Casting Technologies of Cast Steels
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The mechanical capacity of cast steel is higher than that of cast iron. The cast iron has high melting point while the molten steel is apt to oxidize. What is more, the molten steel has poor liquidity and great contraction. The volume and linear shrinkage rate of cast iron are 10-14% and 1.8-2.5% respectively. In order to prevent the steel castings from defects such as misrun, cold shut, shrinkage cavity, shrinkage porosity, crack and adhering sand, manufacturers must adopt technology that is more complex than that of iron castings.
A. Casting Temperatures.
(1) Due to the poor liquidity of molten steel, the thickness of cast steels should not be less than 8mm in case of misrun and cold shut.
(2) The structures of casting systems should be as simple as possible with larger diameter sizes than that of cast iron.
(3) Adopting dry sand cast or hot cast.
(4) Suitably improving the casting temperature. In general, the temperature ranges from 1520℃ to 1600℃. With a high casting temperature, superheat degree of the molten steel will be high. Thus, the steel can remains liquid for a longer period, with improved liquidity. However, coarse grains, hot cracking, porosity and adhering sand might take place when casting steels under an excessive temperature. Therefore, casting temperatures for steel castings are adjusted according to the melting temperature (Tm) of steels. Casting temperatures of small, thin and complex castings are set at Tm plus 150℃, while that of large and thick castings are nearly 100℃ higher than its Tm.
B. The shrinkage degrees of cast steels are higher than cast iron.
In order to guard against shrinkage cavity and shrinkage porosity, most of casting technologies resort to measures like casting head, chilling and supplement to realize progressive solidification.
Besides, in order to generate great uniformity, manufacturers often add saw dusts and cokes into casting sands and cores respectively. This is used to improve deformability and breathability of casting sands and cores. Hollow cores and oil cores are practicable as well.
C. Under high casting temperatures, the interaction between molten steels and mold materials is prone to adhering sands.
Therefore, manufacturers should use artificial quartz sand for casting. Silica flours or zircon flours should also be painted on the surface.
