(1) When cooling ends, i.e., when the surface and core temperatures of the large-diameter steel pipe are completely equal, the elastic deformation of both the surface and core disappears, returning to their original state. Although instantaneous thermal stress is generated during the cooling process, the residual thermal stress after cooling ends is zero.
(2) Of course, this is a special case. Because significant thermal stress is generated in the early stages of rapid cooling, 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 tensile plastic deformation on the surface and compressive deformation on the core, thus alleviating the thermal stress.
(3) As 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 core of the large-diameter steel pipe gradually decreases after a large value, and the thermal stress acting on the surface and core of the large-diameter steel pipe also decreases accordingly. (4) However, due to the pre-induced plastic deformation of the large-diameter steel pipe, the thermal stress is reduced, and when a significant temperature difference still exists, the thermal stress approaches zero. At this time, the core of the large-diameter steel pipe has not yet cooled completely and will continue to shrink during cooling, thus causing the thermal stress to reverse, forming thermal stress with pressure on the surface and tension on the core.
(5) Therefore, after cooling completely, the surface of the large-diameter steel pipe will have a large residual compressive stress, while the core will have residual tensile stress. When the molten steel is poured into the mold, due to the heat absorption of the mold, the temperature of the molten steel gradually decreases, changing from liquid to solid between the liquidus and the solidus line. This process is called solidification, and this transition period is called the solidification period.
(6) Shrinkage cavities, porosity, hot cracks, segregation, various types of porosity, 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.
Post time: Jan-26-2026