Carbo-nitriding is a variation of carburising where both carbon and nitrogen bearing species are used in the gaseous state, usually the inclusion of ammonia in with the carburising gas mixture. Carbo-nitriding is carried out in the austenite state, i.e. temperatures above 850°C, typically 870°C. The case depths are typically lower than those achieved by carburising alone however the surface hardness levels can be higher.