Carbon Steel

What is Carbon Steel?

The majority of the nearly 3, 500 different types of steel produced and available in the global market today is carbon steel. Carbon steel is formed when two elements, iron and carbon, is combined with carbon being used as the alloying element. The carbon is used as a hardening agent to prevent iron atoms in the crystal lattice from sliding around. The carbon steel structure also has ferrite, pearlite and cementite present in varying quantities, depending on the carbon quantity of the steel.


The percentage of carbon in the steel affects the hardness, strength elasticity and ductility of the steel. Low carbon content steel or mild steel has similar properties to iron but it is softer and easy to form. As the carbon content rises, the steel is harder and stronger but it is less ductile. Mild or low carbon steel has a carbon content of 0.05 to 0.26 percent, medium carbon steel has a carbon content of 0.29 to 0.54 percent and high carbon steel has a carbon content of 0.55 to 0.95 percent with very high carbon steel having a carbon content of 0.96 to 2.1 percent.

Carbon Steel Coding

The AISI and the SAE have designed a four digit code to assign to all carbon and alloy steels to show its standard wrought steel composition. In carbon steel grades the last two digits indicate the nominal carbon content. When the code 10 appears in the first two digits of the code (ex. 10xx) the steel is plain carbon steel. If the code is 11xx it is resulfurized, 12xx refers to resulfurized and rephosphorized while 15xx refers to nonresulfurized with a Mn content of over 1 percent.


The presence of the letter L between the second and third digits of the code indicates that it is a leaded steel. The letter B indicates a boron steel. Cast-carbon steels will usually be specified by grade, such as A, B, or C. The A grade (also LCA, WCA, AN, AQ, etc.) has a 0.25 percent carbon content and a maximum of 0.70 percent of Mn. B-grade steel has a 0.30 percent carbon content and a Mn content of 1.00 percent while C-grade steel has a 0.25 percent carbon content and 1.20 percent of Mn. The mixture of carbon and manganese in a steel is used to improve the steel’s strength, toughness, and weldability. Cast carbon steels are specified to ASTM A27, A216, A352, or A487.