Residual Stress and Crack Mitigation for Dissimilar Metal Welded Nuclear Pipes

Welding is a important manufacturing process in the fabrication of small modular reactors. However, welding often produces high residual stress in the weld or heat-affected-zone (HAZ). Many stress mitigation techniques were investigated in nuclear industry including weld overlay and mechanical stress improvement process (MSIP). The MSIP has been used to mitigate the residual stress in the pipe welding such as to control stress corrosion cracking (SCC) in dissimilar metal welded pipes. This paper optimizes this MSIP process with finite element analyses to achieve the maximum residual stress reduction. The effect of three parameters in MSIP, the clamp tool edge to the weld center (a), the clamp width in axial direction (b), and the applied pressure (p) on weld residual stress distributions were studied. A set of optimal process parameters for the dissimilar metal welded joint was predicted. In addition, the action of MSIP was further understood and demonstrated with numerical results and described in contour plots. The plots showed that both axial and hoop weld residual stress on more than half of the pipe wall thickness near the weld became compressive after applying MSIP, although they were highly tensile after welding. MSIP shifted the hoop tensile residual stress from the weld and heat-affected zone to the MSIP locations which are not sensitive to SCC.