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Details of cooling operations in casting processing
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Release time:
2024-05-20
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The use range of castings is getting wider and wider, and the processing technology is getting more and more. Among them, the cooling process is an essential process. Some of them need to experience the solid phase transformation of alloys, and the comparative onset and change of metals during phase transformation. For example, the volume of carbon steel decreases from δ phase to γ phase, and the volume increases when γ phase occurs eutectoid change.
However, if the temperature of each part of the casting is the same, the occurrence of solid phase transformation may not occur microstress, but only microstress. When the phase change temperature is higher than the critical temperature of the plastic-elastic change, the alloy is in a plastic condition when the phase change, even if the temperature of each part of the casting exists, the phase change stress is not large, and will gradually reduce or even disappear.
If the phase change temperature of the casting is lower than the critical temperature, and the temperature difference between the parts of the casting is large, the phase change time of each part is not together, it will cause microscopic phase change stress, because the phase change time is different, the phase change stress may become temporary stress or residual stress.
When the thin-walled part of the casting has a solid phase change, the thick-walled part is still in a plastic condition. If the specific volume of the new phase is greater than that of the old phase during the phase change, the thin-walled part expands during the phase change, while the thick-walled part is subjected to plastic stretching. 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 is continuously cooled, the thick-walled part will undergo phase transformation and become larger in volume. Because it is already in elastic condition, the thin-walled part will be stretched elastically by the inner layer to form tensile stress. The thick-walled part is elastically compressed by the outer layer to form a compressive stress, under which the residual phase change stress and the residual thermal stress have opposite signs and can cancel each other out.
When the thin-walled part of the casting is released into solid phase change, the thick-walled part is already in elastic condition, if the new phase is larger than the old phase, the thick-walled part is elastically stretched to form a tensile stress, while the thin-walled part is elastically compressed to form a temporary compressive stress. At this time, the phase change stress symbol and the thermal stress symbol are the same, that is, the stress is superimposed. When the casting is continuously cooled until the phase change of the thick wall part occurs, the specific volume changes greatly, so that the phase change stress formed by the previous section disappears.
However, if the temperature of each part of the casting is the same, the occurrence of solid phase transformation may not occur microstress, but only microstress. When the phase change temperature is higher than the critical temperature of the plastic-elastic change, the alloy is in a plastic condition when the phase change, even if the temperature of each part of the casting exists, the phase change stress is not large, and will gradually reduce or even disappear.
If the phase change temperature of the casting is lower than the critical temperature, and the temperature difference between the parts of the casting is large, the phase change time of each part is not together, it will cause microscopic phase change stress, because the phase change time is different, the phase change stress may become temporary stress or residual stress.
When the thin-walled part of the casting has a solid phase change, the thick-walled part is still in a plastic condition. If the specific volume of the new phase is greater than that of the old phase during the phase change, the thin-walled part expands during the phase change, while the thick-walled part is subjected to plastic stretching. 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 is continuously cooled, the thick-walled part will undergo phase transformation and become larger in volume. Because it is already in elastic condition, the thin-walled part will be stretched elastically by the inner layer to form tensile stress. The thick-walled part is elastically compressed by the outer layer to form a compressive stress, under which the residual phase change stress and the residual thermal stress have opposite signs and can cancel each other out.
When the thin-walled part of the casting is released into solid phase change, the thick-walled part is already in elastic condition, if the new phase is larger than the old phase, the thick-walled part is elastically stretched to form a tensile stress, while the thin-walled part is elastically compressed to form a temporary compressive stress. At this time, the phase change stress symbol and the thermal stress symbol are the same, that is, the stress is superimposed. When the casting is continuously cooled until the phase change of the thick wall part occurs, the specific volume changes greatly, so that the phase change stress formed by the previous section disappears.
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