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The shrinkage of alloy castings is hindered by various […]
The shrinkage of alloy castings is hindered by various obstacles. Some resistance comes from the mold and core, while others are hindered by the casting structure itself. The casting shrinkage rate varies with the size, wall thickness and structural complexity of the casting, and there are often differences in all directions of the casting. Wear-resistant steel parts are difficult to machine, and they are usually assembled without processing.
The deviation of casting size and flatness error will affect the assembly and use. The castings may be scrapped only due to size deviation and hole size deviation. Therefore, how to obtain castings that meet the dimensional deviation and accurately control the shrinkage of the casting is a very important issue. For this reason, the law of size shrinkage must be studied, and the necessary process parameters must be determined as the basis for the proposed production process.
The casting shrinkage rate of alloy castings under sand casting conditions is 2.6% to 2.7%, but it varies with casting size and wall thickness. The thicker the wall, the stronger the thermal effect of the metal on the mold. The resistance of the mold material to the shrinkage of the casting when it loses its strength after heating will decrease. The casting has more room for shrinkage in the mold, and the shrinkage rate is higher.
On the contrary, the shrinkage value is low. The larger the size of the casting, the greater the hindrance formed by the mold and the casting itself during shrinkage, and the less shrinkage. Conversely, small parts with simple shapes have high shrinkage. Summarizing the production experience data, the relationship between the wall thickness, the contour size of the casting and the casting shrinkage rate is obtained, which is based on the data of medium carbon steel (0.2%-0.5% C) and low alloy steel. When used in alloy steel, the found casting shrinkage value (ie the value of the ordinate) should be multiplied by the correction factor.