The secret of nitriding is not so much the nitriding itself or the forming of the stable nitride case, but the prior core treatment is what will give success to the nitriding process.
The steel core will require a hardened and tempered martensite core to support the formed nitrided case. Without that support, failure may occur when a load is applied to the nitrided surface.
Additionally, the control of the nitrided case is of paramount importance in order to reduce the risk of nitride networks forming at sharp corners and from the compound zone (white layer) into the formed, diffused case.
Nitrogen is soluble in iron up to 6-7% by volume. Therefore, if the ammonia gas is allowed to saturate the furnace atmosphere, there will be an excess of nitrogen diffusing into the steel surface, saturating the compound layer. Needle-like protrusions or fingers will migrate from the interface of the compound layer into the formed nitride diffusion zone. The needle-like structure is a super-saturated nitride that is extremely brittle and is very likely to fracture on an impact or torque load. Even the sharp corners will chip and fracture very easily. This is simply because of high nitrogen content in the steel surface and corner.
Gas nitriding using ammonia can be controlled as to its decomposition ratio and nitride formation. It can be controlled very simply by the burette and water method. It may not be precisely accurate, but it is a process-control method that will work.
The process control for gas nitriding makes use of three control factors: gas decomposition ratio, process temperature and process time at temperature.
Remember that the higher the process temperature selection, the greater the diffusion. If the process gas is uncontrolled, then the surface nitrogen in the steel will be richer.
The core of the processed steel is what will support the formed case under the surface load. Tempered martensite allows the user/heat treater to obtain the most effective case support.
A further benefit of the nitride process and its temperature selection is that the nitride temperature will assist in decomposing any retained austenite present in the tempered martensite core. In addition, the nitride process will produce better dimensional stability and consistent core hardness.
In order to gain controlled core support hardness, an alloy steel with strong nitride formers is best. Cast iron can be nitrided just as well as mild steel, but it will only improve the corrosion resistance. Although it will resist wear with light loads, the formed case will fail if any substantial load is placed on it.