Premature failure of dissimilar metal weld joint at intermediate temperature superheater tube

Case

study

Premature

failure

of

dissimilar

metal

weld

joint

at

intermediate

temperature

superheater

tube

Mohammed

Al

Hajri,

Anees

U.

Malik

,

Abdelkader

Meroufel

*,

Fahd

Al-Muaili

DesalinationTechnologiesResearchInstitute,SalineWaterConversionCorporation,31951Jubail,SaudiArabia

1. Introduction

Forsuperheaterorreheaterboilertubesoperatingathighertemperatures(above5408C),itiscustomarytousestainless steel(SS)inthefinalstagescoveringaportionoftheboilertubingwhichispredominantlycomprisedoflowalloyferritic steel(AS).SSandAStubesarejointbyweldingasinthepresentcaseofITSHtube.Problemsofprematurefailureofwelds betweenthetwodissimilarmaterials(SSandAS)havebeenacommonoccurrenceinboilerindustry.Threemajorcausesof thesecreepfailuresare:carbonmigrationfromtheheat-affectedzone(HAZ)oftheASintotheweldmetal,expansion differencesbetween thetwo varietiesofsteel, andthedifferencesin corrosionresistancetofluegasesleading tothe formationofanoxidewedgeontheoutsidediameteroftheAStubenexttotheweld[1].API571standardconsidermore detailsonthecriticalfactorsleadingtoDMWfailuressuchasweldinggeometry,thermalcycling,etc.[2].DMWfailureof superheatertubeshasbeenreportedinpulverizedcoal-firedpowerplantwherethedifferenceofexpansioncoefficientwas pointedasthemainreason[3].

Ontheotherhand,aninterestingresearchprogramwasconductedbyEPRIinstitutewithobjectivetoprovideguidelines forimprovingdesign andwelding proceduresfor increasingreliabilityandlongevity ofDMW[4].Thefindingsof this programsuggestedthatnickelbaseweldingwithproperheattreatmentformingbandofwelldistributedcarbides(instead ofsharpinterfaceofcarbides)nearthefusionlinewouldimproveDMWlife.Shallowwelddesign,lowtemperatureandlow stressregionoftheDMWwouldalsoprovideadditionalmarginofsafety[4].Asindicationoftherecordedlifetime,nickel baseweldingwassubjectoffailuresinthemid-1970aftergenerally15–17yearsofservice[5].

ARTICLE INFO

Articlehistory:

Received3January2015

Receivedinrevisedform24March2015 Accepted24March2015

Availableonline8April2015

Keywords:

Dissimilarmetalweld Creep

Nickelfiller Superheater PWHT

ABSTRACT

Dissimilarmetalweld(DMW)jointbetweenalloyedsteel(AS)andstainlesssteel(SS) failed at one of intermediate temperature superheater (ITSH) tube in steam/power generationplantboiler.Theprematurefailurewasdetectedafterarelativelyshorttimeof operation(8years)wherethecrackpropagatedcircumferentiallyfromASsidethroughthe ITSHtube.Apartfromphysicalexamination,microstructuralstudiesbasedonoptical microscopy,SEMandEDXanalysiswereperformed.Theresultsoftheinvestigationpoint outthelimitationofCarbidesprecipitationatthealloyedsteel/weldinginterface.Thisis synonymofcreep stageI involvementin thefailureof ITSH.Improper post-welding operationandbendingmomentareconsideredasrootcausesoftheprematurefailure. ß2015TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC

BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Correspondingauthor.Tel.:+96633430333. 1

Retired.

ContentslistsavailableatScienceDirect

Case

Studies

in

Engineering

Failure

Analysis

j o urn a l hom e pa g e : ww w . e l se v i e r. c om / l oca t e / cs e fa

http://dx.doi.org/10.1016/j.csefa.2015.03.006

2213-2902/ß2015TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Inthepresentcase,ASandSSsuperheatertubesarejoinedbynickelbasefillerweldingwhichisknowntohavelow tendencyofcarbidesformation.InadditionithasthermalcoefficientveryclosetothelowalloyferriticsteelssuchasSA213 gradeT11andT22[1].So,theoreticallyweexpectlowstresslevelatelevatedtemperatureforthisweldingmaterial.

