Numerical simulation of stress and deformation of in-service welding onto gas pipeline

CHINA WELDING Vol 15 No, 4 December 2006Numerical simulation of stress and deformation of in-servicewelding onto gas pipelineChen Yuhua, Wang Yong, Han Bin and Wang Zhengfang陈玉华,王勇,韩彬,王正方Abstract SYSWELD was used to simulate in-service welding process of gas pipeline of x70 pipeline steel. Welding thermalcycle, stress and deformation of in-service welded joint were studied. The results show that peak temperature of coarse grainheat-affected zone( CGHAZ)of in-service welding onto gas pipeline is the same with routine welding, but tss, tgg and tg/nrease at certain degree. For the zone near welded seam, axial stress and hoop stress in the inner pipe wall are compressitstress when welding source passes through the cross-section that is studied, but residual axial stress and residual hoop stressafter welded are all tensile stress. Transient deformation and residual deformation are allconvexKey words in-service welding, numerical simulation, welding thermal cycle, stress, deformation0 Introduction1 Numerical simulation modelBecause of corrosion wear and other accident dam- 1.1 Geometric modeloccur wSYSWELD was used to build model and calculate albreakage in local area. Especially for old oil gas pipe- the parameters. Sleeve repair welding technology as shownline, there is much incipient fault and need maintenance in reference [2] was used, and welding thermal cycleir. Repairing oil gas pipeline by in-service stress and deformation of the first bead were studied. Be-welding can keep the running of the pipeline and avoid e- cause the first weld is beading on the surface of the pipeconomic loss. The repair speed is fast and less time is and the pipe is an axisymmetric structure, 1/2 model ofneeded. In-service welding has little effect on the running three-dimension is used. The welded joint type of numeriof pipeline and environment 2-6). Using numerical simula- cal simulation is shown in Fig. 1a and the integral elemention to study in-service welding has irreplaceable superiori- mesh of two-dimensional cross-section model is shown inty because of the risk and expensive experimental cost of Fig 1b. The pipeline model is 200 mm long, 8 mm thickdoing experiment on running gas pipeline with high pres- and the extermal diameter is 508 mmsure. In this paper, SYSWELD was used to build three-dimensional numerical model and simulate in-service weld- 1.2 Model of heat sourceing process onto gas pipeline of X70 pipeline steel. Weld-Double ellipsoidal heat source is adopted because it ising thermal cycle, stress and deformation of in-service more accurate 7. Electrode arc welding is used. Weldingwelded joint were studiedcurrent is 150 A, welding velocity is 3. 4 mm/s and theheat input is 13. 2 kI/cm. Parameters of double ellipsoidal中国煤化工CNMHGChen Yuhua, School of Material Science and Engineering, Nanchang Institute of Aeronautical Technology, Nanchang, 330063. ChenYuhua,Wang Yong, Han Bin and Wang Zhengfang, College of Mechanical and Electronic Engineering, China University of PetroleumDongying,257061.E-mail:ch.yuhu@163.com(ChenYuhua)Numerical simulation of stress and deformation of in-service welding onto gas pipelineheat source are obtained by the depth and width of molten273.15+T1pool, then heat source fitting tool of SYSWELd is used to儿=p273.15check the parameters for several times until the simulateden pool shape is similar with the actual welded jointWhere uo is the kinematic viscosity of gas when its temperature Is0℃Because the content of CHa in natural gas is over90%, the thermal physical properties of ch, are used asthermal physical properties of natural gas listed in refer12 o'clock1.4 Thermal physical properties and mechanicalproperties of X70 pipeline steelCoefficient of thermal conductivity and specific heatof X70 pipeline steel are calculated by the formulas in(a)Integral model (b) Mesh model of 2dcross-sectionerence [9]. Mechanical properties of X70 pipeline steelare gotten from reference [10]Fig 1 Finite element mode2 Results and discussionWelding ofrom1.3 Heat transfer boundary conditions and con-12 o'clock that is the top end of numerical model andstraint conditionsends to 6o clock that is the bottom of numerical modelThe heat exchange form between exterior surface of shown in Fig. la. Welding velocity is uniform and everythe pipe and air is radiation and natural-convection heat cross-section of the whole model is considered to undergotransfer. The total coefficient of heat transfer shows as fol- the same welding process. So thermal cycle, weldingstress and deformation of the representative cross section at9o’ clock are studieda2=0.8×5.67×10-8[(273.15+70)+(273.15+T1)]·[(23.15+T)2+(273.15+7)2]+252. 1 Thermal cycle of in-service weldingIn order to study the regularity ofthe thermal cycle curve of CGHaz of in-service welding isWhere To is the environment temperature( here is 25C) contrasted with that of routine welding as shown in Fig.2and T, is surface temperature of the welded joint (C)The parameters of thermal cycle are listed in Table 1. WeThe heat exchange form between inner surface of the can see from Fig. 2 and Table 1 that peak temperature ofpipe and the gas in the pipe is forced-convection heat CGHAZ of in-service welding is almost the same with rou-transfer, and the coefficient of heat transfer istine welding. But cooling speed of in-service welding isfaster obviously, tg/s, tga and tgn of in-service welding de-a1=0.027凡)0.14(2) crease at a certain degree. The decrease extent of tygreatest of the three and the value is 18% of routine weld-In this formula, A, Re, Pr, u represent the conductivity ing中国煤化工factor, Reynolds number, Prandtl number and kinematicviscosity of gas. The inner diameter of pipe is d, u is the 2.2CNMHGpipe wall of in-serY-kinematic viscosity of gas when its temperature is the same ice welded jointwith the inner surface of the welded jointAxial stress and hoop stress are usually considered forCHINA WELDING VoL 15 No 4 December 20061600ment I coincides with moment IV, that is to say, axial1400I Routine weldingstress at moment l can be seen as residual axial stress12002 In-service weldingThe residual axial stress is tensile stress and achieves max-imum(about 226 MPa) in the inner pipe wall under wel-ded seam center. Residual axial stress decreases with thedistance from welded seam center increasing ande400constant(about 100 MPa) when the distance apart200welded seam center is over 60 mm240200MomentⅡ160Fig 2 Comparison of in-service welding thermal cycleMomentⅣ8040Table 1 Comparison of thermal cycle parameters of0in-service welding with routine weldingThermal cycle Peak temperature ty,//sRoutine welding1419.96.926.2202.7020406080100120Distance from weld center d/mmIn-service welding 1418. 