Cast iron and steel features

Cast steel has higher mechanical properties than cast iron, but its casting performance is worse than cast iron. Because the melting point of the cast steel is high, the molten steel is easily oxidized, the fluidity of the molten steel is poor, and the shrinkage is large, and the body shrinkage is 10 to 14%, and the linear shrinkage is 1.8 to 2.5%. In order to prevent defects such as insufficient pouring, cold separation, shrinkage and shrinkage, cracks and sand sticking of steel castings, it is necessary to adopt complex process measures than cast iron:
1. Due to the poor fluidity of the molten steel, in order to prevent the cold separation and pouring of the steel castings, the wall thickness of the steel castings should not be less than 8 mm; the structure of the casting system is simple and the cross-sectional size is larger than that of cast iron; Or hot-casting type; appropriately increase the pouring temperature, generally 1520 ° ~ 1600 ° C, because the pouring temperature is high, the molten steel has a high degree of superheat, a long time to keep the liquid, and the fluidity can be improved. However, if the pouring temperature is too high, defects such as coarse crystal grains, hot cracks, pores and sand sticks may be caused. Therefore, the casting temperature of a small, thin-walled and complex shape is generally about +150 ° C of the melting point of steel; the casting temperature of large and thick-walled castings is about 100 ° C higher than its melting point.
2. Since the shrinkage of cast steel greatly exceeds that of cast iron, in order to prevent shrinkage and shrinkage defects of castings, most of the casting process uses risers, cold irons and subsidies to achieve sequential solidification.
In addition, in order to prevent shrinkage, shrinkage, porosity and crack defects in the steel castings, the wall thickness should be uniform, avoid sharp corners and right angle structures, add sawdust in the molding sand, add coke in the core, and Hollow cores and oil sand cores are used to improve the retractability and gas permeability of the sand or core.
The cast steel has a high melting point and the corresponding casting temperature is also high. At high temperatures, molten steel interacts with the mold material, which is highly susceptible to sticking sand defects. Therefore, artificial quartz sand with high refractoriness should be used as the mold, and the coating made of quartz powder or zirconium sand powder should be brushed on the surface of the mold. In order to reduce the gas source, improve the fluidity of the molten steel and the strength of the mold, most of the steel castings are cast with a dry or fast drying type, such as a water glass sand type which is hardened by CO2.