Carbon is the most important and influential alloying element in steel. In addition to carbon however, any unalloyed steel will contain silicon, manganese, phosphorus and sulphur, which occur unintentionally during manufacture. The addition of further alloying elements to achieve special effects and intentional increase in the manganese and silicon contents result in alloy steel. With increasing C content, the strength and hardenability of the steel increase, but its ductility, forgeability, weldability and machinability (using cutting machine tools) are reduced. Corrosion resistance to water, acids and hot gases are practically unaffected by the carbon.