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6.4.1 Carbonitriding

Process description
Carbonitriding is similar to gas carburization with the addition of ammonia to the carburizing
atmosphere, which provides a source of nitrogen. Nitrogen is absorbed at the surface and diffuses into
the workpiece along with carbon. Carbonitriding (around 850 °C / 1550 °F) is carried out at
temperatures substantially higher than plain nitriding (around 530 °C / 990 °F) but slightly lower than
those used for carburizing (around 950 °C / 1700 °F) and for shorter times. Carbonitriding tends to be
more economical than carburizing, and also reduces distortion during quenching. The lower
temperature allows oil quenching, or even gas quenching with a protective atmosphere (Fig. 6. 10).

Benefits
According to the comparative study of both processes, carbonitriding and nitrocarburizing develop the
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compressive stress and are associated with the size and shape distortion. Carbonitriding has other
advantages over carburizing. To begin, it has a greater resistance to softening during tempering and
increased fatigue and impact strength. It is possible to use both carbonitriding and carburizing together
to form optimum conditions of deeper case depths and therefore performance of the part in industry.
This method is applied particularly to steels with low case hardenability, such as the seat of the valve.
The process applied is initially carburizing to the required case depth (up to 2.5 mm) at around 900-
955 °C, and then carbonitriding to achieve required carbonitrided case depth.
The parts are then oil quenched, and the resulting part has a harder case than possibly achieved for
carburization, and the addition of the carbonitrided layer increases the residual compressive stresses
in the case such that the contact fatigue resistance and strength gradient are both increased. Studies
are showing that carbonitriding improves corrosion resistance.
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Applications
Typical applications for case hardening are gear teeth, cams, shafts, bearings, fasteners, pins, hydraulic
piston rods, automotive clutch plates, tools, dies and tillage tools.

Fig. 6. 10 Carbonitriding process scheme
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