INTERNAL NITRIDATION OF NICKEL CHROMIUM-ALLOYS

The nitriding behavior of nickel-chromium alloys was studied in ammonia-hydrogen mixtures over the range of 700-900-degrees-C. Nitridation rates decreased with increasing chromium content, but the critical amount of chromium for transition from internal nitridation to continuous-nitride film formation was found to be much greater than the critical value to form a continuous-Cr2O3 film during oxidation. In general, internal-nitridation rates were found to obey the parabolic rate law. Parabolic rate constants and activation energies for the diffusion of nitrogen were measured. Very fine precipitates formed at the lowest temperature, increasing in size with increasing temperature. The precipitate number density was found to vary within the internally nitrided zone, decreasing with distance from the gas/metal surface. The precipitate morphology changed also with temperature and distance, becoming Widmanstatten at higher temperatures and/or increasing distance within the zone. CrN formed for all exposure conditions. No Cr2N was detected under any conditions studied.