After any workpiece is machined, there will remain prestressed on the surface. The residual compressive stress on the rectangular tube can improve the wear resistance of the rectangular tube
and the pre-fatigue of the tensile stress, while the residual tensile stress like the rectangular tube is actually opposite to the compressive stress. Let me introduce to you the reason why the prestress remains on the rectangular tube.
(1) Residual stress due to deformation caused by cold plasticity. In the other side of the pipe in the processing, because of the cutting force, resulting in the surface of the pipe has been processed by a strong cold plasticity and lead to deformation, leading to such a cold plastic deformation of the most prominent of which is: the tool above the knife face. The other side of the tube surface of the squeeze and friction generated deformation, because the rectangular tube of the base metal at this time is in the state of elasticity. After the cutting force disappears, the metal of the base body tends to recover. At this moment, the rectangular tube has been restrained by the shaped surface, and the original shape can not be recovered from the base body. As a result, the pressure on the surface remains, these should be left in the rectangular tube pressure will lead to the rectangular tube wear resistance and fatigue strength decreased.
(2) Residual stress due to thermoplastic deformation. In the other side of the pipe when processing because of cutting heat under the expansion, the temperature of the metal at this time is relatively low, so the surface of the metal will have thermal stress, when the cutting process is completed, the temperature of the metal surface will drop rapidly, the shrinkage deformation is greater than the inside, and the deformation inside is limited by the base metal, so that after the metal base cools down, there will be a corresponding residual tensile stress, which in the metal with residual tensile stress will affect the metal after use.