Research On The Microstructure And Residual Stress Of The Post-Internal-Welding Joint Of Bimetallic Composite Pipe

In oil and gas field surface engineering,long-distance pipelines,and refining and chemical plants to transport corrosive media,the use of bimetallic composite pipes is a relatively safe and economical solution.However,the brittle and hard structure of the fusion zone in the transition welding of composite pipes exists.And the complicated post-weld stress state caused by the welding of dissimilar metals is the main reason that restricts the widespread use of composite pipes.This article takes the bimetal composite pipe（L415/316L）as the research object,adopts two processes of post-internal welding and external welding transition welding.The welding fusion zone of the composite pipe is studied,and through the combination of experiment and simulation,The residual stress of the composite pipe’s post-internal welding process was analyzed,and the optimization plan for the post-welding residual stress of the composite pipe was proposed.The research results provided theoretical guidance for the formulation of post-internal welding process of the composite pipe and promoted the use of composite pipes.SEM,EDS,EBSD and other analysis techniques are used to analyze the dissimilar steel welding fusion zone existing in the welded joint of the composite pipe,revealing the structure difference and the cause of the formation of the fusion zone in different regions.The brittle and hard structure of the fusion zone of the composite pipe’s post-internal welding process exists at the carbon steel base/inner weld layer（ER70S-6/ERNi Cr Mo-3）interface,and the lath martensite structure vertical fusion line to the inner weld layer weld growth,And ends at the type II boundary,with a width of about 20μm.This is due to the mixing of Cr and Ni elements in the molten carbon steel weld,resulting in non-equilibrium cooling in the region.There are two brittle structures in the fusion zone of the external welding transition welding process joint of the composite pipe,one is the transition layer/carbon steel（ER309L/L415）interface,the width is about 40μm,the reason is the same as the former;the other area is the transition layer/Fill layer（ER309L/CHE507）interface,the width of the brittle and hard layer reaches 5 mm,the transition layer weld is diluted,the solute and the subsequent weld bead are fully mixed to form a brittle and hard structure,resulting in brittle joints.The width of the brittle and hard layer in the fusion zone is related to the liquidus temperature(T_LB)of the base material and the liquidus temperature(T_LW)of the weld.The back welding process of the composite pipe can effectively avoid the problem of brittle and hard structures in the fusion zone of large thickness.According to the actual welded joint morphology,a model of the back inner welded joint of the composite pipe was established,and the SYSWELD software was used to numerically simulate the welding process of the composite pipe,analyze the residual stress distribution of the composite pipe after welding,and use the actual weld cross-sectional morphology to simulate the molten pool shape.The appearance comparison realizes the temperature field check,and the blind hole method is used to verify the welding residual stress field.The results show that the simulation is in good agreement with the test,and the simulation results are highly reliable.The simulation results show that,due to the surfacing welding of the pipe ends,there are large axial and circumferential stresses in the composite pipe before butting;there is a large axial tensile stress in the inner lining of the inner welded joint after the composite pipe,and the outer wall is an axial compressive stress;Circumferential stress in the weld and near-seam zone is tensile stress,while away from the weld is compressive stress,which is basically the same as the residual stress distribution of the external welding transition welding process;and the carbon steel welding of the rear inner welded joint has caused pipeline necking.The axial restraint degree is large,and the cooling of the inner welding seam is hindered,which makes the axial tensile stress level of the inner lining layer significantly higher than that of the outer welding transition welding;the transition layer weld of the outer welding transition welding joint has a large axial tensile stress,which is mainly caused by Dissimilar steel welding materials are caused by differences in thermal properties.In view of the large axial tensile stress of the inner lining layer of the composite pipe after the internal welding process,four different welding sequences are designed,namely the sequence of the inner welding layer is advanced in sequence,and the residual stress of the four welding sequences is determined by the SYSWELD software The distribution is simulated and the welding sequence is optimized.The simulation results show that with the advancement of the welding sequence of the inner welding layer,the degree of radial deformation caused by the welding of the base layer before the inner welding decreases successively,and the axial restraint of the inner lining decreases during the inner welding,and the axial tensile stress of the inner wall is reduced accordingly.The level is significantly reduced.From Sequence 1-4,the axial stress and hoop stress of the inner liner are significantly improved;Sequence 4 is the best welding sequence for the inner liner welding after the base layer is bottomed.