Nitrogen is one of the most effective elements to increase not only strength but also corrosion resistance of stainless steels. Since high nitrogen stainless steels have been produced in the commercial scale by the recent pressurized melting methods, the research on high nitrogen stainless steels is now vigorous all over the world. In this work, an experimental low carbon and high nitrogen martensitic stainless steel, 0.004C-0.5N-18Cr-1Mo (mass%) , was melted under 0.9MPa in a pressurized induction furnace and the 20mm round bars were quenched from 1373K to 77K and tempered at 473K to 823K for 3.6ks. The corrosion resistance was evaluated from the pitting potential in 3.5% NaCl aq. solution.
The hardness of the steel increased with tempering temperatures and reached about 56HRC at 623K through 773K. On the other hand its corrosion resistance decreased drastically when it was tempered over 723K, while the bars as-quenched or tempered under 723K exhibited the very high resistance comparable to austenitic Type 316. The TEM observation revealed that the deterioration in corrosion resistance is due to the precipitation of beta-Cr2N at pre-austenite grain boundaries.
