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    Transformation stress in cooling process of copper castings

    Author:Wanhe    Time:2020-02-24     Clicks:14985

    In fact, the cooling process of copper castings is a very unique process. Some of them have to undergo solid-state transformation of the alloy. During the transformation, the metal ratio changes. For example, the volume of carbon steel from δ phase to γ phase decreases, and the volume increases when the γ phase undergoes eutectoid transformation.

    Transformation stress in cooling process of copper castings(圖1)

    However, if the temperature of each part of the copper casting is the same, it is impossible to produce macro stress, but only micro stress. When the transformation temperature is higher than the critical temperature of the plastic elastic transformation, the alloy is in the plastic state during the transformation. Even if the temperature exists in each part of the copper casting, the transformation stress produced is not large, and will gradually reduce or even disappear.

    If the transformation temperature of copper casting is lower than the critical temperature, and the temperature difference of each part of copper casting is large, and the transformation time of each part is different, it will cause the macro transformation stress. Because of the different transformation time, the transformation stress may become temporary stress or residual stress.

    When the thin-walled part of the copper casting undergoes solid transformation, the thick-walled part is still in the plastic state. If the specific volume of the new phase is larger than that of the old phase, the thin-walled part expands while the thick-walled part is subject to plastic stretching. As a result, only a small tensile stress is generated in the copper casting, and gradually disappears with the time.

    In this case, if the copper casting continues to cool, the thick wall part will undergo phase transformation and increase the volume. Because it is already in the elastic state, the thin wall part will be elastically stretched by the inner layer and form tensile stress. Under this condition, the residual phase transformation stress and the residual thermal stress sign are opposite and can cancel each other.

    When the thin-walled part of the copper 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 copper casting continues to cool to the thick wall, the specific volume increases and expands, which makes the transformation stress of the former section disappear.

    It can be seen that for the alloy with solid phase transformation in copper casting, when the tolerance between the old and the new is large and the shear phase transformation stress sign and the thermal stress sign are the same, the superposition of the two kinds of stress results in the cracking and deformation of the copper casting. Therefore, both the temporary phase transformation stress and the residual phase transformation stress should be studied to eliminate the harmful effects as much as possible.