钢的氢裂纹缺陷预防和最佳实践.docx
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钢的氢裂纹缺陷预防和最佳实践
Defects-hydrogencracksinsteels-preventionandbestpractice
钢的氢裂纹缺陷,预防和最佳实践
Defects-hydrogencracksinsteels-preventionandbestpractice
Preheatingofajacketstructuretopreventhydrogencracking
Techniquesandpracticalguidanceontheavoidanceofhydrogencracksaredescribed.
Preheating,interpassandpostheatingtopreventhydrogencracking
Therearethreefactorswhichcombinetocausecrackinginarcwelding:
∙hydrogengeneratedbytheweldingprocess
∙ahardbrittlestructurewhichissusceptibletocracking
∙tensilestressesactingontheweldedjoint
Crackinggenerallyoccurswhenthetemperaturehasreachednormalambient.Inpractice,foragivensituation(materialcomposition,materialthickness,jointtype,electrodecompositionandheatinput),theriskofhydrogencrackingisreducedbyheatingthejoint.
Preheat
Preheat,whichslowsthecoolingrate,allowssomehydrogentodiffuseaway,andpreventsahard,crack-sensitivestructurebeingformed.TherecommendedlevelsofpreheatforcarbonandcarbonmanganesesteelaredetailedinEN1011-2:
2001(whichincorporatesthenomogramsfromBS5135).Thepreheatlevelmaybeashighas200°Cforexample,whenweldingthicksectionsteelswithahighcarbonequivalent(CE)value.
Interpassandpostheating
Ascrackingrarelyoccursattemperaturesaboveambient,maintainingthetemperatureoftheweldmentduringfabricationisequallyimportant.Forsusceptiblesteels,itisusuallyappropriatetomaintainthepreheattemperatureforagivenperiod,typicallybetween2to3hours,toenablethehydrogentodiffuseawayfromtheweldarea.IncracksensitivesituationssuchasweldinghigherCEsteelsorunderhighrestraintconditions,thetemperatureandheatingperiodshouldbeincreased,typically250-300°Cforthreetofourhours.
Post-weldheattreatment(PWHT)maybeusedimmediatelyoncompletionofwelding,i.e.withoutallowingthepreheattemperaturetofall.However,inpractice,asinspectioncanonlybecarriedoutatambienttemperature,thereistheriskthat'rejectable,'defectswillonlybefoundafterPWHT.Also,forhighlyhardenablesteels,asecondheattreatmentmayberequiredtotemperthehardmicrostructurepresentafterthefirstPWHT.
Undercertainconditions,morestringentproceduresareneededtoavoidcrackingthanthosederivedfromthenomogramsforestimatingpreheatinFig.C2ofEN1011-2.SectionC.2.9ofthisstandardmentionsthefollowingconditions:
1.highrestraint,includingweldsinsectionthicknessesaboveapproximately50mm,androotrunsindoublebeveljoints
2.thicksections(
approximately50mm)
3.lowcarbonequivalentsteels(CMnsteelswithC
0.1%andCE
approximately0.42)
4.'clean'orlowsulphursteels(S
approximately0.008%),asalowsulphurandlowoxygencontentwillincreasethehardenabilityofasteel.
5.alloyedweldmetalwherepreheatlevelstoavoidHAZcrackingmaybeinsufficienttoprotecttheweldmetal.Lowhydrogenprocessesandconsumablesshouldbeused.Schemesforpredictingthepreheatrequirementstoavoidweldmetalcrackinggenerallyrequiretheweldmetaldiffusiblehydrogenlevelandtheweldmetaltensilestrengthasinput.
Useofausteniticandnickelalloyweldmetaltopreventcracking
Insituationswherepreheatingisimpractical,ordoesnotpreventcracking,itwillbenecessarytouseanausteniticconsumable.Austeniticstainlesssteelandnickelelectrodeswillproduceaweldmetalwhichatambienttemperature,hasahighersolubilityforhydrogenthanferriticsteel.Thus,anyhydrogenformedduringweldingbecomeslockedintheweldmetal,withverylittlediffusingtotheHAZoncoolingtoambient.
AcommonlyusedausteniticMMAelectrodeis23Cr:
12Ni,e.g.fromEN1600:
1987.However,asnickelalloyshavealowercoefficientofthermalexpansionthanstainlesssteel,nickelausteniticelectrodesarepreferredwhenweldinghighlyrestrainedjoints,toreducetheshrinkagestrain.Figure1isageneralguideonthelevelsofpreheatwhenusingausteniticelectrodes.Whenweldingsteelswithupto0.2%C,apreheatwouldnotnormallyberequired.However,above0.4%Caminimumtemperatureof150°CwillbeneededtopreventHAZcracking.Theinfluenceofhydrogenlevelandthedegreeofrestraintarealsoillustratedinthefigure.
