Details of cooling operation during lost wax casting
The range of use for Lost wax casting is increasing, and more and more processing techniques are used. The cooling process is an essential process. Some of them need to experience the solid phase transition of the alloy. Carbon steel changes in volume from the δ phase to the γ phase, and the volume increases when the eutectic changes to the γ phase.
However, if the temperature of each part of the casting is common, minute stress does not occur during the solid phase transition, and only minute stress may occur. If the phase transition temperature is higher than the critical temperature of plastic elastic change, the alloy will be in a plastic state during the phase transition. Even if there is temperature in each part of the casting, the phase transition stress is not large and gradually decreases or disappears.
If the phase transition temperature of the casting is lower than the critical temperature, the temperature difference of each part of the casting is large, and the phase transition moments of each part are not together, micro phase transition stress occurs. Because the phase transition moments are different, the phase transition stress can be temporary or residual. .
When solid phase transformation occurs in the thin part of the casting, the thick part is still in the plastic state. If the specific volume of the new phase is greater than the specific volume of the old phase during the phase change, the thin part will expand during the phase change and the thick part will be affected. With plastic drawing, only a small amount of tensile stress is generated in the casting, and it gradually disappears over time. In this case, as the Copper Alloy casting to cool, the thick part is already elastic, so it undergoes a phase change to increase its volume, and the thin wall part is elastically stretched by the inner layer to constitute tensile stress. . The thick part is elastically contracted by the outer layer to form a compressive stress. In this state, the residual phase transition stress and residual thermal stress have opposite signs and can cancel each other.
When the thin part of the casting undergoes solid phase transformation, the thick part is already elastic. If the new specific volume is larger than the old phase, the thick part will be elastically stretched to form tensile stress and the thin part will be temporarily compressed to form temporary compression. stress. At this time, the sign of phase transition stress is the same as the sign of thermal stress. In other words, stress is superimposed. When the casting is continuously cooled until the phase transition occurs at the thick wall, the specific volume increases and the expansion expands so that the phase transition stress formed in the previous paragraph disappears. . “