Compared to other types of pipes, ductile iron pipes offer superior resistance to internal forces and pressure, along with excellent corrosion and wear resistance. They also provide advantages such as easy installation, strong sealing at the joints, and a high safety factor during operation. In recent years, there has been significant growth in the market for cast iron materials. However, the production of this material is more prone to defects such as cracks, heavy skin, and slag inclusion.
Cracks often occur due to an unreasonable pouring system in the centrifugal casting process. Heavy skin is related to pouring temperature and speed, while slag inclusion is influenced by the chemical composition of the metal and the effectiveness of slag treatment. To address these issues, several preventive measures have been implemented.
Firstly, improving the centrifugal equipment helps reduce the occurrence of cracks by shortening the time molten iron spends at high temperatures. Enhancing the inoculation process, particularly the effect of the bred inflow, is essential. The amount of inoculant added should be controlled between 0.1% and 0.25% of the molten iron weight. It's also important to ensure even distribution of mold powder and maintain a consistent layer of 20–30 g/m² on the inner surface of the pipe mold. Proper control of the smelting parameters for water-cooled metal centrifuges is crucial.
To prevent splashing of molten iron during casting, the sand core must be properly positioned to avoid burrs. Controlling the silicon content of the molten iron after nodulizer addition is also key—W(Si) should be maintained between 0.8% and 1.2%. Installing circular arc-shaped spray cooling pipes in sections within the centrifuge casing can help manage cooling efficiently without interference between sections.
Secondly, optimizing process parameters plays a vital role in reducing the weight of the pipe body and controlling both the pouring temperature and the inlet and outlet temperatures of the cooling water. The cooling water inlet temperature should be kept between 28°C and 36°C, and the outlet temperature between 50°C and 57°C. Avoiding excessively high casting speeds and adjusting the appropriate rotation and main machine speeds are necessary. Ensuring that the punching depth of the pipe die is between 0.25 mm and 0.3 mm, with a diameter of 4–5 mm, and maintaining a spacing of 0.5–0.7 mm between the points also contributes to better quality.
Thirdly, controlling raw materials and reducing slag-related defects is essential. Lowering the sulfur content in the molten iron (W(S) < 0.02%) helps minimize sulfide formation. Increasing the casting temperature promotes better slag floatation. The rare earth content in the inoculant should not be too high, ideally between 1% and 2%, to enhance slag removal and slag retention. Additionally, the residual magnesium content in the molten iron should be carefully controlled (0.035–0.045%) to avoid excessive levels that could affect the structure. Reducing surface oxidation of iron oxide is also beneficial.
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 final yield rate. These improvements reflect a comprehensive approach to enhancing the performance and reliability of ductile iron pipes in modern applications.
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