Nitridation of Three Austenitic Alloys at 1100 °C

Abstract The nitridation of three austenitic high-temperature alloys in 95% N 2 + 5% H 2 environment at 1100 °C was evaluated in terms of gravimetry and investigated by SEM–EDS, EPMA and STEM. Samples made from Alloy 600, 253 MA and 353 MA were exposed for 1 day, 1 week and 3 weeks. Alloy 600 underwent very little nitridation, while 253 MA and especially 353 MA, were heavily affected by nitride precipitation. The nitridation of all three alloys had reached equilibrium after three weeks; the extent of nitridation depending on the chromium activity in the alloy. The kinetics of nitrogen ingress into the alloy depends on nickel concentration, while the rate-determining step in the nitridation process is the nucleation and growth of the nitride precipitates.