Alloy precision casting has strong shrinkage

Update:06 Dec 2019
Summary:

Shrinkage of alloy castings is hindered in various ways […]

Shrinkage of alloy castings is hindered in various ways. Some resistance comes from the mold and core, while others are caused by the obstacles caused by the casting structure itself. The casting shrinkage 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 are usually assembled without machining. Deviations in casting dimensions and errors in flatness all affect assembly and use. Castings may be scrapped simply because of dimensional deviations, cast hole size deviations, etc. Therefore, how to obtain castings that meet the dimensional deviation and accurately control the shrinkage of casting is an important issue. To this end, the law of dimensional shrinkage must be studied, and necessary process parameters must be determined as the basis for the production process.

The casting shrinkage of alloy castings under sand casting conditions is 2.6% to 2.7%, but it varies with the size and wall thickness of the casting. The thicker the wall, the stronger the metal's thermal effect on the mold. After the mold material loses strength, the resistance to shrinkage of the casting will decrease. The casting has more room for shrinkage in the mold, and the shrinkage rate is high. Conversely, the shrinkage value is low. The larger the size of the casting, the greater the hindrance caused by the mold and the casting itself when shrinking, and the smaller the shrinkage. Conversely, small pieces with simple shapes have high shrinkage. Summarizing the production experience data, it is concluded that the relationship between wall thickness, casting contour size and casting shrinkage is based on the data of medium carbon steel (0.2% -0.5% C) and low alloy steel. When used for alloy steel, the found casting shrinkage value (ie the value on the ordinate) should be multiplied by the correction factor.