Forgings made from one of the four grades of steel are called carbon steel forgings. Carbon steel is simply steel with varying amounts of carbon added to the steel. Low carbon steels contain between 0.05 and 0.25% carbon, while medium carbon steels contain between 0.29 and 0.54% carbon.
Steels that are injected with the highest percentage of carbon are called "high" carbon steels. To withstand strong stresses and forces, high carbon steels contain between 0.55 and 0.95% carbon. The American Iron and Steel Institute (AISI) has developed a system for classifying different grades of steel using a four-digit designation. These numbers indicate the grade and carbon content of carbon steel forgings.
