Steel pipes are not only used to transport fluids and powdery solids, exchange heat energy, and make mechanical parts and containers, but they are also a kind of economical steel. Using steel pipes to make building structural grids, pillars, and mechanical supports can reduce weight, save 20-40% of metal, and enable factory-like mechanized construction. Using steel pipes to make road 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 hollow sections and their length is much larger than the diameter or circumference of steel. According to the cross-sectional shape, it is divided into round, square, rectangular, and special-shaped steel pipes; according to the material, it is divided into carbon structural steel pipe, low alloy structural steel pipe, alloy steel pipe, and composite steel pipe; according to the use, it is divided into transportation pipeline, engineering structure, 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) There are two types, welded steel pipes are divided into straight seam welded steel pipes and spiral seam welded steel pipes.
1. 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 rules of heat treatment stress and to master the methods 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 within the whole or part of the volume of the workpiece, so it is called internal stress. Heat treatment stress is 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 it can be divided into thermal stress and tissue stress according to the cause of its formation.
(4) Thermal stress is caused by the synchronous 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 down slower than the surface when cooled. This is because heat absorption and dissipation are conducted through the surface.
(5) For large-diameter steel pipes whose composition and organizational state do not change, at different temperatures, as long as the linear expansion coefficient is not equal to zero, the specific volume will change. Therefore, during the heating or cooling process, there will be a gap between the surface and the center of the workpiece. Internal stresses that compress each other. Obviously, the greater the temperature difference generated within the workpiece, the greater the thermal stress.
2. How to cool large-diameter steel pipes after the quenching process?
(1) During the quenching process, the workpiece needs to be heated to a higher temperature and cooled at a faster rate. Therefore, during quenching, especially during the quenching cooling process, great thermal stress will be generated. The temperature changes on the surface and center of a steel ball with a diameter of 26 mm when it is cooled in water after being heated at 700°C.
(2) In the early stage of cooling, the cooling speed 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 nearly equal, their temperature difference reaches a large 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 temperature difference disappears when the core is completely cold. 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 contraction, the surface volume must reliably shrink, but the core temperature is still high and the specific volume is large, which will prevent the surface from freely shrinking inward, thus forming a thermal stress in which the surface is stretched and the core is compressed.
(5) As cooling proceeds, the above-mentioned temperature difference continues to increase, and the thermal stress generated 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 corresponding temperature conditions, it will not cause plastic deformation and only produce a trace amount of elastic deformation.
(6) When cooling further, the cooling rate of the surface layer slows down, the cooling rate of the core accelerates accordingly, the temperature difference tends to decrease, and the thermal stress also gradually decreases. As the thermal stress decreases, the above elastic deformation also decreases accordingly.
Post time: Jan-12-2024