Characteristics and remedy of cold crack and warpage in sand casting

Defect characteristics and detection methods

1.Cold crack

It is also a straight or curved crack on the surface of the casting, with slight oxidation on the surface of the crack. Most of them appear in the transition part of the inner sharp corner, the thick and thin sections, and the joint between the inner gate and the casting. It can be found by naked eye, fluorescence inspection and X-ray inspection.

2.warping

The shape and size of the whole casting or its individual parts change, which can be measured by naked eye or measuring instrument.

Causes of formation

The formation of cold crack, warpage and hot crack is different. The cold crack is caused by the internal stress of the cooling shrinkage of the casting blocked by the mold or core at low temperature, which is greater than the strength of the alloy at this temperature. If the internal stress value only exceeds the yield limit of the alloy liquid at this time, there will be no crack and only warping deformation.

Prevention methods and remedial measures

1.Improve the design structure of the casting, eliminate the sudden change of sharp angle and thick section into thin section structure, and try to make all parts of the casting cool evenly and simultaneously

2.Take all kinds of measures to make castings cool evenly and simultaneously

① According to the shape of thick parts, choose the proper shape and size of cold iron and its reasonable setting position

② Correctly select the position of introducing alloy liquid to prevent local overheating

③ Proper reduction of pouring temperature

④ Use riser and chill together to increase temperature shaving of casting

3.Try to make the castings solidify in sequence

① The riser is placed in the thick part of the casting, and the insulation material is pasted to prolong the solidification time

② Place the cold iron or quench material on the thick part of the casting

③ Adopt bottom casting pouring system, match with high mold

④ Properly reduce the pouring temperature of alloy liquid

4.Reduce the strength of sand mold and increase its yield

① Changing dry sand mold to wet sand mold

② Reduce the compactness of sand mold and core

③ Replacing the strong molding sand with the collapsible molding sand

④ Reduce the core frame and outline size to prevent it from hindering the shrinkage of the casting

⑤ Use hollow core to enhance its yielding

5.According to the shape and warping degree of the warped casting, the cold or hot rectification method is adopted to restore the casting to its original shape

6.If none of the above measures can solve the problem, the alloy with high temperature and high strength with narrow crystallization interval (i.e. narrow temperature range of quasi solid phase) can be replaced

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