變形金屬熱處理

Recovery（回復）,Recrystallization（再結晶）andHot-work（熱加工）ofmetals2一、冷變形金屬在加熱時的組織和性能變化

金屬經(jīng)冷變形后,組織處于不穩(wěn)定狀態(tài),有自發(fā)恢復到穩(wěn)定狀態(tài)的傾向。但在常溫下，原子擴散能力小,不穩(wěn)定狀態(tài)可長時間維持。加熱可使原子擴散能力增加，金屬將依次發(fā)生回復、再結晶和晶粒長大。3Theeffectofannealingtemperatureongrainsizeinbrass.Twinboundariescanalsobeobservedinthestructures,(a)annealedat400oC,(b)annealedat650oC,and(c)annealedat800oC(75

).

4Theeffectofannealingtemperatureonthemicrostructureofcold-workedmetals.(a)cold-worked,(b)afterrecovery,(c)afterrecrystallization,and(d)aftergraingrowth.5ThreeStagesofAnnealingRecovery（回復）-Alow-temperatureannealingheattreatmentdesignedtoeliminateresidualstressesintroducedduringdeformationwithoutreducingthestrengthofthecold-workedmaterial.Recrystallization（再結晶）-Amedium-temperatureannealingheattreatmentdesignedtoeliminatealloftheeffectsofthestrainhardeningproducedduringcoldworking.Graingrowth（晶粒長大）-Movementofgrainboundariesbydiffusioninordertoreducetheamountofgrainboundaryarea.6㈠回復回復是指在加熱溫度較低時，由于金屬中的點缺陷及位錯近距離遷移而引起的晶內(nèi)某些變化。如空位與其他缺陷合并、同一滑移面上的異號位錯相遇合并而使缺陷數(shù)量減少等。在回復階段，金屬組織變化不明顯，其強度、硬度略有下降，塑性略有提高，但內(nèi)應力、電阻率等顯著下降。工業(yè)上，常利用回復現(xiàn)象將冷變形金屬低溫加熱，既穩(wěn)定組織又保留加工硬化，這種熱處理方法稱去應力退火。7

Schematicdiagramofpolygonizationinadeformedmetal.(a)deformedmetalcrystalshowingdislocations,(b)afterrecovery,dislocationsmovetoformsmall-anglegrainboundaries.8㈡再結晶當變形金屬被加熱到較高溫度時，由于原子活動能力增大，晶粒的形狀開始發(fā)生變化，由破碎拉長的晶粒變?yōu)橥暾牡容S晶粒。這種冷變形組織在加熱時重新徹底改組的過程稱再結晶。再結晶后，金屬的強度、硬度下降，塑性、韌性提高，加工硬化消失。9Schematicmodelofthegrowthofarecrystallizedgrainduringtherecrystallizationofametal,(a)isolatednucleusexpandedbygrowthwithinadeformedgrain,(b)originalhigh-anglegrainboundarymigratingintoamorehighlydeformedregionofmetal.10

Factorsaffecttherecrystallizationprocessinmetalsandalloysare:

amountofpriordeformationofthemetal

temperature

time

initialgrainsize

compositionofthemetaloralloy11㈢再結晶后的晶粒長大再結晶完成后，若繼續(xù)升高溫度或延長保溫時間，將發(fā)生晶粒長大，這是一個自發(fā)的過程。晶粒的長大是通過晶界遷移進行的，是大晶粒吞并小晶粒的過程。晶粒粗大會使金屬的強度，尤其是塑性和韌性降低。12

Agrainboundaryhasasurfacetension,T(=surface-freeenergyperunitarea)becauseitsatomshaveahigherfreeenergythanthosewithinthegrains.Consequently,toreducethisenergyapolycrystalwilltendtominimizetheareaofitsgrainboundariesandwhenthisoccurstheconfigurationtakenupbyanysetofgrainboundarieswillbegovernedbytheconditionthatTA/sinA=TB/sinB=Tc/sinCGraingrowth13(a)Relationbetweenanglesandsurfacetensionsatagrainboundarytriplepoint;(b)idealizedpolygonalgrainstructure.14The‘cannibalising’ofsmallcrystalsbylargerones.

15AnnealingandMaterialsProcessingHeat-affectedzone-Thevolumeofmaterialadjacenttoaweldthatisheatedduringtheweldingprocessabovesomecriticaltemperatureatwhichachangeinthestructure,suchasgraingrowthorrecrystallization,occurs.16Thestructureandpropertiessurroundingafusionweldinacold-workedmetal.Notethelossinstrengthcausedbyrecrystallizationandgraingrowthintheheat-affectedzone17Grainsizeinadeep-drawncomponentwhichhassubsequentlybeenannealed.Thegrainsizeatthebaseofthecup(a)willbethatoftheoriginalmaterialsincetherehasbeennocold-work,andconsequentlynorecrystallizationduringtheannealingprocess.18Duringhotworking,theelongatedanisotropicgrainsimmediatelyrecrystallize.Ifthehot-workingtemperatureisproperlycontrolled,thefinalhot-workedgrainsizecanbeveryfine.19HotWorkingLackofStrengtheningEliminationofImperfectionsAnisotropicBehaviorSurfaceFinishandDimensionalAccuracy

在金屬學中，冷熱加工的界限是以再結晶溫度來劃分的。低于再結晶溫度的加工稱為冷加工，而高于再結晶溫度的加工稱為熱加工。如Fe的再結晶溫度為451℃，其在400℃以下仍為冷加工。而Sn的再結晶溫度為-71℃，則其在室溫下的加工為熱加工。熱加工時產(chǎn)生的加工硬化很快被再結晶產(chǎn)生的軟化所抵消，因而熱加工不會帶來加工硬化效果。20TheeffectofcoldworkonthepropertiesofaCu-35%Znalloyandthe