Shrinkage cavity and porosity defect of nodular iron main reducer shell cover

The main reducer housing cover is an important component of heavy truck axle, which is matched with the main reducer housing. The outline dimension of the main reducer shell cover produced by the enterprise is 554 mm × 350 mm × 138 mm, the main wall thickness is 7 mm, the wall thickness of large parts is 37 mm, the casting weight is 20 kg, the material is QT400-15, the horizontal molding line is used for molding, the hot core box is used for core making, and the medium frequency induction furnace is used for melting. The casting shall be free of casting defects such as shrinkage cavity and porosity. The casting structure is shown in Figure 1.

The medium frequency induction furnace is adopted for smelting, and the charge ratio is 35% return charge + 65% scrap, and the graphite carburizing agent is used for carburization. In order to reduce the shrinkage tendency of molten iron, the addition amount of spheroidizing agent is strictly controlled to prevent the excessive amount of residual mg; In the later stage of molten iron smelting, 0.6% ~ 0.8% silicon carbide is added into the furnace to increase the graphite nucleation ability of molten iron and reduce the undercooling of molten iron; Spheroidizing, inoculation and pouring inoculation in the ladle in front of the furnace. The total amount of inoculant is 1% of the molten iron, the carbon equivalent is controlled at 4.55% ± 0.05%, and the chemical composition of the molten iron after spheroidization; In order to avoid shrinkage increase caused by excessive pouring temperature, the pouring temperature is 1 380 ~ 1 400 ℃, and the castings after mass production are shown in Figure 2.

Through long-term production practice, the following points should be paid attention to when producing high-quality main reducer shell cover castings on the horizontal line:

(1) The disc face down and the bottom injection open gating system can avoid sand flushing of castings and improve the appearance quality of products.

(2) The inner gate and riser shall be set at the hot joint on the disk surface to avoid the increase of technological hot joint and affecting the feeding of hot joint, resulting in shrinkage cavity and porosity.

(3) For the upper structural isolated hot joint, in addition to designing the cold vent, it is also necessary to place a heating block to eliminate the shrinkage and porosity defects of the isolated hot joint and improve the quality stability of the production process.