Demonstrating the Effect of Precipitation on the Mechanical Stability of Fine-Grained Austenite in Reversion-Treated 301LN Stainless Steel

According to recent investigations, a huge difference exists in the mechanical stability of austenite between the grain-refined structure states obtained in reversion annealing at 800-700 degrees C or at 900 degrees C, in a 301LN type austenitic stainless steel. Precipitation of chromium nitride occurring at these lower temperatures has been argued to be the factor reducing the stability. To prove this argument, a fine-grained, very stable austenitic structure was created at 900 degrees C in 1 s, and subsequently annealed at lower temperatures between 850 and 750 degrees C, up to 1000 s. It was found that the subsequent annealing at 750 and 800 degrees C resulted in prominent gradual decrease of the mechanical stability under tensile straining, detectable after 10 s annealing duration and continued until 1000 s. Only minimal grain growth occurred, which decreased the stability very marginally. The degree of the stability drop followed the predicted kinetics of the Cr2N precipitation with regards as its dependence on annealing duration and temperature. Further, the tensile yield strength of the fine-grained structure increased slightly due to the annealing. The presence of nano-sized Cr2N particles was verified after 1000 s holding at 750 degrees C. These observations and predictions yield firm evidence for the imperative contribution of precipitation to the highly reduced mechanical stability of grain-refined austenite in this steel.