The main “secret” of the process (if it can be considered a secret) is in the pre-heat treatment of the steel to be nitrided. The pre-treatment process necessitates an austenitize (harden) and temper. This treatment is necessary to provide the support for the nitrided case formed. Without this core support, the base material will not support the load on the formed nitrided case if it exceeds the compressive strength of the base steel.
In addition to the core support, the pre-harden and temper will provide a tempered martensite core to enable the nitriding to occur. The nitriding will not be successful if the steel is in the annealed condition.
An advantage of the nitriding process itself is that if there is any retained austenite present, the nitriding process will assist in its final decomposition. It will further assist in stress relieving and dimensionally stabilizing the workpiece being processed.
Do not be under the mistaken illusion that no growth of the workpiece will occur. Naturally, growth will occur, but it will occur uniformly in all directions. The degree of growth will be dependent upon the selected process temperature and the process time at that temperature.
During the heating of the steel for the pre-hardening operation, the phase will change from body centered cubic (BCC) to a face centered cubic (FCC) lattice structure. When the steel is quenched, it further transforms to martensite, which is a body centered tetragonal (BCT) structure.
After successful austenitizing, fresh martensite will be formed. It is necessary then to temper the steel at a temperature that will be slightly above the nitriding process. In other words, if the nitriding process is to be completed at, say, a temperature of 935°F, the tempering temperature should be approximately 50°F above the nitride process temperature. This will ensure that the nitriding process itself will not affect the core hardness property nor will it affect the tempered martensite core.
The appropriate hardness required for the formed core will be determined by the steel chemistry, the austenitizing temperature and the final tempering temperature. The accomplishment of the core metallurgy in terms of hardness and tensile strength will determine the successful performance of the nitrided component.