Carburizing is the addition of carbon to the surface of low-carbon steels. Hardening is accomplished when the high-carbon surface layer is quenched to form marten site so that a high-carbon is superimposed on a tough, low-carbon steel core. Steels for case hardening usually have base-carbon contents. The steel, which has high carbon content, is heated in a carbon free furnace, such as air, the carbon will tend to diffuse out of the steel resulting in decarburization.