Details of cooling operation in casting processing
The use of precision castings is more and more widely, the processing process is more and more, the cooling process is an essential process, some to experience solid phase transformation of alloy, phase transformation of metal comparison change, such as carbon steel from δ to γ phase change volume reduction, γ phase attack eutectoid change, volume increase.
However, if the temperature of each part of the casting is common, the micro-stress may not occur during the solid phase transformation, but only the micro-stress. When the phase change temperature is higher than the critical temperature of plastic-elastic change, the alloy is in a plastic state during the phase change, even if the temperature exists in all parts of the casting, the phase change stress is not large, and will gradually decrease or even disappear.
If the transformation temperature of the casting is lower than the critical temperature, and the temperature difference of each part of the casting is large, and the phase transformation time of each part is not together, it will cause microscopic phase transformation stress, because the phase transformation time is different, the phase transformation stress may become temporary stress or residual stress.
When solid phase transformation occurs in the thin-walled part of the casting, the thick-walled part is already in an elastic state. If the new relative capacity is greater than that of the old phase, the thick-walled part is subjected to elastic tensile stress, while the thin-walled part is subjected to elastic contraction to form temporary compressive stress. In this case, the sign of phase transition stress is the same as that of thermal stress, namely stress superposition. When the casting is cooled continuously until the phase transformation occurs in the thick-walled part, the expansion occurs when the specific volume increases, so that the phase transformation stress formed in the previous section disappears.