(1) When the cooling is completed, that is, when the temperature of the surface and the core is completely the same, the elastic deformation of the surface and the core also disappears and returns to the original state. Although instantaneous thermal stress is generated during the cooling process, the residual thermal stress after the cooling is terminated is equal to zero.

(2) Of course, this is a special case. Since large thermal stress is generated at the beginning of the rapid cooling process, the steel is still at a relatively high temperature and has good plasticity. The thermal stress will exceed the yield strength of the large-diameter steel pipe, resulting in plastic deformation of the surface being tensile and the core being compressive, thereby relieving the thermal stress.

(3) When cooling continues, the cooling rate of the surface slows down, while the cooling rate of the core increases. The temperature difference between the surface and the core gradually decreases after a large value, and the thermal stress acting on the surface and the core also decreases accordingly.

(4) However, due to the above-mentioned pre-generated plastic deformation, the large thermal stress is reduced. When there is still a significant temperature difference, the thermal stress is close to zero. At this time, the core has not cooled completely and will continue to shrink during cooling, thus reversing the thermal stress and forming thermal stress with compression on the surface and tension on the core.

(5) Therefore, after cooling completely, a large compressive stress will remain on the surface, while residual tensile stress will remain on the core. After the molten steel is poured into the mold, the temperature of the molten steel gradually decreases due to the heat absorption of the mold, and it changes from liquid to solid between the liquidus and the phase line. This process is called the solidification process, and this transition period is called the solidification period.

(6) Shrinkage cavities, shrinkage, thermal cracking, segregation, various pores, and inclusions in large-diameter steel pipes are all produced during the solidification period. Therefore, understanding and studying the solidification law and controlling it are of great significance for obtaining excellent and dense castings.