Carbonitriding of Steels,Carbonitriding of Steels,Carbonitriding of Steels,Carbonitriding of Steels
CARBONITRIDING is a modified form of gas carburizing, rather than a form of nitriding. The modification consists of
introducing ammonia into the gas carburizing atmosphere to add nitrogen to the carburized case as it is being produced.
Nascent nitrogen forms at the work surface by the dissociation of ammonia in the furnace atmosphere; the nitrogen
diffuses into the steel simultaneously with carbon. Typically, carbonitriding is carried out at a lower temperature and for a
shorter time than is gas carburizing, producing a shallower case than is usual in production carburizing.
In its effects on steel, carbonitriding is similar to liquid cyaniding. Because of problems in disposing of cyanide-bearing
wastes, carbonitriding is often preferred over liquid cyaniding. In terms of case characteristics, carbonitriding differs from
carburizing and nitriding in that carburized cases normally do not contain nitrogen, and nitrided cases contain nitrogen
primarily, whereas carbonitrided cases contain both.
Carbonitriding is used primarily to impart a hard, wear-resistant case, generally from 0.075 to 0.75 mm (0.003 to 0.030
in.) deep. A carbonitrided case has better hardenability than a carburized case (nitrogen increases the hardenability of
steel; it is also an austenite stabilizer, and high nitrogen levels can result in retained austenite, particularly in alloy steels).
Consequently, by carbonitriding and quenching, a hardened case can be produced at less expense within the case-depth
range indicated, using either carbon or low-alloy steel. Full hardness with less distortion can be achieved with oil
quenching, or, in some instances, even gas quenching, employing a protective atmosphere as the quenching medium.
Steels commonly carbonitrided include those in the 1000, 1100, 1200, 1300, 1500, 4000, 4100, 4600, 5100, 6100, 8600,
and 8700 series, with carbon contents up to about 0.25%. Also, many steels in these same series with a carbon range of
0.30 to 0.50% are carbonitrided to case depths up to about 0.3 mm (0.01 in.) when a combination of a reasonably tough,
through-hardened core and a hard, long-wearing surface is required (shafts and transmission gears are typical examples).
Steels such as 4140, 5130, 5140, 8640, and 4340 for applications like heavy-duty gearing are treated by this method at
845 °C (1550 °F).
Often, carburizing and carbonitriding are used together to achieve much deeper case depths and better engineering
performance for parts than could be obtained using only the carbonitriding process. This process is applicable particularly
with steels with low case hardenability, that is, the 1000, 1100, and 1200 series steels. The process generally consists of
carburizing at 900 to 955 °C (1650 to 1750 °F) to give the desired total case depth (up to 2.5 mm, or 0.100 in.), followed
by carbonitriding for 2 to 6 h in the temperature range of 815 to 900 °C (1500 to 1650 °F) to add the desired carbonitrided
case depth. The subject parts can then be oil quenched to obtain a deeper effective and thus harder case than would have
resulted from the carburizing process alone. The addition of the carbonitrided surface increases the case residual
compressive stress level and thus improves contact fatigue resistance as well as increasing the case strength gradient.
When the carburizing/carbonitriding processes are used together, the effective case depth (50 HRC) to total case depth
ratio may vary from about 0.35 to 0.75 depending on the case hardenability, core hardenability, section size, and
quenchant used. A more shallow effective or total case depth can be achieved with a given carbonitriding process by
using fine grain steels containing higher amounts of aluminum (Ref 1) or titanium. The nitrogen from the process forms
nitrides with the aluminum or titanium. The combined nitrogen does not improve case hardenability.
The fundamental problem in controlling carbonitriding processes is that the rate of nitrogen pick-up depends on the free
ammonia content of the furnace atmosphere and not the percentage of ammonia in the inlet gas. Unfortunately, no stateof-
the-art sensor for monitoring the free ammonia content of the furnace atmosphere has yet been developed Case Composition
The composition of a carbonitrided case depends on the type of steel and on the process variables of temperature, time,
and atmosphere composition. In terms of steel type, the case depth achieved during a given carbonitriding process will be
lower in steels containing higher amounts of strong nitride formers such as aluminum or titanium.
The ammonia system required for carbonitriding often consists of a number of cylinders of liquid ammonia that are
connected to a common manifold. In general, ammonia from only a part of the supply is employed; the remainder is held
in reserve. The flow from each cylinder should be sufficiently low to prevent freezing of the valves. Only stainless steel
valves and schedule 80 or stainless steel piping are recommended.
Outside bulk storage and vaporizing systems are much preferred to cylinder banks, considering the advantage of a
constant, uninterrupted source on the uniformity of work quality. In general, when bulk storage is used, two-stage
pressure regulation is used to ensure a constant and uniform ammonia flow. The first stage regulates the pressure for
delivery to the furnace, whereas the final pressure regulation, at the furnace, controls the pressure to the process.
In terms of process variables, the higher the carbonitriding temperature, the less effective is the ammonia addition to the
atmosphere as a nitrogen source, because the rate of spontaneous decomposition of ammonia to molecular nitrogen and
hydrogen increases as the temperature is raised. At a given temperature, the fraction of the ammonia addition that
spontaneously decomposes is dependent on the residence time of the atmosphere in the furnace: the higher the total flow
of atmosphere gases, the lower the fraction of the ammonia addition that decomposes to nitrogen and hydrogen. Figure 1
shows that lower temperatures favor increased surface nitrogen concentrations. The addition of ammonia to a carburizing
atmosphere has the effect of dilution by the following reaction:
2 NH3 ® N2 + 3 H2

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