Large, heavy welded steel structural components are increasingly vital in modern industry, demanding high dimensional accuracy. This paper explores welding distortion mechanisms, proposes minimization measures, and presents methods for reducing residual stress and correcting distortion.
I. Welding Distortion Mechanism
Welding melts metal, forming a molten pool. Its rapid solidification (~100°C/s, ~10⁴ times faster than casting) causes defects. Key characteristics:
1.Small Pool & Fast Cooling: Promotes porosity, cracks, inclusions, segregation.
2.High Superheat & