Research On Regulation Method Of Welding Distortion And Residual Stress For Large Shell Structure

Large welded shell structures such as steel liner in nuclear and shell plating of ship limited by its huge size and complex structure, easy to cause welding distortion beyond the requirements and residual stress concentration, it is one of the urgent problems need to be solved for manufacture and construction of large welded shell structures. Finite element technology (FEM) can predict welding distortion and residual stress distribution, and provide theoretical basis and guidance for the regulation of welding distortion and stress. While FEM accuracy is directly affected by the welding heat source model, which restricts the guiding effect of simulation to welding technology.A new combined heat source model for welding based on a polynomial curve fit of the experimental fusion line was proposed. This model was composed volumetric component and disc source two parts, the outline of volumetric component is defined by a polynomial fitting curve to reflect the heat flux distribution in the weld pool, while a disc source was employed to account for surface heating effects. Since some parameters of the combined heat source were derived from the fitting process of polynomial curve, parameters need to be determined have been reduced and the simulation accuracy of the combined heat source could be improved. Compared with traditional double ellipsoid and conical heat sources, the accuracies were improved 16.0%,19.0% in shape of fusion zone and 12.7%,15.9% in distortion.Based on the relationship that the shrinkage force and moment of the plastic strain zone should equal with shell element model respectively, one equivalent shell element model was established to predict butt welding distortion by applying the equivalent width of the shrinkage volume region and eccentricity. It can avoid the number limitation of three dimensional (3D) meshes and non-convergence problem of large distortion caused by thermo-mechanical coupling simulation. It was found that the maximum simulation error of one layer steel liner using the shell element model compared with test result is 21.3%.A regulation method for weld stress with two parallel Gaussian heat sources was proposed in this paper. Two oxygen-acetylene flames with Gaussian characteristics were employed to induce tensile stresses at base metal part where is in the vicinity of the weld toes. Because of the initial tensile stress in the weld seam, the superposing stress can be sufficient magnitude during heat-up, and the residual tensile stress around the weld seam could reduce by plastic tension. This regulation method could realize the reduction of welding residual stress and which is suitable for the application of residual stress regulation of large shell structure.Based on the new combined heat source and the parallel Gaussian heating method, the influence parameters of the parallel Gaussian heating process was studied, and the process window for typical weld seam of nuclear steel liner was developed using the obtained influence laws. Residual stress test data showed that the peak longitudinal and transverse stresses at weld surface reduced by 45.0% and 4.3% separately after welded and applied using the parallel Gaussian heating method.Considering the welding characteristic of large shell structures, simulation technology was researched in this paper to predict the welding distortion and residual stress, and then provide technical method for welding process optimization. This research has academic significance and engineering application value to promote the popularization and application of welding simulation technology in large shell structure, and the development of nuclear power technology in China.