The casting factory has more and more processing […]
The casting factory has more and more processing technology, and the use of precision casting is getting wider and wider. Among them, the cooling process is an indispensable process, and some have to experience the solid phase transition of the alloy. During the phase transition, the metal changes relatively. For example, when the carbon steel changes from δ phase to γ phase, the volume decreases, and when the γ phase erupts and changes , The volume increases. When the solid phase transition occurs, it is unlikely that microscopic stress will occur, but if the temperature of each part of the casting is common. There can only be 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 there is a temperature in each part of the casting, the phase transition stress is not large, and it will gradually decrease or even disappear.
The temperature difference of each part of the casting components is large, if the phase transition temperature of the casting is lower than the critical temperature. If the phase transition moments of each part are not together, it will cause microscopic phase transition stress. Because the phase transition moments are different, the phase transition stress may become temporary stress or residual stress. The thick-walled part is still in a plastic state, when the thin-walled part of the casting undergoes a solid phase transition. If the specific volume of the new phase is greater than the specific volume of the old phase during the phase transition, the thin wall will expand locally during the phase transition, while the thick wall part will be plastically stretched. Only a small tensile stress will occur inside the casting, and it will change with time. Extend and gradually disappear. In this case, if the casting continues to cool, the thick-walled part will undergo 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 wall is partially 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.