Annealing must be done in a carefully controlled atmosphere. Annealing often requires a protective atmosphere, either generated, mixed or pure or vacuum (less common). Air atmospheres, or direct fired furnaces using products of combustion are not typically used in the annealing process, since their use may lead to heavy oxidation and scale. Bell furnaces are commonly used for annealing, as well as other types of furnaces (e.g., box, pit, roller hearth, mesh belt, vacuum).

Choosing a gas (or gas mixture) in which to anneal depends on several considerations: the temperature, the type of metal, allowable decarburization/oxidation amounts, part contamination, and the surface requirements of the product. Common atmospheres include: nitrogen and nitrogen-hydrocarbon blends, hydrogen and hydrogen-nitrogen blends, dissociated ammonia, exothermic and endothermic gas and vacuum.

Source: Dan Herring, The Heat Treat Doctor®, Atmosphere Heat Treatment, Volume 1, pages 345–363.