Case Hardening

Case hardening is the process of hardening the surface of a metal, often low carbon steel. Both carbon and alloy steels are suitable for this process; typically mild steels are used, with low carbon content. These mild steels are not normally hard enable due to the low quantity of carbon, so the surface of the steel is chemically altered to increase the harden ability. The hardening process is formed by diffusing carbon into the outer layer of the steel at high temperature, and then heat treating the surface layer to the desired hardness.The term case hardening is derived from the practicalities of the carburization process itself, which is essentially the same as the ancient process. Large work pieces are placed inside a case packed tight with a carbon-based compound. This is known as a carbonizing pack. The pack is put inside a hot furnace for a variable length of time. Time and temperature determines how deep into the surface the hardening extends. However, the depth of hardening is ultimately limited by the inability of carbon to diffuse deeply into solid steel, and a typical depth of surface hardening with this method is up to 1/16”. Small items may be hardened by repeated heating with a torch and quenching in a carbon rich medium, such as the commercial products Kasenit / Casenite or "Cherry Red".

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