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1、Section3VARIABLESINWELDINGFABRICATIONSECTION3.1WELDMENTDISTORTIONPageTheReasonsforDistortion............3.1-1HowPropertiesofMetalsAffectDistortion.........................3.1-3ShrinkageControl....................3.1-4EquationsforCalculatingShrinkage.....3.1-7ExamplesofDistortionC
2、ontrol.........3.1-9ShopTechniquesforDistortionControl..3.1-17CheckListforMinimizingDistortion....3.1-19SECTION3.2ARCBLOWMagneticArcBlow....................3.2-1ThermalArcBlow.....................3.2-3ArcBlowWithMultipleArcs............3.2-3HowtoReduceArcBlow...............3.2-4T
3、heEffectsofFixturingonArcBlow.....3.2-5SECTION3.3PREHEATINGANDSTRESSRELIEVINGPreheating-WhenandWhy...........3.3-1TheAmountofPreheatRequired.......3.3-1MethodsofPreheating................3.3-3InterpassTemperatures.................3.3-4PreheatsforQuenchedandTemperedSteels.........
4、....................3.3-4PointersonPreheat...................3.3-5StressRelief.........................3.3-63.1-1WeldmentDistortionThereareseveralproblemsorvariablescommonbarofsteelshowninFig.3-2.Asthebarisuni•toweldingprocesses.Oneoftheseisdistortion.formlyheated,itexpandsinalld
5、irections,asshownDistortioninaweldmentresultsfromthenon•inFig.3-2(a).Asthemetalcoolstoroomtempera•uniformexpansionandcontractionoftheweldmetaltureitcontractsuniformlytoitsoriginalandadjacentbase·metalduringtheheatinganddimensions.coolingcycleoftheweldingprocess.DuringsuchaButifth
6、esteelbarisrestrained-say,inavisecycle,manyfactorsaffectshrinkageofthemetal-whileitisheated,asshowninFig.3-2(b),lateralandmakeaccuratepredictionsofdistortiondifficult.expansioncannottakeplace.VolumeexpansionPhysicalandmechanicalproperties,uponwhichmustoccur,however,sothebarexpand
7、sagreatercalculationsmustinpartbebased,changeasheatamountintheverticaldirection(thickness).Astheisapplied.Forexample,asthetemperatureofthedeformedbarreturnstoroomtemperature,itwillweldareaincreases,yieldstrength,modulusofstilltendtocontractuniformlyinalldirections,aselasticity,an
8、dthermalconductivityofsteelplateinFig.3-