However,prematurefailureofDMWoccurredonlyafter8yearsofoperationatITSHtube.Allsuperheatertubesare drainableandhorizontaltype.PartofITSHislocatedinsecondpassandthesecondpartislocatedinthefurnacewhereit receivespartorallofheatbyradiation.FailurelocationisindicatedinFig.1andBoilerspecificationsincludingITSHmaterial informationaresummarizedinTables1and2.

Fig.1.SchematicdiagramshowingthefailurelocationintheITSHarea.

Table1

Boilerspecifications.

Boilerdetail Steamcapacity:637t/h;designpressure:106bar;steamtemp.atS.H.outlet5258C Normalfuel Naturalgas

Commissioned 2000 Failurenoticeddate 28/05/2008

Fluegastemp.atITSHinlet 11218Catnormaloperatingloadwithgasfiring SteampressureatITSHinlet 85.0baratnormaloperatingloadwithgasfiring Steamtemp.atITSHinlet 508Catnormaloperatingloadwithgasfiring Approx.tubemetaltemp. 4808C

2. Results

2.1. Visualinspection

Fig.2showstwoboilertubesamples,madefromlowalloysteel(AS)andstainlesssteel(SS)asreceived.Circumferential crackingwasevidentattheareaadjacenttotheweldregionfromtheASsidewithnoplasticdeformation.Therewasno evidenceofbranchingintheobservedcrack.Fig.3showsinnersideviewofSSandASsplittubes.TheASinnersurface appears to have uniform oxide film whereas SS shows some disruption of the oxide film. Table 3 shows chemical compositionsofASandSSsamplesasanalyzedbyOESandcarbonsulfuranalyzer.Thealloycompositionsarematchingthe gradementionedbyplantauthoritiesshowninTable2[6].

2.2. Microstructurestudy

Fig.4ashowslongitudinalviewoftheSSsampleadjacenttotheweld(heataffectedzone).Thephotomicrographshows polygonalferriticcoarsegrainedstructurealong withsomepocketsofcarbideaggregates.In fact,carbideformationis inhibitedbyextra-lowcarboncontent[7].Thiscouldbeexplainedbyanincreaseoftemperatureduringpost-weldingheat

Table2

Materialinformationaboutboilertubeassuppliedbytheplantauthorities.

Basematerials AusteniticstainlesssteelASMESA-213TP347H–OD:50.8mmandthickness4.4mmferriticsteelASMESA-213Gr T12–OD:50.8mmandthickness4.6mm

Weldjoint SingleV

Weldingprocess GTAW(gastungstenarcweldingTIGwelding)bymachine

Weldingfillerwire TGS-70NCb(ofKOBESteelLtd)orWELTIG-m82(ofNipponWeldingRodCo.Ltd).Thisfillerisequivalent to(AmericanWeldingSociety)-AWSA5.14ERNiCr-3andisanInconel-82ROD

Preheattemperature Min.1218C

Fig.2.Boilersuperheatertubessamplesasreceived.AS:alloyedsteel,SS:stainlesssteel.

treatment(PWHT)above5408CwhichisknownasthresholdtemperatureofcreepforStainlesssteelSA213gradeTPH347 alloy[2].InthisSSalloy,carbidecanformat5408Cafter15min,buttheamountofprecipitateistoosmalltoaffectcorrosion behavior[7].Fig.4bshowsphotomicrographofacross-sectionoftheSSsampleawayfromweldingatthecenter.The structureistwinnedausteniticstructurewithvariablegrainsizewhichisquitedifferentfromtheoneviewedlongitudinally neartheweld(Fig.4a).