Fig.1GuidetopreheattemperaturewhenusingausteniticMMAelectrodesat1-2kJ/mm
a)lowrestraint(e.g.materialthickness<30mm)
b)highrestraint(e.g.materialthickness>30mm)
Bestpracticeinavoidinghydrogencracking
Reductioninweldmetalhydrogen
Themosteffectivemeansofavoidinghydrogencrackingistoreducetheamountofhydrogengeneratedbytheconsumable,iebyusingalowhydrogenprocessorlowhydrogenelectrodes.
Weldingprocessescanbeclassifiedashigh,medium,low,verylowandultralow,dependingontheamountofweldmetalhydrogenproduced.Theweldmetaldiffusiblehydrogenlevels(ml/100gofdepositedmetal),andthehydrogenscaledesignationsofEN1011-2:
2001areasfollows:
High
>15
ScaleA
Medium
>10<15
ScaleB
Low
>5<10
ScaleC
Verylow
>3<5
ScaleD
Ultra-low
3
ScaleE
Figure2illustratestherelativeamountsofweldmetalhydrogenproducedbythemajorweldingprocesses.MMA,inparticular,hasthepotentialtogenerateawiderangeofhydrogenlevels.Thus,toachievethelowervalues,itisessentialthatbasicelectrodesareusedandtheyarebakedinaccordancewiththemanufacturer'srecommendations.FortheMIGprocess,cleanerwireswillberequiredtoachieveverylowhydrogenlevels.
Fig.2Generalrelationshipsbetweenpotentialhydrogenandweldmetalhydrogenlevelsforarcweldingprocesses
Generalguidelines
ThefollowinggeneralguidelinesarerecommendedforthevarioustypesofsteelbutrequirementsforspecificsteelsshouldbecheckedaccordingtoEN1011-2:
2001-
Mildsteel(CE<0.4)
-readilyweldable,preheatgenerallynotrequirediflowhydrogenprocessesorelectrodesareused
-preheatmayberequiredwhenweldingthicksectionmaterial,highrestraintandwithhigherlevelsofhydrogenbeinggenerated
C-Mn,mediumcarbon,lowalloysteels(CE0.4to0.5)
-thinsectionscanbeweldedwithoutpreheat,butthickersectionswillrequirelowpreheatlevels,andlowhydrogenprocessesorelectrodesshouldbeused
Highercarbonandalloyedsteels(CE>0.5)
-preheat,lowhydrogenprocessesorelectrodes,postweldheatingandslowcoolingrequired.
MoredetailedguidanceontheavoidanceofhydrogencrackingisdescribedinEN1011-2:
2000.
PracticalTechniques
Thefollowingpracticaltechniquesarerecommendedtoavoidhydrogencracking:
∙cleanthejointfacesandremovecontaminantssuchaspaint,cuttingoils,grease
∙usealowhydrogenprocess,ifpossible
∙drytheelectrodes(MMA)ortheflux(submergedarc)inaccordancewiththemanufacturer'srecommendations
∙reducestressesontheweldbyavoidinglargerootgapsandhighrestraint
∙ifpreheatingisspecifiedintheweldingprocedure,itshouldalsobeappliedwhentackingorusingtemporaryattachments
∙preheatthejointtoadistanceofatleast75mmfromthejointline,ensuringuniformheatingthroughthethicknessofthematerial
∙measurethepreheattemperatureonthefaceoppositethatbeingheated.Wherethisisimpractical,allowtimefortheequalisationoftemperatureafterremovingthepreheatingbeforethetemperatureismeasured
∙adheretotheheatinputrequirements
∙maintainheatforapproximatelytwotofourhoursafterwelding,dependingoncracksensitivity
∙Insituationswhereadequatepreheatingisimpracticable,orcrackingcannotbeavoided,austeniticelectrodesmaybeused
Acceptancestandards
Ashydrogencracksarelinearimperfectionswhichhavesharpedges,theyarenotpermittedforweldsmeetingthequalitylevelsB,CandDinaccordancewiththerequirementsofENISO5817.
Detectionandremedialaction
Ashydrogencracksareoftenveryfineandmaybesub-surface,theycanbedifficulttodetect.Surface-breakinghydrogencrackscanbereadilydetectedusingvisualexamination,liquidpenetrantormagneticparticletestingtechniques.Internalcracksrequireultrasonicorradiographicexaminationtechniques.Ultrasonicexaminationispreferredasradiographyisrestrictedtodetectingrelativelywidecracksparalleltothebeam.
Mostcodeswillspecifythatallcracksshouldberemoved.Acrackedcomponentshouldberepairedbyremovingthecrackswithasafetymarginofapproximately5mmbeyondthevisibleendsofthecrack.Theexcavationisthenre-welded.
Tomakesurethatcrackingdoesnotre-occur,weldingshouldbecarriedoutwiththecorrectprocedure,iepreheatandanadequateheatinputlevelforthematerialtypeandthickness.However,asthelevelofrestraintwillbegreaterandtheinterpasstimeshorterwhenweldingwithinanexcavationcomparedtoweldingtheoriginaljoint,itisrecommendedthatahigherlevelofpreheatisused(typicallyby50°C).