Mass transfer of nitrogen from N2–H2atmospheres into Fe–18Cr–Ni–Mn alloys

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Abstract
Fe–18Cr base alloys containing up to 5%Ni and 15% Mn have been nitrided at different nitriding potentials in the temperature range 1373–1573 K. Under the nitriding conditions employed the nitrogen enters the lattice in solid solution. The rate of uptake of nitrogen into solid solution in austenite is determined by the diffusivity of nitrogen in austenite where, typically, the nitrogen diffusion coefficient is 3·0 ± 1·0 × 10−5 mm2 s−1 at 1473 K. Equilibrium nitrogen solubility across a 1 mm thick sample is established within ∼4 h at 1473 K. The minimum nitrogen content for austenite formation at 1473 K in 5%Ni containing alloys is found to be approximately 0·10%N, which is independent of the manganese concentration. The solubility of nitrogen in austenite increases with nitrogen pressure and manganese concentration, and decreases with temperature and nickel concentration. Some positive deviation from Sievert's law behaviour is observed at high nitriding potentials and in turn with high nitrogen concentrations. However, values for the effective interaction parameters eNMn and eNNi of −0·015 and +0·008 respectively (at ∼101·325 kPa pN2 and 1473 K), may be defined so as to quantify the effect of manganese and nickel on nitrogen solubility. These values of the interaction parameters also allow an estimate to be made, with reasonable accuracy, of the solubility of nitrogen in any Fe–Cr–Ni–Mn alloy containing 0–25%Cr, 0–15%Ni, and 0–25%Mn.

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