Inducing δ-ferrite in 316NG austenitic stainless steel without welding: Microstructure and SCC resistance in simulated BWR environment

• Optimized heat treatment formed island-shaped δ-ferrite at grain boundaries. • Influence of δ-ferrite on SCC growth rate was unclear due to limited distribution. • Secondary crack analysis suggests that δ-ferrite resists crack propagation. • Strategy was proposed for uniform and extensive δ-ferrite distribution. This study aims to develop a stainless-steel microstructure with enhanced resistance to stress corrosion cracking propagation in high-temperature water by uniformly distributing island shaped δ-ferrite along grain boundaries. A partially melted zone near the fusion boundary was reproduced through weld-simulation heat treatments to investigate the formation behavior of δ-ferrite. The key material factors and optimal heat treatment conditions required to obtain a microstructure with uniformly and widely distributed island-shaped δ-ferrite along grain boundaries were examined. Crack growth tests were then conducted in a simulated boiling water reactor environment to evaluate the effect of δ-ferrite on crack propagation. The effect of the δ-ferrite on the crack growth rate was not clear. This was attributed to the low crack growth rate and the insufficient distribution of δ-ferrite along the crack propagation path. An investigation of all secondary cracks observed in the cross sections that encountered δ-ferrite revealed that, although the volume fraction of δ-ferrite along the total crack length was extremely small, >50% of the crack tips were located at δ-ferrite. This result suggests that δ-ferrite possesses resistance to crack propagation.