The application of precision castings is becoming increasingly widespread, and there are various processing techniques involved. Among them, the cooling process is an essential step. Some castings undergo solid-state phase transformations, where changes in the metal occur. For example, carbon steel undergoes a volume reduction when transitioning from the δ phase to the γ phase, and an increase in volume during the occurrence of the γ phase eutectoid transformation.

 

If all parts of the casting have the same temperature during solid-state phase transformations, no significant macroscopic stress will occur, only microscopic stresses. When the phase transformation temperature is higher than the critical temperature for plastic-elastic changes, the alloy is in a plastic state during the phase transformation. Even if there are temperature differences in different parts of the casting, the resulting phase transformation stress is small and gradually decreases or even disappears. If the phase transformation temperature of the casting is lower than the critical temperature and there is a significant temperature difference between different parts, the phase transformations occurring at different times may lead to microstructural phase transformation stresses, which can be temporary or residual stresses.

 

When a thin-walled section of the casting undergoes solid-state phase transformations while the thick-walled section remains in a plastic state, if the specific volume of the new phase is greater than that of the old phase, the thin-walled section expands while the thick-walled section undergoes plastic stretching. As a result, only a small tensile stress occurs within the casting, which gradually diminishes over time. If the casting continues to cool, and the thick-walled section undergoes phase transformation and increases in volume while in an elastic state, the thin-walled section will be subjected to internal elastic stretching, resulting in tensile stress. Meanwhile, the thick-walled section experiences external elastic compression, resulting in compressive stress. Under these conditions, residual phase transformation stresses and residual thermal stresses have opposite signs and can cancel each other out.

 

When the thin-walled section of the casting undergoes solid-state phase transformations while the thick-walled section is already in an elastic state, if the specific volume of the new phase is greater than that of the old phase, the thick-walled section experiences elastic stretching, forming tensile stress, while the thin-walled section undergoes elastic compression, forming temporary compressive stress. In this case, the signs of the phase transformation stress and thermal stress are the same, resulting in stress superposition. When the casting continues to cool until the thick-walled section undergoes phase transformation, the increase in specific volume causes expansion, leading to the disappearance of the previously formed phase transformation stress.