-1-DieLifeandDieFailureProperselectionofthedematerialandofthediemanufacturingtechniquedetermines,toalargeextent,theusefullifeofformingdes.Diesmayhavetobereplacedforanumberofreasons,suchaschangesndimensionsduetowearorplasticdeformation,deteriorationofthesurfacefinish,breakdownoflubrication,andcrackingorbreakage.Inhotimpressiondieforging,theprincipalmodesofdiefailureareerosion,thermalfatigue,mechanicalfatigueandpermanent(plastic)deformation.Inerosion,alsocommonlycalleddiewear,materialisactuallyremovedfromthediesurfacebypressureandslidingofthedeformingmaterial,wearresistanceofthediematerial,diesurfacetemperature,relativeslidingspeedatthedie/materialinterfaceandthenatureoftheinterfacelayerarethemostsignificantfactorsinfluencingabrasivediewear.Thermalfatigueoccursonthesurfaceofthedieimpressioninhotformingandresultsin“heatchecking”.Thermalfatigueresultsfromcyclicyellingofthedesurfaceduetocontactwiththehotdeformingmaterial.Thiscontactcausesthesurfacelayerstoexpend,and,becauseoftheverysteeptemperaturegradients,thesurfacelayersaresubjecttocompressivestresses.Atsufficientlyhightemperatures,thesecompressivestressesmaycausethesurfacelayerstodeform.Whenthedesurfacecools,astressreversalmayoccurandthesurfacelayerswillthenbentension.Afterrepeatedcyclinginthismanner,fatiguewillcauseformationofacrackpatternthatsrecognizedasheatchecking.Diebreakageorcrackingisduetomechanicalfatigueandoccursincaseswherethediesareoverloadedandlocalstressesarehigh.Thediesaresubjecttoalternatingstressesduetoloadingandunloadingduringthedeformationprocessandthiscausescrackinitiationandeventualfailure.Dielifeanddefailurearegreatlyaffectedbythemechanicalpropertiesofthediematerialsundertheconditionsthatexistinagivendeformationprocess.Generally,thepropertiesthataremostsignificantdependontheprocesstemperature.Thus,diematerialsusedincoldformingprocessesarequitedifferent-2-fromthoseusedinhotforming.Thedesignandmanufactureofdiesandtheselectionofdiematerialsareveryimportantintheproductionofdiscretepartsbyuseofmetalformingprocesses.Thediesmustbemadebymodernmanufacturingmethodsfromappropriatediematerialsinordertoprovideacceptabledielifeatareasonablecost.Oftentheeconomysuccessofaformingprocessdependsondielifeanddecostsperpieceproduced.Foragivenapplication,selectionoftheappropriatediematerialdependsonthreetypesofvariables:(a)Variablesrelatedtotheprocessitself,includingfactorssuchassizeofthediecavity,typeofmachineusedanddeformationspeed,initialstocksizeandtemperature,dietemperaturetobeuse,lubrication,productionrataandnumberofpartstobeproduced.(b)Variablesrelatedtothetypeofdieloading,includingspeedofloading,i.e.impactofgradualcontacttimebetweendiesanddeformingmetal(thiscontacttimeisespeciallyimportantinhotforming),maximumloadandpressureonthedies,maximumandminimumdietemperatures,andnumberofloadingcyclestowhichthedieswillbesubjected.(c)Mechanicalpropertiesofthediematerial,includinghardenability,impactstrength,hotstrength(ifhotformingisconsidered)andresistancetothermalandmechanicalfatigue.譯文二：-3-模具的壽命與失效正確的選擇模具材料和模具的制造技術(shù)，在很大程度上決定著成形模具的使用壽命。為著某些原因，模具可能不得不更換。例如，由于磨損或塑性變性而使尺寸發(fā)生改變、表面損壞、光潔度降低、潤(rùn)滑故障和裂紋即破裂。在熱壓模緞中，模具失效的主要模式是腐蝕作用、熱疲勞、機(jī)械疲勞和永久性即塑性變形。腐蝕，通常也叫做模具磨損，實(shí)際上模具由于受到壓力后模具表面上的材料發(fā)生剝落。變形材料的滑移、模具材料的抗磨性，模具表面溫度、模具和材料接觸表面的相對(duì)滑動(dòng)速度以及接觸層的性質(zhì)，都是影響模具磨損的最主要的因素。熱成形加工中會(huì)發(fā)生熱裂效應(yīng)，熱疲勞都發(fā)生在模具模腔的表面。由于跟熱變形材料接觸，就在周期性屈服的模具表面引起了熱疲勞。由于溫度梯度的急劇變化，這種接觸引起的表面層的膨脹，而且表面層受到壓應(yīng)力的影響。在溫度足夠高的時(shí)刻，這些壓應(yīng)力可引起表面層的破壞。當(dāng)模具表面冷卻時(shí)，可發(fā)生反向應(yīng)力，因而表面層將處于拉應(yīng)力狀態(tài)。這種狀態(tài)循環(huán)往復(fù)將引起形成龜裂的模面，那就是作為識(shí)別熱裂紋的特征。模具破裂或產(chǎn)生裂紋是由于機(jī)械疲勞，并且是在模具過(guò)載和局部應(yīng)力高等情況下發(fā)生的。在變形加工過(guò)程中，由于加載、減載、模具承受交變應(yīng)力作用，這就將引起開(kāi)裂并發(fā)生重大破壞。在給定的成形工藝條件下，模具材料的機(jī)械性能對(duì)模具壽命和模具的損壞影響很大。一般而言，最具影響的性能是取決于加工過(guò)程的溫度。這樣，用于冷卻盛開(kāi)加工工藝的模具材料與用于熱成形加工的材料有著極大的區(qū)別。對(duì)于金屬成形加工工藝的小批、單件生產(chǎn)，模具的設(shè)計(jì)、制造和模具材料的選擇是非常重要的。為著提供成本合理和具有令人滿(mǎn)意的壽命的模具，必須用合適的模具材料和用現(xiàn)代的制造方法來(lái)制造模具。成形加工的經(jīng)濟(jì)效益常常是取決于模具壽命和所制造的每件模具的成本。根據(jù)上述應(yīng)用，合適的模具材料的選擇取決于以下三個(gè)方面的因素：(a)與加工工藝本身有關(guān)的因素，包括模腔尺寸、所用機(jī)器形式和