Effect of nitrogen on grain refinement strengthening in austenitic stainless steel at 77 K

Austenitic stainless steels are promising for use in extreme cryogenic environments; however, further strengthening at low temperatures is required. The effects of temperature and nitrogen content on the grain refinement strengthening of 316 L steel were examined. Alloys containing up to 0.45 mass% nitrogen were prepared, and their grain size was varied via rolling–recrystallization. Tensile tests at 293 and 77 K revealed that nitrogen increased yield stress, with a stronger effect at 77 K. Hall–Petch analysis revealed that friction stress and Hall–Petch coefficient rose with increasing nitrogen content and decreasing temperature, indicating enhanced grain refinement strengthening under cryogenic conditions. Pile-up model analysis suggested that these effects originate from an increase in critical grain-boundary shear stress rather than changes in elastic constants or lattice parameters. Thus, nitrogen addition is highly effective for strengthening austenitic stainless steels at low temperatures by simultaneously enhancing solid solution and grain refinement contributions.