The proper selection of a furnace atmosphere is highly dependent on the type of heat treatment being performed. For ferrous alloys, these atmospheres can either be chemical inert (i.e. passive to the surface) or chemically reactive (i.e. reactive to the surface).
Examples of the uses of furnace atmospheres include:
- Purging air (oxygen) from a furnace
- Controlling the surface chemistry to prevent oxidation and/or reduction reactions from occurring
- Controlling the surface chemistry to allow oxidation and/or reduction reactions to take place
- Avoiding decarburization of the surface
- Allowing surface chemistry reactions for the purpose of introducing a chemical species such as carbon (carburizing) or nitrogen (nitriding)
In all cases, it is important to understand the chemical constituents and reactions (i.e. interactions) of the furnace atmosphere gases in order to measure and control them. These gases include:
1. Nitrogen (Chemical Symbol: N or N2)
Molecular nitrogen (N2) is essentially inert to iron in steel and is used for purging and in some instances can be used as a protective atmosphere for low-carbon steels. For use with high-carbon steel, the gas must be completely dry, otherwise decarburization occurs. Atomic nitrogen (N) is not considered an inert or protective atmosphere because it will combine with iron. At high temperatures, nitrogen may show reaction with molybdenum (Mo), titanium (Ti), chromium (Cr) and cobalt (Co). Molecular nitrogen will, under certain circumstances, convert to atomic nitrogen and chemically react with the surface of the material being heat treated, for example when processing stainless steel in the temperature range of 1010-1120°C (1850-2050°F range).
2. Hydrogen (Chemical Symbol: H or H2)
Hydrogen is a reducing gas and is used where reducing atmosphere is required. It may be used to prevent oxidation of iron. The decarburizing effect of hydrogen on steel depends on furnace temperature, moisture content (of the gas and of the furnace), time at temperature and the carbon content of the steel.
FeO + H2 = Fe + H2O
Fe3O4 + H2 = H2O + 3FeO
Hydrogen can also be used to decarburize the steel for certain applications. At material temperature greater than 700°C (1292°F), the following reaction occurs: C + 2H2 = CH4
Atomic hydrogen (H) may be absorbed by the metal at elevated temperatures and cause hydrogen embrittlement. Elements containing titanium (Ti), niobium (Nb) and tantalum (Ta) are especially prone to attack.
This discussion continues in Part Two.
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.