Our discussion concludes with the metallurgy.
The final metallurgy involves the following, in the order of observation (microscopically).
- Compound layer: Also known as the white layer because the immediate surface etches out white with a cross-sectional examination of the formed case). The compound layer is comprised of two phases, gamma prime and epsilon nitride. The compound zone formation can be controlled by adjustment of the process-gas flows (or salt-bath chemistry).
- Diffusion zone: This is where the nitrogen has reacted with the alloying elements to form stable nitrides with the appropriate alloying elements.
- Transition zone: This is the area from the diffusion zone to the core material.
- Core: This is the original core metallurgy of the steel being nitrided.
The exception to the above will be nitriding by plasma techniques and nitriding by dilution techniques.
Steels for Nitriding
Any steel will nitride, simply because of the presence of iron. However, they will not produce the same hardness values because of the steels’ chemistry. The iron will assist in surface corrosion resistance by the formation of iron nitrides. Here is a general group of steels that will nitride. This list is by no means complete.
- Cast irons: Cast iron grades will nitride without any significant difficulty. The problem becomes the cast iron’s porosity and density. The ability to nitride cast iron has been known for many years and is not new. The nitriding of cast iron forming dies for the surface hardening of large auto bodies is a new use that has been developed. This has been pioneered by European and U.S. die manufacturers with commercial heat treaters.
- Alloy steels:Most alloy steels will nitride. Care needs to be taken, however, when considering the choice of steel for nitriding, particularly with the carbon content. It is not generally necessary to have a high carbon percentage in the steel to give high core hardness in order to support the formed case. A carbon content of approximately 0.45% maximum is considered acceptable. Once again, please be aware that the carbon content of the steel will affect the ratio of gamma prime to epsilon phases in the compound layer.
- Tool steels:Typical tool steels for nitriding will be the hot-work series of tool steels. High-speed steels will nitride very satisfactorily as will air-hardening tool steels. There are some applications where the D series are nitrided, but care should be exercised when selecting them.
- Stainless steels:All of the stainless steels will nitride. This is because of the ability of chromium to form high surface hardness values. However, some will nitride easier than others. The martensitic stainless steels are perhaps the easiest to nitride. All of the other stainless steels require some form of surface de-passivation to remove the chrome-oxide layer on the immediate surface of the stainless steels. Once the chrome-oxide surface layer is removed,the stainless steel has lost its corrosion resistance.