Metal workpieces (castings, forgings, and welded parts) will generate residual stress during hot and cold processing. The residual stress is higher (in Pa). Most of the residual stress in the member near the yield limit shows a great harmful effect; For example, reducing the actual strength of the component, reducing the fatigue limit, causing stress corrosion and brittle fracture, the deformation of the residual stress causes the deformation of the component, which greatly affects the dimensional accuracy of the component. Therefore, it is necessary to reduce and eliminate the residual stress of the workpiece, especially in aerospace, shipbuilding, railway and industrial and mining applications, fatigue failure caused by residual stress can not be ignored.
The current different treatment methods for residual stress are: natural aging method and artificial aging method (including heat treatment aging, percussion aging, vibration aging, ultrasonic impact aging, explosion aging)
1. Natural aging - suitable for: thermal stress (residual stress generated during casting forging) cold stress (residual stress generated during machining) welding stress (stress generated during welding)
Natural aging is the oldest method of aging. It is to place the components outdoors in the open air, relying on the power of nature, after several months to several years of wind, sun, rain and seasonal temperature changes, causing repeated temperature stress on the components. Under the overload formed by the temperature stress, the residual stress is relaxed and the dimensional accuracy is stabilized.
The residual stress of natural aging is not large, but the dimensional stability of the workpiece is very good. The reason is that the workpiece is placed for a long time, stress concentration occurs near the tip of the graphite and other wire defects, plastic deformation occurs, and the stress is relaxed. This part of the matrix is ​​strengthened, so the relaxation stiffness of the part is also improved, and the deformation resistance of the material is increased. The natural aging reduces the residual stress, but the relaxation stiffness of the component is improved, and the dimensional stability of the component is better. The method is simple and easy, but the production cycle is long. The occupied space is large, it is difficult to manage, and the defects in the component cannot be found in time, and have been gradually eliminated.
2, heat treatment aging - suitable: thermal stress (residual stress generated during casting forging process) cold stress (residual stress generated during machining) welding stress (stress generated during welding)
Thermal aging is a traditional method of eliminating residual stress. It is to slowly heat the components from room temperature to about 550 ° C, keep warm for 4-8 hours, and then strictly control the cooling rate to below 150 ° C.
The thermal aging process requirements are strict. For example, the temperature difference in the furnace is required to be no more than ±25 ° C, the heating rate is not more than 50 ° C / hour, and the cooling rate is not more than 20 ° C / hour. The maximum temperature in the furnace is not allowed to exceed 570 ° C, and the holding time is not too long. If the temperature is higher than 570 ° C, the holding time is too long, causing graphitization and reducing the strength of the member. If the heating rate is too fast, the heating rate of the member at the thin wall is much faster than that at the thick wall, and the temperature difference of each part of the member increases sharply, which causes additional temperature stress. If the additional stress overlaps with the residual stress of the component itself beyond the strength limit, the component will crack.
If the temperature is improperly cooled, the aging effect is greatly reduced, and even the same temperature stress (secondary stress, stress superposition) as the original residual stress is generated, and remains in the member, thereby destroying the obtained thermal aging effect.
3, tapping aging (hammering method) - suitable: welding stress (stress generated during welding)
Hammering treatment was introduced into the welding field very early, and the initial application was mainly to eliminate welding distortion. The method of hammering is divided into manual hammering method and electric hammer hammering method. Through observation and analysis, it is considered that proper hammering can eliminate and reduce welding cracks, and further conclude that hammering has the effect of eliminating welding residual stress. Therefore, hammering is used in the process to prevent the occurrence of welding cracks. It is generally believed that the hammering treatment eliminates the welding residual stress by causing the metal to be treated to be hammered, locally generating a certain plastic elongation in the body, and releasing the residual tensile elastic strain generated by the welding process, thereby achieving the purpose of releasing the welding residual stress. However, due to the irregularity of the hammering (especially manual hammering) (the size and frequency of the hammering force, the mechanical properties of the matrix and the temperature of the hammering zone, etc.) and the difficulty of accurately testing the welding residual stress, the hammering treatment and There is still no scientific and systematic study on the relationship of residual stress.
Under suitable welding specifications and processes, hammering can not only effectively eliminate the stress of the workpiece weld, but also promote the release of tensile residual stress in the heat-affected zone, and even obtain a certain value of compressive stress.
