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In¯uenceofsulphurcontentandinclusiondistributiononthehydrogeninducedblistercrackinginpressurevesselandpipelinesteels

G.Domizzi*,G.Anteri,J.Ovejero-Garc󰀓õa

󰀓nNacionaldeEnerg󰀓󰀓mica,Av.LibertadorDepto.Materiales,GerenciaDesarollo,C.A.C.,ComisioõaAto

8250,1429BuenosAires,Argentina

Received29December1999;accepted22May2000

Abstract

Inpreviousworks,extensivedesulphurisation,includingshapecontrolandlowhardnessinsegregationzones,werepointedoutase󰂀ectivewaysforincreasingresistancetohydrogeninducedcracking(HIC)insteelsexposedtowetH2Senvironment.Inthisstudy,thebehaviourofeightplateswasevaluatedbyimmersingsamplesinH2S-saturatedNACEsolution.HICsusceptibilitywasassessedbyanultrasonicmethodandcorrelatedwiththeelongatedMnSinclusiondistribution.TheresultsallowustoremarkthatanextremelylowsulphurlevelisnotnecessarytoreachgoodHICresistance,providedthatnohardbandsarepresentinthesteel.Underthiscondition,arelationshiptocalculatetheminimumaveragesulphidelengthwhichprovideslowHICsusceptibilitywasfound.Ó2001ElsevierScienceLtd.Allrightsreserved.

Keywords:Steel;Pipeline;Hydrogeninducedcracking;Sulphide;Sulphur

1.Introduction

SteelsexposedtoaqueousH2SenvironmentsabsorbatomichydrogenproducedonitssurfacebytheH2Scorrosionreaction.Whenthehydrogenatomspenetrateintothesteelandprecipitateinthematrix-inclusionsinterfaces,signi®cantcrackingmayoccurinlowtomediumstrength,lowalloysteelscommonlyusedforpressurevesselsandpipingintheoilandchemicalindustries[1].

*Correspondingauthor.Fax:+-11-47-7362.E-mailaddress:domizzi@cnea.gov.ar(G.Domizzi).

0010-938X/01/$-seefrontmatterÓ2001ElsevierScienceLtd.Allrightsreserved.PII:S0010-938X(00)00084-6

326G.Domizzietal./CorrosionScience43(2001)325±339

Thisformofhydrogendamagehasbeencalledhydrogeninducedcracking(HIC),hydrogeninducedblisteringcrackingorstepwisecracking.HICcanleadtofailureofcomponentsbyreducingthee󰂀ectivethicknessofthematerial.Additionally,expe-riencehasshownthatstablecrackgrowthcanoccurinre®neryequipmentinwetH2Sservicesandthatcanleadtofailure.

ManyresearchprogrammeshavebeenperformedformanyyearstoenhanceHICresistanceoflinepipesteelsinordertoavoidtheoccurrenceoffailuresinlinepipestransportingsourmediums(containingH2S).Theincreasingtendencytowardmoresevereenvironmentalconditionsandhigheroperatingpressurehasledtostrictre-quirementsforHICresistantsteels.

SusceptibilitytoHICisparticularlyrelatedtosteelcomposition,deoxidationpracticeandprocessinghistoryastheseparametersa󰂀ectnonmetallicinclusions(type,size,morphology)andthematerialabilitytoaccommodatehydrogen.Largeinclusionssuchaselongatedmanganesesulphides[1±5]andclustersorstringersofoxide[6],increasetheHICsusceptibility.

Inthepast,twoalternativesolutionswerepresentedinordertoreducethenoxiouse󰂀ectofsulphideinclusions.Theyarechemicalinclusionshapecontrol(Ca,orrareearthtreatment)and/ortheproductionofsteelswithextralowsulphur(lowerthan0.002%).AlthoughsomeauthorshavereportedacorrelationbetweenHICsusceptibilityandsulphurcontent[1,3,7]thee󰂀ectofbothsulphurcontentandin-clusionshapehasnotbeenquanti®ed.

HICbehaviourdoesnotdependonlyonsteelcleanlinessbutalsoonmicro-structure.Crackinginitiatesandpropagatesmoreeasilyinthelessductilemicro-structures.Thus,segregatedzoneswithbainiticormartensiticstructuresareharmful.