84.612.136.6Fig 3 Axial stress distribution of in-service weldinggirth joint of pipe welding. So axial stress and hoop stressat different momentsin the inner pipe wall(OA direction, as shown in Fig 1b)of the cross-section in Fig. 1b are studied. Stress at fourFig 4 shows the distribution of hoop stress in the in-moments such as moment I (one second after welding ) ner pipe wall. The distribution of hoop stress is differentmoment I (120.9 seconds after welding, when welding from axial stress. Hoop stress is about 75 MPa andheat source passes through the cross-section), moment Il doesnt change with the distance from welded seam center(241 8 seconds after welding, when welding is just fin- increasing at moment I. At moment I, compressiveished)and moment IV(1 000 seconds after welding) is hoop stress produces in zone near weld seam and increasesstudiedThestressatmomentI,IandilLrepresentswiththedistanceapartfromweldedseamcenter.com-transient stress during welding procedure and the stress at pressive hoop stress achieves maximum( about 55 MPa)moment IV represents residual stress after weldingwhen the distance is about 4 mm and then decreasesThe distribution of axial stress in the inner pipe wall Hoop stress changes to tensile stress when the distance a-(OA direction) of the cross-section( Fig 1b)is shown in part from welded seam center is about 9 mm and achievesFig 3. Axial stress in the inner pipe wall begins to change to maximum( about 121 MPa)when the distance is aboutat moment I and tensile stress is produced in zone near 37 mm. Hoop stress keeps stationary after moment I asweld seam. Tensile stress in the inner pipe wall under the same with axial stress. Residual hoop stress is tensilewelded seam center is the biggest and is about 40 MPa. At stress and achieves maximum( about 260 MPa)in zoneis produced in zone near nea中国煤化工 tress decreases with theweld seam and increasesCNMHGIncreasing and keeps 75seam center increasing. Compressive stress achieves maxiMPmum(about 123 MPa)when the distance is about 6 mmIn brief, for in-service welding onto gas pipelineand then decreases. Axial stress distribution curve at mo- when welding source passes through the cross-sectionNumerical simulation of stress and deformation of in-service welding onto gas pipelinehich is studied at moment I, the axial stress and hoop ation increases with the approaching of welding thermalstress in zone near weld seam changes to compressive source and achieves maximum when welding thenstress because weld seam is fused and the thermal action is source passes through. Deformation decreases during theintensive at this moment. Stress in inner pipe wall keeps cooling stage after welding and residual deformation isstationary after moment Ill because the cooling speed dur- smaller than that of moment Iing the cooling stage after welding is fast. Residual axialstress and hoop stress are all tensile stress and decreasewhen the distance from welded seam center increases.Moment I06omentⅡMomentⅣ250MomentMomentⅡMomentⅢ0.21000020406080100120Distance from weld center d/mm020406080100120Distance from weld center d/mmFig 5 Deformation of in-service welding at different momentsFig 4 Hoop stress distribution of in-service weldingat different moments(1)Peak temperature of CGHAZ of in-service weld-ing onto gas pipeline is almost the same with routine weld-2.3 Deformation of in-service welded jointing. But cooling speed of in-service welding is faster obvi-Deformation along radial direction of in-service wel- ously, tys, tsa and t of in-service welding decrease at aded joint is the important factor affecting burm-through. certain degree. The decrease extent of ty is greatest ofFig. 5 shows the deformation along radial direction in thehe three and the value is 18% of routine welderinner pipe wall of the cross-section studied at moment I(2)When welding source passes through the cross-Ⅱ,ⅢandⅣ. At moment I, deformation about0.2msection which is studied at moment I, the axial stress andhas produced because of the pressure in pipe. At moment hoop stress in zone near weld seam are compressive stressI, deformation achieves maximum and the maximum de- Stress keeps stationary quickly because of the fast coolingformation of the inner pipe wall under welded seam center speed after welding. Residual axial stress and hoop stressis 0. 52 mm because the molten pool is the deepest and theare all tensile stress and decrease when the distance frostrength of pipe wall is lowest when welding thermal source welded seam centre increasespasses through the cross-section. Deformation decreases(3)Transient deformation and residual deformationwith the distance from welded seam center increasing. At of in-service welding onto gas pipeline are all convex de-moment Il and I, deformation of the inner pipe wall un- formation compared with the original pipe diameter size.der welded seam center is smallest and is almost zeroDefor中国煤化工 n increases with the ap-In a word, transient deformation and residual deform-CNMHe. achieves maximumation of in-service welding onto gas pipeline are all convex when welding thermal source passes through and then de-deformation compared with the original pipe diameter size. creases during the cooling process after weldingDeformation process of zone near weld seam is that deform50CHINA WELDING VoL 15 No 4 December 2006burn-through during in-service welding of gas pipelines. In-[1]Chen Yuhua, Wang Yong, Dong Lixian, et al. 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