(1) When the cooling is over, that is, when the temperature of the surface layer and the core are all the same, 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 is equal to zero after the cooling is terminated.
(2) Of course, this is a rather special case. Due to the large thermal stress generated in the early stage of the quenching 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, resulting in tension on the surface and compression on the center. The plastic deformation, so that the thermal stress is eased.
(3) When 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 a large value, and the thermal stress acting on the surface layer and the core also corresponds decrease.
(4) However, the above-mentioned pre-generated plastic deformation, plays a role in reducing the large thermal stress. When there is still a significant temperature difference, the thermal stress has approached zero. At this time, the heart has not yet cooled completely, and it will continue to shrink when cooling so that the thermal stress is reversed, forming a thermal stress in which the surface is compressed and the heart is stretched.
(5) Therefore, after cooling through, the surface layer will have a relatively large residual compressive stress, while the core will have a residual tensile stress. After the molten steel is poured into the mold, due to the heat absorption of the mold, the temperature of the molten steel gradually drops, and it changes from liquid to solid between the liquidus line and the garden phase line. This process is called the solidification process, and this transition period is called solidification. Expect.
(6) Shrinkage cavity, shrinkage porosity, thermal cracking, segregation, various pores, and inclusions in large-diameter steel pipes all occur during the solidification period. Therefore, it is of great significance to understand and study the solidification law and control it for obtaining excellent and dense castings.
Post time: Feb-16-2023