Steel castings are mainly used for machine parts which require higher strength, plasticity and toughness. Its output is second only to cast iron, accounting for about 15% of total castings.
The mechanical property of cast steel is higher than that of cast iron, but its casting performance is worse than that of cast iron. Because the melting point of cast steel is higher, the molten steel is easy to oxidize, the fluidity is poor, the shrinkage is big, the shrinkage rate is 10~14%, the line shrinks to 1.8~2.5%. In order to prevent cast steel casting defects such as insufficient pouring, cold insulation, shrinkage and shrinkage, crack and sticky sand, it is necessary to take more complicated technological measures than cast iron:
1. Due to the poor fluidity of molten steel, in order to prevent the steel castings from producing cold insulation and insufficient pouring, the wall thickness of the cast steel can not be less than 8mm, the structure of the gating system is simple, and the section size is larger than that of cast iron, the dry cast or hot mold is used, and the pouring temperature is 1520°~1600℃, because the pouring temperature The fluidity of molten steel can be improved by its high superheat and long time of keeping liquid. But the pouring temperature is too high, it will cause defects such as coarse grain, hot crack, pore and sticky sand.
Therefore, the general small, thin-walled and complex shape castings, the pouring temperature of the steel is about the melting point temperature +150℃, large, thick-walled castings casting temperature than its melting point of 100 ℃ or so.
2, because the shrinkage of cast steel is much more than cast iron, in order to prevent the casting shrinkage, shrinkage defects, most of the casting process using riser and, chill and subsidies and other measures to achieve sequential solidification.
In addition, in order to prevent shrinkage, shrinkage and porosity of steel castings, the porosity and crack defects should be uniform wall thickness, avoid sharp angle and right angle structure, add sawdust in mould type sand, add coke in core, and adopt hollow core and oil sand core to improve the yield and permeability of sand-type or core.