We understand that the application range of fine castings is more and more wide, and the processing technology is also more and more. The cooling process is essential, and some also undergo solid phase transformation of the alloy. When a phase transition occurs, the metal changes relative, such as carbon steel decreases in volume when it changes from phase to phase and increases in volume when it changes from phase to eutectoid phase.
However, if the temperature of all parts of the casting is the same, the solid phase transition may not produce microstresses, but only microstresses. When the transition temperature is higher than the critical temperature of elastoplastic transition, the alloy is in a plastic state during the transition process. Even if the temperature exists in all parts of the casting, the phase transition stress is not large and will gradually decrease or even disappear.
We say that if the phase transition temperature of the casting is lower than the critical temperature, and the temperature difference between the parts of the casting is large, the phase transition time of each part is not the same, it will cause microscopic phase transition stress, because the phase transition time is not the same, the phase transition stress may become temporary stress or residual stress.
When the thin - wall part of the casting underwent solid phase transition, the thick - wall part of the casting remained in the plastic state. If the specific volume of the new phase is greater than that of the old phase during the phase transition, the thin-walled part expands during the phase transition, while the thick-walled part stretches plastically. As a result, only a small tensile stress is generated in the casting, which gradually disappears with the extension of time. In this case, if the casting is kept cool, the thick-walled part will undergo a phase transition and increase its volume. Because it is already in an elastic state, the thin wall is stretched by the inner layer, creating tensile stress. And the thick wall part is contracted by the elastic outer layer, forming compressive stress. In this case, the residual phase change stress and the residual thermal stress have opposite symbols and can cancel each other out.
We say that when the thin-walled part of the casting is released into the solid phase transition, the thick-walled part is already in an elastic state. If the new specific volume is larger than the old phase, the tensile stress is formed by the elastic stretching of the thick wall part, and the temporary compressive stress is formed by the elastic contraction of the thin wall part. In this case, the symbol of phase change stress is the same as that of thermal stress, namely stress superposition. When the castings are continuously cooled until the thick-walled parts undergo phase transformation, the specific volume becomes larger and expands, so that the phase transformation stress formed in the previous section disappears.
I believe that through the above introduction, we also have a lot of understanding of us. If you want to know more about this, please contact us. We can introduce you from different aspects, we have many years of experience in this industry.
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