Steel is an alloy of metals that consists primarily of iron and contains 0.2 to 2.1 percent carbon. All steel contains carbon, but the term “carbon steel” applies specifically to steel that contains carbon as the main alloying constituent. Medium carbon steel is carbon steel that contains between 0.30 and 0.60 percent carbon. It also has a manganese content between 0.6 and 1.65 percent. This type of steel provides a good balance between strength and ductility, and it is common in many types of steel parts.
Iron consists of a crystal lattice of iron atoms that allow the atoms to slide past each other, making pure iron relatively soft. The carbon in steel reduces this tendency, making medium carbon steel harder than iron. Additional elements such as chromium, manganese, tungsten and vanadium can also act as hardening agents in steel. The precise proportion of these elements determines the specific properties of the steel.
Additional carbon makes the steel harder but also more brittle, so manufacturing carbon steel requires a balance between hardness and ductility. The most common uses of medium carbon steel are in heavy machinery, such as axles, crankshafts, couplings and gears. Steel with a carbon content between 0.4 and 0.6 percent is commonly used in the railroad industry to make axles, rails and wheels.
The treatment of medium carbon steel with heat significantly changes the mechanical properties, such as ductility, hardness and strength. Heat treatment of steel slightly affects other properties such as its ability to conduct heat and electricity as well. A variety of methods exist for treating steel with heat.
The carbon and manganese content in medium carbon steel make quenching and tempering the most common method of heat treatment for this type of steel. This process generally involves repeatedly heating the steel to less than 1,333°F (about 723°C), and cooling it rapidly by quenching it in a liquid such as oil or water. The temperature and time of this process allows the manufacturer to precisely control the final properties of the steel.
Case hardening is a process for hardening steel that only affects the exterior of the steel. This produces a hard, water-resistant exterior with a more ductile interior. Carbon steel is frequently case-hardened because it’s difficult to harden a thick carbon steel part completely. Steel with more alloying agents than that of medium carbon steel has a greater ability to be hardened and may not need to be case-hardened.