Residual stress-Generation and elimination of welding residual stress
* 來源: 論壇 * 作者: admin * 發表時間: 2020-03-12 16:32:25 * 瀏覽: 30
The mechanism of residual stress is as follows
Various machining processes, such as casting, cutting, welding, heat treatment and assembly, will produce different degrees of residual stress.
1、 Residual stress caused by machining
2、 Residual stress caused by temperature inhomogeneity
There are two main reasons for this kind of residual stress: the first is the local thermal plastic deformation caused by the non-uniform temperature; the second is the local plastic deformation caused by the non-uniform volume expansion caused by phase transformation.
3、 Residual stress caused by dimensional tolerance of components
In welding, riveting and screw connection, there are often tolerance problems. If the steel plates must be pulled together when the hull sections are connected, the residual stress will appear in the whole system after the external force is removed. Generally speaking, this kind of stress belongs to structural stress and is in elastic state in most cases.
Influence of residual stress
The residual stress of metal components (castings, weldments, forgings) is produced in the process of processing, and the higher one is near the yield limit. Most of the residual stresses in components have great harmful effects, such as reducing the strength of components, reducing the fatigue limit of workpieces, causing stress corrosion and brittle fracture. Due to the relaxation of residual stress, the components are deformed and the dimensional accuracy of components is affected. Therefore, it is necessary to reduce and eliminate the residual stress.
1. Influence on yield limit of metal materials
If the material has tensile residual stress, it is equivalent to reducing the tensile yield limit of the material. If the material has compressive residual stress, the tensile yield limit increases and the compressive yield limit decreases.
2. Effect of residual stress on fatigue life
It has been known for a long time that when there is compressive residual stress in the component subjected to alternating stress, the fatigue strength of the component will be improved, while the fatigue strength will be decreased when there is tensile residual stress.
The distribution and magnitude of residual stress in electroplating treatment are very different due to the difference of process current, type of plating solution and temperature, so the influence of electroplating residual stress on fatigue strength is also great. Most metals produce tensile residual stress on the surface after electroplating, so the fatigue strength will be greatly reduced.
3. Influence of residual stress on deformation of components
The residual stress is an unstable stress state. When a member is subjected to an external force, the interaction between the applied stress and the residual stress will cause plastic deformation in some parts and redistribute the internal stress in the section. When the external force is removed, the whole component will deform. Therefore, the residual stress obviously affects the accuracy of the machined components.
4. Effect of residual stress on brittle failure of metals
The residual stress exists in the member as the initial stress, especially the superposition of tensile residual stress and applied tensile stress accelerates the brittle failure.
5. Effect of residual stress on stress corrosion cracking
The damage caused by chemical action caused by contact between metal and surrounding medium is called corrosion. If there is stress effect at the same time of corrosion, it will accelerate the corrosion damage, which is stress corrosion cracking. It is characterized by the coexistence of tensile stress and corrosion. The other is that the sensitivity to stress corrosion is different due to the difference of material composition, microstructure and medium. Sometimes in the medium without corrosion, some metals also occur stress corrosion under the action of stress. Third, in the process of stress corrosion cracking, pitting corrosion first occurs, and then gradually expands into cracks. The crack propagation is mainly along the vertical direction of the maximum principal stress, and in the micro level, it is along the grain boundary or through the grain.
Methods of eliminating residual stress
1. Mechanical impact method: including hammer (hammer, air hammer, etc.), ultrasonic impact, shot peening, etc
a. Elimination of welding residual stress (elimination of tensile residual stress and generation of beneficial compressive residual stress): the metal surface and a certain depth are subjected to plastic deformation by impact gun, so as to eliminate tensile residual stress.
b. Prolonging the fatigue life of welded joint: a smooth and continuous groove is processed on the weld toe to reduce the stress concentration factor, at the same time, the compressive stress is formed in the weld toe area, and the grain size in the weld toe area is refined, which can improve the fatigue life of welded joint by tens or even hundreds of times. Because more than 90% of the fatigue fracture of the welded structure is caused by the fatigue crack of the weld toe, the fatigue life of the weld toe is increased, so is the life of the welded structure.
c. Strengthening the stress corrosion resistance of metals: refining metal grains and improving comprehensive mechanical properties
2. Vibratory stress relief method
An eccentric motor is installed on the component, and the residual stress is eliminated by controlling the motor speed to make the component and the motor resonate. The stress relief is about 30%. Suitable for large size parts. However, the VSR is not to eliminate the residual stress, but to homogenize the residual stress. Moreover, if the vibration is too strong during the VSR operation, it may crack the weld. Disadvantages: high energy consumption, high cost, high pollution, long cycle.
3. Heat treatment aging
It is a traditional method to eliminate residual stress. With the help of heat treatment facilities, the workpiece is slowly and evenly heated from room temperature to about 600 ℃ and kept at this temperature for 4-8h. Then the temperature is slowly cooled to below 120 ℃ and then cooled out of the furnace to room temperature. Advantages: it can eliminate the whole stress, not only can eliminate the tensile stress, but also can eliminate the compression stress. 80 ~ 90% residual stress can be eliminated. Small workpieces can be processed in batch.
4. Natural prescription
Put the workpiece in the outdoor and other natural conditions to make the internal stress of the workpiece