At least four different, yet basic, methods of applying the nitride process to steel are:
- Gaseous nitriding (using ammonia as the nitrogen source)
- Pack nitriding
- Salt-bath nitriding (using a cyanide-based salt such as KCN and NaCN)
- Plasma nitriding process techniques can also be known also as glow-discharge nitriding, continuous DC nitriding or pulsed plasma nitriding
The decomposition of the ammonia to release both nitrogen and hydrogen diffusion is very similar with each of the above methods except with plasma nitriding. Time, temperature and material chemistry will also influence the surface metallurgy, surface hardness, core hardness, diffusion zone enrichment and nitride networks.
Plasma nitriding has the distinct advantage of being able to control the ratio of nitrogen to hydrogen in order to control the resulting surface metallurgy of the nitriding process. The formed compound zone can be constructed of dual phase, single (mono) phase or no compound layer. This is accomplished simply by control and the ratios of nitrogen to hydrogen.
Dilution Nitriding (aka Precision Nitriding)
This method of gaseous nitriding can control the phase formation in the surface compound zone. In addition, carbon that is present in the steel analysis will also influence the compound-layer formation.
The basis of this nitriding method is the measurement and analysis of insoluble exhaust gases from the process. The method analyzes the insoluble hydrogen and the insoluble nitrogen and adjusts the gas flow appropriately to construct the compound-layer formation. Therefore, the basis of the control is by gas analysis and not by volumetric methods as is with conventional gas nitriding.
Next time, we will address metallurgy and metals.