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The application of precision casting is more and more extensive, and the processing technology is more and more. Among them, the cooling process is an essential process. Some of them need to experience the solid-state transformation of alloy, and the metal changes during the transformation. For example, the volume of carbon steel changes from δ phase to γ phase is reduced, and the volume increases when the γ phase changes.
However, if the temperature of each part of the casting is the same, it is impossible to have micro stress in solid phase transformation, but only micro stress. When the transformation temperature is higher than the critical temperature of plastic elastic change, the alloy is in the plastic state during transformation. Even if there is temperature in each part of the casting, the transformation stress is not large, and will gradually reduce or even disappear.
If the transformation temperature of the casting is lower than the critical temperature, and the temperature difference of each part of the casting is large, and the Transformation Times of each part are different, the micro transformation stress will be caused. Because the transformation times are different, the transformation stress may become temporary stress or residual stress.
When the thin-walled part of the casting undergoes solid transformation, the thick-walled part is still in a plastic state. If the specific volume of the new phase is larger than that of the old phase during transformation, the thin-walled part will swell during transformation, while the thick-walled part will be subject to plastic stretching. As a result, only a small tensile stress will occur in the inner part of the casting, and gradually disappear with the extension of time. In this case, if the casting continues to cool down, the thick wall will undergo phase transformation and increase its volume. Because it is already in the elastic state, the thin wall will be elastically stretched by the inner layer to form the tensile stress. In this case, the residual transformation stress and the residual thermal stress are opposite, which can cancel each other.
When the thin-walled part of the casting is released into solid phase transformation, the thick-walled part is already in elastic state. If the new specific volume is larger than the old phase, the thick-walled part is elastically stretched to form tensile stress, while the thin-walled part is elastically compressed to form temporary compressive stress. At this time, the sign of transformation stress is the same as that of thermal stress, i.e. stress superposition. When the casting is cooled to the thick wall, the specific volume increases and the transformation stress of the former section disappears.
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