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Furnace atmospheres play a vital role in the success of the heat-treating process. It is important for the heat treater to understand why we use them and what the best atmosphere is for a specific application. Many different types of atmospheres are used. It is critical to understand how a particular atmosphere is chosen as well as its advantages and disadvantages and to learn how to control them safely.

    In simplest terms, the purpose of a furnace atmosphere varies with the desired end result of the heat-treating process. The atmospheres used in the heat-treating industry have one of two common purposes:

•   To protect the material being processed from surface reactions, i.e., to be chemically inert (or protective)

•   To allow the surface of the material being processed to change, i.e., to be chemically active (or reactive)

    Let’s take a look at several examples to understand what we mean.

When the Role of the Furnace Atmosphere  is to be Chemically Inert

A high-carbon steel AISI 52100 bearing race is hardened at 845°C (1550°F) in an endothermic gas atmosphere and held “neutral” to a 1.00% carbon steel so that the surface of the bearing race will not be subjected to decarburization (the removal of carbon from the surface of the steel) or carburization (the addition of carbon into the surface of the steel). By the use of this type of heat treatment, a soft race can be machined to close tolerances and then hardened to prevent excessive wear in service while maintaining an excellent surface finish.

When the Role of the  Furnace Atmosphere is to be Chemically Active

A low-carbon steel AISI 12L14 retaining ring is gas carbonitrided at 845°C (1550°F) in an endothermic gas atmosphere enriched with natural gas and ammonia to provide a source of both carbon and nitrogen for absorption into the surface of this 0.14% carbon steel. By the use of this type of heat treatment, a shallow “case” is imparted onto the surface of the retaining ring to prevent excessive wear in service while maintaining an excellent surface finish.

    Note the similarities in the heat-treatment parameters (temperature, basic atmosphere type) and desired end use in each of these examples. The choice of material, 52100 versus 12L14, is based on the design and service demands of the product as well as the cost of the raw material and associated manufacturing steps required, all of which dictate the type of heat treatment to be used. Once all these variables are known, the purpose of the atmosphere (inert or active) can be determined.

    Thus, the proper selection of a furnace atmosphere is highly dependent on both the desired purpose of the atmosphere and/or the type of heat treatment being performed.

    Additional 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. The reaction gases include the following.

Nitrogen (N or N₂)

Molecular nitrogen (N₂) is essentially inert to iron in steel. It is used for purging and in some instances can be used as a protective atmosphere for low-carbon steels. The gas must be completely dry for use with high-carbon steel, 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).

Hydrogen(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.

(1)     FeO + H₂ = Fe + H₂O

(2)     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:

(3)     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.

Oxygen(O₂)

Oxygen reacts with the iron in steel to form iron oxide (Fe_xO_y) and also reacts with carbon to lower the carbon content of steel. Oxidation of iron occurs in the presence of oxygen and increases in severity as the temperature is raised. In addition, oxygen will decarburize steel. If steel is to be kept bright during heat treatment and free of decarburization, oxygen (O₂) should be avoided.

    Oxidation in air involves reactions that are irreversible and, in general, not controllable due in part to the fact that the relative humidity changes constantly, affecting the oxygen potential. The result of thermal oxidation in air is almost always unsatisfactory with respect to thickness and composition uniformity.

(4)      2Fe + O₂ = 2FeO

(5)      4Fe + 3O₂ = 2Fe₂O₃

(6)      3Fe + 2O₂ = Fe₃O₄

Water Vapor (H₂O)

Dew point is a temperature at which gas is saturated with water vapor at 100% relative humidity. In other words, dew point is a measure of the amount of moisture present in a gas and quantifies the concentration of water vapor in the atmosphere. In a furnace atmosphere, the water-gas reaction (Eq. 7) relates the concentration of H₂, H₂O, CO and CO₂ in the atmosphere.

(7)     CO + H₂O = CO₂ + H₂

         (water-gas reaction)

Dew-point analyzers look at the H₂O/H₂ ratio in the water-gas reaction. Infrared and oxygen-probe analyzers look at the CO/CO₂ ratio in the water-gas reaction.

