In my present position as an Engineering Manager, I've spec'ed in nitriding on a number of different applications. There are several different nitriding processes, the three I'm most familiar with are: salt bath(or liquid) nitriding, gas nitriding, and ion(or plasma) nitriding. All three do the exact same thing, the diffuse nitrogen ions into the surface of the metal so that iron nitride (Fe3N or Fe4N) can form, either of which are extremely hard. Before doing nitriding it's important to understand the heat treat history of the metal being case hardened. The nitriding process temperture can be as high as 1100 1200 degrees F, if your metal's tempering temperature was lower than the nitiding process temperature, then the steel's mechnical properties will be reduced as a result of being exposed to the the higher nitriding temperature.
PS
Melonite is a salt bath nitriding process