During operation, the volume of protective atmosphere required for safe use in a particular heat-treating furnace depends to a great extent on the:
  • Type and size of furnace
  • Presence or absence of doors and/or curtains
  • Environment (especially drafts)
  • Size, loading, orientation and nature of the work being processed
  • Metallurgical process involved
In all cases, the manufacturer’s recommendations should be followed since they have taken these factors into account during the design of the equipment. Remember that to purge air out of a furnace prior to introduction of a combustible furnace atmosphere requires a minimum of five volume changes of the chamber. This is to ensure that the oxygen content of the chamber is below 1% prior to the introduction of the atmosphere.

Generated atmospheres are classified according to the relative amounts of the individual gases produced. Table 4 provides a list of these classifications according to the American Gas Association (AGA). The gases are divided into six main classes.

Exothermic reactions are heat-producing, while endothermic reactions require heat to promote the reaction. The composition of the atmospheres produced can be changed in a number of ways. Varying the gas/air ratio or using a different “feed” stock (natural gas or propane, for example) will cause the chemistry of the gas to change.