The application range of castings is becoming more and more extensive, and there are also more and more processing techniques. The cooling process is essential, and some also undergo solid phase transformation of the alloy. When phase transition occurs, the metal undergoes relative changes. For example, when carbon steel is removed from δ Phase transition to γ When in phase, its volume decreases, and when γ When the phase transforms into an eutectoid phase, its volume increases.
But if the working temperatures of various parts of the casting can be common, it is impossible to experience microstructural stress during solid-state phase transformation, and only microscopic environmental stress can occur. When the phase change temperature is significantly higher than the critical temperature of constantly changing plasticity and elasticity, the alloy development is in a plastic state during phase change. Even if temperature control exists in various parts of the casting, the phase change stress that occurs is not significant and will gradually reduce or even disappear.
If the phase change temperature of the casting is significantly lower than the critical temperature, and the temperature difference between different parts of the casting has a significant impact, and the phase change time of each part is not the same, it will cause micro phase change stress in the enterprise, because there is no phase change time. Depending on the difference, phase change stress may have become a temporary stress or residual stress.
When the new volume is larger than the old volume, the thin-walled part expands during the phase transformation process, while the thick wall part undergoes plastic stretching. The results indicate that the tensile stress in the casting is very small, and gradually disappears with time. In this case, if the casting is continuously cooled, the thick walled part undergoes phase transformation and becomes large, and since the thin walled part is already in an elastic state, it will be elastically stretched by the inner layer to form tensile stress. In the case of compressive stress formed by the elastic contraction of the outer layer on the thick wall, the residual transformation stress and residual thermal stress have opposite symbols and can cancel each other out.
If the specific volume of the new phase is greater than that of the old phase, the thick part of the casting will be elastically stretched to form tensile stress, while the thin part will be elastically compressed to form temporary compressive stress. The symbol of phase change stress is the same as that of thermal stress, that is, stress superposition. When the casting is continuously cooled to the thick walled part, the specific volume increases and the phase transformation stress in the front section disappears.