Alloy Ductile iron casting is characterized by having all of its graphite occur in microscopic spheroids. These graphite consists of about 10% by volume of ductile iron, its compact spherical shape minimizes the effect on mechanical properties. The graphite, which is commercially produced ductile iron is always not a perfect spheres. It can occur in a somewhat irregular form. The difference between the various grades of ductile iron is in the microstructure of the metal around the graphite, which is called the matrix. This microstructure varies with composition and the cooling rate of the casting. It can be slowly cooled in the sand mold for a minimum hardness as-cast or, if the casting has sufficiently uniform sections, it can be freed of molding sand while still at a temperature above the critical and normalized. The matrix structure and hardness also can be changed by heat treatment.