structural steel with a low carbon content (usually 0.1–0.25 percent), used to manufacture parts that are to be carburized. Carbon steels are used for small parts that must withstand wear but that need not have a high core strength; alloy steels are used for very large and heavily loaded parts. In some cases, as in the manufacture of gear wheels that are to be carburized or carbonitrided, the carbon content of the steel is increased to 0.25–0.3 percent, which increases the core strength and makes it possible to decrease the depth of case hardening, thus decreasing the time required for treatment.