KC: case (pack) hardening is accomplished at red heat. The parts to be hardened are packed in a carbon rich material consisting of bone, wood, and leather charcoal, in a crucible with a lid. The crucible is placed in a fire or furnace and held at red heat for a period of time, several hours sometimes. then the crucible is removed and dumped all at once into water at room temperature, sometimes having been aerated. The carbon rich environmemnt in the crucible allows carbon to be absorbed into the surface of the lock parts, which when cooled quickly in water, have a glass hard surface, only a few thousandths of an inch deep. But the advantages are profound...wear resistant, rust resistant, and interesting colours. There is much more to it than I have mentioned, and a search will come up with lots of good reading.
Good luck.