外文翻译结构钢的焊接性精品.docx

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