Nitriding is a diffusion process just like carburizing and carbonitriding. There are significant differences in these surface-hardening techniques, however, which are: lower process temperature and surface metallurgy and mechanical properties (different surface phases and diffusion zone metallurgy).
Nitriding is traced back to Adolph Machlet of Elizabeth, N.J. in 1903 (patent granted in 1908). The process was further developed in Germany in 1917 (patented in 1923), ultimately leading to the manufacture of nitriding steel by Krupp Steelmakers and the marketing (and subsequent licensed manufacturing) of the Nitralloy group of nitriding steels.
The principle of the process is based on the simple premise of the limits of solubility of nitrogen in iron. In other words, nitrogen will diffuse in iron or steel very easily with the application of heat and will interact with some of the alloying elements.
The chemistry of the process is extremely simple and uncomplicated. The source of nitrogen is ammonia for gaseous nitriding. The surface reaction and the decomposition are as follows: 2NH3 + heat = 2N + 3H2
The initial decomposition is to separate the nitrogen from the hydrogen of the ammonia (nitrogen source). For a fraction of a second, the nitrogen is atomic, which will react with the steel being treated and diffuse into the steel surface. The hydrogen acts as a dilutant gas and also a reducing gas to assist in reducing surface-oxide contaminants. The steel surface acts as the catalyst to assist in the gas decomposition.
The ratio of nitrogen to hydrogen in the ammonia decomposes to a ratio of 1 part nitrogen to 3 parts hydrogen. This is a very important observation to note when defining what type of formed immediate surface metallurgy is required for the application of the part being nitride.
From the decomposition of the gas, atomic nitrogen will react with the steel surface and begin to diffuse into the steel. The nitride-forming elements that will react with the atomic nitrogen and form stable nitrides are iron, chromium, aluminum, molybdenum, vanadium, tungsten, titanium and silicon.
It can be seen from above that iron is also a nitride former, which is often forgotten. The resulting nitrides do not exhibit high hardness values, but they do assist in the improvement of the steels resistance to corrosion.
Check back next time for part 2 of this discussion.