IGSCC of Non-Sensitised Austenitic Stainless Steels in BWR Environment – Effect of Deformation Mode on Grain Boundary Susceptibility

Abstract

Intergranular Stress Corrosion Cracking (IGSCC) of austenitic Stainless Steels (SS) in Boiling Water Reactor (BWR) environment is generic in nature in both the sensitised and the non-sensitized conditions. IGSCC in non-sensitized austenitic SS in the strain hardened condition has been reported without any grain boundary chromium depletion or impurity segregation. The present study ascertains the reason for IGSCC in BWR environment in non-sensitized condition and investigates the effect of nitrogen content in SS on the susceptibility to IGSCC. Two heats of type 304LN stainless steel with 0.08 and 0.16 wt. % nitrogen were used. Strain hardening was done by cross rolling at 200 °C to 20 % thickness reduction (warm rolling) to simulate the weld induced strain in constrained welds. Subsequently, Transmission Electron Microscopic (TEM) examination was carried out on the rolled SS. The deformation mode observed due to warm rolling was predominantly elongated twin and shear band (SB) formation in both the SS, terminating at the grain boundary regions. This resulted in higher stresses and strains making grain boundary susceptible to IGSCC. Presence of more dislocations at grain boundaries is a key feature for such enhancement in the susceptibility of non-sensitized SS to IGSCC. Formation of twins and SB occurred to a greater extent in the SS with higher nitrogen content indicating greater susceptibility to IGSCC in BWR environment. Crack growth studies done in simulated BWR environment at different Dissolved Oxygen (DO) levels showed higher crack growth rates in the SS with higher nitrogen content in the non-sensitised and strain hardened condition, confirming the higher susceptibility of SS with a higher level of nitrogen.