As a substitute for steel, cast iron was developed in 1949. Cast steel contains less than 0.3% carbon, while cast iron and cast iron contain at least 3% carbon. The low carbon content in cast steel prevents the carbon present as free graphite from forming structural flakes. The natural form of carbon within cast iron is in the form of free graphite flakes. In ductile iron, this graphite flake is transformed into tiny spheres through special processing methods. This improved sphere makes ductile iron have better physical properties than cast iron and steel. It is this spherical microstructure of carbon that makes ductile iron have better ductility and impact resistance, while the thin flake form inside the cast iron causes the cast iron to have no ductility. Optimum ductility is achieved through a ferritic matrix, so all pressure-loaded parts of American NIBCO ductile iron are processed through a ferritizing annealing cycle. The spherical structure inside the ductile iron can also eliminate the cracks that are easily caused by the flake graphite inside the cast iron. In microscopic photographs of ductile iron, cracks can be seen traveling to the graphite nodules and terminating. Within the ductile iron industry, these graphite balls are called "crack terminators" because of their ability to stop fractures.

Ductile iron is sometimes referred to as the "best of both worlds" metal, meaning that it has the strength of cast steel but also the excellent corrosion resistance of cast iron.

Comparison of ductile iron and cast iron (grey cast iron)

Compared with cast iron, ductile iron has an absolute advantage in strength. The tensile strength of ductile iron is 60k, while the tensile strength of cast iron is only 31k. The yield strength of ductile iron is 40k, while cast iron shows no yield strength and eventually breaks. The strength-to-cost ratio of ductile iron is far superior to that of cast iron. (See page 83 for a complete comparison of mechanical properties). Ductile iron is identical to cast iron in terms of corrosion resistance.

Comparison of Ductile Iron and Cast Steel

The strength of ductile iron and cast steel are comparable. Ductile iron has a higher yield strength, with a minimum yield strength of 40k, while the yield strength of cast steel is only 36k. (See page 83 for a full comparison of mechanical properties). In most municipal applications, such as water, salt water, steam, etc., the corrosion resistance and oxidation resistance of ductile iron exceed that of cast steel. Due to the spheroidal graphite microstructure of ductile iron, ductile iron is superior to cast steel in terms of attenuating vibration, so it is more conducive to reducing stress. An important reason for choosing ductile iron is that the cost of ductile iron is lower than that of cast steel. The low cost of ductile iron makes this material more popular, allows for more efficient casting, and reduces the machining costs of ductile iron.