Steel is a chemical compound of iron and carbon, but contains no carbon in the free state as cast iron does. Its tensile strength is greater than that of wrought iron and its compressive strength greater than that of cast iron. It is by far the strongest metal used in the mechanical arts and its strength varies greatly with its purity and the amount of carbon it contains. Steel is divided into high medium, and low grade. The high-grade steel contains the most carbon, and, unlike wrought iron, is fusible. Unlike cast iron, steel can be forged, and, with the exception of the higher grades, can be welded by heating and hammering, although care must be exercised in performing this operation.