Parts to be hardened by case hardening process are made from a steel with a carbon content of 0.15% so that they will not respond to direct hardening.
The steel is subjected to treatment in which the carbon content of the surface layer is increased to about 0.9%.
When the carburised steel is heated and quenched, only the surface layer will respond, and the core will remain soft and tough as required.

Gas nitriding process consists of heating the parts at 500°C in a constant circulation of ammonia gas for up to 100 hours
The completely stress-relieved workpieces are preheated (about 400°C) before being put in the salt bath (about 520-570°C).