In gravity casting, precision castings are used more and more widely, and there are more and more processing technologies. Among them, the cooling process is an essential process. Some have to experience the solid-state phase transformation of the alloy, and the metal changes during the phase transformation, such as carbon steel δ Opposite γ Phase change, volume reduction, γ When the phase eutectoid changes, the volume increases.
If the temperature of each part of the casting is the same, the micro stress may not occur during the solid-state phase transformation, but only the micro stress. When the transformation temperature is higher than the critical temperature of plastic elastic change, the alloy is in plastic state during transformation. Even if there is temperature in each part of the casting, the transformation stress is small, and will gradually decrease or even disappear.
If the phase change temperature of gravity casting casting is lower than the critical temperature, and the temperature difference of each part of the casting is large, and the phase change time of each part is different, it will cause micro phase change stress. Because the phase change time is different, the phase change stress may become temporary stress or residual stress.
When the thin-walled part of the casting has solid-state phase transformation, the thick-walled part is still in plastic state. If the specific volume of the new phase is greater than that of the old phase during phase transformation, the thin-walled part expands during phase transformation, while the thick-walled part is subjected to plastic tension. As a result, only a small tensile stress occurs in the casting and gradually disappears with the extension of time.
In this case, if the casting continues to cool, the thick wall part will undergo phase transformation and increase the volume. Because it is already in an elastic state, the thin wall part will be elastically stretched by the inner layer to form a tensile stress. Under this condition, the sign of residual phase transformation stress and residual thermal stress is opposite and can offset each other.
The above are the problems encountered during cooling of gravity casting. These problems should not be underestimated. We must pay attention to them, otherwise the quality of the whole casting will be affected.
Editing: WMM