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The cooling process of castings is actually a very unique one. Some need to undergo solid-state phase transformation of the alloy. During the phase transformation, the comparison of metals changes. For example, when carbon steel transforms from δ to γ phase, its volume decreases. When the γ phase undergoes eutectoid transformation, its volume increases.

However, if the temperatures of all parts of the casting are consistent, when solid-state phase transformation occurs, macroscopic stress cannot be generated but only microscopic stress. When the phase transformation temperature is higher than the critical temperature of the plastic-elastic transition, the alloy is in a plastic state during phase transformation. Even if there is temperature in each part of the casting, the phase transformation stress generated is not large and will gradually decrease or even disappear.

If the phase transformation temperature of the casting is lower than the critical temperature and the temperature difference among different parts of the casting is large, and the phase transformation times of each part are different, it will cause macroscopic phase transformation stress. Due to the different phase transformation times, 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 remains in a plastic state. If the specific volume of the new phase is greater than that of the old phase during the phase transformation, the thin-walled part expands during the phase transformation, while the thick-walled part is subjected to plastic stretching. As a result, only a very small tensile stress is generated inside the casting, and it gradually disappears over time.

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, thus forming tensile stress. The thick-walled part is elastically compressed by the outer layer to form compressive stress. Under such conditions, the residual phase transformation stress and the residual thermal stress have opposite signs and can cancel each other out.

When solid-state phase transformation occurs in the thin-walled part of the casting, the thick-walled part 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 part will be elastically stretched to form tensile stress, while the thin-walled part will be elastically compressed to form temporary compressive stress. At this time, the phase transition stress symbol and the thermal stress symbol are the same, that is, stress superposition. When the casting continues to cool until the thick-walled part undergoes phase transformation, the specific volume increases and expansion occurs, causing the phase transformation stress formed in the previous section to disappear.

It can be seen from this that for alloys with solid-state phase transformation in castings, when the volume difference between new and old is large and the stress signs of shear phase transformation and thermal stress are the same, the result of the superposition of the two stresses may cause cracking and deformation of the castings in quenched and tempered round steel. Therefore, both temporary phase transformation stress and residual phase transformation stress should be studied carefully to eliminate harmful effects as much as possible.