Inthiscontext,thee󰂀ectsofmetallurgicalfactorsonHICwereinvestigated.Inparticular,theHICsusceptibilitywascorrelatedwiththesulphurcontentandsul-phidedistribution.2.Experimental

PlatesofeightcommercialpipelinesteelsandpressurevesselsteelsweretestedforHICresistance,microstructureandinclusioncharacterisation.Thechemicalcom-positionandthemechanicalpropertiesareshowninTables1and2,respectively.Themicrostructurewasobservedbyopticalandscanningelectronmicroscopy(SEM).ThebandswerehardnesstestedwithaVickersindenter,25gload.Inclusionassessmentofeachsteelwascarriedoutonlongitudinalsectionsofthreesamples.Oxideinclusionsofglobulartypewerequanti®edaccordingtotheASTME45standard.Sulphurparticlesandaluminastringerswereobservedatamagni®cationofÂ320.Thethicknessandlengthofeachparticle(orstringers)weremeasuredon60®eldsof0.5mm2.Theaveragelengthofinclusionsandthetotallengthperunitareawerecalculatedforeachspecimen.

HICtestswereperformedaccordingtoNACETM0284-96standard,insolutionA(5%NaClplus0.5%aceticacidsolutionsaturatedwithH2S,pH3±3.5)[8],during

Table1Chemicalcomposition(wt.%)Mn1.301.241.141.070.951.241.511.470.0240.0160.0140.280.250.230.210.170.250.200.140.0070.0070.0150.0150.0050.0090.0100.0100.0170.0150.0110.0120.0200.022±±±±0.050.05±±±±±±0.080.08SiSPTiVNb0.0450.0430.0490.052±0.040±±HeattreatmentAsrolledAsrolledAsrolledAsrolledNormalisedAsrolledAsrolledAsrolledNo.Steel/speci®cationCG.Domizzietal./CorrosionScience43(2001)325±339

ABCDEFGHAPI5LÂ60API5LÂ60API5LÂ52API5LÂ52ASTM516g70±±±0.100.090.140.150.1970.080.100.095327

328G.Domizzietal./CorrosionScience43(2001)325±339

Table2

MechanicalpropertiesofsteelsmeasuredonASTMcylindricaltransversespecimensSteelno.ABCDEFGH

YS(MPa)5195023803±±347330

UTS(MPa)612605506523±±527507

ein2in.(%)2727.328.128.4±±24.825.8

RA(%)66.66769.769.9±±63.567.4

96h.Threespecimens(platethicknessÂ55mmÂ55mm)weretestedforeachplate.BeforetheHICtest,thesurfacesweregroundwithSiCabrasivepaperdownto320mesh.

Crackingsusceptibilitywasassessedbyanultrasonicmethodbasedonthemeasurementofthedecreaseofbackwallechoduetocracks[9].Thepercentageofspecimensurface,wherethedecreaseislargerthan12dB(referencelevel),isre-portedastheultrasonicattenuationlevelUALanditisanindicationoftheextentofcracking.

Aftertesting,typicalhydrogeninducedcrackswereexaminedbyopticalmi-croscopyandSEMinordertoknowthecrackpathanditscorrelationwiththemicrostructure.

3.Results

3.1.Microstructure

Table3showsthemainmicrostructuralcharacteristicsoftheplates.Themicro-structureofsteelplateBconsistedofuniformlydistributedferrite±pearlite.Alltheotherplatesexhibiteddi󰂀erentdegreesofbandingthatcamefromMnandPseg-regation(asitwasveri®edbyelectronprobemicroanalysisorenergydispersiveX-rayanalysis).InplateA,someisolatedpearliticbandswereobserved,mainlyatmid-thickness(Fig.1(a)).PlatesC,D,andEpresentedauniformpearliticbandingthroughoutthethickness(Fig.1(b)).AmuchlessseverebandingwasobservedinplateF(Fig.1(c)).InsteelG,awideandcontinuousbandexistedatmid-thicknessoftheplate,whileinsteelHthebandingwasuniformlydistributedthroughouttheplatethickness.ThemicrostructureofthebandsinsteelsGandHwasmorecomplexanditconsistedofbainite,pearliteandprobablyislandsofmartensite,Figs.1(d)and2(b)and(c).

Vickersmicrohardnesswasmeasuredinthepearliticislandsandinthebands.Thehighestvalues(reportedinTable3)wereobtainedinthebandslocalisedinthemiddleoftheplate.Asmightbeexpected,steelsGandHpresentedthehighesthardnessaccordingtothemartensitepresence.

G.Domizzietal./CorrosionScience43(2001)325±339

Table3

Microstructuralcharacteristicsofthedi󰂀erentplatesNo.ABCDEFGH

a329

ASTMgrainsize121210±1110±11119

Bandingdistribution

Isolatedpearliticbandsinthemid-thickness

Pearlite±ferrite.NobandingPearliticuniformbandingPearliticuniformbandingPearliticuniformbandingPearliticuniformbanding

Widepearlitic±bainiticbandinthemid-thicknesswithmartensiteislandsPearlitic±bainiticuniformbandingwithmartensiteislands.

