Study On Multi-wire Tandem Submerged Arc Welding Of High-strength Steel Linepipe

Multi-wire tandem Submerged Arc Welding process is one of the key technologies in the production of steel linepipe with thick wall.The quality of weld joint directly determines the quality of steel pipe.After the longitudinal weld seams of high strength steel linepipe have been welded by multi-wire tandem SAW,grains become coarsening in heat affected zone,and the weld joints get softening.In this study,the effect of the heat input on the microstructure and mechanical properties of the weld joints are investigated by means of numerical simulation and welding experiments,so that the optimized welding procedure and technical measures for improving the performance of joints are put forward.Thus,it is of important theoretical significance and practical application value.The grade X80 pipeline steel with thickness 22 mm was selected as base metal to carry out the welding tests.X-type groove with root face was designed,and four-wire tandem submerged arc welding process was used to join the inner and outer weld seams.For the welded specimen,metallographic observations were made,and microhardness test,tensile test,Charpy impact test,etc.were conducted to measure the mechanical properties of the weld joints.The results show that the microstructure of the weld zone mainly consists of acicular ferrite,and also contains a certain amount of polygonal ferrite and elongated strip bainite composition.The coarsened grain heat affected zone consists of granular bainite and coarsened polygonal ferrite.The fine grain heat affected zone is mainly composed of small bulk of pro-eutectoid ferrite and granular bainite,which contains a small amount of quasi-polygonal ferrite and few of M-A constituents.The hardness of the inner and outer weld joints is in the distribution of capital letter W.The mean value of hardness in heat affected zone is between the base metal and the weld metal,and there is a certain degree of softening.The tensile fracture appears in the base metal,indicating that the tensile strength of the weld joint is greater than that of the base metal,which is consistent with the Selection principle of welding materials with high strength matching.The impact energy of the inner bead and outer bead are beyond 200 J,and there are many dimples on the fracture section which indicates that the impact toughness of the weld joint is very well.According to the standard of SPEC API 5L,the various test results meet its technical requirements.Based on the characteristics of four-wire LSAW welding process,four double-ellipsoid volumetric heat source models were used to integrate/superimpose the heat flux density,and the factor of wire inclination was considered.A mathematical model for thermal process in multi-wire submerged arc welding was established.Taking the electromagnetic force,arc pressure,surface tension,buoyancy and so on into consideration,the model for liquid metal flow in weld pool was developed.The welding temperature field,the molten pool flow field and the welding thermal cycle of the inner bead and the outer bead were numerically analyzed and studied.Firstly,the dynamic evolution of the temperature field and molten flow field during the stages of initial welding,quasi-steady state and cooling stage in the four-wire LSAW welding process were analyzed.For the inner weld,a single molten pool is formed at instant 1.0 s,and quasi-steady state is achieved at 3.7 s.In numerical simulation,the liquid metal is set to start cooling process at 6.0 s.Then it enters the liquid-solid phases cooling stage at 7.9 s,and finally enters solid-state cooling stage at 11.7 s.During the whole process?the initial welding;quasi-steady state,cooling stage?,the depth and width of weld pool and the peak temperature all go through a process of firstly ascending to a platform and then descending.The shape of the molten pool changes from the increasing of multi pools to a bigger single pool and then from a contracting single pool to multi pools state.During the stage of quasi-steady state,the peak temperature is about 2613 K,and the maximum length,width and depth of the molten pool are respectively 132 mm,24.2 mm and 13.9 mm.The location of the maximum depth and width is behind the fourth welding heat source,at the transverse cross-section corresponding to this location there are four symmetrical flow loops,and four clockwise flow loops at the longitudinal cross-section.According to the calculated welding thermal cycle curves,for a series of points along a line perpendicular to the welding direction,the farther away from the fusion line,the lower is the peak temperature,and the longer is the cooling time.During the cooling stage,a substage with very slow cooling rate is emerged,which is caused by the latent heat of liquid-solid phase transformation.The cooling time t_8/5 can be calculated by the analysis of welding thermal cycle curve.Compared with the SH-CCT curve of grade X80 pipeline steel,the microstructure consisted of 5.7%ferrite and 94.3%bainite and the microhardness of about230 HV10 can be predicted.The results are consistent with the hardness test of the weld joint.In addition,the effects of different welding parameters on the thermal cycle parameters in HAZ and the molten pool shape in the quasi-steady state are discussed in this study.With the increasement of welding speed or the distance between welding wires,the weld penetration and cooling time t_8/5 are significantly reduced,but the weld width and peak temperature are slightly reduced.With the increasing of the preheating temperature,the weld penetration and cooling time t_8/5 are substantially increased,while the weld width and peak temperature are slightly increased.