Steel pipes are not only used for transporting fluids and powdery solids, exchanging heat, and manufacturing mechanical parts and containers, but they are also an economical steel material. Using steel pipes to construct building structural frames, columns, and mechanical supports can reduce weight, save 20-40% of metal, and enable factory-based mechanized construction. Using steel pipes to construct highway bridges not only saves steel and simplifies construction, but also significantly reduces the area requiring protective coating, saving investment and maintenance costs. Large-diameter steel pipes have a hollow cross-section, with a length much greater than the diameter or circumference of the steel. Steel pipes are classified according to their cross-sectional shape into circular, square, rectangular, and irregular shapes; according to their material into carbon structural steel pipes, low-alloy structural steel pipes, alloy steel pipes, and composite steel pipes; according to their application into pipes for transportation, engineering structures, thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, and high-pressure equipment; and according to their production process into seamless steel pipes and welded steel pipes. Seamless steel pipes are further divided into hot-rolled and cold-rolled (drawn) types, while welded steel pipes are divided into straight-seam welded steel pipes and spiral-seam welded steel pipes.
First, what is the heat treatment process for large-diameter steel pipes?
(1) The reason for the change in the geometric shape of large-diameter steel pipes during heat treatment is the effect of heat treatment stress. Heat treatment stress is a complex issue; it is both the cause of defects such as deformation and cracks and an important means to improve the fatigue strength and service life of workpieces.
(2) Therefore, it is very important to understand the mechanism and variation law of heat treatment stress and to master the methods of controlling internal stress. Heat treatment stress refers to the stress generated within a workpiece due to heat treatment factors (thermal processes and structural transformation processes).
(3) It is self-equilibrium within the entire workpiece or a portion of its volume, hence it is called internal stress. Heat treatment stress is classified into tensile stress and compressive stress according to its nature; into instantaneous stress and residual stress according to its duration; and into thermal stress and structural stress according to its cause.
(4) Thermal stress is formed due to the asynchronous temperature changes in different 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, while the core cools down slower than the surface during cooling. This is because heat absorption and dissipation are conducted through the surface.
(5) For large-diameter steel pipes whose composition and microstructure do not change, changes in specific volume will occur at different temperatures, provided the coefficient of linear expansion is not zero. Therefore, during heating or cooling, mutual tensile and compressive internal stresses will be generated between the surface and the core of the workpiece. Obviously, the greater the temperature difference generated within the workpiece, the greater the thermal stress.
Second, how is the large-diameter steel pipe cooled after quenching?
(1) During the quenching process, the workpiece is heated to a high temperature and cooled at a relatively fast rate. Therefore, a large thermal stress is generated during quenching, especially during the cooling process. The temperature changes of the surface and core of a 26 mm diameter steel ball after heating to 700°C and then cooling in water are illustrated.
(2) In the initial cooling phase, the surface cooling rate significantly exceeds that of the core, and the temperature difference between the surface and core continuously increases. As cooling continues, the surface cooling rate slows down, while the core cooling rate relatively increases. When the cooling rates of the surface and core are nearly equal, their temperature difference reaches its maximum.
(3) Subsequently, the core cooling rate is greater than the surface cooling rate, and the temperature difference between the surface and core gradually decreases until the core is completely cooled, at which point the temperature difference disappears entirely. This describes the process of generating thermal stress under rapid cooling conditions.
(4) In the initial cooling phase, the surface cools rapidly, and a temperature difference begins to form between it and the core. Due to the physical property of thermal expansion and contraction, the surface layer needs to reliably shrink in volume, while the core temperature is still high and its specific volume is large, which hinders the free inward contraction of the surface layer, thus creating thermal stress where the surface layer is under tension, and the core layer is under compression.
(5) As cooling proceeds, the 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 steel under the corresponding temperature conditions, it will not cause plastic deformation, but only a small amount of elastic deformation.
(6) With further cooling, the cooling rate of the surface layer slows down, while the cooling rate of the core accelerates accordingly, the temperature difference tends to decrease, and the thermal stress gradually decreases. As the thermal stress decreases, the aforementioned elastic deformation also decreases accordingly.
Post time: Dec-04-2025