Transactions of JWRI, Special Issue on WSE2011 (2011) 9 Study on Burn-through Prediction of In-service Welding Tao HAN*, Yong WANG* and Wei LIU* *China University of Petroleum (East China), Qingdao 266555, China Abstract The pressure of media flowing in the pipe is one of the most important factors that has greatly influence onburn-throughduringin-servicewelding.Theburn-throughphenomenonwasstudiedontheradial displacementwithnumercialsimulationmethodsupplementedbymeansofactualweldingtestsunder differentinnerpressure.Theresultsshowthattheradialdisplacementofthenodebeneaththemolten poolincreaseswiththeincreasingtemperatureofwelding.Whenthetemperatureisclosetothepeak temperature,thedeformationofthenodehasamutation.Futhermore,themutationdegreebecomes larger with higher inner pressure. Under a specific temperature field, the radial displacement of the node withmaximumtemperatureattheinnerwallchangeslinearlywiththeincreasingpressureatthefirst stage,butwhentheinnerpressureisuptoafixedvalue,thetrendcannotbecontinued,andamutation appearsandindictsthattheburu-throughshouldoccurifwedonttakepreventivemeasures.Sothe radial displacement can be seen as the burn-through criterion of in-service welding. KEY WORDS: (in-service welding), (burn-through), (radical displacement), (SYSWELD) 1. Introduction In-serviceweldingisanadvancedtechniquethatis frequentlyemployedintherepair,modificationor extension of high-pressure gas pipelines[1]. When replacing thebrokenpipecausedbycorrosionorlocaldamagein ordertomaintainthesafeoperationoflong-distancegas pipelines,in-serviceweldingrepairtechnologyhas significanteconomicandenvironmentaladvantageswhile comparedwithtraditionalrepairmethods[2-4].Oneofthe primaryconcernswithweldingontopipelinesinactive operationisburn-through[5].Onceburn-throughoccurs, thegasleakislikelytocausepressureburstinginthe pipe-wall and even explosion, which will seriously threaten thesafetyofworkersandthecontinuousoperationof high-pressure pipelines. Domesticandforeignscholarsareputtingmoreand moreresearchemphasisonburn-throughpredictionduring in-servicewelding.Becausehowtopredictandprevent burn-throughisthekeytechniquetoguaranteethesafe in-serviceweldingrepair.PreviousresearchbyBMI (BattelleMemorialInstitute)concludedthatburn-through willnotoccurunlessthemaximuminnerwalltemperature exceeds982 [6].However,Bruceetal.didsome in-serviceweldingexperimentsonAPIX55pipelines,and provedthatburn-throughdidnotoccurwhentheinside surface temperature exceeded 1260which had the safety marginof278 whilecomparedwith982 burn-throughlimit[7].Belangeretal.analyzed burn-through conditions by using water as flowing medium. Hefoundthatthemaximuminnerwalltemperaturedoes notchangewiththeincreaseofinnerpressure[8].In addition,throughexperimentalmethods,theinsidesurface temperatureisverydifficulttomeasure.Sabapathyetal. studiedtheeffectofinternalpressureonburn-throughby translatingthetemperaturefieldintoaneffectivecavityin thepipe-wall[9].Goldaketal.analyzedcircumferential welds during sleeve repair welding and found that the given electrodediameterandtheshapeofthefilletweldhavea signification effect on burn-through[10]. Inthispaper,three-dimensionfiniteelement(FE) modelswereestablishedtosimulateburn-through phenomenonduringin-serviceweldingprocess.Theeffect ofpressureonweldingdeformationhasbeenstudied mainly,aimingatdevelopingaproceduretopredict burn-throughandprovidingthetheoreticalsupporttothe in-service welding repair technique. 2. Numerical Modeling of In-service Welding The3-DFEMmodelandmeshgenerationare established in Visual Mesh. The diameter and the thickness ofthepipeare75and4mm,respectively.Thematerialof pipeis16Mnanditsthermalphysicalpropertiesbasedon temperaturearecalculatedbytheformulain[11].The modeladoptseight-nodehexahedralelementsmesh including23717notesand26934elements.Inorderto reducethecalculatingtimeandimprovethenumerically computational precision, the mesh densification was used in the hardened zone and HAZ which have a high temperature changinggradients.Intheplacefarfrommeltingarea, temperaturechanginggradientsisnotsoobviousandthe mesh is sparsely divided. TheDoubleEllipsoidalHeatSource(DEHS)is adopted, since the result of this model is at better agreement withtheexperiment[12].Thedoubleellipsoidmodelis basedontheworkofGoldak[13].Themodelusesthe following equation to define the volumetric heat flux inside thefronthalfandrearhalfoftheheatsource,asshownin Fig. 1. Study on Burn-through Prediction of In-service Welding 10 Fig. 1Double ellipsoidal heat source configuration Thefronthalfofthesourceisthequadrantofone ellipsoid, determined as following in Eq. (1): 2 2 2 13/ 26 3q exp{ 3[( ) ( ) ( ) ]}ff Q x y zabc a b c 1Fortherearquadrantofthesource,thepowerdensity distribution calculated as following in Eq. (2): 2 2 2 23/ 26 3q exp{ 3[( ) ( ) ( ) ]}rf Q x y zabc a b c 2In Eq. (1) and (2), Q is the heat input; the parameters a, b, c are the semi-major axis of ellipsoid. Comparedwiththetraditionalweldingprocess, in-serviceweldingisatechniquethatappliesweldingonto apipelinewithinwhichthereisflowingmedium.The internalmediumpressurehaveanimportanteffecton in-serviceweldingsafetyandburn-throughsusceptibility. Thus,thenumericalsimulationonin-servicewelding processmustconsidertheeffectofinternalpressure. SoftwareSYSWELDcouldachievethisresearchpurpose. Fig.2showsthewayhowinnerpressureloadsonthe