I use it frequently in gunsmithing work. It's a coarse carbon powder that the part is dunked into after heating to a cherry-red color, then the heating continues until the surface of the metal absorbs some of the carbon. After a time, the red-hot metal is dunked into icewater, which results in a glass-hard outer surface with a tough, but somewhat flexible under layer. A little polishing of some slight surface roughness is required, but a hard, wear-resistant layer several thousandths of an inch remains after polishing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!