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As an alternative to steel, humans developed ductile iron in 1949. Cast steel contains less than 0.3% carbon, while cast iron and ductile iron contains at least 3% carbon. The low carbon content in cast steel so that carbon present as free graphite does not form structural sheets. The natural form of carbon in cast iron is a free graphite sheet form. Within ductile iron, these graphite flakes change into tiny spheres by special treatments. This improved sphere gives ductile iron a much better physical performance than cast iron and steel. It is this spherical microstructure of carbon that provides better ductility and impact resistance for ductile iron. But the thin form inside causes the cast iron no ductility .
Therefore, all the pressure load components of the ductile iron are treated by the ferric bulk annealing cycle, and the spherical structure inside the ductile iron can also eliminate the easy cracks of the thin graphite inside the cast iron. In microscopic photographs of ductile iron, the cracks can be seen terminating after marching to the graphite ball. In the ductile iron industry, these graphite spheres are called "crack finishers" because of their ability to stop breaking.
Compared with cast iron ,ductile iron has an absolute advantage in strength.The tensile strength of ductile iron is 60k, and the cast iron is only 31k.The yield strength of the ductile iron is 40k, and the cast iron did not show the yield strength and eventually broke.
The strength of the ductile iron is comparable to the strength of the cast steel. Dutile iron has higher yield strength at a minimum of 40k, and cast steel is only 36k.In most municipal applications, such as water, salt water, steam, etc. , ductile iron corrosion resistance and oxidation resistance exceed cast steel. Due to the spherical graphite microstructure of the ductile iron, the ductile iron is better than the cast steel in terms of weakening the vibration ability, so it is more conducive to reducing the stress. An important reason for choosing ductile iron is that ductile iron is less expensive than cast steel. The low cost of ductile iron made this material more popular, with higher casting efficiency, and less machining cost of ductile iron.