Carburization is the name of the process by which carbon is introduced into a metal. The carbon diffused into the surface is intended to make the surface harder and more abrasion resistant. Since too great a concentration of carbon makes metal brittle and unworkable, metals are also often decarburized .
Carburization of steel generally involves a heat treatment of the surface iron using a gaseous or solid source of carbon. Early carburization used a direct application of charcoal packed onto the metal, but modern techniques apply carbon-bearing gases (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.
In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process.