As defined by the American Iron and Steel Institute, any steel is considered to be carbon steel when there is no specified minimum content for any other alloying element other than carbon. Carbon steels contain a carbon content between 0.05 and 3 percent, and trace amounts of other elements, such as manganese. Low-carbon steel contains a maximum carbon content of 0.35 percent; medium-carbon steel, maximum 0.6 percent; and high-carbon steels, up to 2.5 percent.
When present as an alloying element, carbon causes steel to become harder and more brittle when it is quenched. These affects intensify as the carbon content in the steel becomes greater. Due to their brittle nature after the quenching process, higher-carbon steels must be heat treated to obtain a balance between hardness and strength. Higher-carbon contents also cause steel to have a reduced ductility and weldability.
Despite the adverse effects that carbon can have on steel, carbon steels account for approximately 90 percent of all steels produced. Medium-carbon steels are frequently used in applications that require a balance between strength, ductility and wear resistance, such as automobile parts. High-carbon steels are used for applications that require a high level of strength, such as high-strength wiring and springs. Ultra-high-carbon steel, a classification designated for steels containing between 2.5 and 3 percent carbon, are solely used for applications that require a very high level of hardness, such as punches and axles.