The application scope of precision castings is more and more extensive, and there are more and more manufacturing processes. Among them, the cooling process is an indispensable process. Some need to experience the solid-state phase transformation of alloys. During the phase transformation, metal materials change more frequently, such as carbon steel δ go in opposite directions γ The phase changes and the volume becomes smaller, γ When the phase eutectoid changes, the volume increases.
However, if the temperature of each part of the casting is the same, the external economic stress may not occur during the solid-state phase transformation, but only the external economic stress. When the transformation temperature is higher than the critical pressure of plastic elastic change, the alloy is in 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 subside.
If the phase change temperature of the casting is less than the critical pressure, and the temperature difference of each part of the casting is very large, and the phase change times of each part are different, it will cause external economic phase change stress. Because the phase change times are different, the phase change stress may become temporary stress or residual stress.
When a part of the thin wall of the casting undergoes solid-state phase transformation, a part of the thick wall is still in plastic condition. If the specific volume of the new phase exceeds that of the old phase during phase transformation, a part of the thin wall will expand during phase transformation, while a part of the thick wall will suffer plastic tension. As a result, only small tensile stress will occur inside the casting and gradually subside with the extension of time. Under such conditions, if the casting continues to cool, a part of the thick wall will undergo phase transformation and increase its volume. Because it is already in elastic condition, a part of the thin wall will be elastically stretched by the inner layer to form tensile stress. Under this kind of standard, the remaining phase transformation stress and the remaining thermal stress mark, in turn, can offset each other.
When a part of the thin wall of the casting releases solid phase transformation, the part of the thick wall is already in elastic condition. If the new specific volume exceeds the old phase, a part of the thick wall is elastically stretched to form tensile stress, while a part of the thin wall is elastically compressed to form temporary compressive stress. At this time, the phase transformation stress mark is the same as the thermal stress mark, that is, stress superposition. When the casting is continuously cooled until a part of the thick wall has phase transformation, the specific volume increases and expands, so that the phase transformation stress composed of the previous section disappears
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