Cyaniding is a case-hardening process that is fast and efficient; it is mainly used on low-carbon steels. The part is heated to 871-954 °C in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide. Reactions are as follows:
2NaCN + O2 → 2NaCNO
2NaCNO + O2 → Na2CO3 +CO + N2
2CO → CO2 + C
This process produces a thin, hard shell (between 0.25 - 0.75 mm, 0.01 and 0.03 inches) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted.
It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous.
Nitriding heats the steel part to 482–621 °C in an atmosphere of ammonia gas and dissociated ammonia.
The time the part spends in this environment dictates the depth of the case. The hardness is achieved by the formation of nitrides.
Nitride forming elements must be present for this method to work; these elements include chromium, molybdenum, and aluminium.
The advantage of this process is that it causes little distortion, so the part can be case-hardened after being quenched, tempered and machined. No quenching is done after nitriding