The first, second and third type of internal stress, i.e., the macroscopic internal stress (the first type of stress) and the stress in the grain range (the second type of stress) and the internal stress of the crystal lattice (the third type of stress), are generated when the metal steel is subjected to Metal heat treatment. Only the first internal stress is sufficient to cause distortion and crack of the metal workpiece, which is mainly about the first internal stress.
In the process of heating and cooling, the volume change of thermal expansion and cold shrinkage will occur, as well as the volume change due to the tolerance between the new and the old when the microstructure changes. Due to the heat conduction process, the surface of the workpiece is heated or cooled before the center, and there is a temperature difference between the parts of the cross section, which prevents the above volume change from the surface and the core of the steel part at the same time. The internal stress is formed by the mutual restraint of the volume change of each part. The higher the heating or cooling rate is, the greater the temperature difference on the cross section of the workpiece is, and the greater the internal stress is.
The internal stress formed during the heat treatment is an ever-changing transient stress. The internal stress present in the workpiece after heat treatment is the residual internal stress. The instantaneous stress constantly changes its size and direction with the change of temperature and the process of the tissue transformation, and the distribution state is also changed.
In fact, the thermal stress has been active, which is one of the important factors of deformation and cracking of steel parts. However, it also has its advantages, such as surface strengthening heat treatment, shot peening, rolling and other processes, resulting in beneficial residual compressive stress distribution on the surface of the workpiece, which can effectively improve the mechanical properties, especially the fatigue strength. Extend the service life of the workpiece.
Overseas manager: Tom Wang
Phone: 0086-13303078975(whatsapp, wechat,line)
Specialist of bar heat treatment furnace in China; Glad to be your business partner in induction heating field.
Post time: 06-11-2019