搜索
MaterialsTechnology.3PropertiesandApplicationofMaterialsreleasingconstituentsaswellasnitrogen.Theprocesscanbecar-riedoutinpowder.saltbath,gasorplasma.Thegasn~tmcarburisatinnprocesses,rrfrrrrdtohytheblankettern1shonttrnegasnitrid-ing,needconaderablyshortertreatmenttimesascomparedwiththeothergasnrrn+gprocessesThextimerareofthesameorderasforsaltbathnitiriding.attemperaturesfrom570to590C.Alloyingelementswithanespeciallyhighaffinitytonitrogen.suchaschmmium,molybdenum,aluminium,titaniumorvanadium,yieldpart~cularlyhardboundarylayerswithhighrrxstancetowearbyslidingfriction(lutndesteels).Inthecaseoftemperedsteelsw~thlowretentionrrfhardnesscaremusthetakentoensurethatlongtunenitridingdoesnotreducematendlstrengthatthecore.Hardnessrrtmtioncanbeunprovedbyalloy~ngelementssuchaschromemdmolybdcnom,suthath~ghcoresuengthcanbecom-binedwithhighboundarylayerhardne\rinlow-alloyCrMosteels.Aluminisation.ThisgenerallyreferstotheproductionofA1coatings.Ofthediffusionprocesses,calorisingandalitisinghaveprovedparticularlyusefillInculorisationtheworkpieces(usudllysn~dllparts)areheatedinAlpowderw-ithcrndlnadditivesinarotatingreactiondrumat450CThe).Irrthmheatedforashortperiodoutsidethedrumto700to800CtoIncreasediffus~onAbrittle,tightlybondedFe-Alalloylayer(A1I(%)isformedbelowahardlayer01AI,O,.whlchbdsgoodresistancetoscallngAImabrittleprotectivelayerwithbetterformabilityandrhesamescalingrrsistdoce1sproducedbyaNIisingInthisprocesstheanicleishratrdmapowderofFe-A1alloyat800to1200°(:Bothprocessesarealsoapplicabletometallicmaterialsotherthansteel,e.g.calorisingwithcopperandbrassandalitisingwithnickelalloysforgasturbineblades.Siliconising.Abrittlebutveryscale-resistantsurfaceisobtainedforlow-carbonsteelbytreatmentwithhotSiCI,vapour.TheSicontentofthelayercanbeasmuchas20%.Sherardisation.Thisprocessiscarriedoutinasimilarwaytocalorisation.Aftermordurantingorsandblasting,theworkpiecesareheatedat370to400OCinzincdustwithcertainadditives.Apartfromincreasedcorrosionprotection.agoodkeyingsurfaceforpainr~ngisobtained.Boronising.Byboronisinghardandwrar-resistantboundarylayersareproduced.Theprocesscanbecarriedoutinpowder(950to1050OC),gasandsaltbathsChromising.Theprocessiscarriedoutatabout1000to1200'(1withchrome-releasingsubstancesinthegasormoltenphasr.Theboundarylayeroftheworkpieceisenrichrdtoasmuchas3iUc2Cr.Itthusbecomesscale-rrmtantuptotrmprraturesofover800C.Becauseofthecorrosionresistanceofthelayerthisprocesscanobvi-atethenerdiornoncomodingmaterialinthework-pieceitself.SpecialHeatTreatmentsIsothermalTransformationintheBdniteStage.Inthisprocessaworkpieceisrapidlycooledafteraustenitisingtoatemperatureatwh~ch.ifthistempera-tureismaintained.bainiteconversiontakesplace.Thetemperatureappn~priatetoapanicldrmatenalcanbeascertainedfromtheisothermal'ITTcurvc.Thebeststrengthandtoughnesspropertiesresdtfromconversioninthelowertemperaturebandofthebainitestage.Apartfromgoodmechanicalpropertiestheprocessoffersecon-omicadvantagesovertempering,sinceasecondheatingstageisnotrequired.Inparticular,smallpartsmadefromstructuralsteelaretreatedbythisprocess.Patenting.Thisreferstoaheattreatmentofwireandstnp111which,afteraustenitisation,theworkpieceisrapidlycooledtoatemperatureaboveM,,toobtainafavour-ablestructureforsubsequentcoldforming.Inwirepro-ductionthematerialisusuallycooledinahotbathattemperaturesthatgiverisetoclose-barredpearlite,sincethisstructureisespeciallysuitedtotensionloads.MartensiteHardening.Incarbon-poorFe-Nialloyswithmorethan6to7%nickeltheconversionoftheysolidsolutiontakesplaceevenunderslowcoolingfromtheausteniterange(820to850'C);itnolongerinvolvesdiffus~onintoferrite,butdiffusionlessshearingintonickelntanensite,ametastablesolidsolutionsupersaturatedwithnickel(insteadofcarbon).Inthesubsequenthot-harden-ingprocessbelowthere-austenitisingtemperature(450to500Y.),alloylngelementssuchasTi,Nb.A1andespeciallyMobringaboutaconsiderdbleincreaseinstrengthcoupledwithgoodtoughnessthroughthesrgrr-gationoffinelydividedintermetallicphasesmdthepro-ductionofslip-resistantordrringpphases.ThermomechanicalTreatmentsThermomechanicaltreatmentsareacombinationoffor-mingprocesseswithheattreatments,toobtaincertainpropertiesinthematmal.AusteniticFormHardening.Inthisprocessasteelissubjectedtoformingaftercr)olingfromtheaustenitisingtemperatureandbeforeordunngaustenitetransform-ation.Inthiswaystrengthcanbeincreasedwhiletough-nessisimprovedbyafinerbainiteandmartensitestruc-ture.Temperature-ControlledHotForming.Bymanipul-atingthetemperatureinthelaststagesofahotformingprocessinvolvingasufficientchangeofformandsub-sequentcooling,asimilarstructuretothatproducedbynomalisingisaimedat.Hot-ColdCompacting.Anincreaseinstrengthisachievedbyfomngatahightemperaturebelowtherecrystall~sationthresholdusingreducedformingforceascomparedwithroomtemperature.Thisprocessisespeciallysuitedtoausteniticmaterials.3.1.4SteelsIDesignationofSteelsTocreateaclearmeansofclassifyingthemultiplicityofsteelvarietiesnowknown,asystemofabbreviatednamesisneeded:DIN17006andEU-Norm27-74.ThemostIrecentsurveyofabbreviatednamesandmaterialnun