钢的氢裂纹缺陷预防和最佳实践Word文件下载.docx

钢的氢裂纹缺陷预防和最佳实践Word文件下载.docx

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∙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<

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）.