(Chemical symbol: C). A nonmetallic element that occurs in many inorganic and all organic compounds. An element of prehistoric discovery, carbon is widely distributed in nature. It is found native in diamond and graphite, and as a constituent of coal, petroleum, asphalt, limestone and other carbonates. In combination, it occurs as carbon dioxide and as a constituent of all living things. Carbon is unique in forming an almost infinite number of compounds. It has an atomic weight of 12; atomic number, 6; melting point, above 3500°C (6300°F); specific gravity, amorphous 1.88, graphitic 2.25, diamond 3.51.

The addition of carbon to iron produces steel; carbon is the principal hardening agent in steel. In most cases, alloy steels containing carbon up to about 0.20% are considered easily weldable. Alloy steels containing over 0.20% carbon are generally considered heat-treatable steels, and are heat treated by quenching and tempering to obtain the best combinations of strength and toughness or ductility. In some cases, these steels are used in the as-rolled condition.

As the principal hardening element in most alloy steels, carbon controls the strength and hardness of the steel, while alloying additions are used to increase hardenability and improve toughness.