Hardening metal requires expensive machinery, not a thing you can do at home. You can "surface Harden" steel at home by heating it till its glowing red then quenching it in oil. But this process only hardens the surface and leaves the rest of the metal untouched.

The liquid nitrogen treatment (cryo treating) you mentioned doesn't actually harden metal. It actually realigns the molecules of the metal into their natural state, strengthening their bond to each other. Machining, casting, forging, etc can push or pull the molecules apart, decreasing the strength of their bond to each other. Cryo treating doesn't strengthen metal, it makes it as strong as it can possibly be after you've cast, forged, machined and/or heat treated it.