Case hardening

Case hardening

Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy. Case hardening is usually done after the part in question has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour.

Modern use

Both carbon and steel/" target="_blank" class="keylink">alloy steels are suitable for case-hardening; typically mild steels are used, with low carbon content, usually less than 0.3% (see plain-carbon steel for more information). These mild steels are not normally hardenable due to the low quantity of carbon, so the surface of the steel is chemically altered to increase the hardenability. Case hardened steel is formed by diffusing carbon (carburization), nitrogen (nitriding) and/or boron (boriding) 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. The steel work piece is placed inside a case packed tight with a carbon-based case hardening compound. This is collectively known as a carburizing 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 1.5 mm. Other techniques are also used in modern carburizing, such as heating in a carbon-rich atmosphere. Small items may be case hardened by repeated heating with a torch and quenching in a carbon rich medium, such as the commercial products Kasenit / Casenite or "Cherry Red". Older formulations of these compounds contain potentially toxic cyanide compounds, such as ferrocyanide compounds, while the more recent types such as Cherry Red do not.