Cooling operation details in precision casting processing
Cooling operation details in precision casting processing
The use of precision castings is becoming wider and wider, and the processing technology is becoming more and more. The cooling process is an indispensable process. Some have to experience the solid phase transformation of the alloy, and the relative changes of the metal during the phase change, for example. The carbon steel is reduced in volume from the δ phase to the γ phase, and the volume increases when the γ phase seizure changes.
However, if the temperature of each part of the Precision casting is common, the onset of solid phase transformation will not cause microscopic stress, but only microscopic stress. When the phase transition temperature is higher than the critical temperature at which the elastic change is made, the alloy is in a plastic state during the phase change. Even if the temperature of each part of the casting exists, the phase transformation stress is not large and will gradually decrease or even disappear.
If the phase transition temperature of the casting is lower than the critical temperature, and the temperature difference between the various parts of the casting is large, the phase transition time of each part is not together, which will cause microscopic phase transition stress. Because the phase transition time is different, the phase transition stress may become temporary stress or residual stress. .
When the thin-walled part of the casting has a solid phase transition, the thick-walled part is still in a 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-walled portion is swollen during the phase change, and the thick-walled portion is affected. By plastic stretching, only a small tensile stress is generated inside the casting, and it gradually disappears as time goes by. In this case, if the casting continues to cool, the thick-walled portion undergoes a phase change to increase the volume, and since it is already in an elastic state, the thin-walled portion is elastically stretched by the inner layer to constitute a tensile stress. The thick-walled portion is elastically contracted by the outer layer to form a compressive stress. Under these conditions, the residual phase-change stress and the residual thermal stress are opposite to each other and can cancel each other out.
When the thin-walled part of the casting is released into a solid phase transition, the thick-walled portion is already in an elastic state. If the new comparative volume is larger than the old phase, the thick-walled portion is elastically stretched to form a tensile stress, and the thin-walled portion is elastically contracted to constitute a temporary pressure. stress. At this time, the phase change stress symbol and the thermal stress symbol are the same, that is, the stress is superimposed. When the casting continues to cool to the thick-walled part, the specific volume increases and the swelling increases, so that the phase transition stress formed in the previous section disappears.
