Controlled Atmosphere

A gas or mixture of gases having specific or general properties, known either to cause or prevent certain gas-to-metal reactions occurring during heat treatment. Most controlled or prepared atmospheres are produced during heat treatment from a consistent source of hydrocarbon gas, i.e., natural gas (methane), vaporized propane or butane, by a process of total or partial combustion in air in the case of the exothermic type atmospheres or by passing a mixture of the reaction fuel and air over a heated catalyst in the case of the endothermic type atmospheres. An annealing atmosphere can also be prepared for a typical bell annealer by mixing two or more delivered gases, such as using bulk nitrogen with a small “spike” gas like propylene. In metal forming operations such as continuous casting, inert gas atmospheres are maintained to prevent metal oxides from forming.