During Heat treatment furnace of metal steel, the first, second and third kinds of internal stress will be produced, that is, the macroscopic internal stress (the first kind of internal stress) and the stress within the grain range (the second kind of stress) between the various parts of the steel part. And the internal stress of the lattice (the third kind of stress). Just the first internal stress is enough to cause distortion and cracks in metal workpieces. This article focuses on 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 in the process of heat treatment is a constantly changing instantaneous stress. The internal stress in the workpiece after heat treatment is residual internal stress. With the change of temperature and the process of tissue transformation, the instantaneous stress constantly changes its size and direction, and also changes the distribution state.
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Post time: 06-06-2019