However, the local high temperatures during welding and the subsequent non-uniform cooling process can easily induce welding residual stresses within the components, leading to a series of hidden dangers such as structural deformation, cracking, and shortened fatigue life, seriously affecting the service safety and durability of the bridge. To this end, this article will systematically explore the prevention and control technologies for welding residual stress in steel structure bridges from three aspects: structural design, manufacturing process, and post-weld treatment. At the same time, it will also introduce specific measures and applicable scenarios for stress relief through methods such as heat treatment, mechanical vibration, and hammering when residual stress has already formed.
For more information on the definition of welding stress and its manifestation in production, it is recommended to read the previous blog post:
Analysis of Welding Stress and Deformation Problems in Steel Structure Bridges.
Key Strategies for Preventing and Controlling Welding Stress
Structural Design Measures
During the initial structural design of steel components, technical personnel must consider the actual circumstances and select appropriate methods to reduce the number and form of welds in the structure. Specifically, each weld that is eliminated corresponds to one fewer source of residual stress. When weld dimensions are small, the heated area during welding also decreases; conversely, excessively large weld dimensions result in a correspondingly large heated area during welding. This not only causes welding deformation of the component but also significantly increases residual stress in the plastically deformed areas. Therefore, to avoid excessive weld concentration, a certain distance should be maintained between welds.
Furthermore, excessively concentrated welds in a component can lead to more uneven weld stress distribution and potentially complex weld stress concentrations. It is worth noting that high weld joint stiffness can induce significant restraint stresses during welding, which increases the likelihood of cracking. Therefore, the obvious solution is to use weld joints with lower stiffness to avoid longitudinal and transverse weld residual stresses.
Regarding the welding process for steel components, technical departments should adopt a reasonable pairing and welding sequence. A proper assembly welding sequence ensures that each weld in the component is in a state of maximum free expansion after welding. It should be noted that when welding on the same plane of a component, space should be left for free expansion both horizontally and vertically. For example, when welding butt welds, a proper welding sequence (welding short welds first, then long welds) ensures orderly expansion of the welds and reduces residual stress. Furthermore, welds with greater shrinkage should be welded first, followed by welds with less shrinkage. This is primarily because welds welded first experience less resistance to shrinkage, resulting in less residual stress.
When welding I-beams with covers, the butt weld on the cover should be welded first, followed by the fillet welds between the I-beam and the cover. This is because fillet welds shrink less after welding than butt welds. When welding heavily stressed welds, such as large I-beams, the stressed butt weld should be welded first, followed by the unstressed butt weld, and finally the reserved fillet weld. Furthermore, when welding cross-plane welds, significant residual stresses are easily generated at the intersections, placing a greater strain on the welding team leader's welding sequence.
Furthermore, before welding, the component must be heated, either partially or entirely, to a certain temperature (a reference value of 150°C to 300°C). For metal materials that are brittle or relatively rigid, preheating is generally recommended. Alternatively, cold welding (using minimal heat input) can be employed. This method reduces the temperature difference between the weld and the rest of the component by minimizing the heated surface area. Finally, during welding, the weld restraint should be minimized. This is because closed welds typically experience significant restraint, and the resulting tensile stresses in the transverse and longitudinal directions of the weld are high, making cracks more likely to form. Therefore, to reduce residual stresses, welders must take measures to minimize the restraint of closed welds.
Selection of welding method
According to the above theory, to prevent residual stress in component welding, workers should try to use some high-energy density welding methods during the actual welding process. The welding line energy of these welders is relatively small, and the welding stress generated is also very small:
- Electron beam welding
- Laser welding
- Narrow gap welding
In general manufacturing processes,
CO2 gas shielded welding is increasingly used, not only for its high efficiency but also for its ability to minimize weld distortion. When welding thin plate components, pulsed tungsten inert gas arc welding or resistance welding can be used to prevent buckling. If low-energy methods are unavailable, direct water or air cooling can be used to alter the heat field distribution. This approach minimizes weld distortion without compromising welding specifications.
Solutions to eliminate welding stress
As is well known, due to the complexity of welding, significant residual stresses may remain in steel components after welding. Some structures are also prone to developing residual stresses during assembly, which inevitably impacts the performance of the steel components. Whether stress relief treatment is necessary after welding is generally determined based on a comprehensive analysis of the component's material properties, manufacturing process, and operational history. This article only considers common methods for eliminating and preventing residual stresses.
Heat treatment
It is a common method in metalworking, eliminating residual stresses through bulk, surface, or chemical means. In the early days, thermal effects were used to mitigate deformation and residual stresses before and during welding to address structural deformation caused by welding thin-walled panels. Pre-weld preheating methods for steel components include bulk heating in a furnace, localized far-infrared heating, localized power frequency heating, and flame heating. These methods involve heating the area around the weld seam to a specific temperature through various methods, then maintaining the temperature and allowing for slow, natural cooling.
With continuous technological advancements, modern steel structure bridge production primarily employs two methods to eliminate welding residual stresses: overall heat treatment and localized heat treatment. Overall, heat treatment involves slowly heating the entire steel component in a furnace to a specific temperature, maintaining that temperature for a certain period, and then allowing it to cool naturally in the furnace. Generally, the effectiveness of eliminating welding residual stresses using overall heat treatment is significantly better than that of local heat treatment. However, for large steel components that cannot be subjected to overall heat treatment, local heat treatment is more appropriate for eliminating residual stresses. This primarily depends on the heating temperature, holding time, cooling rate, heating method, and heating range of the component, typically eliminating 50% to 70% of the welding residual stresses.
