Development of Stress Reduction Method in the Pipe Welded Zone

Stress Corrosion Cracking (SCC) failures have occurred in the vicinity of austenitic stainless steel pipe welds used in boiling water reactors, since the late 70s. One of the initial countermeasures against SCC has been to use low-carbon stainless steel. However, in older plants, SCC failures in low-carbon stainless steel pipe were still observed in recent years. It is well understood that residual tensile stress due to welding largely affects occurrence and growth of SCC in low-carbon stainless steel. Because of this, it is important to reduce the residual tensile stress in the welded zone in addition to utilizing less susceptible material. However, a countermeasure to reduce residual tensile stress in small-bore pipe has not been established. In this study, a new stress reduction method is developed in order to reduce residual tensile stress in the welded zone, even for small-bore pipe. This method is applied to the butt-welded zone while the pipe is filled with water. First, the pipe is frozen at two points centering around the welded zone by cooling the outer surface of the pipe to transform the water to plugs of ice. The volume inside the pipe between the ice plugs becomes watertight and pressurized by the expansion of the plugs up-on freezing. The pipe expands near the zone of the weld groove, because the wall thickness of the butt-welded zone is thinner locally because of the weld counter bore. In this process, tensile strain occurs near the welded zone due to the pipe expansion beyond its plastic range, and then hoop tensile plastic strain of inner surface is larger than that of outer surface. Once the pressure load is removed, the difference in these tensile strains induces the hoop residual tensile stress on the inner surface to reverse to a compressive stress. On the other hand, the local bending deformation that occurs in the welded zone results in an axial residual compressive stress on inner surface. Both experimental studies and finite element analysis confirmed that the hoop and axial residual tensile stress at the inner surface of the welded zone are reversed to compressive stress by the pipe expansion.