Nitrocarburizing is similar to nitriding but, in addition to nitrogen, carbon is transferred to the surface of the steel from a carbon containing medium. The temperature is somewhat higher than for nitriding, about 550-580 ºC. Nitrocarburizing can be applied using gas, plasma or a salt bath

Nitrocarburizing in gas is often preceded by preheating/pre-oxidation, either in a separate furnace or in the nitrocarburizing furnace. Since carbon is transferred to the surface of the steel, the atmosphere must contain, in addition to ammonia, a carbon containing medium. Endogas or exogas have for a long time been used as the carbon source medium; they both contain carbon monoxide, which is the active carbon transferring gas. Other sources are also used today, primarily carbon dioxide, which reacts with hydrogen during the forming of carbon monoxide according to: CO2 + H2  --> CO + H2O.

Hydrogen can either be provided through the breakdown of ammonia, equation above, or as a separate addition. The nitrocarburizing process involves cleaning, loading into the furnace and heating to process temperature 560 – 580 ºC followed by a nitrocarburizing time to provide the desired compound layer thickness and/or diffusion zone depth. A uniform temperature is required throughout the furnace to ensure the compound layer thickness and diffusion zone depth.

Ref. Steel and its Heat Treatment – A Handbook (8.12.8)