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Unit2SelectionofConstructionMaterials工程材料的选择Thereisnotagreatdifferencebetween“this”steeland“that”steel;allareverysimilarinmechanicalproperties.Selectionmustbemadeonfactorssuchas,price,andavailability,andnotwiththeideathat“this”steelcandosomethingnoothercandobecauseitcontains2percentinsteadof1percentofacertainalloyingelement,orbecauseithasamysteriousname.Atremendousrangeofpropertiesisavailableinanysteelafterheattreatment;thisisparticularlytrueofalloysteels.在钢之间没有太大的区别；所有的钢在机械性能方面都是近似的。它们的选取标准是诸如脆硬性，价格，和可用性等。不仅仅是因为这种钢含有2％的合金元素另一种钢含有1％而使前者具有了后者没有的某些能力，或者是某种钢具有神奇的名字。经过热处理后，任何一种钢都具有大范围的特性；这种性质同样在合金钢中存在。ConsiderationsinfabricationThepropertiesofthefinalpart(hardness,strength,and),ratherthanpropertiesrequiredbyforging,governtheselectionofmaterial.Thepropertiesrequiredforforginghaveverylittlerelationtothefinalpropertiesofthematerial;therefore,notmuchcanbedonetoimproveits.Higher-carbonsteelisdifficulttoforge.Largegrainsizeisbestifsubsequentheattreatmentwillrefinethegrainsize.关于加工的考虑最后零件的特性（硬度、强度和可加工性）而不是锻造特性决定了材料的选择。可锻性与材料的最后特性联系不大；因此，提高金属的可锻造性价值不大。高碳钢很难锻造。如果在随后的热处理过程进行细化，大尺寸晶粒是最好的。Low-carbon,nickel-chromiumsteelsarejustaboutasplasticathightemperatureunderasingle520-ft·lb(1ft.1b=1.355,82J)asplainsteelsofsimilarcarboncontent.Nickeldecreasesofmedium-carbonsteels,buthaslittleeffectonlow-carbonsteels.Chromiumseemstohardensteelatforgingtemperatures,butvanadiumhasnodiscernibleeffect;neitherhasthemethodofmanufactureanyeffectonhigh-carbonsteel.在高温下低碳，镍铬合金钢在受到520-ft·lb的冲击下表现出与相同碳含量普通钢几乎同样的塑性。镍减少了中碳钢的可锻性，但对低碳钢影响不大。铬在锻造温度下时使钢硬化，但钒没有明显的效果；两种加工方法对高碳钢没有影响成型。Thecold-ofsteelisafunctionofitstensilestrengthcombinedwithductility.Thetensilestrengthandyieldpointmustnotbehighortoomuchworkwillberequiredinbending;likewise,thesteelmusthavesufficientductilitytoflowtotherequiredshapewithoutcracking.Theforcerequireddependsontheyieldpoint,becausedeformationstartsintheplasticrangeabovetheyieldpointofsteel.Work-hardeningalsooccurshere,progressivelystiffeningthemetalandcausingdifficulty,particularlyinthelow-carbonsteels钢的冷成形是它的拉伸强度和延展性相结合的结果。拉伸强度和屈服点不能太高否则在发生弯曲时需要做很多工作；与之相类似，钢应该有高延展性，使其在没有断裂的情况下成形。加工力的大小取决于屈服点，因为钢在屈服点之上才开始变形。与此同时，加工硬化也同时发生，金属变得越来越硬，增加加工难度，尤其在低碳钢中容易发生。Itisquiteinterestinginthisconnectiontodiscoverthatdeepdrawscansometimesbemadeinonerapidoperationthatcouldnotpossiblybedoneleisurelyintwoorthree.Ifadrawishalfmadeandthenstopped,itmaybenecessarytoannealbeforeproceeding,thatis,ifthepieceisgiventimetowork-harden.Thismaynotbeascientificstatement,butitisactuallywhatseemstohappen在这方面，相当有趣的是你将发现有时可通过一次快速加载完成大拉伸，但以缓慢的方式两三次加载却不能实现。