What should be done at the end of cooling of large-diameter steel pipes
(1)When the cooling is over, that is, when the temperature 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 return 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 rather special situation. Due to the large thermal stress generated in the early stage of the quenching process, the steel is still at a 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. The plastic deformation and thermal stress are relaxed.
(3)When cooling continues, the cooling rate of the surface layer slows down, and the cooling rate of the core part increases faster. The temperature difference between the surface layer and the core part gradually decreases after a larger value, and the thermal stress acting on the surface layer and the core part is also the corresponding Decrease.
(4)However, due to the above-mentioned pre-produced plastic deformation, it has the effect of reducing the large thermal stress. When there is still a significant temperature difference, the thermal stress has approached zero. At this time, the core is not completely cold, and it will continue to shrink when cooled 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 about, the surface layer will have larger compressive stress, and the core will have 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 decreases, and it changes from liquid to solid between the liquidize line and the field line. This process is called solidification, and this transition period is called solidification. period.
(6) The shrinkage cavity, shrinkage porosity, thermal cracking, segregation, various pores and inclusions in the large-diameter steel pipe are all produced during the solidification period. Therefore, understanding and studying the law of solidification and controlling it is of great significance for obtaining fine and dense castings.