The spheroidal graphite irons, aka ductile irons and nodular irons are the strongest. There are several governing specifications from the various standardization groups that describe this material, but it is common to refer to the grades by yield strength and elongation. One common generic name is "5506", which means 55,000 psi min. yield and 6% min. elongation. There are as cast grades, heat treated grades and austenitic corrosion/heat resisting grades.