Carburizing

Carburizing, also called case hardening, is a heat treatment process that introduces carbon into the surface of a low-carbon iron or steel object, producing a hard, wear resistant outer case around a softer, tougher core, typically creating a hardened layer around half a millimeter deep. Historically the technique solved a practical problem in early ironworking: wrought iron, the dominant early form of iron, was too soft for sharp tools such as knives, while fully carbonized cast iron was too brittle, so smiths packed finished iron objects in carbon-rich materials such as ground bone and charcoal, or a mixture of leather, hooves, salt and urine, sealed the package in a container, and heated it below iron's melting point, with longer heating producing deeper carbon penetration. This traditional process often left a distinctive mottled black, blue and purple surface pattern that came to be prized decoratively, especially on firearms. At carburizing temperatures the carbon itself does not migrate directly through the case; it is transported by gaseous carbon monoxide generated inside the sealed container, in what is known as the CO cycle.