Effect of cyclic loading on the relaxation of residual stress in the butt-weld joints of nuclear reactor piping

Weld residual stress is among the most important factors in stress corrosion cracking (SCC) of the austenitic stainless steels used for pressure boundary piping in nuclear power plants. To assess the integrity of piping, particularly over long-term operation, it is necessary to understand the effects of cyclic loading, such as that caused by an earthquake, on residual stress. In this study, finite element analyses were performed using an axisymmetric model of a 250A pipe butt weld composed of low-carbon Type 316L stainless steel. The moving heat source was simulated by a double ellipsoid model. The accuracy of the method was verified by comparing the calculated results with experimental measurements. Subsequent to the welding simulation and residual stress analysis, the effects of cyclic loading were studied by applying several axial cyclic loading patterns to the model, varying the maximum load. Higher loading caused greater relaxation of the weld residual stress near the piping welds. It was concluded that cyclic loading on piping butt welds suppresses the SCC growth by reducing the tensile residual stress at the inner surface.