Carbon steel, also called plain-carbon steel, is a metal alloy, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are: manganese (1.65% max), silicon (0.60% max), and copper (0.60% max). Steel with a low carbon content has the same properties as iron, soft but easily formed. As carbon content rises the metal becomes harder and stronger but less ductile and more difficult to weld. Higher carbon content lowers steel's melting point and its temperature resistance in general.

• Mild (low carbon) steel: approximately 0.05% to 0.25% carbon content with up to 0.4% manganese content (e.g. AISI 1018 steel). Less strong but cheap and easy to shape; surface hardness can be increased through carburizing.
• Medium carbon steel: approximately 0.29% to 0.54% carbon content with 0.60 to 1.65% manganese content e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and car parts.
• High carbon steel: approximately 0.55% to 0.95% carbon content with 0.30 to 0.90% manganese content.. Very strong, used for springs and high-strength wires.
• Very high carbon steel: approximately 0.96% to 2.1% carbon content, specially processed to produce specific atomic and molecular microstructures.