The latest addition to the family of cast irons is ductile iron, which was discovered in the 1940's. It is sometimes referred to as nodular iron and is called SG or spherulitic graphite iron in England. An unusual combination of properties is obtained in ductile iron because the graphite occurs in spheres or spherulites rather than as individual flakes. This mode of solidification is obtained by alloying a very small amount of magnesium with the molten iron. The metallurgy of this addition is quite involved because magnesium is a very reactive metal and vaporizes at the temperature of molten iron.
The high carbon and silicon content of ductile iron retain the casting process advantages and the excellent machinability of gray iron. Ductile iron has a high modulus of elasticity with a linear stress-strain relation, a very good range of yield strengths, and ductility. The majority of applications of ductile iron have been made to utilize its steel-like mechanical properties in combination with the castability, machinability, and corrosion resistance of gray iron.