Nitrogen-Enhanced Temperature Dependence of Grain Refinement Strengthening in Austenitic Stainless Steel

Hall–Petch relationships at room temperature and lower temperatures are investigated for 316L steel (low-N steel) and 0.43% nitrogen-added 316L steel (high-N steel) produced by solution nitriding. The yield stress is separated into thermal and athermal components, and the effects of nitrogen on the former are clarified. In the case of low-N steel, the yield stress increases as the temperature decreases, and the degree of the increase is greater for fine-grained steels. This is not only due to the increase in friction stress (including solid solution strengthening) but also results from the increase in grain refinement strengthening caused by the increase in the Hall–Petch coefficient at low temperatures. By contrast, in high-N steel, the temperature dependence of the yield stress is more remarkable than in low-N steel. However, the amount of increase in friction stress at low temperatures is not very different between the low-N and high-N steels. Instead, the increase in grain refinement strengthening caused by nitrogen is greatly enhanced at lower temperatures. This result suggests that the combination of nitrogen addition and grain refining is very effective at strengthening austenitic steels under low temperatures.