Nitriding entails the transfer of nitrogen from a nitrogen-source medium to the surface of the steel at a temperature of 500 – 550 ºC.

During the process a compound layer about 0 – 20 µm thick forms, comprising ɣ´-phase as well as some ɛ-phase, depending on the grade of steel. In addition, a diffusion zone about 0.5 – 0.8 mm thick forms below the compound layer. Because of the slow diffusion rate of nitrogen at the temperatures in question, very long nitriding times, from tens to hudnreds of hours, are needed to obtain an extensive surface hardening depth.

Nitriding can be applied using gas, plasma or a salt bath.

Ref. Steel and its Heat Treatment – A Handbook (8.12.7)