Carbon

A nonmetallic element that is the principal hardening element present in all steels. In general, increased carbon content reduces ductility but increases tensile strength and the ability of the steel to harden when cooled rapidly from elevated temperatures. At temperatures below 700°C, carbon is present in steel as iron carbide, cementite, Fe3C. The cementite forms lamellae, which “reinforces” the iron. This explains why a steel with a high-carbon content is harder than one with a low-carbon content. Generally, 0.05 to 0.25% is considered low-carbon steel, 0.25 to 0.5% is medium-carbon and 0.5 to 0.9% is high-carbon. The mixture of ferrite and cementite in a grain is called pearlite. Ductility and weldability decrease with increasing carbon content. Austenite can dissolve up to 2% carbon. When cooled in water or oil, carbon stays in a supersaturated solution of martensite.