Ductile iron is known as the "best of the two worlds" metal, meaning that ductile iron has the strength of cast steel, as well as the excellent corrosion resistance of cast 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 ductile iron to have superior physical properties compared to cast iron and steel. It is this spherical microstructure of carbon that makes ductile castings have better ductility and impact resistance, 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, so all pressure-loaded components of Nibel's ductile iron are subjected to a ferrite annealing cycle. The spherical structure inside the ductile iron can also eliminate the crack phenomenon easily caused 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.
Compared with cast iron, ductile iron has an absolute advantage in terms of strength. Ductile iron has a tensile strength of 60k, while cast iron has a tensile strength of only 31k. The yield strength of ductile iron is 40k, while cast iron does not show yield strength and eventually breaks. The strength of the ductile iron - the cost ratio is far superior to that of cast iron. Ductile iron is the same as cast iron in terms of corrosion resistance.
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, casting more efficient, and reduces the machining cost of ductile iron.