What is the difference in process between large cast steel and cast iron?

Update:25 Dec 2020

The mechanical properties of cast steel are higher than Auto parts aluminum die casting, but its casting properties are worse than cast iron. Because of the high melting point of cast steel, molten steel is easy to oxidize, the fluidity of molten steel is poor, and the shrinkage is large. Its body shrinkage rate is 10-14%, and linear shrinkage is 1.8-2.5%. In order to prevent defects such as insufficient casting, cold isolation, shrinkage and porosity, cracks and sand sticking of steel castings, more complicated process measures than cast iron must be adopted:

1. Due to the poor fluidity of molten steel, in order to prevent cold barriers and insufficient pouring of steel castings, the wall thickness of steel castings should not be less than 8mm; the structure of the gating system should be simple and the cross-sectional size is larger than that of cast iron; dry casting is used Or hot casting mold; appropriately increase the pouring temperature, generally 1520℃~1600℃, because the pouring temperature is high, the molten steel has a high degree of overheating, and the retention time is too long, and the fluidity can wait to be improved. However, if the pouring temperature is too high, it will cause defects such as coarse grains, heat, pores and sand sticking. Therefore, the casting temperature of small, thin-walled and complex-shaped castings is generally about the melting point temperature of steel + 150℃; the pouring temperature of large, thick-walled castings is about 100℃ higher than the melting point.

2. Since the shrinkage of large cast steel greatly exceeds that of cast iron, in order to prevent shrinkage cavities and shrinkage defects in castings, risers, chilled iron and subsidies are mostly used in the casting process to achieve sequential solidification.

In addition, in order to prevent the occurrence of shrinkage cavities, shrinkage porosity, pores and cracks in steel castings, the wall thickness should be uniform, sharp corners and right-angle structures should be avoided, sawdust is added to the casting sand, coke is added to the core, and Hollow cores and oil sand cores are used to improve the retreatability and permeability of sand molds or cores.

Steel castings have a high melting point and correspondingly their pouring temperature is also high. The interaction between molten steel and mold material at high temperature can easily produce sticky sand defects. Therefore, artificial quartz sand with high refractoriness should be used as the mold, and the surface of the mold should be brushed with a coating made of quartz powder or zircon sand powder. In order to reduce the body source, improve the fluidity of molten steel and the strength of the mold, most of the steel castings are cast in dry or quick-drying molds, such as CO2 hardened sodium silicate sand molds.