Carbon steels are steels in which the main alloying additive is carbon. Alloy steels are alloyed with other metals or materials, in addition to carbon, to improve properties. Carbon steels are classified based on the amount of carbon content in the steel. The four main classes of carbon steels are mild and low carbon steel, medium carbon steel, high carbon steel, and ultra-high carbon steel. Alloy steels contain varying amounts of different metals and materials to specialize their properties. Some of the most common materials added include chromium, molybdenum, nickel, and silicon.