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 geometry of the
large-diameter steel pipe is the effect of heat treatment stress. Heat treatment stress is a relatively complex problem. 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 the workpiece.
(2) Therefore, it is very 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 microstructure transformation process).
(3) It is self-phase 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 y according to the time of its action, and 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 faster than the core when heating, and the core cools slower than the surface when cooling because the absorption and dissipation of heat are conducted through the surface.
(5)
Large-diameter steel pipes that do not change in composition and structure state, at different temperatures, as long as the linear expansion coefficient is not equal to zero, will cause changes in specific volume, so in the process of heating or cooling, between the workpiece surfaces will be generated. Internal stress in mutual tension. Obviously, the greater the temperature difference 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 should be heated to a higher temperature and cooled at a faster rate. Therefore, during quenching, especially during the quenching and cooling process, large thermal stress will be generated. Temperature changes on the surface and core of a 26 mm diameter steel ball heated at 700°C and cooled in water.
(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 the 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 center of the watch gradually decreases until the core is completely cold, and the temperature difference also disappears. The process of generating thermal stress under quench conditions.
(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 be reliably contracted, while the core temperature is still high and the specific volume is large, which will prevent the surface from freely shrinking inward, thus forming thermal stress in which the surface is stretched and the core is compressed.
(5) With the progress of cooling, the above temperature difference continues to increase, and the generated thermal stress also increases accordingly. When the temperature difference reaches a larger 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 layer slows down, the cooling rate of the core increases accordingly, the temperature difference tends to decrease, and the thermal stress also decreases gradually. As the thermal stress decreases, the above elastic deformation also decreases accordingly.
