It is one of the products of nodular cast iron. Spherical graphite is obtained from nodular cast iron through spheroidizing and inoculation treatment, which effectively improves the mechanical properties of cast iron, especially the plasticity and toughness, thus obtaining higher strength than carbon steel. Ductile iron is a kind of high-strength cast iron material developed in the 1950s. Its comprehensive performance is close to that of steel. It is precisely based on its excellent performance that it has been successfully used to cast some parts with complex stress and high requirements for strength, toughness and wear resistance. Ductile iron has rapidly developed into a widely used cast iron material, which is second only to gray iron. The so-called "replacing steel with iron" mainly refers to nodular cast iron.

Ductile iron manhole covers are generally divided into round and square ones. In terms of urban road administration, round manhole covers are generally used, because round manhole covers are not easy to tilt, which can better protect the safety of pedestrians and vehicles. The use of circle is mainly based on the fact that the length of each diameter of the round manhole cover passing through its center is the same. In this way, if the manhole cover is rolled up by passing vehicles, its diameter will be slightly wider than the lower wellhead no matter how it is rolled up, and the manhole cover will not fall into the wellhead.