Figure 1. Case depth plotted against time for various temperatures (data based on information from about 30 plants)[1]

I can’t find a lot of information on times, temperatures and case compositions for gas carbonitriding. Can you help?

Figure 2. Effect of ammonia concentration on case composition[2]

Carbonitriding is considered a modified form of gas carburizing in which both carbon (C) and nitrogen (N) are introduced into the surface of the steel.

Case depth is a function of time at temperature (Fig. 1). The data collected in Figure 1 shows considerable spread and is represented as a band width, explained by the original authors[1] 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.”

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. 2).

Figure 3. Effect of temperature on case composition[2]

Carbonitriding is usually carried out in the temperature range of 800-900ºC (1475-1650ºF), but temperatures as low as 700ºC (1300ºF) are sometimes used (Fig. 3). The optimum range appears to be between 845-870ºC (1550-1600ºF). Penetration rates are up to 50% faster than carburizing. For example, a given effective case depth is produced in about the same time with carbonitriding at 850ºC (1560ºF) as with carburizing at 900ºC (1650ºF).

Since carbonitriding is typically done at lower temperatures and for shorter times than gas carburizing, it produces shallower but harder case depths (Fig. 4), usually no greater than about 0.030 inches maximum. Reasons for shallower case depths in carbonitrided parts are:
  • 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.

Figure 4. Hardness, carbon and nitrogen profiles of a typical case[2]

Specific advantages of carbonitriding compared to carburizing are:
  • A carbonitrided case has better hardenability than a carburized case. This is due to the addition of nitrogen into the surface layer. Nitrogen produces
  • A more rapid enrichment in carbon that results in a shorter treatment time at lower temperature
  • A better resistance to wear and to softening (at elevated service temperatures)
  • Carbonitrided parts have better fatigue properties (higher fatigue limits) than carburized parts.
  • The costs of operating a carbonitriding process are lower than for a carburizing process.
  • Carbonitriding can be used for case hardening of powder-metal iron and steel-sintered components.