Ductile iron pipes offer superior resistance to internal forces and pressure compared to other types of pipes, along with excellent corrosion and wear resistance. They are also known for their ease of installation, strong sealing performance at the joints, and a high safety factor during operation. In recent years, the market for cast iron materials has seen rapid growth. However, the production of ductile iron pipes is prone to various quality issues such as cracks, heavy skin, and slag inclusion. Cracks often occur due to improper pouring systems in the centrifugal casting process, while heavy skin is related to pouring temperature and speed. Slag defects are influenced by the chemical composition of the metal and the efficiency of slag treatment.
To address these problems, several preventive measures can be implemented:
1. Improve the centrifugal equipment to reduce the occurrence of cracks by minimizing the time molten iron spends at high temperatures. Enhance the inoculation process, particularly focusing on the effect of the bred inflow. The amount of inoculant added should be between 0.1% and 0.25% of the molten iron weight. Ensure even distribution of mold powder, with a controlled amount of 20–30 g/m² on the inner surface of the pipe mold. Monitor the smelting parameters of water-cooled metal centrifuges carefully.
To prevent splashing caused by molten iron, ensure that the sand core is placed correctly to avoid burrs. Control the composition of the molten iron and reduce its silicon content to 0.8–1.2% after nodulizer addition. Install circular arc-shaped spray cooling pipes evenly inside the centrifuge casing, divided into sections to prevent interference between cooling zones.
2. Optimize process parameters to reduce the weight of the pipe body. Control the pouring temperature and the inlet and outlet temperatures of the cooling water, and shorten the interval between castings. Keep the inlet temperature of the cooling water between 28–36°C and the outlet temperature between 50–57°C.
Avoid excessive casting speeds in the water-cooled metal centrifugal casting process. Adjust the appropriate turn-up speed and main machine operating speed. Ensure that the depth of the punching points on the pipe die is 0.25–0.3 mm, with a diameter of 4–5 mm, and the spacing between the points is 0.5–0.7 mm.
3. Control raw material input to minimize slag and slag inclusion defects. Reduce the sulfur content in molten iron to below 0.02% to limit sulfide formation. Increase the casting temperature appropriately to help slag float. The rare earth content in the inoculant should not be too high, ideally kept between 1–2%. Enhance the slag removal and slag retention effects in the molten iron. Ensure that the residual magnesium content in the molten iron is not too high, preferably maintained at 0.035–0.045%. Reduce surface oxidation of iron oxide.
By implementing these measures, the overall quality of ductile iron pipes has significantly improved. Defects such as cracks, heavy skin, and slag inclusion have been greatly reduced, leading to fewer defective products and a substantial increase in the overall finished product rate.
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