Effect of Size of Butt Weld Repairs on Weld Overlay Residual Stresses

Abstract

Weld overlays have been used to provide repair and mitigation to stress corrosion cracking (SCC) susceptible butt welds in nuclear power plant piping. Among the several advantages associated with weld overlays are the beneficial compressive residual stresses that are developed in the inner portion of the component after application of the overlay. These compressive stresses can provide significant mitigation against SCC in these welds. To determine the residual stresses resulting from the weld overlay process in analytical modeling, a weld repair during original fabrication of the butt weld is typically assumed before application of the weld overlay. If the fabrication records are available, the details of the weld repair can be simulated in the analysis. However, in most cases, the weld records are not easily accessible and in instances where they are available, the quality and completeness of the information are questionable. As such, various conservative assumptions are made on the extent of the weld repair to be simulated in the analytical modeling. In this paper, the residual stress results of an axisymmetric finite element simulation of a bimetallic weld subjected to an inside surface weld repair followed by a weld overlay repair are presented. Three through-wall weld repair sizes (25%, 50% and 75% of the wall thickness without the overlay) assumed to be full 360° around the circumference were considered in the study. The results indicate that for all three weld repair cases, the inside of the configuration is very tensile after the weld repair indicating that regardless of the size of the weld repair, SCC is a possibility. The post weld repair stress distribution of the 50% and the 75% repair cases are similar indicating that an assumed 50% repair is fairly representative of repairs that can be assumed for analysis purposes. The application of the overlay resulted in favorable compressive stresses on the inside portion of the configuration for all the three weld repair cases indicating that regardless of the size of the initial weld repair, the application of the weld overlay provides mitigation against SCC.