-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