(1) When the cooling is completed, that is, when the temperatures of the surface layer and the core part are all the same, the elastic deformation of the surface layer and the core part also disappears and returns to the original state. Although transient 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 in the early stage of the quenching 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 surface tension and core compression. plastic deformation and the thermal stress is relieved.
(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 larger value, and the thermal stress acting on the surface layer and the core is also corresponding. decrease.
(4) However, due to 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 core has not yet cooled completely, and it will continue to shrink when it cools so that the thermal stress is reversed, forming thermal stress in which the surface is compressed and the core is stretched.
(5) Therefore, after cooling through, there will be a large residual compressive stress on the surface layer, and residual tensile stress in the core. After the molten steel is poured into the mold, due to the heat absorption of the mold, the temperature of the molten steel gradually decreases, and the liquid 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 solidification. Expect.
(6) Shrinkage holes, shrinkage porosity, thermal cracking, segregation, various types of 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.
