Cast or grey iron is an alloy characterized by its relatively high content of carbon flakes (2% to 4%). In contrast, the carbon in ductile iron is in the form of spherical nodules. The formation of such nodules is achieved by the addition of nodulizer like magnesium or cerium into the castings melt. Due to its inherent properties, such nodules resist the creation of cracks and augment its ductility. That is why this process is called as ductile iron casting. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Ductility and impact properties are principally determined by the proportions of ferrite and pearlite in the matrix.
The mechanical properties of ductile iron are controlled by the presence of graphite nodules. The Different grades of Ductile Iron Castings are produced by obtaining different matrix microstructures in the Iron. Alloying elements may be added to enhance as cast properties of Ductile Iron. In some special cases Heat Treatment can be employed to achieve the higher properties. The Grades of Ductile Iron Castings are based on the Mechanical Properties of the casting.