Enhancing the Intergranular Corrosion Resistance of High-Nitrogen-Containing 316L Stainless Steels by Grain Boundary Engineering via Thermomechanical Treatment

Abstract

High-nitrogen-containing Type 316L stainless steels (SS) with 0.12% to 0.22% N are being developed as future structural material of fast breeder reactors because of their improved hardness and resistance to localized corrosion. However, stainless steels with higher nitrogen content are prone to intergranular corrosion (IGC) due to their tendency to get sensitized by enhanced precipitation of Cr2N. Thermomechanical treatment (TMT) of 6.5% cold-work and heat-treatment (1,323 K for 30 min) is evaluated in this study to enhance IGC resistance of 0.07%, 0.12%, 0.14%, and 0.22% nitrogen-containing Type 316L SS. The frequency of coincident site lattice (CSL) boundaries is found to increase with increase in nitrogen content in Type 316L SS. A maximum CSL increase of 35% was seen in 0.22% nitrogen containing stainless steel, as compared to samples containing 0.07% to 0.12% N. The effective grain boundary energy was the least (<0.1 μm−1) for Type 316L SS containing 0.22% N, which is attributed to the higher percentage of Σ3 boundaries. Double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests conducted on the sensitized as-received and TMT samples showed a clear decrease in sensitization for TMT samples. The improved resistance to IGC visualized in the post-DL-EPR optical micrographs of TMT samples is attributed to the breakdown in the connectivity of attacked boundaries. The role of nitrogen in austenitic SS on twinning and generation of CSL boundaries is also discussed.