Stress Analysis Of In-service Welding Joint Onto Pipeline Steel

In-service welding is performed while the pipe lines are in the working condition, which has effect on the safety of welding and operating characteristic after welding as there are hyperbaric flowing mediators in the pipes when welding. This paper used non-linearity analytical software SYSWELD, analyzed the process and the characteristics of the heat exchange process between the mediators, amended the criterion function of the coefficient of forced-convection heat transfer, and simulated the three-dimensional model of in-service welding by pressing evenly on the pipe wall in view of the changing of material performance on the basis of thermal-elastoplasticity theory. The researches in this paper provide theoretical supporting for the repairing of in-service welding, are helpful for the development and application of the repairing technology in-service welding of gas-oil pipes.The cooling rate of the in-service is faster than the routine welding in the course of cooling comparing differences of the thermal cycles between in-service and conventional welding. The regularities of the residual stress distribution in both conditions are similar, both the axial and circumferential residual stress near the weld bead inwall are tensile stress, but on the ectotheca of the pipe the axial stress is compressive stress, while the circumferential residual stress is tensile stress. Residual stress of in-service welding increases with the increasing of welding heat input and decreases with the reducing of flow rate of the mediator in the pipe; pressure inside the pipe increases will result in the decreasing of axial residual stress and the increasing of the circumferential residual stress; and the axial residual stress near the welded joint decreases while the circumferential residual stress increases as the increasing of the radius. The residual stress distribution of sleeving with multi-pass welding was obtained by building repairing model in-service sleeving welding, and also the effects of the in-service condition on the residual stress with multi-pass welding was researched.The results of the numerical modeling show that the compression deformation generating near the weld bead on the high temperature and the shrinkage while cooling are the prime reason for the residual stress. As in-service welding, the mediators flowing fast accelerate the cooling rate, increase the shrinkage, and decrease the effect of phase transformation on the residual stress; the restriction of the press in the pipe on the deformation"convex inside and concave outside"is an important influencing factor of the residual stress distribution as in-service welding.