The application range of precision castings is getting wider and wider, and there are more and more processing techniques. Among them, the cooling process is an essential process, and some also undergo the solid phase transformation of the alloy. During this process, the contrast of metals changes, such as The volume of carbon steel changes from δ phase to γ phase, and the volume increases when γ phase changes to eutectoid.
However, if the temperature of each part of the precision casting is the same, there may be no micro-stress during the solid phase transformation, only micro-stress. When the phase transition temperature is higher than the critical temperature of plastic-elastic transition, the alloy is in a plastic state during the phase transition. Even if there is temperature in each part of the casting, the phase transformation stress is small and will gradually decrease or even disappear.
If the phase transition temperature of the precision casting is lower than the critical temperature, the temperature difference of each part of the casting is large, and the phase transition time of each part is different, which will cause microscopic phase transition stress. Due to the different phase transition time, the phase transition stress may become temporary stress or residual. stress.
When the thin-walled part of the precision casting undergoes solid phase transformation, the thick-walled part is still in a plastic state. During the phase transition, if the specific volume of the new phase is greater than that of the old phase, the thin-walled part expands while the thick-walled part stretches plastically. As a result, only small tensile stresses appear in the casting and gradually disappear over time. In this case, if the casting continues to cool, the thick-walled part will undergo a phase transformation and increase its volume. Since it is already in an elastic state, the thin-walled part will be elastically stretched by the inner layer, resulting in tensile stress. However, the thick-walled portion is elastically compressed by the outer layer to create compressive stress. In this case, residual phase transformation stress and residual thermal stress have opposite signs and can cancel each other out.