(1) When the cooling is completed, that is, when the temperatures of the surface layer and the core are all consistent, the elastic deformation of the surface layer and the core will also disappear and return to the original state. Although instantaneous thermal stress is generated during the cooling process, the residual thermal stress after cooling is terminated is equal to zero.

(2) Of course, this is a relatively special situation. Since large thermal stress is generated in the early stage of the rapid cooling process, when 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, causing tension on the surface and compression on the core. plastic deformation, thereby relaxing the thermal stress.

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

(4) However, due to the above-mentioned pre-generated plastic deformation, it plays a role in reducing large thermal stress. When there is still a significant temperature difference, the thermal stress has approached zero. At this time, the core has not yet cooled down and will continue to shrink when cooled, thus causing the thermal stress to reverse, forming a thermal stress in which the surface layer is compressed and the core is stretched.

(5) Therefore, after cooling, the surface layer will have residual compressive stress, while the core will have residual tensile stress. When the molten steel is poured into the mold, the temperature of the molten steel gradually drops due to the heat absorption of the mold, and changes from liquid to solid between the liquidus line and the phase line. This process is called the solidification process, and this transition period is called solidification. 

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