Geno is 100% correct. Unless the part is quenched from above its critical temp absolutely no hardening takes place. This is the same for cyanide as charcoal hardening. In the Cyanide process the part is submerged in a molten cyanide bath above its critical temp & quenched from there.
I do not know how low a temp the cyanide can be used to still produce color, but if below that critical temp no hardening takes place. In fact a heat treated part will be tempered to whatever temp is used, the higher the temp used the softer the steel.
There are chemicals which can be used to put on an oxide coloring which operate below 200°F. These are often used for putting a "Faux Finish" on alloy steel parts which have been heat treated by a through hardening method.
