What are the causes of casting stress? Are there any solutions?

Factors affecting casting stress

During solidification and cooling, the total stress of the casting is the algebraic sum of thermal stress, phase change stress and mechanical hindrance stress.

Thermal stress and phase transformation stress are often the residual stresses in castings. Residual stress is mainly related to the following four factors:

1. Metal properties

The residual stress of metal is directly proportional to the elastic modulus of metals. For example, the residual stress of metals with large elastic modulus such as cast steel and white iron is larger than that of gray cast iron with small elastic modulus, which confirms this theory.

2. The residual stress of the casting is proportional to the free line contraction coefficient of the alloy used.

The thermal conductivity of the metal will affect the temperature difference between the inside and outside of the casting. The thermal conductivity of carbon steel is higher than that of carbon steel, so the casting stress of carbon steel will be less than that of alloy steel  under the same standard.

2. Mold properties;

The heat storage coefficient represents the cooling rate of the casting, and the cooling of the casting will cause the temperature difference between inside and outside to increase. Large temperature difference will lead to greater casting stress. Therefore, the stress of metal castings is greater than that of sand castings.

3. Casting condition

Increasing the pouring temperature makes the temperature distribution more uniform after pouring, and also makes the cooling rate of the castings quicken, which can eliminate the casting stress.

Positive influence.

4. Casting structure

The greater the difference between the inner and outer wall thickness of a casting, the greater the difference between the cooling rate of the surface during casting, and the greater the casting stress.

Measures to reduce and eliminate casting stress

1. Rationally design the structure of the casting.

In the process of casting design, the wall thickness should be made as uniform as possible, and the phenomenon of protruding sharp angle should be avoided. This design method can effectively reduce the production of casting stress. In order to solve the problem of casting deformation, the following measures are put forward:

(1) When designing the castings, the inner and outer wall thickness of the castings should be basically the same, and the inner and outer walls of the castings should be cooled together with the castings as far as possible.

(2) To make the overall structure of the castings symmetrical in the design of the castings, even if the internal stress exists to offset the stress in the opposite direction

(3)  Reverse denaturation.

 When casting the shape of the castings with the same casting stress, condensation is to take some measures to make the internal and external cooling rate of the castings basically the same. The aim is to make the temperature difference between inside and outside of the casting small, so as to avoid the deformation of the casting. Therefore, the casting gate can be set skillfully in the casting process.

2. Adopt simultaneous solidification process

At the same time, condensation means taking some measures to make the casting inside and outside the cooling rate basically the same. The aim is to make the temperature difference between inside and outside of the casting small, so as to avoid the deformation of the casting. Therefore, the casting gate can be set skillfully in the casting process.

3. Aging treatment

3.1 Artificial aging

Temperature and retention time for different metal materials in eliminating residual stresses should be determined according to specific conditions. In the casting process, the usual method is heating to the elastic-plastic state, and cooling for a period of time to eliminate the stress.

In order to make the temperature distribution uniform in the process of heating and cooling, the temperature control should be carried out uniformly. In the actual production process for the casting cooling or heating speed and retention time should be set according to specific circumstances.

3.2 Natural aging

Natural aging refers to the casting after casting in the open air environment for a long time, casting stress will naturally disappear over time. The residual casting stress will cause the lattice deformation of the casting and the internal instability. This method is the simplest and does not require any cost. But in modern society where time is scarce, this primitive method is seldom adopted.