How to cool the large diameter steel pipe after the quenching process

The steel pipe is used to transport fluid and powder, exchange heat and make mechanical parts and containers, what’s more, it’s a kind of economical steel. Using steel pipes to make building structure grids, pillars, and mechanical supports can reduce weight and save 20-40% of metal, and can realize factory-like and mechanized construction. The use of steel pipes to make highway bridges can not only save steel and simplify construction but also greatly reduce the area coated with protective layers, saving investment and maintenance costs. Large-diameter steel pipes have a hollow section whose length is much greater than the diameter or circumference of steel. According to the cross-sectional shape, it can be divided into circular, square, rectangular, and special-shaped steel pipes; according to the material, it can be divided into carbon structural steel pipes, low alloy structural steel pipes, alloy steel pipes, and composite steel pipes; Steel pipes for thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, high-pressure equipment, etc.; according to the production process, they are divided into seamless steel pipes and welded steel pipes, among which seamless steel pipes are divided into hot-rolled and cold-rolled (drawn) Two kinds, welded steel pipe is divided into straight seam welded steel pipe and spiral seam welded steel pipe.

1. What is the heat treatment process of large-diameter steel pipes?
(1) During the heat treatment process, the cause of the geometric change of the large-diameter steel pipe is the heat treatment stress. Heat treatment stress is a relatively complicated issue. It is not only the cause of defects such as deformation and cracks but also an important means to improve the fatigue strength and service life of workpieces.
(2) Therefore, it is very important to understand the mechanism and change law of heat treatment stress and to master the method of controlling internal stress. Heat treatment stress refers to the stress generated inside the workpiece due to heat treatment factors (thermal process and tissue transformation process).
(3) It is self-equilibrium in the whole or part of the volume of the workpiece, so it is called internal stress. Heat treatment stress can be divided into tensile stress and compressive stress according to the nature of its action; it can be divided into instantaneous stress and residual stress according to its action time and can be divided into thermal stress and tissue stress according to the cause of its formation.
(4) Thermal stress is formed due to the asynchrony of temperature changes in various parts of the workpiece during the heating or cooling process. For example, for a solid workpiece, the surface always heats up faster than the core when heated, and the core cools slower than the surface when cooled because heat is absorbed and dissipated through the surface.
(5) For large-diameter steel pipes that do not change in composition and organizational state, when they are at different temperatures, as long as the coefficient of linear expansion is not equal to zero, the specific volume will change. Therefore, during the heating or cooling process, there will be Mutual tension and internal stress. Obviously, the greater the temperature difference generated in the workpiece, the greater the thermal stress.

2. How to cool the large-diameter steel pipe after the quenching process?
(1) During the quenching process, the workpiece must be heated to a higher temperature and cooled at a faster rate. Therefore, during quenching, especially during the quenching and cooling process, a large thermal stress will be generated. When a steel ball with a diameter of 26 mm is cooled in water after being heated at 700°C, the temperature change of the surface and core.
(2) In the initial stage of cooling, the cooling rate of the surface is significantly higher than that of the core, and the temperature difference between the surface and the core is continuously increasing. When cooling continues, the cooling rate of the surface slows down, while the cooling rate of the core increases relatively. When the cooling rates of the surface and the core are nearly equal, their temperature difference reaches a large value.
(3) Subsequently, the cooling rate of the core is greater than that of the surface, and the temperature difference between the surface and the core gradually decreases until the core is completely cooled, and the temperature difference also disappears. The process of generating thermal stress during rapid cooling.
(4) In the early stage of cooling, the surface layer cools rapidly, and a temperature difference begins to occur between it and the core. Due to the physical characteristics of thermal expansion and cold contraction, the volume of the surface layer must be reliably contracted, while the temperature of the core is high and the specific volume is large, which will hinder the free contraction of the surface layer inward, thus forming thermal stress in which the surface layer is stretched and the heart is compressed.
(5) As the cooling proceeds, the above-mentioned temperature difference continues to increase, and the resulting thermal stress also increases accordingly. When the temperature difference reaches a large value, the thermal stress is also large. If the thermal stress at this time is lower than the yield strength of the steel at the corresponding temperature, it will not cause plastic deformation, but only a small amount of elastic deformation.
(6) When further cooling, the cooling rate of the surface slows down, and the cooling rate of the core increases accordingly, the temperature difference tends to decrease, and the thermal stress gradually decreases. As the thermal stress decreases, the above elastic deformation also decreases accordingly.