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d) Charcoal base atmospheres (AGA 400)
It is produced by following reaction
 .
Theoretically atmosphere would consist of  and  . Due to moisture and volatiles in charcoal and incomplete combustion of carbon, the reaction would produce  and  .
Normally the atmosphere is neutral to higher carbon steels but the carbon potential may be increased by adding natural gas.
The charcoal based atmosphere is used for hardening, annealing and normalizing high carbon steels without scale formation or decarburization.
e) Exothermic-Endothermic base atmospheres (AGA500)
They are prepared by combusting a mixture of air and fuel .POC is dehydrated, and a predetermined quantity of hydrocarbon fuel is added. The mixture is made to react in presence of a catalyst.
Typical application includes carburizing and carbonitriding. Due to the cost of production, these atmospheres are not very often used.
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