The process chemistry of nitriding is consistent with the process medium being used, be it gas or salt-bath nitriding. When using the gas nitriding process, for example, the source of nitrogen is anhydrous ammonia, which decomposes to a fixed ratio of one part nitrogen and three parts hydrogen. This means that with fixed gas chemistry, the results will be a fixed surface metallurgy. However, this decomposition ratio can be varied by dilution of the process gas with a supplemental gas of nitrogen or hydrogen.
The immediate surface metallurgy is seen as a compound zone (also known as the white layer). This layer will consist of two phases known as gamma prime and epsilon nitride. This next phase is a direct result of the ratio of the process gas – hydrogen to nitrogen (3:1 ratio). As previously stated, it is possible to manipulate that ratio by dilution. When this is accomplished, the surface metallurgy will be changed.
The gamma-prime phase within the compound zone is a ductile phase, which will accomplish hardness values of approximately 800 VPN. The epsilon phase is very much less ductile, and it is extremely brittle. This phase is a very hard phase, having a very low impact value. The epsilon phase has extremely good abrasion resistance to aluminum oxide formed during the hot extrusion of aluminum.
The carbon content of the steel is a contributing factor to the formation of the epsilon phase in the treated steel. Medium- to high-carbon steels tend to cause the promotion of the epsilon phase, whereas low-carbon steels will tend to promote the gamma-prime phase in the compound layer.