Steel pipes are not only used to transport fluids and powdered solids, exchange heat energy, and make mechanical parts and containers, but they are also economical steel. Using steel pipes to make building structure grids, pillars, and mechanical supports can reduce weight, save 20-40% of metal, and realize factory-based mechanized construction. Using steel pipes to make highway bridges can not only save steel and simplify construction but also greatly reduce the area of protective coating, saving investment and maintenance costs. Large-diameter steel pipes have a hollow section, and their length is much greater than the diameter or circumference of steel. According to the cross-sectional shape, it can be divided into round, 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; according to the purpose, it can be divided into steel pipes for transportation pipelines, engineering structures, thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, high-pressure equipment, etc.; according to the production process, it can be divided into seamless steel pipes and welded steel pipes, among which seamless steel pipes can be divided into hot rolling and cold rolling (drawing), and welded steel pipes can be divided into straight seam welded steel pipes and spiral seam welded steel pipes.
First, what is the heat treatment process of large-diameter steel pipes?
(1) During the heat treatment process, the reason for the change in the geometric shape of large-diameter steel pipes is the effect of heat treatment stress. Heat treatment stress is a relatively complex 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 important to understand the mechanism and change the law of heat treatment stress and 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 structural transformation process).
(3) It is self-balanced 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 the time of its action; and it can be divided into thermal stress and structural stress according to the cause of its formation.
(4) Thermal stress is formed by the asynchronicity of temperature changes in various parts of the workpiece during heating or cooling. For example, for a solid workpiece, the surface always heats up faster than the core during heating, and the core cools down slower than the surface during cooling. This is because the absorption and dissipation of heat are both conducted through the surface.
(5) For large-diameter steel pipes whose composition and structural state do not change, as long as the linear expansion coefficient is not equal to zero when at different temperatures, it will cause changes in specific volume. Therefore, during the heating or cooling process, internal stresses of tension and compression will be generated between the surface and the core of the workpiece. Obviously, the greater the temperature difference generated in the workpiece, the greater the thermal stress.
Second, how to cool large-diameter steel pipes 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 cooling process, a large thermal stress will be generated. When a steel ball with a diameter of 26 mm is heated to 700°C and then cooled in water, the temperature changes of the surface and the core.
(2) In the early stage of cooling, the cooling rate of the surface significantly exceeds that of the core, and the temperature difference between the surface and the core continues to increase. 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 almost equal, their temperature difference reaches a maximum value.
(3) Subsequently, the cooling rate of the core is greater than the cooling rate 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 disappears completely. The process of generating thermal stress under rapid cooling.
(4) In the early stage of cooling, the surface cools rapidly, and a temperature difference begins to occur between it and the core. Due to the physical characteristics of thermal expansion and contraction, the volume of the surface layer must shrink reliably, while the temperature of the core is still high and the specific volume is large, which will hinder the free contraction of the surface layer inward, thus forming thermal stress with tension on the surface layer and compression on the core.
(5) As cooling proceeds, the above temperature difference continues to increase, and the generated 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 under the corresponding temperature conditions, it will not cause plastic deformation, but only a slight elastic deformation.
(6) During further cooling, the cooling rate of the surface layer 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.
Post time: Dec-26-2024