MicrohardnessHV311Æ27290263253257277409

ÆÆÆÆÆÆ23a2021212139

323Æ27

Measuredinpearliticbands.

Fig.1.Microstructureofthesteels:(a)plateA,isolatedpearliticband,(b)plateC,uniformpearliticbanding,(c)plateF,lightbandingand(d)plateG,hardbandswithpearlite,bainiteandprobablemartensite.

330G.Domizzietal./CorrosionScience43(2001)325±339

Fig.1(continued)

3.2.Inclusiondistribution

ElongatedMnSparticleswerethemaintypeofinclusionsobservedinalltheplates.Table4showstheaveragelength(LM),thetotallengthperunitarea(LA),andthetotalnumberofparticlesperunitarea(NA).Thesevaluesweremeasuredthroughoutthethickness.

PlatesA,CandEexhibitedthehighestvaluesofsulphideaveragelength.Nev-ertheless,itisinterestingtopointoutthattheplateEwascleanerthantheothers,asitisevidentfromitslowerLAvalue.Ontheotherhand,sulphideinclusionsaslongas550and900lmweremeasuredinsteelsAandC,respectively,whileinsteelEthemaximumsulphidelengthwas180lm.

G.Domizzietal./CorrosionScience43(2001)325±339331

Fig.2.HICcrackingalongbandinthemid-thickness:(a)PearlitebandinplateA,(b)and(c)hardbandsinplateG.

332

Table4Quanti®cationofsulphideinclusionsSulphur(wt.%)NA(1/mm2)11X2Æ2X912X7Æ0X148X8Æ14X561X1Æ10X65X9Æ1X39X0Æ1X1X6Æ6X952X2Æ18X5±±39X0Æ1218X1Æ5X731X5Æ4X120X0Æ2X114X1Æ1X510X7Æ2X8±±22X5Æ4X618X2Æ3X730X5Æ6X712X8Æ1X110X9Æ1X08X9Æ1X7Atmid-thicknessLMm(lm)NearthesurfaceLMs(lm)0.0070.0070.0150.0150.0050.0090.0100.01022X7Æ6X40X17Æ0X463X0Æ5X021X0Æ11X011X3Æ5X72Æ172X5Æ9X619X3Æ9X5UAL(%)Steelno.SulphideinclusionsAverageinthethicknessLM(lm)LA(mm/mm2)G.Domizzietal./CorrosionScience43(2001)325±339

ABCDEFGH32X4Æ14X016X7Æ4X329X5Æ8X318X7Æ4X231X8Æ1X815X9Æ1X412X2Æ0X99X8Æ2X30X34Æ0X070X21Æ0X051X36Æ0X031X12Æ0X070X19Æ0X040X14Æ0X020X50Æ0X080X48Æ0X07G.Domizzietal./CorrosionScience43(2001)325±339

Table5

Quanti®cationofaluminastringersSteelno.ABCDEFGH

AluminastringersLM(lm)39X6Æ16X923X0Æ15X4133X8Æ56X8155X4Æ108X90

19X7Æ16X300

LA(mm/mm2)0X05Æ0X030X06Æ0X050X08Æ0X060X08Æ0X060

0X06Æ0X0700

333

ExceptingplatesAandB,testedatthebeginningofthework,inalltheotherplatesthequanti®cationofsulphidewasmadeseparatelyinthemiddleofthicknessandnearthesurfaceoftheplate.Theaveragelengths(LMmandLMs)arereportedinTable4.InsteelsC,FandG,thesulphideparticleswerelongeratmid-thicknessthannearthesurface.

TheLMandLAvaluesofaluminastringersarereportedinTable5.ThelongeststringerswereobservedinplatesCandD,butevenintheseonesthein-clusiondensitywasverylow(LA󰂈0X08mm/mm2).Noelongatedsilicateswerefound.

Duetothelowcontentofglobularoxides(typeD)theywereclassi®edbythemethodDoftheASTME45standard.Inalltheplates,theparticlesratedwerelessthan8lmwide(thinseries).ForsteelsA±Ftheratingnumberwaslowerthan2(minimuminclusioninone®eld:14),whileinsteelsGandHtheworst®eldspre-sentedinclusionratingnumber3.5(35inclusioninone®eld).

