The application range of precision castings is becoming wider and wider, and the processing technology is also increasing. Cooling process is an essential process, and some also undergo solid phase transformation of alloys. During this process, the comparison of metals changes, such as the volume of carbon steel changing from δ Phase transition γ Xiang, γ When the phase transition is to eutectoid, the volume increases.
But if the temperature of each part of the precision casting is the same, there may not be micro stress during solid phase transformation, only micro stress. When the phase transition temperature is higher than the critical temperature for plastic elastic transition, the alloy is in a plastic state during the phase transition process. Even if there is temperature in various parts of the casting, the phase transformation stress is very small and will gradually decrease or even disappear.
If the phase transition temperature of precision castings is lower than the critical temperature, the temperature difference between different parts of the casting is large, and the phase transition time of each part is different, it will cause micro phase transition stress. Due to different phase transition times, phase transition stress may become temporary stress or residual stress.
When the thin-walled part of precision castings undergoes solid phase transformation, the thick walled part is still in a plastic state. During the phase transition process, 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 undergoes plastic stretching. As a result, only small tensile stresses appeared in the casting and gradually disappeared over time. In this case, if the casting continues to cool, the thick walled part will undergo phase transformation and increase its volume. Due to being in an elastic state, the thin-walled part will be stretched by the inner layer, forming tensile stress. However, the thick walled portion is compressed by the outer layer to form compressive stress. In this case, residual phase transition stress and residual thermal stress have opposite signs and can cancel each other out.
When the thin-walled part of a precision casting releases solid phase transformation, the thick walled part is already in an elastic state. If the new specific volume is greater than the old phase, the thick walled part is subjected to elastic tension to form tensile stress, and the thin walled part is subjected to elastic contraction to form temporary compressive stress. At this point, the symbol of phase change stress is the same as that of thermal stress, that is, stress superposition. When the precision casting is continuously cooled until the thick walled part undergoes phase transformation, the specific volume increases and expands, causing the phase transformation stress formed in the previous section to disappear.
