Carbonitriding
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Carbonitriding is a modification of gas carburizing, as opposed to a form of nitriding. This modification describes the addition of ammonia molecules into the gas carburizing atmosphere, and will therefore add nitrogen to the carburized case. Atomic Nitrogen is formed at the surface which thereafter diffuses into the steel with the carbon. Carbonitriding is carried out at lower temperatures and for shorter times than carburizing, which not only is more economical, but also reduces distortion during quenching. The lower temperature allows oil quenching, or even gas quenching with a protective atmosphere. The carbonitriding also allows the use of low carbon steels, which are more economical and easier to machine.
Carbonitriding forms a hard, wear resistant case that is approximately 0.075 – 0.75 mm thick, and generally has a better hardenability than a carburized case. The preferred case depth is determined by the service application, and the thicker the case, the greater the increase in wear resistance and hardness the part will have. Because the carbonitriding is affecting only the top layers of the steel, and not depositing an additional layer, it does not significantly alter the dimensions of the part. The case depth is restricted to 0.75mm for two reasons: the lower temperature of the treatment which restricts the diffusion of the two species, and the shorter times of the treatment. The primary reason that short times are employed is to restrict the concentration of nitrogen as nitrogen addition is more difficult to control than carbon; an excess of nitrogen can cause high levels of retained austenite and porosity, which are undesirable in producing a part of high hardness.
Carbonitriding also has other advantages over carburizing; 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 2.5mm at around 900-955oC, 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.
