Details of cooling operation in precision casting processing


Release time:

2023-06-20

The scope of application of precision castings is getting wider and wider, and there are more and more processing techniques. Among them, the cooling process is an indispensable process. Some of them have to undergo solid-state phase transformation of alloys. During the phase transformation, the comparison of metals changes, such as The volume of carbon steel decreases from the δ phase to the γ phase, and the volume increases when the γ phase undergoes eutectoid change.

Details of cooling operation in precision casting processing

  The scope of application of precision castings is getting wider and wider, and there are more and more processing techniques. Among them, the cooling process is an indispensable process. Some of them have to undergo solid-state phase transformation of alloys. During the phase transformation, the comparison of metals changes, such as The volume of carbon steel decreases from the δ phase to the γ phase, and the volume increases when the γ phase undergoes eutectoid change.

  However, if the temperature of each part of the casting is the same, it is impossible to produce microscopic stress when the solid-state phase transition occurs, but only microscopic stress. 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 that occurs 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 each part of the casting is large, and the phase transition time of each part is different, it will cause microscopic phase transition stress. Because of the different phase transition time, the phase transition stress may become temporary stress or residual stress .

  When the solid-state phase transition occurs in the thin-walled part of the 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 transition, the thin-walled part swells during the phase transition, while the thick-walled part suffers From plastic stretching, as a result, only a small tensile stress occurs inside the casting, and it gradually disappears as time goes by. In this case, if the casting continues to cool, the thick-walled part undergoes phase transformation and increases in volume. Since it is already in an elastic state, the thin-walled part will be elastically stretched by the inner layer to form tensile stress. The thick-walled part is elastically compressed by the outer layer to form compressive stress. Under this condition, the residual phase transition stress and residual thermal stress have opposite signs and can cancel each other out.

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