(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.
