The ductile iron is ideal for valve materials, for it has lots of merits. As a substitute for steel, ductile iron was developed in 1949. The carbon content of cast steel is less than 0.3%, while that of cast iron and ductile iron is at least 3%. The low carbon content of cast steel makes the carbon existing as free graphite not form flakes. The natural form of carbon in cast iron is free graphite flakes. In ductile iron, graphite is in the form of nodules rather than flakes as in cast iron. Compared with cast iron and cast steel, ductile iron has better physical properties. It is the rounded nodules that inhibit the creation of cracks, thus providing the enhanced ductility that gives the alloy its name. However, the flake in cast iron leads to the lack of ductility of the iron. The best ductility can be obtained by the ferrite matrix.

Compared with cast iron, ductile iron has absolute advantages in strength. The tensile strength of ductile iron is 60k, while that of cast iron is only 31k. The yield strength of ductile iron is 40k, but the cast iron does not show the yield strength and will crack finally.  

The strength of ductile iron is comparable to that of cast steel. Ductile iron has higher yield strength. The lowest yield strength of ductile iron is 40k, while the yield strength of cast steel is only 36k. In most municipal applications, such as water, salt water, steam, the corrosion resistance and oxidation resistance of ductile iron are superior to those of cast steel. Ductile iron is also known as spheroidal graphite iron. Because of the spheroidal graphite microstructure, ductile iron is superior to cast steel in dampening vibration, so it is more conducive to reducing stress. An important reason for choosing ductile iron as the valve material is that it has lower cost than cast steel. The low cost of ductile iron makes this material more popular. Besides, selecting the ductile iron can reduce the machining cost.