The annealing process in post-welding heat treatment is frequently used in the manufacturing of pressure vessels and pipelines, but rarely applied to large civil engineering structures such as steel bridges. Typically, standard units are placed into large annealing furnaces for annealing treatment, and the residual stresses in the welded joints are significantly reduced after this process. Research indicates that adding two movable heating sources on either side of the welding gun to heat the area near the weld seam, and creating a uniform temperature field during the cooling process of the weld fusion zone and adjacent base material can reduce the formation of residual stress. The results show that peak residual stress parallel to the weld seam direction was reduced by approximately 21%. Similarly, when using parallel heating technology to perform post-welding heat treatment on the weld near the weld area, the results showed that residual stress in the direction parallel to the weld decreased by approximately 37%. Therefore, while movable local heating devices can effectively control welding distortion, the results indicate that their effect on eliminating residual stress is not significant.
In 2014, a specialist initially employed a linear heating device to heat the weld seam area on the top surface of a steel bridge deck to 625°C and hold the temperature for three hours. The test results showed a significant reduction in residual stress parallel to the weld seam. The specialist also employed a sheet-shaped ceramic heating device to heat a single U-rib specimen to 600°C and hold the temperature for one hour, obtaining similar experimental results demonstrating that heat treatment significantly reduced weld residual stress in the weld seam between the top plate and the U-rib. Furthermore, to investigate the effects of heating and cooling on the mechanical properties of bridge steel, a UK welding laboratory conducted a series of further tests. The results showed virtually no change in the material's elastic modulus, yield strength, and ultimate tensile strength. Using a high-frequency induction heating device to locally heat the weld toe reduces residual stress by nearly 90%. Fatigue test results show a significant improvement in fatigue life at the same stress amplitude.
In summary, post-weld heat treatment (PWHT) is a highly effective, mature, and easily industrialized method for eliminating weld residual stress. It significantly impacts the fatigue performance of critical welds in integral welds of steel bridge structures.
Mechanical hammering
involves mechanically impacting the weld toe to induce plastic deformation, which offsets some of the pressure and plastic deformation, thereby reducing weld residual stress. Its production practices typically include hammering, shot peening, and sand blasting. Numerous studies have been conducted on this method, both domestically and internationally. Research by Borg et al. demonstrated the presence of a compressive residual stress field on the surface of hammered specimens. Furthermore, experimental results by American researchers indicate that the number of loading cycles for crack initiation and crack propagation in hammered test materials is significantly increased. Furthermore, Yamada discovered that hammering cracks creates a compressive residual stress field that not only closes existing cracks but also improves fatigue resistance.
However, due to the complex structure and numerous welds in
steel bridge decks, hammering every weld in complex structures is currently difficult to perform in steel bridge deck manufacturing. Long-term practice has shown that this technique not only requires high operator skill and experience but is also sensitive to the shape parameters of the hammer head, making it difficult to adapt to all conditions and steel components.
The vibration component method
uses vibration aging to eliminate weld residual stress during steel component manufacturing. Its effectiveness depends on the vibrator, the location of the steel component's fulcrum, the vibration frequency, and the duration. Therefore, for rigid and complex steel components, multiple vibrations at multiple locations can be used, with the optimal vibration duration being 45 minutes. Furthermore, this method is suitable for vibrating large steel components, offering the advantages of simplicity, flexibility, time efficiency, energy savings, and low cost.
Temperature Difference Stretching Method
In the production and manufacturing of steel structure components, the temperature difference stretching method is also a commonly used method for eliminating residual welding stress. Its principle is essentially the same as that of the mechanical stretching method. The primary method involves using a flame gun to heat both sides of the weld on the component, raising the surface temperature to a specific level. At the same time, a water spray nozzle is used behind the flame gun for cooling. This creates a special temperature field where the temperatures on both sides of the component are high, while the temperatures of the weld and heat-affected zone are low. It is important to note that the heating area and temperature must be appropriately controlled to effectively eliminate and prevent residual stresses in steel components caused by welding.
Conclusion
In summary, welding residual stress is a significant structural hazard during the fabrication of steel bridges. Its generation mechanisms are complex, and its manifestations are diverse. This can not only lead to geometric deviations in components and assembly difficulties, but can also directly impact the load-bearing performance and lifespan of steel bridges. Therefore, developing a systematic and efficient welding residual stress control solution, from the design stage to the manufacturing process, and from prevention and control to post-stress relief, is crucial to ensuring the safety and durability of bridge structures.
On the preventative level, optimizing structural design, rationally arranging welding sequences, and selecting appropriate welding methods and preheating processes can minimize the formation and accumulation of welding stress at the source. Regarding stress relief, methods such as post-weld heat treatment, with its mature and effective technology, as well as mechanical hammering, vibration aging, and temperature differential stretching, with their convenient and applicable localized treatment options, all demonstrate significant potential.
In the future, with the diversification of steel bridge structures and the continuous improvement of manufacturing precision requirements, welding residual stress control technologies will continue to evolve towards more efficient, intelligent, and controllable approaches. In short, as a world-class one-stop prefabricated building expert, driven by our professional theoretical research and engineering practice, the welding quality control system of XTD Steel Structure's unique bridges will continue to provide customers with comprehensive services!
