In Part 4, we continue our discussion of furnace atmospheres.

1. Ammonia (NH₃)

Atomic nitrogen (N) does not normally occur in a furnace atmosphere unless it is purposely introduced by the addition ammonia (NH₃). Most atomic nitrogen immediately reacts with itself to reform molecular nitrogen, which is then inert to iron.

(3) 2NH₃ →  2N + 6H → N₂ + 3H₂

2. Steam

Steam can be used to produce a protective oxide on the surface of component parts as well as create a blueing effect on steel between 573K and 923K. This effect is due to the formation of one of the oxides of iron: Fe₂O₃ (ferric oxide or iron oxide or hematite or red iron oxide), Fe₃O₄ (iron oxide or magnetite or black iron oxide) or FeO (ferrous oxide or iron oxide or wustite). The iron-oxide formation depends on temperature and ratio by partial pressure of water vapor to the partial pressure of hydrogen in the atmosphere.

Iron has an equilibrium constant that favors complete Fe₃O₄ formation below 565°C (1050°F) in a steam atmosphere Above this temperature, a mixture of Fe₃O₄ and FeO is obtained, depending on the H₂O content and the H₂O/H₂ ratio. At temperatures below 830°C (1525°F), H₂O is a stronger oxidizing medium than CO. The reactions are as follows:

(5)        Fe + H₂O = FeO + H₂

(6)        3Fe + 4 H₂O = Fe₃O₄ + 4 H₂

3. Argon (Ar)

Argon is a totally inert gas used in some applications for purging or as a replacement for nitrogen where reaction to the steel surface is of concern.

4. Helium (He)

Helium is a totally inert gas that can be used for purging (uncommon), as a protective atmosphere (uncommon) or as a quenchant gas.

This concludes our discussion of the basic constituents of furnace atmospheres.

References

1. Herring, D. H., Heat Treating and Atmosphere Generation, Chartered Institute Gas Consultancy Program, Institute of Gas Technology, 1979.

2. Korla. S.C., Atmosphere in Furnaces, Lecture 35, NPTEL (National Programme on Technology Enhanced Learning), IIT Kanpur, India.