The carbonitriding of fasteners is of critical importance to their functionality and one of the most common case-hardening heat treatments. Studying how the process works in this application will shed a great deal of light on all carbonitriding processes.

Gas carbonitriding is a modified form of the carburizing process and is not a form of nitriding. In this process, both carbon (C) and nitrogen (N) are introduced into the surface of the steel by introducing ammonia (NH₃) and a hydrocarbon enriching gas into the furnace atmosphere in order to add nitrogen to the carburized case as it is being produced (Fig. 1).

Figure 1.  Surface layers produced by carbonitriding of steel at 850°C (1560°F), where carbon predominates in the formation of a martensitic layer¹

The Carbonitriding Process

Typically, carbonitriding is done at a lower temperature than carburizing – 700-900°C (1300-1650°F) – and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon and what generally results is a shallower case than is typical for carburized parts. A carbonitrided case is usually 0.075-0.75 mm (0.003-0.030 inch) deep.

It is important to note that a common contributor to non-uniform case depths is to begin the ammonia addition before the load is stabilized at temperature. This mistake often occurs in batch furnaces that start gas additions as soon as the setpoint recovers or in continuous furnaces where the parts are not up to temperature. It is better to introduce a time delay so that the entire load reaches temperature. Remember, too, that when the ammonia addition ends, desorption of nitrogen begins.

The temperature range for carbonitriding is not arbitrary. The thermal decomposition of ammonia is too rapid at higher austenitizing temperatures, which limits nitrogen availability. A brittle structure is formed at lower temperatures. Operating furnaces below 760°C (1400°F) can also pose a safety concern.

Carbonitriding has been known by many names over the years, including “dry cyaniding,” “gas cyaniding,” “nicarbing” and “nitrocarburizing.” (Today, nitrocarburizing is used to describe another type of case-hardening process.)

Nitrogen Content of the Case

The nitrogen in carbonitrided steel enhances hardenability and makes it possible to form martensite in plain-carbon and low-alloy steels that initially have low hardenability. Examples include SAE 1018, 12L14 and 1117 that can be transformed in the case to martensite due to the lowering of the critical-cooling rate required to transform the steel. The nitrides formed contribute to a high surface hardness.

Carbonitrided steels cover a broad cross section, including those in the SAE 10xx, 11xx, 12xx, 13xx, 15xx, 40xx, 41xx, 46xx, 51xx, 61xx, 86xx and 87xx series. Carbon contents up to about 0.25% are typical. However, carbon contents as high as 0.50% in these same series are carbonitrided to case depths up to about 0.3 mm (0.010 inch).

Nitrogen – like carbon, manganese and nickel – is an austenite stabilizer. So, retained austenite is a concern after quenching. Lowering the ammonia percentage will reduce the amount of retained austenite and should be done if decreases in hardness or wear resistance cannot be tolerated. Another consequence of too-high nitrogen is the formation of voids or porosity. In general, the nitrogen content at the surface should be no greater than 0.40%.

A common carbonitriding variation is to introduce ammonia near the end of the carburizing cycle, typically 15-30 minutes before the load is quenched. Any loss of hardenability that might occur due to internal (or intergranular) oxidation is partially offset by the nitrogen absorption.

Temper Resistance

Several other points are worth mentioning. The nitrogen in the carbonitrided case increases the steel’s resistance to softening on tempering, just like some alloying elements do. In addition, the greater the nitrogen content, the greater the resistance to softening. This is why higher tempering temperatures up to 225°C (440°F) are often used on carbonitrided parts.

The resistance to tempering manifests itself in improved wear resistance. Carbonitrided gears, for example, often exhibit better wear properties than carburized gears. In addition, many shallow-case, thin-section parts made of unalloyed steel, such as die-cutting punches, can be used without tempering.

