Just to set the record straight, Cyanide "Hardening" is not a chemical process but a carburizing process as is the charcoal method. It is used when a very thin case is satisfactory & operates by immersion of the part in molten Sodium Cyanide at temps on the order of 1450°-1650°F. The metal absorbs a thin carbon rich "Skin" which is hardened upon quenching with the colors formed by oxidation. One might say, "It is the same, but different" as Charcoal hardening. Cyanide hardening is I believe quicker & generally less expensive than "Pack Hardening" (Charcoal) but normally does not give as deep a case & colors which in the eyes of many are not as attractive.
Ref; "Machinery's Handbook" (not a direct quote)

Miller/TN
I Didn't Say Everything I Said, Yogi Berra