The formed surface metallurgy will be determined by:
- The chemistry of the steel being processed
- The process-gas analysis (nitrogen-to-hydrogen ratio and sometimes a hydrocarbon gas for ferritic nitro-carburizing). This will be determined by the gas decomposition and analysis method selected.
- The selected process temperature
- The Kn factor or nitride potential (derived from the Lehrer diagram)
Generally, I have found that the immediate surface metallurgy – approximately 50% gamma-prime and 50% epsilon phase – of the formed case has been found to be, and will be dependent on, the carbon content of the steel (assuming a gaseous flow process of ammonia for the nitrogen source).
The above combined phases are categorized as the “white layer” or “compound zone.” The total thickness of the compound layer will be dependent on the time at temperature, the selected process temperature and the chemistry analysis of the steel being processed.
Generally (but not in all cases), the total thickness of the compound layer can be approximately 10% of the total formed case (if the case is formed using ammonia and a gas dissociation of approximately 30%) at a process temperature of 500°C (930°F). It must be emphasized that the above will change dependent on the steel chemistry and is meant as a guide only in a general sense.
The core hardness of the steel being processed is determined by the previous heat treatment given to it before the nitriding process is introduced. The core hardness is achieved by:
- Prior austenitizing temperature
- Prior cool-down rate from the austenitize temperature
- Prior tempering temperature
- Prior stress-relieving temperature
Please note: The stress-relieving procedure will be used prior to the final nitride process as an intermediate and final stress relieving before final machine followed by nitriding.
The final nitriding process selection temperature will be at least 35°C (50°F) below the final tempering temperature. Otherwise, the core hardness will be tempered back to a lower resulting hardness value. Likewise, the selected stress-relieving temperature should be below the final tempering temperature of the steel.
Another process step prior to the final tempering procedure could be considered, and that is the procedure of cryogenic (cold) treatment. The reason for this consideration is to finally decompose any retained austenite that may be present as a result of the austenitize and quench and to ensure good dimensional stability of the finished component. Please also be aware that if you have introduced the cryogenic treatment, it is necessary to follow it with a temper. If retained austenite was present, it will decompose (by the cold treatment) into untempered martensite, which is extremely brittle and could possibly lead to cracking if the freshly precipitated martensite is not tempered.