In the process of solidification and cooling, the stress of the casting is the algebraic sum of thermal stress, phase change stress and mechanical hindering stress. This stress value is greater than the strength of the metal at this temperature, and the casting will crack.
The mechanical hindrance stress generally disappears after the casting falls out of sand, which is a temporary stress. Residual stress is often thermal stress and phase change stress. The residual stress is related to the following factors.
1. Metal properties
1) The greater the modulus of elasticity of the metal, the greater the residual stress in the casting. For example, the residual stress of cast steel, white iron and ductile iron is larger than that of gray cast iron. One of the reasons is related to the elastic modulus of the metal.
2) The residual stress of the casting is proportional to the free line shrinkage coefficient of the alloy. When other conditions are the same, the residual stress of austenitic stainless steel is 50% larger than that of ferritic stainless steel due to the large α value.
3) The thermal conductivity of the alloy directly affects the temperature difference between the thickness of the casting. Alloy value has lower thermal conductivity than carbon steel, so when other conditions are the same, alloy steel has larger residual stress.
2. In terms of mold properties
The greater the heat storage coefficient of the mold, the greater the cooling rate of the casting, the greater the temperature difference between the inside and outside of the casting, and the greater the stress produced. Metal molds are easier to cause greater residual stress in castings than sand molds.
3. Pouring conditions
Increasing the pouring temperature is equivalent to increasing the temperature of the casting mold, delaying the cooling rate of the casting, making the temperature of each part of the casting uniform, and reducing the residual stress.
4. Casting structure
The greater the difference in wall thickness of the casting, the greater the temperature difference between the thick and thin walls during cooling, and the greater the heat caused.
