Ammonia gas nitriding of Fe-18Cr-9Ni alloy at lower than 823 K

An Fe-18Cr-9Ni alloy, which it had not previously been possible to nitride at temperatures below 873 K, was found to form nitrides in an ammonia gas atmosphere at temperatures as low as 823 K after annealing at low hydrogen pressure at 1473 K. Microstructure and hardness were examined on cross-sections of the nitrided specimens. An internal nitriding layer had formed beneath an external nitriding layer on the specimen surface. Vickers hardness was above 1000 throughout the internal nitriding layer. The nitrides formed at the specimen surface and in the internal nitriding layer were identified using grazing incidence X-ray diffraction and ordinary X-ray diffraction methods, respectively. The external nitriding layer, which was about 6 to 10 m thick, formed on the surface, which consisted of -Fe_2–3N, -Fe₄N, and CrN. Two types of chromium nitride were precipitated by ammonia gas nitriding of the present alloy: CrN in the external nitriding layer and Cr₂N in the internal nitriding layer.