外文翻译结构钢的焊接性精品.docx
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外文翻译结构钢的焊接性精品.docx
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外文翻译结构钢的焊接性精品
附录1
英文原文
Lecture2.6:
WeldabilityofStructuralSteels
Thelecturebrieflydiscussesthebasicsoftheweldingprocessandthenexaminesthefactorsgoverningtheweldabilityofstructuralsteels.
SUMMARY
Thefundamentalaspectsoftheweldingprocessarediscussed.Thelecturethenfocusesonthemetallurgicalparametersaffectingtheweldabilityofstructuralsteels.Asteelisconsideredtoexhibitgoodweldabilityifjointsinthesteelpossessadequatestrengthandtoughnessinservice.
Solidificationcracking,heataffectedzone-liquationcracking,hydrogen-inducedcracking,lamellartearing,andre-heatcrackingaredescribed.Theseeffectsaredetrimentaltotheperformanceofweldedjoints.Measuresrequiredtoavoidthemareexamined.
1.INTRODUCTION
1.1ABriefDescriptionoftheWeldingProcess
Weldingisajoiningprocessinwhichjointproductioncanbeachievedwiththeuseofhightemperatures,highpressuresorboth.Inthislecture,onlytheuseofhightemperaturestoproduceajointisdiscussedsincethisis,byfar,themostcommonmethodofweldingstructuralsteels.Itisessentiallyaprocessinwhichanintenseheatsourceisappliedtothesurfacestobejoinedtoachievelocalmelting.Itiscommonforfurther"fillermetal"tobeaddedtothemoltenweldpooltobridgethegapbetweenthesurfacesandtoproducetherequiredweldshapeanddimensionsoncooling.Themostcommonweldingprocessesforstructuralsteelworkuseanelectricarcmaintainedbetweenthefillermetalrodandtheworkpiecetoprovidetheintenseheatsource.
Ifunprotected,themoltenmetalintheweldpoolcanreadilyabsorboxygenandnitrogenfromtheatmosphere.Thisabsorptionwouldleadtoporosityandbrittlenessinthesolidifiedweldmetal.Thetechniquesusedtoavoidgasabsorptionintheweldpoolvaryaccordingtotheweldingprocess.Themainweldingprocessesusedtojoinstructuralsteelsareconsideredinmoredetailbelow.
1.2TheMainWeldingProcesses
a.ManualMetalArcwelding(MMA)
Inthisprocess,thewelderusesametalstickelectrodewithafusiblemineralcoating,inaholderconnectedtoanelectricalsupply.Anarcisstruckbetweentheelectrodeandtheweldareawhichcompletesthereturncircuittotheelectricitysupply.Thearcmeltsboththeelectrodeandthesurfaceregionoftheworkpiece.Electromagneticforcescreatedinthearchelptothrowdropsofthemoltenelectrodeontothemoltenareaoftheworkpiecewherethetwometalsfusetoformtheweldpool.
Theelectrodecoatingoffluxcontributestothecontentoftheweldpoolbydirectadditionofmetalandbymetallurgicalreactionswhichrefinethemoltenmetal.Thefluxalsoprovidesalocalgaseousatmospherewhichpreventsabsorptionofatmosphericgasesbytheweldmetal.
Therearemanytypesofelectrodes.Themaindifferencesbetweenthemareinthefluxcoating.Thethreemainclassesofelectrodeareshownbelow:
1.Rutile:
Generalpurposeelectrodesforapplicationswhichdonotrequirestrictcontrolofmechanicalproperties.Theseelectrodescontainahighproportionoftitaniumoxideinthefluxcoating.
2.Basic:
Theseelectrodesproduceweldswithbetterstrengthandnotchtoughnessthanrutile.Theelectrodeshaveacoatingwhichcontainscalciumcarbonateandothercarbonatesandfluorspar.
3.Cellulosic:
Thearcproducedbythistypeofelectrodeisverypenetrating.Theseelectrodeshaveahighproportionofcombustibleorganicmaterialsintheircoating.
b.SubmergedArcWelding(SAW)
Thisprocessusesabarewireelectrodeandafluxaddedseparatelyasgranulesorpowderoverthearcandweldpool.Thefluxprotectsthemoltenmetalbyformingalayerofslaganditalsostabilisesthearc.
Theprocessisusedmainlyinamechanicalsystemfeedingacontinuouslengthofwirefromacoilwhilsttheweldingleadismovedalongthejoint.ASAWmachinemayfeedseveralwires,onebehindtheother,sothatamulti-runweldcanbemade.Submergedarcweldingproducesmoreconsistentjointsthanmanualwelding,butitisnotsuitableforareasofdifficultaccess.
c.Gasshieldedwelding
Inthisprocess,abarewireelectrodeisusedandashieldinggasisfedaroundthearcandweldpool.Thisgaspreventscontaminationoftheelectrodeandweldpoolbyair.Therearethreemainvariationsofthisprocessasshownbelow:
1.MIG(metal-inertgas)welding-Argonorheliumgasisusedforshielding.Thisprocessisgenerallyusedfornon-ferrousmetals.
2.MAG(metal-activegas)welding-Carbondioxide(usuallymixedwithargon)isusedforshielding.Thisprocessisgenerallyusedforcarbonandcarbon-manganesesteels.
