1. Ductile iron boasts high strength and toughness, approximately 30% lighter than gray cast iron. The grade of ductile iron is QT500-7, with a melt temperature of no less than 1453°C. Rare earth silicon magnesium alloy is added during spheroidization, utilizing high sulfur coke, and the spheroidizing grade exceeds level 3. The physical properties of ductile iron include: compressive strength of 500N/mm² and elongation of 7%.

2. The surface of the fracture (non-machined surface) can be easily distinguished between ductile iron and gray cast iron. Upon visual inspection, the fracture surface of gray cast iron appears coarse, grayish-white, with a metallic texture in the grain direction. The fracture surface of ductile iron is finer, showing a gray-black color. If the spheroidization is good, it basically lacks a metallic feel. If there is a white luster, it is generally indicative of a white cast appearance.

3. Graphite in gray cast iron is flat and flake-like, whereas in ductile iron, it is spherical. If ductile iron emits a sound similar to carbon steel, it indicates good spheroidization. The sound of knocking on gray iron sounds very dull.

4. In the melting process, the addition of deoxidizing and nodulizing agents during gray cast iron melting transforms the graphite from dispersed fibrous matter into spherical shapes, thereby reducing the differentiation of ferrite and enhancing strength.

5. On the issue of graphitization rate, it can be said that the same product can have varying graphitization rates, which in turn affects the quality and the price can differ significantly. A graphitization rate of 85% is generally considered to be the standard for a qualified product. The reason for a graphitization rate below 85% is due to inadequate graphitization during the casting process, leading to low iron liquid temperature and products that do not meet the required pressure resistance.

6. The gray cast iron manhole covers are produced without any auxiliary raw materials; pig iron is directly melted into iron water and then poured into the manhole covers. However, the covers are made with rare earth nodulizing agents and other nodulizing agents. Before casting, a certain amount of nodulizing agents is added to the iron water for nodulizing treatment, and the addition of a small amount of deoxidizer is beneficial for graphitization. After casting, ductile iron is obtained directly. Therefore, the market and relevant departments all require the use of ductile iron manhole covers. This is because the load-bearing capacity of ductile iron manhole covers far exceeds that of gray cast iron manhole covers.