The internal stress that persists after the end of welding and complete cooling is called welding residual stress. Welding residual stresses are classified as follows:
1. According to the cause of stress
(1) Thermal stress: Welding is a process of uneven heating and cooling. The stress inside the weldment is mainly caused by uneven heating and temperature difference, which is called thermal stress, also known as temperature stress.
(2) Restraint stress: The stress caused mainly by the structure itself or by the external restraint is called restraint stress.
(3) Phase transformation stress: The stress mainly caused by the uneven microstructure transformation in the welded joint area is called phase transformation stress, also known as microstructure stress.
(4) Hydrogen-induced concentrated stress: The stress mainly caused by the accumulation of diffusible hydrogen at the microscopic defects is called hydrogen-induced concentrated stress.
Among these four residual stresses, thermal stress is dominant. Therefore, according to the causes of stress, it can be divided into two categories: thermal stress (temperature stress) and phase transformation stress (tissue stress).
2. According to the direction of stress in space
It can be divided into one-way stress, two-way stress and three-way stress
(1) Unidirectional stress: The stress existing in one direction in the weldment is called unidirectional stress, also known as line stress. For example, the butt welds of welded sheets and the stress generated when surfacing on the surface of the weldment.
(2) Bidirectional stress: The stress acting on two mutually perpendicular directions in a plane of the weldment is called bidirectional stress, also known as plane stress. It usually occurs in welded structures of medium and heavy plates with a thickness of 15-20mm.
(3) Three-way stress: The stress acting in three directions perpendicular to each other in the weldment is called the three-way stress, also known as the volume stress. For example, the stress at the intersection of the butt weld of the welded thick plate and the welds in three directions perpendicular to each other.
The volume expansion and contraction of the metal when it is heated and cooled are in three directions, so strictly speaking, the residual stress generated in the weldment is always a three-way stress. But when the stress value in one or two directions is very small and can be ignored, it can be considered as bidirectional stress or unidirectional stress, and the above is the case of the type of welding residual stress.
In the production process of welded pipes, strip steel needs to be extruded, bent, formed and welded. There will definitely be stress during that time. In order to obtain industrial welded pipes with superior performance, these stresses must be eliminated. At the same time, taking into account the long-term cost pressure, it is necessary to find an efficient and energy-saving way. Hangao Tech (SEKO Machinery) single-tube energy-saving bright annealing induction heater machine can not only remove the stress generated during the forming process of welded tubes, but also have the characteristics of energy saving and environmental protection. Compared with similar products, the effective utilization of energy is 20%-30% higher. The cooling water circulation system can realize the recycling of water resources and effectively control the long-term cost.
