Causes:

A. The composition of molten iron is unreasonable.

B. Shrinkage cavity and porosity at the tensile position of the test bar.

C. The finish of the test bar is not enough.

D. The cooling rate of casting is too fast.

Improvement measures:

A. Increase riser at shrinkage position to improve shrinkage.

B. The ideal temperature gradient and solidification sequence can be obtained by improving the heat distribution. Thus, the casting with dense structure can be obtained.

C. It is suggested to improve the composition of molten iron.