Mechanism Study on IGSCC by Analyzing Residual Stress and Work Hardening in Welds of Low-Carbon Austenitic Stainless Steel With Surface Machining

To make clear the effects of residual stress and work hardening on intergranular stress corrosion cracking behavior in the welds of the austenitic stainless steel with surface hardening, residual stress and hardness in butt joints of pipes are evaluated by 3-dimentional thermal elastic-plastic analysis and the grain boundary sliding behavior is examined using constant strain rate tensile test. Based on these results, the mechanism of IGSCC has been clarified by the integrated knowledge between metallurgy and mechanics. The relationship between plastic strain and hardness in hard-machined surface near welds is clarified from the experimented relationship and the thermal elastic-plastic analysis. The distributions of hardness and residual stress with the actual surface machining could be evaluated. It was made clear that grain boundary sliding occurred in the steel at 561K by a constant strain rate tensile test with the numerically simulated tensile residual stress due to welding and surface machining. From the comparison of grain boundary sliding behavior between solution treated specimen and cold-rolled one, the grain boundary sliding in cold-rolled one occurs in smaller strain conditions than that in solution treated one, and the amount of grain boundary sliding in cold-rolled one increases remarkably with increase in added strain. In addition, it is clarified that the grain boundary energy is raised by the grain boundary sliding. On the basis of these results, it is concluded that the cause of intergranular stress corrosion cracking in the welds of Type 316L austenitic stainless steel with surface hardening is the increase in grain boundary energy due to grain boundary sliding accelerated by residual stress of multi pass welding and surface hardening.