The raw materials for producing ductile iron manhole covers include pig iron, recycled materials, and rare earth magnesium-silicon ferroalloys. Pig iron entering the plant must be tested for the content of elements such as carbon, silicon, manganese, phosphorus, and sulfur; only qualified pig iron can be used. The proportion of recycled materials is usually strictly controlled within 30% to ensure material stability. A dual-frequency electric furnace is used for smelting, with the tapping temperature controlled at 1400-1500℃. Each ladle of molten iron weighs approximately 1500 kg. The rare earth magnesium alloy content is 1.4%-1.6%. Pouring must be completed within 10 minutes after the spheroidization reaction, and batch markings must be used to prevent confusion. The required clearance between the manhole cover and the manhole ring is 3 mm.
Inspection standards include visual inspection; the casting surface should be free of obvious casting defects, and patterns and lettering should be clear. Dimensional and fit inspection uses a special jig to check the flatness of the manhole cover and the diameter of the manhole ring. After assembly, the manhole cover and ring should be flat, without shaking or abnormal noise, with a height misalignment error not exceeding 1 mm, and the clearance between the T-shaped rubber ring and the groove body not exceeding 1 mm.