Figure 2.  Case depth plotted against time for various temperatures (data based on information from about 30 plants)²

Case depth is a function of time at temperature (Fig. 2).  The data collected in Figure 2 shows considerable spread and is represented as a bandwidth, which is explained by the original authors¹ as “not at all surprising due to differences in definitions of case depth and methods of measuring it, lack of precise knowledge of the length of time for which a given part is actually at the furnace temperature and the effect of variation of heating-up time.”

Figure 3.  Effect of ammonia concentration on case composition³

During carbonitriding, nitrogen is added in the form of ammonia since the dissociation, or breakdown, of ammonia will produce atomic (or nascent) nitrogen as opposed to molecular nitrogen. Atomic nitrogen will combine to form molecular nitrogen, but if the dissociation occurs at the surface of the steel, the nascent nitrogen can diffuse into the steel simultaneously with carbon (Fig. 3).

Figure 4.  Effect of temperature on case composition³

Carbonitriding is usually carried out in the temperature range of 800-900˚C (1475-1650˚F), but temperatures as low as 700˚C (1300˚F) are sometimes used (Fig. 4). The optimum range appears to be between 845-870˚C (1550-1600˚C). Penetration rates are up to 50% faster than carburizing. For example, carbonitriding at 850˚C (1560˚F) and carburizing at 900˚C (1650˚F) produce about the same case depth in the same amount of time.

Figure 5.  Hardness, carbon and nitrogen profiles of a typical case³

Since carbonitriding is typically done at lower temperatures and for shorter times than gas carburizing, it produces shallower but harder case depths (Fig. 5); usually no greater than about 0.030 inch maximum. Carbonitrided fasteners are typically mounted (Fig. 6a and etched Fig. 6b) to reveal their case-depth pattern and microstructure.

Figure 6.  Typical screw case profile after shallow-case carbonitriding

The reasons for shallower case depths in carbonitrided parts include:

• Carbonitriding is generally done at lower temperatures and for shorter times than gas carburizing.
• The nitrogen addition is less-readily controlled than the carbon addition. This leads to an excess of nitrogen in the steel and, consequently, to high levels of retained austenite. Increased porosity may occur when cycle times are too long.

Carbonitriding, like carburizing, is used primarily to produce a hard, wear-resistant case. This case has higher hardenability than a carburized case. Consequently, carbonitriding creates a harder case on low-carbon or low-alloy steels.

The specific advantages of carbonitriding for fasteners compared to carburizing include:

• A carbonitrided case has better hardenability than a carburized case. This is due to the addition of nitrogen into the surface layer.
• More rapid enrichment in carbon, which results in a shorter treatment time at lower temperature
• Better resistance to wear and to softening (at elevated service temperatures)
• Better fatigue properties (i.e., higher fatigue limits) than carburized parts
• Lower cost of operation due to the lower temperatures and shorter cycle times involved

Summary

Carbonitriding is an excellent choice for low-carbon fasteners and similar materials that require a uniform but shallow case with good wear properties. In general, heat treaters performing these processes have a great deal of experience and the practical know-how to control the time, temperature and furnace atmospheres to achieve the required case depths and properties.

References

1. Slycke J., and T. Ericsson, “A Study of Reactions Occurring During the Carbonitriding Process,” Journal of Heat Treating, Vol. 2, No. 1, 1981, p. 3-19
2. Bever, M. R., and C. F. Floe and W. G. Zarbuda, “A Survey of Industrial Carbonitriding Practice,” Armour and Company, white paper
3. Hancock, P. F., “Modern Case Hardening Processes,” Iron and Steel, 1956
4. Herring, D. H., “Gas Carbonitriding,” The Experts Speak (Blog), Industrial Heating, 2010
5. Steels Heat Treatment and Processing Principles, by G. Krauss: ASM International, Materials Park, Ohio, 1990, p. 310-317
6. Key-to-Metals (www.keytometals.com)
7. ASM Handbook, Vol. 4, Heat Treating: ASM International, Materials Park, Ohio, 1991, p. 376-386 and 425-436