Finite element analyses of the effect of weld overlay sizing on residual stresses of the dissimilar metal weld in PWRs

摘要

Dissimilar metal welds are commonly used in various types of reactors to connect low alloy steel nozzles and austenitic stainless steel piping systems. To mitigate the degradation of stainless steel pipe welds that are affected by intergranular stress corrosion cracking in BWRs, weld overlays have been implemented to provide the structural reinforcement. They have also been applied to repair dissimilar metal welds at nozzles in BWRs and PWRs. A weld overlay can favorable reverse residual stresses from tensile to compressive around interior of the susceptible materials. The ASME Code Case N-504 series suggest that the evaluation of repaired welds should consider residual stresses that are generated by the weld overlay with other applied loads on the system. This study considers a dissimilar metal weld that joins the low alloy steel nozzle to the stainless steel safe end in a typical PWR, and is to calculate the post-overlay residual stress states using ANSYS finite element analysis software. Based on the stress report and site survey of the hot leg nozzle, models for analyzing residual stresses were developed on various assumptions. Computational results demonstrate that the weld overlay process probably provides compressive distributions of axial and hoop residual stresses at the inside surface of the dissimilar metal weld. The residual stress distributions that are induced by weld overlays of various sizes are compared to determine the effect of weld overlay sizing on the residual stresses in the original weld. The comparison reveals that weld overlay length has a greater effect on residual stresses than weld overlay thickness. As the applied overlay length increases, the compressive residual stresses increase markedly. Each nozzle/safe end design is somewhat unique; using a feasible weld overlay dimension in consideration of technical and economic is necessary. This investigation has provided a reference for optimizing the weld overlay design of dissimilar metal welds in PWRs.