In the welding process of steel box beams, controlling welding deformation and residual stress is crucial for ensuring structural accuracy and safety. Welding deformation stems from uneven material shrinkage caused by localized heating, while residual stress arises from the combined effects of thermal expansion and contraction during welding and constraint conditions. Failure to effectively control these factors can lead to structural instability, fatigue cracks, and even overall failure. Therefore, a comprehensive approach is needed, encompassing process design, operational procedures, and post-processing.
Optimizing the welding sequence and direction is a fundamental strategy for controlling deformation. The complex structure of steel box beams requires adherence to the principle of "symmetrical welding, layered welding." For example, the longitudinal butt welds between the top and bottom plates must proceed synchronously from the center outwards to avoid the accumulation of localized shrinkage caused by unidirectional welding. The connection between the diaphragms and web plates should follow a "standing first, then tilting" sequence to reduce bending deformation caused by gravity. Furthermore, during circumferential welding, the top and bottom plates should be welded alternately to balance shrinkage forces and counteract deformation trends.
Selecting appropriate welding methods and parameters is key to reducing heat input. For steel box beam welding, low-energy welding processes such as CO₂ gas shielded welding or submerged arc welding should be prioritized. The former, due to its concentrated arc and shallow penetration, significantly reduces the heat-affected zone; the latter achieves uniform heating through multi-layer, multi-pass welding, avoiding severe shrinkage caused by excessively thick single-pass welds. Simultaneously, welding speed, current, and voltage parameters must be strictly controlled to ensure the heat input matches the plate thickness, preventing material degradation due to localized overheating.
Groove design and assembly accuracy directly affect weld quality. Groove dimensions must be precisely designed based on plate thickness and welding position. For example, the groove angle between the U-rib and the top plate should be controlled between 45° and 55° to ensure weld penetration while preventing deformation due to excessive filler. During assembly, tooling fixtures should be used to fix components, ensuring that alignment errors do not exceed allowable limits and preventing stress concentration caused by misalignment or uneven gaps. Furthermore, codeless welding technologies (such as magnetic clamping) can reduce damage to the base material caused by traditional plate clamps, further improving assembly accuracy.
Preheating and slow cooling are effective means to alleviate residual stress. Preheating both sides of the weld before welding reduces the temperature gradient and minimizes shrinkage stress caused by rapid cooling. After welding, covering with insulation or slow cooling in a furnace ensures uniform cooling of the material, preventing cracks caused by localized rapid cooling. For example, when welding thick plates, the preheating temperature should be controlled between 100℃ and 150℃, and the slow cooling time after welding should be no less than 2 hours to ensure full stress release.
Reverse deformation and rigid fixing methods can actively compensate for deformation. Based on welding deformation test data, a reverse deformation amount is applied to the component beforehand, so that post-weld shrinkage cancels out the preset deformation. For example, before welding U-ribs, the curvature of the top plate can be adjusted using a reverse deformation jig to compensate for angular deformation after welding. Rigid fixing methods constrain the freedom of the component through temporary supports or clamps, limiting displacement during welding, but care must be taken to avoid excessive restraint leading to a surge in residual stress.
Post-weld heat treatment and mechanical straightening are the ultimate guarantees for eliminating residual stress. For high-strength steel or complex joints, overall or partial annealing can be used to relax stress through high-temperature tempering. Mechanical straightening applies a reverse force to the deformed area using hydraulic jacks or flame heating, but the straightening amount and temperature must be strictly controlled to prevent material overload or performance degradation.
Controlling welding deformation and residual stress in steel box beams needs to be integrated throughout the entire design, manufacturing, and installation process. Through process optimization, precise parameter control, and multi-stage synergy, structural quality can be significantly improved, ensuring the long-term safe operation of bridges and other infrastructure.
