Steels and cast irons are both primarily iron with carbon as the main alloying element. Two percent is about the maximum carbon content at which iron can solidify as a single-phase alloy with all the carbon solution in austenite. Thus, the cast irons, by definition, solidify as heterogeneous alloys and always have more than one constituent in their microstructure. High-carbon content and silicon in cast irons give them excellent castability. Their melting temperatures are appreciably lower than those of steel.
Molten iron is more fluid than molten steel and less reactive with molding materials.
Formation of lower density graphite during solidification makes production of complex shapes possible. Cast irons, however, do not have sufficient ductility to be rolled or forged. Iron's carbon content is the key to its distinctive properties. But the ferrite of iron is different because it contains sufficient dissolved silicon to eliminate the characteristic gummy nature of low-carbon steel. Thus, cast irons containing ferrite do not require sulfur or lead additions in order to be free machining.