Variation of mechnical properties with nitrogen content in low activity martensitic steels

The effect of nitrogen on mechanical properties in low activation martensitic steel has been studies. Hardness, tensile, and impact test were performed at room temperature and high temperature. Hardness increased with nitrogen content, and hardness showed maximum value when the nitrogen content is 0.08wt.%. But the secondary hardening due to the precipitation of carbide and nitride appeared at about 500 .deg. C regardless of nitrogen content. Tensile strength and yield strength also increased with nitrogen content, but elongation decreased with nitrogen content. The maximum strength was obtained at 0.08wt.% nitrogen content. The increase of nitrogen content decreased the DBTT, and upper shelf energy was higher at low nitrogen steels. But the maximum DBTT was about -30 .deg. C in steels contained higher nitrogen. The average size of martensite lath width was decreased with nitrogen content. The minimum lath width was obtained at 0.08wt.% nitrogen content. The results showed the similar tendency with hardness and tensile test results. So it seemed that the hardness and tensile strength had a relationship with martensite lath width. It seems that the optimum nitrogen content is 0.08wt.% as considered from the impact and tensile test results