Fig.5ashowsmicrostructureatthejunctionofASandwelding.TwophasesarepresentinwhichAShasintermetallic dispersion in a matrix (S1)and theweld exhibitsa single homogeneous phase(S2). Fig.5b shows closeviewof the microstructureattherootofweldingwhereelongated(islandtype)grainscoexistwithdispersedparticles.Fig.6shows microstructureofacross-sectionofASboilertubesample(farawayfromwelding)atthecenterlocationinwhichislandsof pearlitearepresentinaferriticmatrix.

Fig.7a–dshowsmicrostructuresofheataffectedzones(HAZ)at4locationsfromtheASmatrixtotheweldingareanamed HAZ1,HAZ2,HAZ3andHAZ4respectively.AtlocationHAZ1,thestructureiscoarsenedpolygonalferriticinwhichatthe interstitialspearlitegrainsarepacked(Fig.7a).AtlocationHAZ2,afinesecondarybainitestructureisobserved(Fig.7b).The microstructureofHAZ3appearsasafinestructureresemblingtosecondarybainite(Fig.7c).HAZ4showsatypicaltempered martensiticstructuremicrostructureinwhichfreecarbidesaggregationisexhibited(Fig.7d).

Table3

ChemicalcompositionsofASandSSsamplesasanalyzedbyOES.

Element AS SS Fe Balance Balance Cr 0.0625 18.687 Ni – 12.306 Mo 0.452 0.241 C 0.143 0.087 S 0.0035 0.003 Si 0.189 1.57 Mn 0.369 1.567 Cu 0.003 0.148 W 0.009 0.036 V 0.012 0.063

Fig.4.MicrostructureofSSsample,(a)neartheweldjunction;(b)awayfromtheweldjunction(200).

2.3. Hardnesstesting

Thehardnessvalues(inBHN)oftheweldedASandSSsamplesshowincreasingtrendsofhardnessatlocationsawayfrom weldtothelocationsattheweld(Table4).Thevaluesobtainedawayfromtheweldingaresignificantlyreducedcomparedto thevaluesrecommendedinASMEboilercodeparticularlyforAS[6].Thisisduemainlytothelossofcarboninfavortothe junctionareawherewenoteanincreaseofhardnessduetothecarbideprecipitationobservedinthemicrostructurestudy. ThehighhardnessvaluecorrespondstothetemperedmartensiteobservedinHAZ4(Fig.7d).

2.4. Scanningelectronmicroscopy

TheweldingareaobservedbySEM(Fig.8)showscrackprogressingwhichisconsideredassourceofweldingfailures.This crackhasnobranchingandseemsjoiningthegrainboundary.Thiscrackcouldbeduetothepost-weldingtreatmentwhich

Fig.6.MicrostructureatthecenterofAStubefarawayfromwelding(200).

inducesresidualinternalstresses.CrackingofweldmetalcanbecriticalevenifEPRIconsideritassecondorderfactorwhich hasledtofrequentservicefailures[8].

Threelocations(regions)areselectedtobeanalyzedbyEDXi.e.regionS1iscarbidedispersedmatrixclosetotheHAZ,S2 showstheweldingareaandS3showsgrainboundaries.

EDXprofileofthematrix(regionS1)showsveryhighconcentrationsofNi(48%)followedbyFe(25%),Cr(18%)andC (3.5%)indicatingthepresenceof(Fe,Cr)carbidesinthematrix(Fig.9).

Table4

HardnessmeasurementsofASandSSsuperheatertubeparts.

HRB EquivalentHBN Location

ASsuperheatertubepart

75.4 138.2 Awayfromweld

80.1 150 NeartheHAZ

92.0 195 Atthewelding

SSsuperheatertubepart

83.8 161.9 Awayfromweld

85.6 167 NeartheHAZ

90.3 186.3 Atthewelding

Fig.8.SEMimageoftheareaaroundthejunctionofASwelding.

EDXprofileshowninFig.10indicatesastrongpresenceofNi(43%),Fe(31%),Cr(16%),andC(4.9%).Thisresultconfirms thenickel-chromebasecompositionofthefillerasmentionedinTable2.Thistypeofweldingalloyhasverylowtendencyto formcarbidesandthermalcoefficientveryclosetoferriticASsteel[1].

