C45

C45 is the European designation for a plain carbon steel grade, equivalent to SAE 1045. It has a medium carbon content, typically around 0.42–0.50% carbon, with small additions of manganese and silicon. The material offers a balance of strength, hardness, and machinability, and can be hardened and tempered to higher strength levels. Its properties vary with heat treatment: annealed C45 is relatively soft and machinable; heat-treated variants, such as quenched and tempered, achieve higher hardness and strength but reduced ductility. C45 is not alloyed with significant alloying elements, which keeps costs lower and generally allows good weldability, though welding a high-carbon section requires care to manage cracking risk.

Douglas C-45 Expeditor is a twin-engined transport aircraft derived from the civilian DC-3, produced in the late 1930s. It was used by the United States Army Air Forces and other Allied services during World War II for cargo and personnel transport. Designated C-45 in U.S. service, it served in various support roles, including training and liaison. After the war, many were sold into civilian service or scrapped.

Other uses of the designation C45 may appear in product codes, model numbers, or service designations, but the two meanings above are among the most commonly encountered.