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 Cr 2 N. 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.
Here the effect of nitrogen on the intergranular stress corrosion cracking (SCC) resistance of sensitized Type 316LN stainless steel containing different amounts of nitrogen is reported. SCC studies were performed at 70% of yield strength. Double-loop electrochemical potentiokinetic reactivation technique was used to quantify degree of sensitization (DOS) that was correlated with SCC resistance. SCC time to failure increased from 220 h to 285 h with increasing nitrogen content from 0.07 wt% to 0.14 wt%, but decreased drastically to approximately 120 h at 0.22 wt% nitrogen (i.e., beyond N solubility limit), due to excessive precipitation of Cr 23 C 6 and Cr 2 N and drastic reduction in the coincidence site lattice (CSL) boundary distribution from 48% to approximately 32%. Scanning electron microscope images showed mixed mode of failure.