4. Vibration aging - suitable for: thermal stress (residual stress generated during casting forging) cold stress (residual stress generated during machining) welding stress (stress generated during welding)
The vibration aging (VSR) is to resonate the metal workpiece at a certain frequency under the action of the excitation-period excitation force. The dynamic stress is generated to make the workpiece tens of thousands of times of large amplitude sub-resonant vibration in half an hour. . The internal residual stress is superimposed, and after reaching a certain value, at the most concentrated stress, the yield limit will be exceeded and a slight plastic deformation will occur, which will reduce the residual stress and strengthen the metal matrix; then the vibration will have the same effect in the remaining stress concentration. Until it can not cause any partial plastic deformation, so that the residual stress in the member is reduced and redistributed, in an equilibrium state, and the strength of the material is improved. In the subsequent installation and use of the component, since it is no longer in a resonance state, it does not bear a larger external force than the resonance force, and the post-vibration member does not undergo stress deformation. The vibration aging can also be regarded as the cyclic strain under the cyclic dynamic stress. The crystal dislocation motion inside the metal material increases the micro stress and reaches the process of adjusting the stress stabilization member size.
Vibration aging, known abroad as VSR technology, is the abbreviation of Vibratory Stress Reliele. It is caused by the periodic external force (excitation force) of the exciter, causing the workpiece itself to resonate, and then the internal lattice is distorted, causing slippage to restore balance, lifting the workpiece's relaxation stiffness, eliminating and even Residual stresses make it dimensionally stable. In the aging method for the purpose of eliminating residual stress, the vibration aging can completely replace the thermal aging. The six ministries and commissions, including the former Ministry of Electrical and Mechanical Services, set the vibration aging as the energy-saving project promoted by the seventh five-year plan, and replaced such products with imported products. This vibration-relieving technology has been applied abroad for decades. experience.
5, ultrasonic impact - suitable: welding stress (stress generated during welding)
Ultrasonic shock is the development of tapping aging.
The ultrasonic shock technology is characterized by high input energy per unit time and a large specific energy (output energy to device mass ratio) of the device. The vibration processing frequency can be as high as 18KHZ-27KHZ, the vibration speed can reach 2m/s-3m/s, the acceleration is up to 30,000 times of the gravitational acceleration, and the high-speed instantaneous impact energy makes the surface temperature of the treated weld zone extremely high. It rises to 600 ° C and cools at a very fast rate. After the high-frequency energy is input to the surface of the weld zone, the phase structure of the surface metal of the energy-active zone is changed to some extent.
1) The tensile residual stress in the metal surface layer of the weld zone is changed to compressive stress, so that the fatigue life of the structure can be greatly improved.
2) The metal grains in the surface layer are thinned to produce a plastic deformation layer, so that the strength and hardness of the metal surface layer are correspondingly improved.
3) Improve the geometry of the weld toe and reduce stress concentration.
4) Change the welding stress field and significantly reduce the welding deformation.
6, explosion aging - suitable: welding stress (stress generated during welding)
The new technology of explosion treatment to eliminate welding residual stress is to treat the welded joint by the blasting shock wave when the special explosive is detonated, which can effectively reduce the welding residual stress, thereby greatly improving the stress corrosion cracking resistance of the welded joint and reducing fatigue. The crack growth rate reduces the brittle transition temperature and improves the service safety reliability of the welded structure. The principle is that the original welding residual stress is transformed by the shock wave induction into the corresponding plastic strain (εp), ie εp=εe+C, and C is a constant depending on the explosion condition. Adjust by controlling the explosion treatment process. Since the explosion treatment eliminates the welding residual stress is a high degree of local treatment, for practical use conditions, C is a constant greater than zero, that is, the explosion zone can form a compressive stress after the explosion treatment. Compared with the annealing heat treatment, the explosion treatment eliminates the welding residual stress and has the advantages of less energy consumption, low processing cost, no internal brittleness of the material, and no limitation on the size of the component [10]. The technology is quite convenient to construct. A series of energy-grade special explosives can be obtained based on the detonation speed, which can meet the requirements of eliminating residual stress under different conditions. At the same time, the explosive can be adsorbed or adhered to the steel parts to meet the requirements of all-round medicine. The flexibility, continuity, stability, safety of the explosive and the ablation protection of the treated component after the detonation of the high temperature and high pressure products have been considered and solved.
After more than ten years of theoretical research and industrial application practice, this technology has been used in China's metallurgical machinery equipment, hydroelectric power station pressure steel (fork) pipe and turbine volute to maintain the pouring head, aluminum plant storage alkali tank and alkali pipeline It is used on more than 160 large and medium-sized structures such as chemical containers and reaction towers, and has achieved great economic and social benefits.
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