The heat treater has a great deal to worry about in relation to phase changes, etc. (particularly in relation to elevated austenitizing temperatures). But distortion with nitriding? No phase changes occur, so how does distortion occur?
Distortion during the nitriding process can occur. This is simply because the process temperature will stress relieve the surface of the processed steel. This in turn will relieve induced machining stress, residual forging stresses and stresses from prior manufacturing operations.
It is most important that the manufacturer of the part being nitrided considers intermediate stress-relieving operations during the manufacturing procedure and prior to the nitriding process. Do not wait to allow the nitriding process itself to do the stress relieving. The final stress relieve must be prior to the last machining operation before nitriding.
Another phenomenon of distortion is the process control of the process gas and an understanding of what type of surface metallurgy is necessary for the part to function satisfactorily. That is the formation of the compound layer (white layer). Process gas flow control and the monitoring of that process gas flow is mandatory and should not be left to chance. That is even if the process gas decomposition is being monitored by the solubility of ammonia in water method. If the process gas decomposition is not monitored, there is a serious risk of nitride networks forming. Networks can be caused by:
- Nitriding at too high a process temperature
- Incorrect decomposition of the ammonia gas
- Not monitoring the gas dissociation
A phenomenon that can occur with the pre-harden and temper is that there could be the potential for retained austenite if the steel has been quenched from too high an austenitizing temperature or quenched too slowly. The nitriding process will act as a “stress relieving” or decomposition process for the retained austenite, and retained austenite will decompose into tempered, fresh martensite as well as give additional dimensional stability to the finished part.
Growth will occur due to the diffusion of nitrogen into the surface of the steel being nitrided. However, the growth will be dependent on the time at temperature, the selected process temperature (case-depth requirement – the deeper the case, the greater the growth) and the gas ratio decomposition.
Do not be under any illusion that distortion will not occur with the nitriding process. There are reports from industry, however, that the growth is infinitesimal. That may be so, but, nevertheless, it will occur.