pipe-wall.Thevaluesofinnerpressureare0.3MPa, 0.4MPa,0.5MPa,0.6MPa,0.7MPa,0.8MPa,0.9MPa, 1MPa and 1.2MPa, respectively. Fig. 2Internal pressure loads on pipe model 3. Results and Discussion Hightemperaturemakesthemateriallossesstrength. Whenthereductionofpipe-wallstrengthistoogreat, burn-throughofin-serviceweldingwilloccur.Thus,this paperstudiedtheweldingdeformationonlywheninner wall surface is heated to high temperature. Fig. 3 shows the radialdisplacementcurveatthepeaktemperatureunder 0.7MPa at measured inner points across themolten pool. It can be seen that the center point undermolten pool has the highestweldingdeformation.Whenthedeformation reachestoacertaindegreeandexceedstheplasticlimitof pipe-wall,pipe-wallcannotresisttheloadofinternal mediumpressure,andthenburn-throughoccurs.Sothe most danger point is the one just under the molten pool. Fig. 3Welding deformation curve of the inner points under 0.7MPa across the molten pool at high temperature Fig.4isthechangecurveofradialdeflectionat differentinnerpressures.Itcanbeseenthatasinternal pressureraises,radialdeflectionincreaseslinearly.Atthis stage,radialdeflectionisthefunctionofinternalpressure. Whenmediumpressureraisesto 0.8MPa,thecurveclimbs suddenlyandthecurveslopeincreasessharply.Afterthis threshold value of internal pressure, radial deflection grows gentlyagainwiththeincreaseofinternalpressure.Thus,it can be concluded that thesharp growth of radial deflection underinnerpressureof0.8MPacouldberegardedasthe indication of the occurrence of burn-through. Fig. 4Curve of radial deflection at different pressures Burn-through during in-service welding is actually the convexdeformationofpipe-wallcausedbythecombined actionofweldingstressandinternalpressure.Theauthor thoughtthatthelatterisevenmoreimportantforthe burn-throughsusceptibility.Whiletheplasticdeformation ofpipe-wallexceedstheplasticlimitbearingcapacity,the deformation grows sharply, metal fails, the flowing medium leaksatthefailureposition,andthenburn-throughoccurs. Fig.5isthemacrographoftestspecimenofin-service welding.Itisclearthatburn-throughoccursatthe high-temperatureregionjustbelowmoltenpool,asshown in Fig. 5a and c. Burn-through mainly depends on the local reduction of pipe-wall strength caused by localized heating duringin-serviceweldingprocess.Thelossofmaterial Transactions of JWRI, Special Issue on WSE2011 (2011) 11 strengthattheregionjustbelowweldpoolisthegreatest and the effective thickness is the least. When the remaining strengthcannotresisttheloadofinternalpressure, destabilizationwilloccur.AsshowninFig.5b,theinside surfaceofpipe-wallappearsburn-throughhole.Around burn-throughhole,metalproducesplasticdeformation, whichmakes pipe-wall bendoutward, as shown in Fig. 5c. Fig.6showsthepredictedradialdeflectionofacertain cross-section in the pipe under internal pressure of 0.7MPa. The simulation result is in a good agreementwith practical deformation. Accordingtoaboveanalysis,theconclusioncanbe drawnthatasinternalpressureincreases,theconvex deformationatthehigh-temperatureregionofpipe-wall willgraduallygrow.Thechangecurveofradial deformationwithinternalpressurehasamutationpoint where the deformation has a sharp growth. At this mutation point, burn-through occurs. Internal pressure of this point is regarded as threshold value of predicting burn-through. (a) the top surface of molten pool (b) the back surface of molten pool (c) the cross-section of molten pool when burn-through occurs Fig. 5The morphology of burn-through during in-service welding Fig. 6Pipe-wall deformation profile under 0.7MPa 4. Conclusion Thepressureofflowingmediuminthepipehasa significantinfluenceonburn-throughduringin-service welding.Inthispaper,theburn-throughphenomenonwas studied by analyzing radial displacement of pipe-wall using numericalsimulationsoftwareSYSWELD.Thesimulation resultsatdifferentinternalpressuresareverifiedthrough in-serviceweldingtests.Itcanbeconcludedthatatthe measured point just blow molten pool, welding deformation gradually increases as inner wall surface temperature raises. Whentemperatureisclosetothepeaktemperature,radial deformationofthemeasuredpointhasasharpgrowth. Furthermore, the extent of mutation raises with the increase of internal pressure. Under a given temperature field, radial deflectionattheregionwiththemaximuminnerwall temperatureincreaseslinearlywiththeincreasinginternal pressure.Wheninternalpressureisuptoafixedvalue, weldingdeformationraisessharplyandamutationpoint appears.Theradialdisplacementatthispointcanbe regardedastheburn-throughcriterionofin-service welding. Acknowledgements TheauthorswouldliketothankNationalNatural Science Foundation of China for its funding (51074174 ) of the current project. References [1]P.N.Sabapathy,M.A.Wahab,andM.J.Painter, Numericalmethodsofin-serviceweldingofgas pipelines,J.JournalofMaterialsTechnology.118 (2001) 14-21. [2]M.A.Wahab,P.N.Sabapathy,andM.J.Painter,The onset of pipewall failure during in-service welding of gas pipelines,J.JournalofMaterialsTechnology.168 (2005) 414-442. [3]BruceW.A.,HoldrenR.L.andKiefnerJ.F.,Repair ofPipelinesbyDirectDepositionofWeldMetal Further Studies, PR-185-9515. Edison Welding Institute, 1996, pp. 1-63. 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