Pay attention to details in the cooling process of precision castings

Update:27 Dec 2019
Summary:

The range of use of precision castings is getting wider […]

The range of use of precision castings is getting wider and wider, and more and more processing techniques are being used. The cooling process is an indispensable process. Some of them need to experience the solid phase transition of the alloy, and the comparative changes of the metal during the phase transition, such as It is said that the volume of carbon steel decreases from the change of δ phase to γ ​​phase. When the eutectoid change of γ phase occurs, the volume increases. However, if the temperature of each part of the casting is common, micro-stresses may not occur during the solid-state phase transition, but only micro-stresses. When the phase transition temperature is higher than the critical temperature of plastic-elastic change, the alloy is 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 it 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, and the phase transition moments of the various parts are not together, it will cause micro-phase transition stress. Because the phase transition moments are different, the phase transition stress may become temporary stress or residual stress. . When solid-phase transformation occurs in the thin-walled part of the precision casting, the thick-walled part is still in a plastic state. If the specific volume of the new phase is greater than that of the old phase during the phase change, the thin-walled part swells during the phase change, and the thick-walled part Part of it was plastically stretched, and only a small amount of tensile stress occurred in the resulting casting, and it gradually disappeared with time. In this case, if the casting continues to cool, the thick-walled part will undergo a phase change to increase its volume, because it is already in an elastic state, and the thin-walled part will be elastically stretched by the inner layer to constitute a tensile stress. The thick-walled part is elastically contracted by the outer layer to form a compressive stress. Under this condition, the residual phase transition stress and the residual thermal stress have opposite signs and can cancel each other.

When the thin-walled part of the casting undergoes solid phase transformation, the thick-walled part is already in an elastic state. If the new specific volume is greater than the old phase, the thick-walled part is elastically stretched to form tensile stress, and the thin-walled part is temporarily compressed to form temporary compression. stress. At this time, the phase transition stress sign is the same as the thermal stress sign, that is, the stress is superimposed. When the precision casting is continuously cooled until the phase transition occurs in the thick wall part, the specific volume increases and the expansion swells, so that the phase transition stress formed in the previous paragraph disappears.