Controlled atmosphere

A mixture of gases, the composition of which can be varied to match the required surface carbon content of the material being treated.

Controlled atmospheres normally consist of a neutral or inert carrier gas, which can be used for hardening, and can have additions of active gasses which produce carburising or carbonitriding as required.

Since, when heated in air, steel readily scales and the sub-surface region can suffer decarburisation due to the oxidation of the steel surface, and the loss of oxygen atoms from the sub-surface, hardening must be carried out in a protective or controlled environment, if expensive finishing operations are to be avoided. There are many protective ‘atmospheres’ available, ranging from endothermic and exothermic gas mixtures, to inert gases, such as nitrogen or argon, and molten salt or treatment in vacuum may be employed. Carburising conditions can be obtained when required by the addition of a hydrocarbon gas such as propane to a carrier gas, generally an endothermic gas mixture. Carbonitriding or nitrocarburising conditions can be obtained by the extra addition of ammonia gas to the carburising gas mixture.