Study on the Mechanism of Intergranular Stress Corrosion Cracking and Analysis of Residual Stress and Work Hardening in Welds of Low-carbon Austenitic Stainless Steel with Hard Surface Machining

摘要

In order to clarify the effects of residual stress and hardening on intergranular stress corrosion cracking (IGSCC) behavior in the welds of Type 316L low-carbon austenitic stainless steel with surface hardening, the residual stress and hardness in the butt joint of pipes as a typical example of the actual structure were estimated; grain boundary sliding was analyzed from the viewpoint of microdeformation. On the basis of these results, the mechanism of IGSCC was discussed by the integrated knowledge between metallurgy and mechanics. The relationship between plastic strain and hardness in hard-machined surfaces near welds was clarified from the experimented relationship and the analysis method by the thermal elastoplastic analysis. Distributions of hardness and residual stress with the actual surface machining could be simulated. It was made clear that grain boundary sliding occurred in the steel at 561 K with the constant strain rate tensile test. By comparing grain boundary sliding behavior between a solution treated specimen and a cold-rolled one, we found that grain boundary sliding in the cold-rolled specimen occurs in smaller strain conditions than that in as-received specimen; the amount of grain boundary sliding in the cold-rolled specimen increases remarkably with an increase in rolling reduction. In addition, we clarified that grain boundary energy is raised by grain boundary sliding. On the basis of these results, we concluded that the cause of IGSCC in the welds of Type 316L low-carbon austenitic stainless steel with surface hardening is the increase in grain boundary energy due to grain boundary sliding induced by residual stress of multipass welding and surface hardening.