The production process of ductile iron manhole covers is rigorous and complex, with each step being crucial. First, high-quality cast iron blocks are selected as raw materials, with strict requirements on chemical composition. This ensures the carbon and silicon content of the molten iron is higher than that of gray cast iron, while reducing the content of manganese, phosphorus, and sulfur. During molten iron melting, the tapping temperature is typically around 1450℃-1500℃ to compensate for the temperature loss during spheroidization and inoculation treatments.
Next is the spheroidization treatment of the molten iron. By adding a spheroidizing agent, the graphite in the cast iron crystallizes into spherical shapes, thereby improving the strength and toughness of the cast iron. The inoculation treatment involves adding an inoculant to promote graphitization, improve the morphology and distribution of graphite, and further enhance the performance of ductile iron. Because ductile iron has poor fluidity and significant shrinkage, a higher pouring temperature and a larger gating system size are required. Simultaneously, risers and chills must be used appropriately, and the principle of sequential solidification must be followed to ensure the quality of the castings.
After casting, the molten iron is allowed to cool and solidify naturally in the mold before demolding to obtain the blank of the manhole cover. The blank is then shot-blasted to remove surface sand and impurities, followed by grinding to improve surface quality and dimensional accuracy. Finally, the finished cast iron manhole cover is treated with asphalt or electrostatic spraying for corrosion protection, and after drying, it is ready for storage.
