Analysis of welding residual stress and methods for eliminating welding stress

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I. What is welding stress? Welding stress is the stress that occurs in welded components as a result of welding. The internal stresses generated in the welded parts during the welding process, as well as the changes in the shape and size of those parts caused by the welding heat. The non-uniform temperature field during the welding process, as well as the local plastic deformation and microstructures with different specific volumes resulting therefrom, are the fundamental causes of welding stresses and deformations. When the uneven temperature field caused by welding has not yet dissipated, such stress and deformation in the welded part are referred to as transient welding stress and deformation ； The stress and deformation that remain after the welding temperature field disappears are known as residual welding stress and deformation. In the absence of external forces, the welding stresses are balanced within the welded joint. Welding stress and deformation can affect the functionality and appearance of the welded parts under certain conditions. II. Hazards of welding stress Welding residual stress has six effects on the welded parts: ① Effect on strength: If there are severe defects in areas with high residual tensile stress, and the welded part operates at temperatures below its brittle transition temperature, then the welding residual stress will reduce its static strength. Under cyclic stress, if there is residual tensile stress at stress concentration sites, the welding residual tensile stress will reduce the fatigue strength of the welded joint. The fatigue strength of a welded joint depends not only on the magnitude of the residual stress but also on the stress concentration factor, the characteristic coefficient of stress cycling, and the maximum value of the cyclic stress. Its influence decreases as the stress concentration factor decreases, increases as / decreases, and decreases as increases. As the stress approaches the yield strength, the effect of residual stress gradually disappears. ② Effect on stiffness: The welding residual stresses, combined with the stresses induced by external loads, may cause localized premature yielding and plastic deformation of the welded part. The stiffness of the welded part will decrease as a result. ③ Effect on the stability of compression-welded members: When welded members are under compression, the residual welding stresses combine with the stresses induced by external loads, which may cause local yielding or instability in the members, thereby reducing their overall stability. The effect of residual stress on stability depends on the geometric shape of the member and the distribution of internal stresses. Residual stress has a greater effect on members with non-closed cross-sections (such as I-shaped sections) than on those with closed cross-sections (such as box sections). ④ Impact on machining accuracy: The presence of welding residual stresses has varying effects on the machining accuracy of welded parts. The lower the stiffness of the welded part, the greater the amount of work required for processing, and the greater the impact on precision. ⑤ Effect on dimensional stability: Welding residual stresses change over time, and as a result, the dimensions of the welded parts also change. The dimensional stability of the welded parts is also affected by the stability of residual stresses. ⑥ Impact on corrosion resistance: Welding residual stresses, just like service stresses, can also cause stress corrosion cracking. The effect of welding residual stresses on structures and components: Welding residual stresses are the initial stresses present in the cross-section of a component before any loads are applied. During the service life of the component, these stresses combine with the working stresses induced by external loads, resulting in secondary deformation and a redistribution of residual stresses. This not only reduces the stiffness and stability of the structure but also, under the combined influence of temperature and environmental factors, severely impacts the structure’s fatigue strength, resistance to brittle fracture, as well as its ability to resist stress corrosion cracking and high-temperature creep cracking. III. Methods for eliminating welding stress. The currently used methods for reducing stress include vibration aging (which eliminates 30%~50% of the stress), thermal aging (which eliminates 40%~70% of the stress), and Hoek energy PT aging (which eliminates 80%~100% of the stress). Preheating: The cladding welding process, which involves first subjecting the important welding components to overall thermal aging and then welding them together on-site with other components, is a commonly used method in the manufacturing of building steel structures. It has three functions: dehydrogenating welds, restoring plasticity, and relieving stress. It is generally believed that the stress-relieving effect of thermal aging is over 40%. The remelting weld toe defect is a small, sharp, and continuous defect that is inevitable at the fusion line of a weld bead; it often serves as a source of cracks leading to structural fatigue failure. The TIG remelting process is commonly used to refine the weld toe, restoring it to its condition prior to crack initiation and reducing the stress concentration caused by weld toe defects, thereby extending the fatigue life. At the same time, TIG remelting can also reduce the transverse residual stress in the weld zone ； Remelting has little effect on reducing the longitudinal residual stress in welds, and the absolute value of the residual stress does not decrease significantly ； However, it has a certain effect on the uniform distribution of longitudinal residual stress. However, it has a significant improvement effect on the transverse residual stress; the absolute value of the residual stress decreases markedly and its distribution becomes more uniform. Vibration aging involves applying alternating stresses to a component; these stresses combine with the residual stresses present in the component to reach the material’s yield stress, resulting in local macroscopic and microscopic plastic deformation ； This plastic deformation usually occurs first at the points where the residual stress is highest and where stress concentration exists in the component, thereby releasing the residual stress there and achieving the effect of reducing and homogenizing it. Although vibration aging equipment does not have the functions of dehydrogenation and restoring plasticity, in terms of dimensional stability, it has reached and even surpassed the level of thermal aging ； Vibration aging is an advanced process that replaces thermal aging with the aim of eliminating stress and improving dimensional stability. Vibration welding is also known as vibration-modulated welding or in-process vibration ； In the appendix to the vibration aging standards, it has been identified as one of the processes that can be combined with vibration aging. It does not alter the existing welding process; rather, during welding, vibrations with controllable frequency and amplitude are applied to the components using an exciter, thereby enabling vibration welding. Such vibrations of this amplitude will inevitably have an effect on the welding pool and the heat-affected zone: (1) When the weld is in a molten state, the vibrations facilitate the upward movement and removal of bubbles, impurities, etc. (2) Vibrating the grains during recrystallization facilitates grain refinement ； (3) In areas where the temperature is above 600 degrees Celsius, as the strength of the material gradually recovers during cooling, thermoplastic deformation accompanied by vibration occurs, which helps to reduce and homogenize the residual welding stresses resulting therefrom, thereby minimizing welding deformation and the formation of welding cracks. Welding stress relief equipment: This type of equipment applies impact to the weld toe, allowing for the rapid correction of defects in that area; it reduces stress concentration. Additionally, through the creation of a compressive stress zone, it can help decrease the residual stresses in the unaffected weld areas surrounding the weld toe ； Weld stress relief equipment can impact the weld toe at a frequency of 20,000 times per second along the weld direction, causing significant compressive plastic deformation and resulting in a smooth geometric transition at the weld toe, thereby **reducing stress concentration ； It eliminates the minor cracks and slag defects on the surface of the weld toe, prevents the premature initiation of welding cracks, adjusts the stress field, and generates a certain amount of compressive stress to strengthen the weld toe area, which has a significant effect on improving the fatigue life of the welded joint. Explosive process: Special adhesive materials are applied along the weld path and near the weld. **After detonation, successive shock waves are generated, forcing the regions of peak stress in the structure to undergo plastic deformation, thereby achieving stress relief. It is completed in an instant and is suitable for large and extra-large structures. Safety measures are of paramount importance when using the explosion method for stress relief, and its application in urban buildings presents certain difficulties.