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Heat treatment change tracking of precision castings

2020-10-09 15:35:52

The use of precision castings is becoming wider and wider, and the processing technology is becoming more and more. The cooling process is an indispensable process. Some have to experience the solid phase transition of the alloy. The metal changes during the phase transition. For example, The volume of carbon steel changes from the δ phase to the γ phase, and the volume increases when the γ phase undergoes eutectoid change. However, if the temperature of all parts of the casting is the same, microscopic stress is unlikely to occur when the solid phase transition occurs, but only microscopic stress. When the phase transition temperature is higher than the critical temperature for plastic-elastic change, the alloy is in a plastic state during the phase transition. Even if the temperature of each part of the casting exists, the resulting phase transition stress 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 of each part of the casting is large, the phase transition time of each part is not the same, it will cause the microscopic phase transition stress, because the phase transition time is different, the phase transition stress may become temporary stress or residual stress .

When the thin-walled part of the casting undergoes a solid phase change, 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 will expand during the phase change, and the thick-walled part will suffer When it reaches plastic stretching, only a small tensile stress occurs inside the casting, and it gradually disappears with time. In this case, if the casting continues to cool, the thick-walled part undergoes a phase change to increase its volume. Because it is already in an elastic state, the thin-walled part will be stretched elastically by the inner layer to form a tensile stress. The thick-walled part is elastically compressed by the outer layer to form a compressive stress. Under this condition, the residual phase change stress and the residual thermal stress have opposite signs and can cancel each other out.

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