguidanceastohowtheycanbeeliminated.Whilefollowingtheverybestmethodsinmaterialselectionmaynoteliminateallproductliabilityclaims,theuseofproperproceduresbydesignersandindustriescangreatlyreducetheirnumbers.Fromthepreviousdiscussion,itisapparentthatthosewhoselectmaterialsshouldhaveabroad,basicunderstandingofthenatureandpropertiesofmaterialsandtheirprocessing.Thegenerallyaccepteddefinitionforheattreatingmetalsandmetalalloysis"heatingandcoolingasolidmentaloralloyinawaysoastoobtainspecificconditionsand/orproperties".Heatingforthesolepurposeofhotworking(asinforgingoperations)isexcludedfromthisdefinition.Likewise,thetypesofheattreatmentthataresometimesusedforproductssuchasglassorplasticsarealsoexcludedfromcoveragebythisdefinition.TransformationCurvesThebasisisthetime-temperature-transformationcurvesorTTTcurveswhere,inasinglediagramallthethreeparametersareplotted.Becauseoftheshapeofthecurves,theyarealsosometimescalledC-curvesorS-curves.ToplotTITcurves,theparticularsteelisheldatagiventemperatureandthestructureisexaminedatpredeterminedintervalstorecordtheamountoftransformationtakenplace.Itisknownthattheeutectoidsteel(C80)underequilibriumconditionscontains,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.1IsothermalDecompositionDiagramofC80SteelNormalizingconsistsofheatingaferrousalloytoasuitabletemperature(usually50Fto100For28Cto56C)aboveitsspecificuppertransformationtemperature.Thisisfollowedbycoolinginstillairtoatleastsometemperaturewellbelowitstransformationtemperaturerange.Forlow-carbonsteels,theresultingstructureandpropertiesarethesameasthoseachievedbyfullannealing；formostferrousalloys,normalizingandannealingarenotsynonymous.Normalizingusuallyisusedasaconditioningtreatment,notablyforrefiningthegrainsofsteelsthathavebeensubjectedtohightemperaturesforforgingorotherhotworkingoperations.Thenormalizingprocessusuallyissucceededbyanotherheattreatingoperationsuchasausteniticforhardening,annealing,ortempering.Annealingisagenerictermdenotingaheattreatmentthatconsistsofheatingtoandholdingatasuitabletemperaturefollowedbycoolingatasuitablerate.Itisusedprimarilytosoftenmetallicmaterials,butalsotosimultaneouslyproducedesiredchangesinotherpropertiesorinmicrostructure.Thepurposeofsuchchangesmaybe.