Fig.11showsEDXprofileofregionS3representinggrainboundariesalsoindicatesthehighcontentsofcarbon.This carbideprecipitationisknowntobeinducedbytemperatureincreaseleadingtothecarbondiffusiontothegrainboundaries.

3. Discussion

Based on the above investigation results, it appears that dissimilar metal welds (DMW) failed on the AS side. Microstructurestudyshowedcarbonmigratedfromalloysteelintotheweldmetal.Thismigrationresultsintheformationof chromiumironcarbidesalongthewelding/ASinterfaceasobservedbymicroscopeanddetectedbyEDXanalysis.Two consequencesofthiscarbidesprecipitationcouldbementioned:

Fig.10.EDXprofileofpositionS2(weldedarea).

1.ThedecreaseofthecreepstrengthofAS(whichisafunctionofcarboncontent)neartheweldinginterface. 2.Theincreaseoftheweldinghardness.

ThefailureofDMWintheformofmicrocrackobservedinthefillerweldingmaterialjoiningtheASgrainboundarycould beduemainlytothreereasons:

a.Highhardnessoftheweldingcombinedtothethermalfatigue, b.ResidualthermalstressescausedbyimproperPWHT,

c.BendingstressesduetothehorizontalpositionoftheweldingintheITSHtubeswhichisnotrecommendedbyEPRI guidelinesasdiscussedbyFrench[9].

Thethermalfatiguemechanismpossibilitycouldbeexcludedformanyreasonsi.e.thegoodthermalstabilityofthefiller anditsthermalexpansioncoefficientwhichisclosetotheoneofAS.Inaddition,theboilerwasincontinuousserviceto supplynecessarysteamforthermaldesalinationplant(duetothehighwaterdemand).Therefore,thenumberofthermal cyclesismuchreduced.

The possibilityof animproperPWHT is reinforcedbythedetection ofcreep stageI (carbidesspheroidization and precipitation)intheareaclosetotheweldingwhileitwasnotobservedfarfromthefailurearea(basematerial).Itiswell establishedthathightemperaturesencounteredduringeitherpost-weldingheattreatment(PWHT)orserviceprovidethe activationenergyforcarbondiffusiontooccur[10].ToillustratethatimproperweldingcouldbetherootcauseofDMWin shorttimecouldbefoundintheworkdonebyUl-Hamidetal.[11]wherethelifetimewaslessthan2.5years.

OntheotherhandandaccordingtoEPRIrecommendations[12],theusedfiller(Inconel82)doesnotneedPWHT. Thethirdreasonrelatedtothebendingstresses,isdependentonthestart-upprocedureandthehorizontalpositionof ITSHtubesandclosetothestringertubes.AsdepictedinFig.1,theimpactofbendingmomentcouldbeconsideredsincethe failurelocationveryclosetoastringertube.

4. Conclusions

1.Thefailuremechanismofintermediatetemperaturesuperheater(ITSH)appearstobedissimilarmetal(SSandAS)weld (DMW).

2.ThemicrostructuralstudiesoftheweldsamplessupportedbyEDXanalysisshowtheprecipitationofcarbideattheweld/ metalinterfacewhichisastrongevidenceofcreepstageIprocessinvolvement.

3.Bendingstressesand/orimproperweldingoperationappearstoplaysomerolecrackformationintheweldstructure. 4.Themiss-applicationofrecommendedNi-baseweldingfillerplaysaneffectiveroleinshorteningthelifetimeofthis

junction.

5. Recommendations

1.Thereappearstobeshortcomingsduringtheweldingofdissimilarmetalboilertubesresultinginthefailureoftubes. Therefore,thestandardprocedureforproperweldingshouldbestrictlyfollowed.

2.Strictcontroloftheboilerstart-upprocedureshouldreducethepossibilityofbendingstressesonhorizontalDMW.

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