Thermochemical production of nitrogen stainless steels

Abstract

Laboratory studies have shown that nickel-free fully austenitic stainless steel strip can be produced via a simple thermochemical route. Fe-18Cr alloys containing 0 and 5% Ni and 0 to 25% Mn have been nitrided in nitrogen-hydrogen atmospheres in the temperature range 1373–1573K. Provided the gas atmosphere is sufficiently pure, the rate of uptake of nitrogen into solid solution in austenite is determined by the diffusivity of nitrogen in austenite where, typically it has been found that the nitrogen diffusion coefficient is 2·5 × 10−11 m2s−1 at 1473K. Equilibrium within a 1mm thick sample is established within 4h at 1473K. The solubility of nitrogen in austenite increases with nitrogen pressure and manganese concentration, and decreases with temperature and nickel concentration. Fe-18Cr alloys containing manganese are fully austenitic at 1473K if the nitrogen concentration exceeds 0·4%. However, the minimum partial pressure of nitrogen necessary to give an austenitic structure is lowered by increasing the manganese concentration. The addition of 5% Ni reduces this nitrogen concentration by 0·25% demonstrating that 0·05% N is equivalent to about l% Ni in promoting the formation of austenite. The condition by which the surface of substantial section sizes can be transformed into an austenitic material in nickel-free alloys has been established.