如果拉伸进行了一半就停止了，那么在再加工之前应先退火，也就是说工件是否有时间进行加工硬化。这不是一种科学的叙述方法，但确实是发生了。InternalstressesColdformingisdoneabovetheyieldpointinthework-hardeningrange,sointernalstressescanbebuiltupeasily.Evidenceofthisistheastheworkleavestheformingoperationandthe，inanysubsequentheattreatment.Evenasimplewashermight,byvirtueoftheinternalstressesresultingfrompunchingandthenflattening,warpseverelyduringheattreating.believedinthevirtueofprayer.内应力在高于屈服点的加工硬化区进行冷加工很容易产生内应力。例如工件停止成型加工后会发生回弹，在随后的热处理后，工件会发生翘曲。即使是一个简单的垫圈，由于打孔和随后的平整加工中产生内应力，也会在热处理中呈现严重的翘曲。Whendoubtexistsastowhetherinternalstresseswillcause,apiececanbecheckedbyheatingittoabout1100andthenlettingitcool.Ifthereareinternalstresses,thepieceislikelytodeform.Piecesthatwillwarpseverelywhilebeingheatedhavebeenseen,yettheheat-wasexpectedtoputthemthroughandbringthemoutbetterthantheywereinthefirstplace.当是否内应力会引起翘曲的怀疑存在时，可以通过将工件加工至1100然后进行冷却来验证。如果存在内应力，工件会发生变形。经过热处理的工件像我们看到的那样会发生严重的翘曲，但是我们仍然希望工件被扔到热处理炉中被处理，这样好过它存在内应力的状态。WeldingThemaximumcarboncontentofplaincarbonsteelsafeforweldingwithoutpreheatingorsubsequentheattreatmentis0.3%.higher-carbonsteelisweldedeveryday,butonlywithproperpreheating.Therearetwoimportantfactors:theamountofheatsthatisputin;therateatwhichitisremoved.焊接不需要预热或之后进行热处理就能安全焊接的最高碳含量为0.3％。高碳钢通过合适的预热通常也可焊接。有两点值得注意：吸收热量的多少；移除速度。Weldingataslowerrateputsinmoreheatandheatsalargevolumeofmetal,sothecoolingrateduetolossofheattothebasemetalisdecreased.Apreheatwilldothesamething.Forexample,sae4150steel,preheatedto600or800,canbeweldedreadily.Whentheflameorarcistakenawayfromtheweld,thecoolingrateisnotsogreat,owingtothehighertemperatureofthesurroundingmetalandslowercoolingresults.Eventhemostrapidair-hardeningsteelsareweldableifpreheatedandweldedataslowrate.低速焊接带来了更多的热量，这对金属的大量体积进行了加热，所以冷却速度降低。预热可以取得与之相当的效果。例如当被预热至或时可以很好的焊接。由于周围金属的较高温度，当焊接弧移开焊接点后，冷却速度不会太快，产生了低速冷却的结果。即使是冷作硬化速度最快的金属也可以通过预热和慢速焊接达到良好的焊接效果。MachinabilityMachinabilitymeansseveralthings.Toproductionmenitgenerallymeansbeingabletoremovemetalatthefastestrate,leavethebestpossiblefinish,andobtainthelongestpossibletoollife.appliesto（应用于）thetool-work（工具，零件）combination.可加工性可加工性意味着几件事情。对于加工者来说，它意味着可以快速的移除金属，取得最好的加工效果，得到最长的刀具寿命。可加工性是刀具和零件的结合。Itisnotdeterminedbyhardnessalone,butbythetoughness,chemicalcomposition,andtendencyofametaltohardenundercoldwork.Inthemisleadingexpression“toohardtomachine”,theword“hard”isusuallymeanttobesynonymouswith“difficult”.Manytimesamaterialisactuallytoosofttomachinereadily.Softnessandtoughnessmaycausethemetaltotearandflow