Carburization (often also called carburizing) is how carbon is introduced into a metal. It is done to make the surface harder and more abrasion resistant.
Depending on the amount of time and temperature, the affected area can vary in carbon content. In earlier times, direct application of charcoal was packed onto the metal for carburization. Modern techniques use carbon-bearing gases or plasmas (such as carbon dioxide or methane). Carburization is mainly used to harden the surface of low alloy steel.
Too great a concentration of carbon makes metal brittle and unworkable, so then metals may be decarburized.