The decomposition of ammonia by heat is as follows:
2NH₃⇄ 2N +3H₂
As can be seen, the equation of decomposition is balanced. It is often misunderstood that one simply needs to load the nitriding furnace with the work to be processed, close up the furnace, purge, apply the heat and open the ammonia valve. This is then followed by soaking at the appropriate temperature with the ammonia gas flowing. On completion of the cycle, the furnace is allowed to cool down to ambient temperature followed by purging and opening of the furnace.
When gas carburizing, it is important to control the gas decomposition during a carburizing process. It is equally important during the nitriding process to control the ammonia decomposition.
This is usually accomplished by a volumetric measurement using water to dissolve the process exhaust gas, which will leave a volume of undissociated gas. The volume of gas is the measurement and control factor of the process effectiveness. This technique measures only the volume of gas but not the composition of that undissociated gas. In other words, we have no idea of the composition of the undissociated gas.
It is most important to at least know the volume of the undissociated gas. This control factor will determine both the thickness and composition of the formed surface-compound layer of the nitrided case.
An uncontrolled ammonia gas flow will very easily cause nitride networks at the corners or at the interface between the compound layer and the nitride diffusion layer. It is most important to control the ammonia gas flow because of the limit of solubility of nitrogen in iron, which is a maximum of 7% (dependent on steel chemistry). One can very easily exceed the limit of solubility if the ammonia process-gas flow is not controlled.
The methods of controlling the process gas are:
- Volumetric analysis (water)
- Analysis and measurement of the individual insoluble gases of nitrogen and hydrogen.