    Water vapor (H₂O) is a strongly decarburizing gas. Therefore, any constituent (such as CO₂) will have a tendency to form water vapor. Thus, CO₂ must also be closely controlled. In addition, to prevent decarburization by water vapor, the carbon monoxide (CO) and hydrogen (H₂) must be present in amounts to satisfy the equilibrium condition at each temperature.

    Water vapor and CO₂ oxidize and decarburize steel. Hydrogen is formed when H₂O vapor oxidizes iron. To prevent oxidation and to keep iron bright, therefore, a definite excess of H₂ over H₂O vapor is required for each temperature. Water vapor present in a furnace atmosphere is oxidizing to iron and also combines with carbon in the steel.

Carbon Dioxide (CO₂)

CO₂ is one of the reaction products when a fuel is burned in air. CO₂ oxides iron at elevated temperatures. To prevent oxidation, it is necessary to have an excess of carbon monoxide (CO). Therefore, CO is a desirable constituent to prevent oxidation. CO₂ is not only oxidizing to steel, but it is extremely decarburizing. To prevent decarburization, CO₂ must be controlled very closely. The actual amount depends upon the CO content, temperature and the carbon content of the steel. Carbon dioxide is a mildly oxidizing gas and forms oxides with iron at elevated temperatures. At temperatures greater than 540°C (1000°F), the following reaction may occur:

(8)      Fe + CO₂ = FeO + CO

    At temperatures lower than 540°C (1000°F), the following reaction may occur:

(9)      3FeO + CO₂ = Fe₃O₄ + 3CO
Decarburization may also result by a reaction like:

(10)    Fe₃C + CO₂ = 3 Fe + 2CO

(11)    C + CO₂ = 2CO

    At temperatures above 760°C (1400°F), CO₂ can react with carbon to produce CO.

Carbon Monoxide (CO)

Carbon monoxide is a highly reducing gas. The reversible reaction of CO to form carbon and CO₂ is of particular interest in a furnace atmosphere. CO has a high carbon potential and becomes increasingly more stable at elevated temperatures. It is only at lower temperatures in the range of 480-730^?C (900-1350^?F) that CO will create carbon in the so-called “carbon reversal” reaction (Eq. 12). Carbon in the form of soot can have a negative influence on the heat-treatment process.

(12)    2CO = C + CO₂

Hydrocarbons

Methane (CH₄), propane (C₃H₈), butane (C₄H₁₀) and propylene (C₃H₆) are common hydrocarbons.

    Hydrocarbons are used both as fuel gas and introduced directly into the atmosphere as a source of carbon supply in the furnace. Their chemical activity depends on their thermal decomposition and tendency to form nascent carbon at the surface of the steel. Reactions involving these gases are also dependent on the temperature of the furnace and of the workload.

    Under certain circumstances when hydrocarbons dissociate, free carbon in the form of soot can be created. This free carbon acts both as a barrier to slow or prevent carburization (i.e., carbon pickup by the steel) and as an insulator retarding the cooling rate during quenching.

Ammonia (NH₃)

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

(13)   2NH₃ = 2N + 6H → N₂ + 3H₂

Steam

Steam can be used to produce a protective oxide on the surface of component parts as well as create a bluing effect on steel between 300-650°C (572-1202°F). 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:

(14)        Fe + H₂O = FeO + H₂

(15)        3Fe + 4 H₂O = Fe₃O₄ + 4 H₂

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.

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.

Summary

We trust you have found this discussion of the basic constituents of furnace atmospheres helpful. Our experts regularly speak on topics of interest in “The Experts Speak” blog at www.industrialheating.com. IH

For more information:  Contact Daniel H. Herring, “The Heat Treat Doctor,” The HERRING GROUP, Inc., P.O. Box 884, Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: dherring@heat-treat-doctor.com.

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.

This article is adapted from several blog posts from The Experts Speak. Each month, new and web-exclusive content is posted to provide additional and helpful information for our readers. Check out our expert blogs at www.industrialheating.com/blogs.