3.3.Hydrogeninducedcrackingresistance

ValuesofUALarereportedinthelastcolumnofTable4.Metallographicob-servationofHICspecimensaftertestingshowedthatthecracksextendedalongthepearliticbandsorpearlitic±martensiticbands(steelsGandH).Moreover,manycrackswereassociatedwithMnSinclusionasshowninFig.3(a)and(b).

WhenelongatedparticlesofMnSwerepredominantlylocalisedinthemid-thicknesssectionoftheplate,(steelsA,C,FandG)thecrackswerealsoprefer-entiallylocatedthere.InplatesA,CandG,whichhadahighUALlevel,longandcontinuouscrackswereobservedparalleltotherollingplane(Fig.2(a)and(b)).WhenamoreuniformdistributionofMnSparticlesexisted(D,E,H),thecracksweredispersedthroughoutthethicknessandthecrackpatterntendedtobemorestepwise,Fig.4.

Fig.5isaplotofUALversustheScontentofthesteels,whileinFig.6theUALvalueswereplottedversusLM.

334G.Domizzietal./CorrosionScience43(2001)325±339

Fig.3.Cracksassociatedwithelongatedsulphides.

4.Discussion

Contrarytothatobservedbyotherauthors[1,3,7],Fig.5showsthatnoclearcorrelationexistsbetweenHICsusceptibilityandScontentofthesteelswithinthepresentrangeofScontents.PlatesBandFweremoreHICresistantthanplateEinspiteoftheirhigherScontent:thesameisvalidwhencomparingsteelsFwithA.Anothervariablethata󰂀ectstheHICresistanceistheaveragelengthofinclu-sions,neverthelessnocorrelationhasbeenfoundinthepresentwork(Fig.6).

TheresultsshowninFigs.5and6canbeexplainedifthee󰂀ectofbothvariables(LMandS)isconsideredtogether.EvenwhensteelEhadalowersulphurcontentthansteelsBandF,itsresistancetoHICwasworseduetoitshighersulphideav-eragelength.Inthesameway,fromcomparisonofsteelshavingthesamesulphur

G.Domizzietal./CorrosionScience43(2001)325±339335

Fig.4.Step-wisecrackinginplateD.

Fig.5.HICsusceptibility(UAL,%)vs.sulphurconcentration.Fig.6.HICsusceptibility(UAL,%)vs.sulphideaveragelength.336G.Domizzietal./CorrosionScience43(2001)325±339

content(AwithBandCwithD)itisclearthatthehighertheLMtheworsetheHICresistance.

ThesusceptibilitytoHICofsteelA,whichhaslowsulphurcontent(0.007%)butlongsulphides(upto550lmandLM󰂈32X4lm),wassimilartothesusceptibilityofsteelD(0.015%SandLM󰂈18X7lm).ItcanthenbeconcludedthatasmallnumberofveryelongatedinclusionsmayreducetheHICresistanceinthesamewayasagreaternumberofshorterparticles.

SteelCcombineshighsulphurcontentandhighLM(withelongatedMnSstringersreachingupto900lm)andleadingtoahighHICsusceptibility.

SteelGwasthemostsusceptibletoHICinspiteofhavingshortsulphideparticles(LM󰂈12X2lm)andnottoohighsulphurcontent.Thisfactcanbeexplainedbythepresenceofhardphasesinthebands(409HV).Thisresultisingoodagreementwiththeharmfule󰂀ectofhardbandsreportedbyotherauthors[5,6,10].SteelHhadamuchbetterbehaviourthansteelGandthiscanbeascribedtothelowerhardnessofbanding(323HV)andthelowerLM.However,thebandsarehardenoughtoin-creasetheUALnearthelevelcorrespondingtosteelD,whichhadhighersulphurcontentandlongersulphides.TheresultsinsteelsHandGclearlydemonstratethatthepresenceofbandswithhighmicrohardnessimpairsconsiderablytheHICre-sistanceevenifthesteelhasagoodlevelofinclusionshapecontrolandlowsulphurcontent.Eventhoughmoresteelsshouldbetestedtodeterminethemicrohardnessthresholdabovewhichthisimpairmentoccurs,accordingtoourresultsitseemstobenear300HV.

