Nitriding
Nitriding turns up in 2 fields: heat treating, gears. Here is what it means to the people in each one.
Nitriding in heat treating
Diffusing nitrogen into steel below its critical temperature to form a hard case with no quench.
Case hardening at about 495 to 565 °C in which nitrogen from cracked ammonia, plasma or salt diffuses into the surface and forms hard nitrides with chromium, aluminum, molybdenum and vanadium. No quench is needed and the core is hardened and tempered beforehand, so distortion is small. Cases run about 0.1 to 0.6 mm after cycles of 10 to 80 hours, with surface hardness above 900 HV on aluminum-bearing nitriding steels.
"4140 crank gets nitrided after Q&T, 48 hours in ammonia, and comes out with about 0.015 total case."
Nitriding in gears
A low-temperature surface hardening that diffuses nitrogen into alloy steel gears with almost no distortion.
A case-hardening process carried out around 500 to 550 C in ammonia gas or plasma, well below the steel's transformation temperature, so there is no quench and very little distortion. Gears can be finish-machined or ground before nitriding. The case is hard but shallow, typically a few tenths of a millimeter, so nitriding suits large thin gears and moderately loaded teeth, while the heaviest loads go to carburized gears.
"Ring gear is too big and thin to carburize without potato-chipping it, so we're nitriding it after finish grind."