Compared with cast iron, ductile iron has an absolute advantage in terms of 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 does not show yield strength and eventually breaks. The strength-cost ratio of ductile iron is much better than that of cast iron.
The strength of ductile iron is comparable to the strength of cast steel. Ductile iron has a higher yield strength with a yield strength of at least 40k, while cast steel has a yield strength of only 36k. In most municipal applications, such as water, brine, steam, etc., ductile iron has more corrosion resistance and oxidation resistance than cast steel. Due to the spherical graphite microstructure of ductile iron, ductile iron is superior to cast steel in reducing vibration capacity, so it is more conducive to reducing stress. An important reason for choosing ductile iron is that ductile iron is less expensive than cast steel. The low cost of ductile iron makes this material more popular, with higher casting efficiency and less machining costs for ductile iron.
As a substitute for steel, spheroidal graphite cast iron was developed in 1949. Cast steel has a carbon content of less than 0.3%, while cast iron and ductile iron have a carbon content of at least 3%. The low carbon content in the cast steel allows the carbon present as free graphite to form no structural flakes. The natural form of carbon in cast iron is in the form of free graphite flakes. In ductile iron, this graphite flakes are transformed into tiny spheres by special treatment methods. This improved sphere allows the ductile iron to have superior physical properties compared to cast iron and steel. It is this spherical microstructure of carbon that makes ductile iron more ductile and impact resistant, while the flake form inside the cast iron results in no ductility of the cast iron. The best ductility is obtained by the ferrite matrix.
Therefore, after the pressure load component of the ductile iron is subjected to the ferrite annealing cycle process, the spherical structure inside the ductile iron can also eliminate the crack phenomenon easily generated by the flake graphite inside the cast iron. In the microphotograph of ductile iron, it can be seen that the cracks are terminated after the parade to the graphite ball. In the ductile iron industry, these graphite balls are known as "crack terminators" because of their ability to resist fracture.