Excludingthesteelswithhardbands,theexperimentalUALvalueswerecorre-latedwithLMandLAorwithLMandthesulphurcontent(S).Theobtainedex-pressionsare:

UAL󰂅%󰂆󰂈5X43Â10À2LM2LAYUAL󰂅%󰂆󰂈38X118LM2S1X5X

󰂅1󰂆󰂅2󰂆

Figs.7and8showthecalculatedvaluesvs.theexperimentalones.Thecontin-uouslinescorrespondtothelinear®t.Thecorrelationfactorsare0.996and0.998,respectively.FromthegoodcorrelationobtainedbetweentheexperimentalvaluesandEq.(1),itcouldbeconcludedthatMnSinclusionsin¯uenceHICresistancethroughboth,theiraveragelengthandtheirtotallengthperunitarea.

AgoodcorrelationhasalsobeenobtainedwhenUAL(%)isexpressedasafunctionofLMandS.Thethicknessofthesulphidesfoundinthestudiedsteelswasbelow2lm.TheHICcrackspropagatemainlyalongtherollingdirectionoftheproduct.Then,thecrackspropagationisin¯uencedbytheinclusionlengthandnotbytheinclusionthickness[1].Inconsequence,ifsteelswiththickerinclusionsweretesteditissupposedthatEq.(1)wouldstillbevalidbutnotEq.(2)becausethesulphurcontenttakesintoaccountboththelengthandthethicknessoftheinclu-sions.

Consideringthedi󰂀erentbandingdistributionsobservedinsteelsA±F,itseemsthatwhenthemicrohardnessislowerthan300HV,thebandinghaslittlee󰂀ectonHIC.

G.Domizzietal./CorrosionScience43(2001)325±339337

Fig.7.CorrelationbetweenexperimentalvaluesofUAL(%)andcalculatedvaluesasfunctionofsulphideaveragelengthandsulphurcontent(±±).Thedashedlinesrepresentthe95%con®denceband.Fig.8.CorrelationbetweenexperimentalvaluesofUAL(%)andcalculatedvaluesasfunctionofsulphideaveragelengthandthetotallengthperunitarea(±±).Thedashedlinesrepresentthe95%con®denceband.Inallthetestedsteels,aluminastringerswereoccasionallyfoundreachingupto800lmbuttheywereveryisolatedandthevaluesofLAforthiskindofparticleswerelessthan0.08mm/mm2.Oxideinclusionscontentwasalsoverylow.Conse-quently,inthesesteels,themaintypeofinclusionthatin¯uencestheHICresistancewasMnS;thisbeinginagreementwiththemetallographicobservationsofcrackspath.

Insomestandardspeci®cationsforpipelinesintendedforsourgasoroiltrans-port,HICtestsaccordingtoNACEstandardTM02-84aremandatoryandmaxi-mumvaluesforcracklengthratio(CLR)orcrackthicknessratio(CTR)arerequired[11].AccordingtotheworkofBridouxetal.[10]thesemaximumvaluesforCLRorCTRcorrespondtoanUALlevelof20%.Takingthisvalueasaboundarybetween

338G.Domizzietal./CorrosionScience43(2001)325±339

acceptableandunacceptableHICresistance,itisinterestingtoremarkthatsteelsBandFpresentedverygoodHICresistance(UAL󰂈0X17and2%),althoughitssulphurcontentisnotextralow(e.g.below0.002%)andthiscanbeattributedtothelowaveragelengthofMnSparticles.

FromEq.(1)itisinferredthatagoodresistancetoHIC(UAL`20%)canbeobtainedinsteelshavingsulphurcontentashighas0.015%providedthattheav-eragelengthofinclusionsislowerthan17lmandthatnohardbandsarepresentinthemicrostructure.

5.Conclusions

1.NoclearcorrelationcouldbefoundbetweenthesulphurcontentortheaveragelengthofinclusionsandtheHICresistance.

2.HICsusceptibilitywasempiricallycorrelatedtoLM,andLA,ortoLMandsul-phurcontent.Thecorrelationswereobtainedforsteelswithasulphurcontentbe-tween0.005and0.015%andabandingmicrostructurehavingmicrohardnesslowerthan300HV25.

3.Wheneverthebandmicrohardnessdoesnotovercome300HV,thebandingdis-tributionhaslittlee󰂀ectonHICresistance.

4.Fromthesteelmakerpointofview,itisimportanttonotethatitisnotnecessarytoreachcostlyextralowvaluesofsulphurinordertoobtainagoodHICre-sistanceinsourenvironments.

Acknowledgements

TheauthorswishtoexpresstheirrecognitiontoDr.M.IpohorskiandDr.A.Hey

󰀓nCient󰀓formanyusefuldiscussionsandtotheAgenciaNacionaldePromocioõ®cay

󰀓gicaofArgentinaforpartial®nancialsupporttothepresentinvestigation,Tecnolo

PICT'97No.12-00006-00143.

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