The steel surface (and the core) needs to be in the tempered martensite condition rather than an annealed condition. The reason for and advantage of tempered martensite is that in the annealed condition the steel surface is predominantly ferrite, which will significantly inhibit the nitrogen diffusion. On the other hand, the tempered martensite core will assist in the diffusion of the nitrogen.
If the steel surface becomes decarburized as a result of the pre-heat treatment, the steel surface will not readily accept the nitrogen diffusion. It is, therefore, necessary to ensure a completely decarburized-free surface in order to accomplish successful nitriding.
Steels that contain the elements of aluminum and/or vanadium will assist in the formation of very hard but stable nitrides in the surface. However, the surface does tend to be very hard with some brittleness associated with the high hardness, particularly if higher nitride process temperatures are utilized.
It should be noted that the steels that do contain aluminum, chromium and molybdenum are prone to surface decarburization during the pre-harden and temper procedure. Care needs to be taken with either the dissociation rate (if gas nitriding) or the ratio of nitrogen to hydrogen (if plasma/ion nitriding). Hydrogen has a natural decarburizing action on the steel surface (particularly if high nitride process temperatures are needed). Therefore, good atmosphere control is necessary at pre-nitride hardening as well as with the nitride process gas or ammonia dissociation.
The question of “re-nitriding” often crops up. First, one cannot really “de-nitride” the diffused nitrided case once it has been nitrided by salt, gas or plasma. The nitriding (particularly if it is to be precision nitriding on a precision-machined component) should be performed correctly the first time, otherwise the component could possibly be scrapped.
The next concern is the often-made comment of “nitriding will not distort the component.”
This statement cannot be further away from the truth. Any time that one diffuses another element into the surface of the steel, there will be a change in volume of that surface. However, this type of growth distortion will be uniform in all directions. If the component has not been correctly stress relieved prior to nitriding, the heat up to the nitride temperature will act as a stress-relieving operation and distortion will occur. Further, the nitride process will act as a “stabilizing” procedure. Any retained austenite that might be present resulting from the pre-heat treatment will be decomposed and give phase stability by reducing the retained austenite.