The use range of casting parts is becoming wider and wider, and there are more and more processing techniques. Cooling process is an essential process, and some of them need to experience the solid phase transformation of alloys. During the phase transformation, the comparison of metals changes, such as carbon steel δ Opposite direction γ Phase change reduces volume, γ When the phase undergoes eutectoid changes, the volume increases.

But if the temperatures of all parts of the casting are the same, micro stress cannot occur during solid-state phase transformation, and only micro stress can occur. When the phase change temperature is higher than the critical temperature for plastic elastic change, the alloy is in a plastic state during phase change. Even if there is temperature in various parts of the casting, the phase change stress that occurs is not significant and will gradually decrease or even disappear.
If the phase change temperature of the casting is lower than the critical temperature, and the temperature difference between different parts of the casting is large, and the phase change times of each part are different, it will cause micro phase change stress. Because the phase change times are different, the phase change stress may become temporary stress or residual stress.
When the thin-walled part of the casting undergoes a solid phase transformation, the thick walled part is still in a plastic state. If the specific volume of the new phase during the transformation is greater than that of the old phase, the thin-walled part swells during the transformation, while the thick walled part undergoes plastic stretching. As a result, only a small tensile stress occurs inside the casting and gradually disappears with time. In this case, if the casting continues to cool, the thick walled part undergoes phase transformation and becomes larger in volume, as it is already in an elastic state. The thin walled part will be elastically stretched by the inner layer, forming tensile stress. The thick walled part is compressed by the outer elastic layer to form compressive stress. Under this condition, the residual phase change stress and residual thermal stress have opposite symbols and can cancel each other out.
When the thin-walled part of the casting undergoes solid phase transformation, the thick walled part is already in an elastic state. If the new phase volume is greater than the old phase, the thick walled part is subjected to elastic tension to form tensile stress, while the thin walled part is subjected to elastic compression to form temporary compressive stress. At this point, the phase change stress symbol and the thermal stress symbol are the same, that is, stress superposition. When the casting continues to cool until the thick walled part undergoes phase transformation, the specific volume increases and swells, causing the phase transformation stress formed by the previous section to disappear.