The use of precision castings is getting wider and wider, and the processing technology is more and more. The cooling process is an indispensable process. Some of them need to experience the solid phase transition of the alloy, and the metal changes during the phase transition, such as The volume of carbon steel changes from δ phase to γ phase, and the volume increases when the eutectoid of γ phase changes.
But if the temperature of all parts of the casting is common, it is impossible to have microscopic stress when the solid phase transition occurs, but only microscopic stress. When the phase transition temperature is higher than the critical temperature of plastic-elastic change, the alloy is in a plastic state during the phase transition. Even if the temperature of each part of the casting is present, the phase transition stress is not large and will gradually decrease or disappear.
If the phase transition temperature of the casting is lower than the critical temperature, and the temperature difference of each part of the casting is large, and the phase transition time of each part is not together, it will cause microscopic phase transition stress. Because the phase transition time is different, the phase transition stress may become temporary stress or residual stress .
When the solid phase transition occurs in the thin-walled part of the casting, the thick-walled part is still in a plastic state. If the specific volume of the new phase is greater than the specific volume of the old phase during the phase transition, the thin-walled part swells during the phase transition, while the thick-walled part To plastic stretching, only a small tensile stress occurs inside the casting, and it gradually disappears with time. In this case, if the casting continues to cool, the thick-walled part undergoes a phase change to increase the volume. Because it is already in an elastic state, the thin-walled part will be elastically stretched by the inner layer, which constitutes a tensile stress. The thick-walled part is elastically contracted by the outer layer to form compressive stress. Under this condition, the signs of the residual phase transition stress and the residual thermal stress are opposite and can cancel each other out.
When the thin-walled part of the casting undergoes a solid phase transition, the thick-walled part is already in an elastic state. If the new volume is greater than the old phase, the thick-walled part is elastically stretched to form a tensile stress, and the thin-walled part is elastically contracted to form a temporary compression 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 continues to cool until the thick-walled part undergoes a phase change, the specific volume increases and expands, causing the phase change stress formed by the previous section to disappear.