Precision castings are used more and more widely, processing technology to more and more, the cooling process is an essential process, some to experience the solid phase transformation of alloy, phase transformation of the metal compared to change, such as carbon steel by δ to γ phase change volume reduction, γ phase attack eutectoid change, volume increase.

But if the temperature of all parts of the casting is common, there may be no microscopic stress when the solid phase transition occurs, but only microscopic stress. When the temperature of phase transition is higher than the critical temperature of plastic-elastic change, the alloy is in a plastic condition during the phase transition. Even if the temperature exists in each part of the casting, the phase transition stress is not large, and will gradually reduce or even disappear.

If the phase transition temperature of the casting is lower than the critical temperature, and the temperature difference of each part of the casting is large, and the phase transition time of each part is not together, the microscopic phase transition stress will be caused, because the phase transition time is different, the phase transition stress may become temporary stress or residual stress.

If the specific volume of the new phase is larger than that of the old phase, the thin-wall part expands during the phase transition, while the thick-wall part is subjected to plastic tension. As a result, only a small tensile stress occurs inside the casting and gradually disappears with the extension of time. In this case, if the casting continues to cool, the thick wall part of the phase transition and increase the volume, because it has been in an elastic condition, the thin wall part will be elastic stretched by the inner layer, and constitute tensile stress. The thick part of the wall is compressed by the elasticity of the outer layer to form the compressive stress. In this condition, the residual phase transition stress and the residual thermal stress have opposite signs and can cancel each other.

When the thin wall part of the casting is released into the solid phase transformation, the thick wall part is already in the elastic state. If the new relative volume is greater than the old phase, the thick wall part is formed by elastic tensile stress, while the thin wall part is formed by elastic compression stress. In this case, the phase transition stress symbol is the same as the thermal stress symbol, namely the stress superposition. When the casting is cooled continuously until the phase transition occurs in the thick-walled part, the specific volume increases and expands, making the phase transition stress of the previous section disappear.