Causes and solutions for casting cracks

Cracks are the most destructive defects in precision castings. Cracks are not allowed in all precision casting acceptance standards. Most of the precision castings are hot cracks, and cold cracks are relatively rare.

All cracks are caused by internal stress in the casting. When the internal stress of the casting and the plastic deformation caused by it exceed the strength limit and elongation at the temperature, cracks will occur. When the cooling of the various parts of the casting is uneven, thermal stress will be formed. 

When the solid phase change time and degree of each part are different during the cooling process, it will cause phase change stress.When the casting solidifies and shrinks, the mechanical obstruction stress is caused by the poor yield of the shell mold.

After understanding the above reasons, you can take the following measures

2.1. Influence of casting structure: The wall thickness of the casting should not change too much. Try to avoid right angles. It is best to change to rounded corners to reduce stress. The connection between the nozzle and the casting should also be made into rounded corners as much as possible.

2.2. Influence of material type. Generally, low alloy steel, medium carbon steel, high carbon steel and 420, 430 are prone to cracks. Pay special attention to this type of steel.

2.3. Reasonably design the position of the inner gate and set the pouring system, change the temperature field of the casting during solidification to ensure the sequential solidification diagram, avoid the large difference in cooling speed of each part and the formation of relatively large internal stress, because the structure of the casting is generally difficult to change.

2.4. Change the temperature field during shell mold cooling to reduce stress, such as cotton wrapping the fast-cooling part to reduce the cooling temperature.

2.5. Reduce slag inclusions in the casting, because slag holes will cut the matrix structure of the casting, reduce the strength performance of the casting, and make it more prone to cracks.

In addition, some cracks are caused by insufficient shrinkage compensation. To strengthen the shrinkage compensation of the casting, it is a good method to use the riser insulation when the process remains unchanged, but it cannot affect the chemical composition of the die head and introduce impurities into the die head material.

For example, once our employees were irresponsible and did not cover a part of the same casting in the same furnace with insulation, and a shrinkage hole as big as a finger appeared inside the casting, while most of the castings covered with insulation were all qualified, so don't underestimate the use of Changkou insulation, which is actually very important.

I remember that 20 years ago, one of our customers needed to produce a rod-shaped casting. Strangely, there was a shrinkage at a location far from the ingate, but not at the hot spot. Some of the shrinkage locations had obvious cracks, but not all of them had cracks. The location was not fixed, and there was no way to solve it.

Our engineer suggested checking whether the temporary hot spot was formed because the trees were too close together during assembly. It turned out that this was the reason. After changing the distance between the wax molds during assembly, the problem was completely solved. 

Finally, we stipulated that when assembling trees, all wax parts should be at least one finger width apart.