Gear-Materials-and-their-Heat-Treatment(1)

2014年3月21日GearMaterialsandtheirHeatTreatment|2012-09-12|IndustrialHeating(PhotographcourtesyofAichelinUSA)HomeGearMaterialsandtheirHeatTreatmentByDanielH.Herring,FrederickJ.Otto,andFredR.SpechtMarch4,2013Gearsplayanessentialroleintheperformanceofmanyproductsthatwerelyoninoureverydaylives.Whenwethinkaboutgearswegenerallyseparatethemintotwocategories–motion-carryingandpower-transmission.Motion-carryinggearsaregenerallynonferrousorplastics,whileload-bearingpower-transmissiongears(Fig.1)areusuallymanufacturedfromferrousalloysandareintendedforheavy-dutyserviceapplications.GearMaterialsPower-transmissiongearsinvolveawidevarietyofsteelsandcastirons.Inallgears,thechoiceofmaterialmustbemadeonlyaftercarefulconsiderationoftheperformancedemandedbytheapplicationend-useandtotalmanufacturedcost,takingintoconsiderationsuchissuesaspre-andpost-machiningeconomics.Keydesignconsiderationsrequireananalysisofthetypeofappliedload–whethergradualorinstantaneous–andthedesiredmechanicalproperties,suchasbendingfatiguestrengthorwearresistance,allofwhichwilldefinecorestrengthandheat-treatingrequirements.Itisimportantforthedesignertounderstandthateachareainthegear-tooth2014年3月21日GearMaterialsandtheirHeatTreatment|2012-09-12|IndustrialHeating(PhotographcourtesyofMidwestThermal-Vac)EnlargedImageFig.4.Typicalvacuumcarburizingmicrostructure–M50NiL:0.090-inchprofileseesdifferentservicedemands.Considerationmustbegiventotheforcesthatwillactonthegearteeth,withtoothbendingandcontactstress,resistancetoscoringandwear,andfatigueissuesbeingparamount.Intherootarea,forexample,goodsurfacehardnessandhighresidualcompressivestressaredesiredtoimproveenduranceorbendingfatiguelife.Atthepitchdiameter,acombinationofhighhardnessandadequatesubsurfacestrengtharenecessarytohandlecontactstressandwearandtopreventspalling.Asanexample,someofthefactorsthatinfluencefatiguestrengthare:Hardnessdistribution,afunctionof:CasehardnessCasedepthCorehardnessMicrostructure,afunctionof:Retained-austenitepercentageGrainsizeCarbidesize,typeanddistributionNon-martensiticphasesDefectcontrol,afunctionof:ResidualcompressivestressSurfacefinishandgeometryIntergranulartoughnessThus,inthetotalmanufacturingscheme,asynergisticrelationshipmustexistbetweenthematerialselectionprocess,engineeringdesignandmanufacturing(includingheattreatment).Abalanceoftheprioritiesineachdisciplinemustbereachedinordertoachievetheoptimizationnecessaryfortheultimateperformanceofthegeardesign.Thisisoftennotaneasytask.AtmosphereHeat-TreatmentMethodsCarburizingAtmospherecarburizingisthemostcommonofthecase-hardeningmethodsinusetodayandcanhandleadiverserangeofpartsizesandloadconfigurations(Fig.2).Ingeneral,aproperlycarburizedgearwillbeabletohandlesomewherebetween30-50%moreloadthanathrough-hardenedgear.ExamplesofcommonlycarburizedgearsteelsincludeSAEgrades1018,4320,5120,8620and93102014年3月21日GearMaterialsandtheirHeatTreatment|2012-09-12|IndustrialHeating(50HRC);61HRC@0.035inch;5%retainedaustenite;500X(PhotographcourtesyofMidwestThermal-Vac)aswellasinternationalgradessuchas20MnCr5,17CrNiMo6,18CrNiMo7-6and20MoCr4.Atmospherecarburizingistypicallyperformedinthetemperaturerangeof870-955°C(1600-1750°F),althoughtemperaturesapproaching1010°C(1800°F)areusedfordeep-casework.Carburizingcasedepthsvaryoverabroadrange,typically0.13-8.25mm(0.005-0.325inches).CarbonitridingCarbonitridingisamodificationofthecarburizingprocess,notaformofnitriding.Thismodificationconsistsofintroducingammoniaintothecarburizingatmosphereinordertoaddnitrogentothecarburizedcaseasitisbeingproduced.ExamplesofgearsteelsthatarecommonlycarbonitridedincludeAISI1018,1117and12L14.Typically,carbonitridingisdoneatalowertemperaturethancarburizing–between700-900°C(1300-1650°F)–andforashortertime.Combinethiswiththefactthatnitrogeninhibitsthediffusionofcarbonandwhatgenerallyresultsisashallowercasethanistypicalforcarburizedparts.Acarbonitridedcaseisusuallybetween0.075-0.75mm(0.003-0.030inches)deep.NitridingNitridingisanothersurface-treatmentprocessthathasincreasingsurfacehardnessasitsobjective.Oneoftheappealsofthisprocessisthatrapidquenchingisnotrequired.Hence,dimensionalchangesarekepttoaminimum.Itisnotsuitableforallgearapplications.Oneofitslimitationsisthattheextremelyhighsurface-hardnesscaseproducedhasamorebrittlenaturethanthatproducedbythecarburizingprocess.Despitethisfact,nitridinghasprovedtobeaviablealternativeinanumberofapplications.ExamplesofcommonlynitridedgearsteelsincludeSAE4140,4150,4340andNitralloy135M.Nitridingistypicallydoneinthe495-565°C(925-1050°F)temperaturerange.Case-depthan