Cyaniding
Cyaniding is a case hardening process that is fast and efficient; it is mainly used on low carbon steels. The part is heated to 1600-1750 °F in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.
This process produces a thin, hard shell (between 0.010 and 0.030 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.
Carbonitriding
Main article: Carbonitriding
Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be quenched then the part is heated to 775”“885 °C (1,427”“1,625 °F); if not then the part is heated to 649”“788 °C (1,200”“1,450 °F).
http://en.wikipedia.org/wiki/Case_hardening[/quote]
Thanks to all the people who have described the detail of their use of this stuff. I haven't used it yet, though I need to.