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First, let’s learn about the material properties of ductile iron! Although the manufacturing cost of ductile iron is higher than that of gray iron, it possesses much higher strength and toughness, making it a material suitable for casting applications that can compete with steel castings. The tensile strength of ductile iron is higher than that of gray cast iron and cast steel; with appropriate heat treatment, it can reach the level of forged carbon steel. Moreover, for ductile iron with the same composition, its tensile strength can vary over a wide range (400–1500 MPa) depending on the heat treatment process used. Under the same tensile strength conditions, the yield strength of ductile iron is higher than that of steel; this is a valuable property of ductile iron that makes it suitable as a structural material, and it is also one of the reasons why it is increasingly used as an important component in mechanical and electrical products. Although the elongation of ductile iron is lower than that of cast steel, it is better than that of gray cast iron. It can meet the general requirements for most mechanical and electrical product components during operation, as these components allow only very slight plastic deformation while in use (based on a yield strength of 0.2, the elongation should be less than 0.2%). The elastic modulus of ductile iron is higher than that of gray cast iron, indicating that its stiffness is greater than that of gray cast iron, but slightly lower than that of cast steel. Although the impact toughness of ductile iron is not as high as that of cast steel, it is much higher than that of gray iron, which is sufficient to meet the requirements of parts operating under dynamic loads. The fatigue strength of ductile iron is more than twice that of gray cast iron, approaching that of 45 steel. Moreover, since steel is more sensitive to notches than ductile iron, its fatigue strength decreases more rapidly; therefore, ductile iron can be used to manufacture complex-shaped parts such as crankshafts in place of steel. Ductile iron also exhibits good wear resistance, heat resistance, and corrosion resistance. Although its vibration damping property is inferior to that of gray cast iron, it is much better than that of steel; the vibration attenuation ratios for these three materials are approximately 1:1.8:4.3. Although ductile cast iron has poorer casting properties than gray cast iron and is prone to defects, it offers better machinability, and ferritic ductile cast iron also has good weldability. Furthermore, heat treatment enables the mechanical properties of ductile iron to vary over a wide range; under the same microstructural conditions, heat-treated ductile iron exhibits significant differences from as-cast ductile iron in certain properties. By comparing these three materials – gray cast iron, ductile iron, and cast steel – it is easy to see that gray cast iron valves are well-suited for use in HVAC systems for chilled water, cooling water, and hot water applications. They can fully meet the requirements regarding the strength and toughness of valves under such operating conditions, and they are also cost-effective. For applications involving high-temperature hot water or steam in urban heating systems, we should choose ductile iron valves that exhibit better strength and toughness in order to handle media under high temperature and pressure conditions. On the other hand, cast steel valves are generally used in environments with higher steam pressure and temperature, such as steam at temperatures above 200°C.