The process of gas nitriding relies on a very simple premise – the solubility of nitrogen in iron. Gas nitriding was developed and patented approximately 110 years ago by Adolph Machlet of Elizabeth, N.J. This was followed in 1921 by Dr. Adolph Fry at Krupp Steel in Germany.
The interesting aspect of the patent is that it describes the nitride-forming elements but misses one element. Believe it or not, that element is named in the patent heading. The nitride-forming elements are aluminum, chromium, molybdenum, vanadium, titanium and silicon. One element that is never discussed is iron (a strong nitride former).
When iron is gas nitrided, however, it develops two properties that are usually not considered: corrosion and hardness. Hardness is usually disregarded simply because the resulting hardness value is very low, at approximately 35 HRC scale. It is interpreted as not hard enough.
Hardness is, of course, relevant to the material that it would perhaps be abrading against. So, it can be stated that if the material that is in contact with the iron is lower in hardness than the iron, the iron will resist abrasion. Its corrosion resistance will increase dramatically, although it will not challenge stainless steels.
When one considers the alloy elements that can form hard and stable nitrides (including iron and cast irons), it opens the doors to having the ability to process many different steels, including tool steels.
The gas nitriding process is perhaps the easiest of the surface treatments to understand. The process is reliant on the decomposition of ammonia under heat: 2NH3 + Heat ↔2N + 3H2
This further decomposes to: N + N + 3H2
This leaves the residual nascent (atomic) nitrogen. Because the steel is at temperature and the nitrogen in the process atmosphere is not in equilibrium with the steel, the steel will absorb the nitrogen and the nitrogen will react with the nitride-forming alloys at the immediate surface of the steel.
The major disadvantage of the process is, of course, the time taken for the nitrogen to both diffuse and react with the nitride-forming alloys. Yet the diffusion rate is still governed by the Harris Formula of diffusion or Fick’s 1st and 2nd laws of diffusion.
We will conclude our nitriding discussion next week.