The influence of pouring temperature on casting

1. Excessive pouring temperature will significantly increase the proportion of defective products.

Excessive pouring temperature can cause the sand mold to expand, especially for gray cast iron parts with complex sand cores. When the pouring temperature is ≥ 1420℃, the number of defective products increases, and when the pouring temperature is 1460℃, the defective rate reaches 50%. In production, using an induction furnace for melting can better control the temperature of the molten iron.

2. Defects that may occur when the pouring temperature is too low

(1) Manganese sulfide pores: These pores are located beneath the surface of gray cast iron parts and are mostly found on the upper surface. They often become apparent after processing. The diameter of these pores is approximately 2 to 6 mm. Sometimes, the pores contain a small amount of slag. Microscopic examination indicates that this defect is formed by the segregation of MnS and the mixture with slag. The reason is the low pouring temperature, along with the high content of Mn and S in the molten iron.

Such a content of S and an appropriate content of Mn (0.5% – 0.65%) can significantly improve the purity of the iron liquid, thereby effectively preventing such defects.

(2) Gas pores caused by sand cores Gas pores and porous gas pores are often caused by poor exhaust of the sand cores. Since the sand cores are mostly hardened in the core box during core making, this often results in insufficient number of exhaust holes on the sand cores. To form exhaust holes, drilling can be carried out after the core has hardened.

(3) Liquid slag inclusion After processing, small individual holes can be found beneath the surface of the gray cast iron part. The diameter of these holes is generally 1 to 3 mm. In some cases, there are only 1 to 2 holes. Microscopic analysis shows that these small holes occur together with a small amount of liquid slag inclusion, but no segregation of S is found at this location. It has been studied that this defect is related to the pouring temperature. When the pouring temperature is higher than 1380℃, this defect is not found in the castings. Therefore, the pouring temperature should be controlled within 1380 – 1420℃. It is worth noting that changing the design of the pouring system failed to eliminate this defect, so this defect can be considered to be caused by the low pouring temperature and the casting of the molten iron under a trace reducing atmosphere.

The most common reason for low pouring temperature is that the molten iron cools down during long transportation and storage in an open ladle before pouring. Using a ladle cover with insulating material can significantly reduce heat loss.