3.TIG(tungsten-inertgas)-Argonorheliumgasisusedforshieldingandthearcstruckbetweentheworkpieceandanon-consumabletungstenelectrode.Thisprocessisgenerallyusedforthinsheetworkandprecisionwelding.
1.3WeldedJointDesignandPreparation
Therearetwobasictypesofweldedjointsknownasbuttandfilletwelds[1].SchematicviewsofthesetwoweldtypesareshowninFigure1.Theactualshapeofaweldisdeterminedbythepreparationoftheareatobejoined.Thetypeofweldpreparationdependsontheweldingprocessandthefabricationprocedure.ExamplesofdifferentweldpreparationsareshowninFigure2.Theweldjointhastobelocatedandshapedinsuchawaythatitiseasilyaccessibleintermsofboththeweldingprocessandweldingposition.Thedetailedweldshapeisdesignedtodistributetheavailableheatadequatelyandtoassistthecontrolofweldmetalpenetrationandthustoproduceasoundjoint.Operatorinduceddefectssuchaslackofpenetrationandlackoffusioncanbedifficulttoavoidifthejointpreparationanddesignpreventgoodaccessforwelding.
1.4TheEffectoftheWeldingThermalCycleontheMicrostructure
Theintenseheatinvolvedintheweldingprocessinfluencesthemicrostructureofboththeweldmetalandtheparentmetalclosetothefusionboundary(theboundarybetweensolidandliquidmetal).Assuch,theweldingcycleinfluencesthemechanicalpropertiesofthejoint.
Themoltenweldpoolisrapidlycooledsincethemetalsbeingjoinedactasanefficientheatsink.Thiscoolingresultsintheweldmetalhavingachillcastmicrostructure.Intheweldingofstructuralsteels,theweldfillermetaldoesnotusuallyhavethesamecompositionastheparentmetal.Ifthecompositionswerethesame,therapidcoolingcouldresultinhardandbrittlephases,e.g.martensite,intheweldmetalmicrostructure.Thisproblemisavoidedbyusingweldfillermetalswithalowercarboncontentthantheparentsteel.
Theparentmetalclosetothemoltenweldpoolisheatedrapidlytoatemperaturewhichdependsonthedistancefromthefusionboundary.Closetothefusionboundary,peaktemperaturesnearthemeltingpointarereached,whilstmaterialonlyafewmillimetresawaymayonlyreachafewhundreddegreesCelsius.Theparentmaterialclosetothefusionboundaryisheatedintotheaustenitephasefield.Oncooling,thisregiontransformstoamicrostructurewhichisdifferentfromtherestoftheparentmaterial.Inthisregionthecoolingrateisusuallyrapid,andhencethereisatendencytowardstheformationoflowtemperaturetransformationstructures,suchasbainiteandmartensite,whichareharderandmorebrittlethanthebulkoftheparentmetal.Thisregionisknownastheheataffectedzone(HAZ).
ThemicrostructureoftheHAZisinfluencedbythreefactors:
Thechemicalcompositionoftheparentmetal.
Theheatinputrateduringwelding.
ThecoolingrateintheHAZafterwelding.
ThechemicalcompositionoftheparentmetalisimportantsinceitdeterminesthehardenabilityoftheHAZ.TheheatinputrateissignificantsinceitdirectlyaffectsthegrainsizeintheHAZ.Thelongerthetimespentabovethegraincoarseningtemperatureoftheparentmetalduringwelding,thecoarserthestructureintheHAZ.Generally,ahighheatinputrateleadstoalongerthermalcycleandthusacoarserHAZmicrostructure.ItshouldbenotedthattheheatinputratealsoaffectsthecoolingrateintheHAZ.Asageneralrule,thehighertheheatinputratethelowerthecoolingrate.Thevalueofheatinputrateisafunctionoftheweldingprocessparameters:
arcvoltage,arccurrentandweldingspeed.Inadditiontoheatinputrate,thecoolingrateintheHAZisinfluencedbytwootherfactors.First,thejointdesignandthicknessareimportantsincetheydeterminetherateofheatflowawayfromtheweldduringcooling.Secondly,thetemperatureofthepartsbeingjoined,i.e.anypre-heat,issignificantsinceitdeterminesthetemperaturegradientwhichexistsbetweentheweldandparentmetal.
1.5ResidualWeldingStressesandDistortion
Theintenseheatassociatedwithweldingcausestheregionoftheweldtoexpand.Oncoolingcontractionoccurs.Thisexpansionandsubsequentcontractionisresistedbythesurroundingcoldmaterialleadingtoaresidualstressfieldbeingsetupinthevicinityoftheweld.Withintheweldmetaltheresidualstresstendstobepredominantlytensileinnature.Thistensileresidualstressisbalancedbyacompressivestressinducedintheparentmetal[2].AschematicviewoftheresidualstressfieldobtainedforlongitudinalweldshrinkageisshowninFigure3.ThetensileresidualstressesareuptoyieldpointinmagnitudeintheweldmetalandHAZ.Itisimportanttonotethattheresidualstressesarisebecausethematerialundergoeslocalplasticstrain.ThisstrainmayresultincrackingoftheweldmetalandHAZduringwelding,distortionofthepartstobejoinedorencouragementofbrittlefailureduringservice.
Transverseandlongitudinalcontractionsresultingfromweldingcanleadt
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