In the gravity casting process, precision castings are used more and more widely, and there are more and more processing technologies. The cooling process is an essential process. Some of them have to experience the solid-state phase transformation of the alloy. During the phase transformation, the metal changes. For example, carbon steel δ Opposite γ Phase change volume reduction, γ When the eutectoid phase 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 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 a plastic state during transformation. Even if there is temperature in each part of the casting, the transformation stress is small, and will gradually reduce or even disappear.
If the phase transformation 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 transformation time of each part is different, it will cause micro phase transformation stress. Because the phase transformation time is different, the phase transformation stress may become temporary stress or residual stress.
When the thin-walled part of the casting undergoes solid-state phase transformation, the thick-walled part is still in a plastic state. If the specific volume of the new phase during phase transformation is greater than that of the old phase, the thin-walled part swells during phase transformation, while the thick-walled part undergoes plastic tension, resulting in only a small tensile stress in the casting, which gradually disappears with time extension.
In this case, if the casting continues to cool, the thick wall part will undergo phase transformation and increase its 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 residual phase transformation stress and residual thermal stress have opposite signs and can offset each other.
The above are the problems encountered by gravity casting during cooling. These problems should not be underestimated. We must pay attention to them, otherwise the quality of the whole casting will be affected.
