Dave

Basically case hardening is diffusing carbon into a steel surface to get
a high carbon alloy on the surface. There are lots of ways of doing
that, so there are lots of methods that work. My grandfather, who grew
up as a blacksmith, used sodium cyanide. He'd melt it and put the metal
in. 1/32" depth per minute was his rule of thumb. He always did it
outside and stayed upwind (or did it in the forge) so he didn't poison
himself. Frankly, I'm not that confident and happy to pay others to do it.

Jim