Hot joint analysis and riser design of nodular iron main reducer shell cover to avoid shrinkage defects

Casting hot joints are generally divided into structural hot joints and process hot joints. Structural hot spot refers to the node with the largest metal accumulation volume and the most heat released during solidification, which is composed of the geometric structure of the casting; Due to the needs of the casting process, the hot joint formed on the casting or upgraded the form of the hot joint of the original structure due to the setting of risers, ingates and vent holes is called process hot joint. Due to the long solidification time of hot spot and large volume loss caused by volume shrinkage, shrinkage cavity and porosity are easy to occur under the condition of defect feeding measures, which affects the quality of castings.

According to the structural characteristics of the developed main reducer shell cover and the selection of pouring position, the position of the casting hot joint is analyzed. As shown in Figure 1, there are mainly two structural hot joints, namely the position 1 of the boss on the bottom disk surface and the upper part of the casting ϕ 125 mm round hole position 2, where the shrinkage tendency is the largest.

The hot spot at position 1 is relatively small, and can be fed by gravity, and the shrinkage tendency is small. According to the production practice, the inner gate needs to be set at position 1, which will form a process hot joint near the inner gate on the panel. Combined with the structural hot joint, the shrinkage tendency of the hot joint is increased, which is easy to produce shrinkage defects, as shown in Figure 2.

According to the theoretical calculation of hot section riser and multi wheel trial production, the diameter is set at the inner gate ϕ 55 mm and 85 mm high hot risers are used for feeding, and the shrinkage cavity defect of the hot joint is eliminated.

The hot joint at position 2 is located at the upper part of the product. The molten iron is filled at last during pouring and filling. The molten iron filling temperature at this place is relatively low. An attempt has been made to use the equilibrium solidification theory without riser for test. The verification results show that there are shrinkage pits in the appearance and serious shrinkage defects in the interior, as shown in Figure 3.

Due to process constraints, only one ordinary riser can be placed near position 2. The disadvantage of the ordinary riser is that the temperature of molten iron in the riser is relatively low. There may be that the riser solidifies before the hot spot of the casting, and the riser cannot play the role of feeding, resulting in shrinkage defects of the casting. According to the preliminary trial production experience, the module is twice the module and diameter of the hot joint ϕ 80 mm and 110 mm high ordinary risers. Meanwhile, the riser neck shall be misplaced away from the hot joint to reduce the influence of the process on the hot joint at position 2. However, the verification results show that 20% of the castings still have shrinkage defects, as shown in Figure 4.

Due to the particularity of the product structure, based on the previous test, in the casting ϕ Place one in a 125 mm round hole ϕ The 80 mm riser (as shown in Figure 5) forms a sharp angle sand structure near the riser neck. Due to the sharp angle sand effect, it has the effect of slow heat dissipation, which can prolong the solidification time of the riser neck and finally solidify the area near the riser neck; At the same time, in order to realize local sequential solidification, a heating block is placed in the riser to prolong the solidification time of the riser and effectively feed and shrink the hot joint. It is found in the test that the shrinkage defect at the hot joint is eliminated (as shown in Fig. 6), which has achieved good results in production practice.

The sand box size is 700 mm × 600 mm × 250 mm / 250 mm, one for each type. Based on the preliminary test, the gating system scheme and pouring placement of mass production process are shown in Figure 7. The semi closed and semi open gating system is adopted, and the gate cross-sectional area ratio at each position is, Σ S section: Σ S horizontal: Σ S = 1 ∶ 1.15 ∶ 1.3, use a piece of 60 mm × 60 mm × 15 mm, 10PPI foam ceramic filter, designed 2 internal gates to disperse heat.

1 sprue, 2 filter screen, 3 runner, 4 hot riser, 5 cold riser, 6 castings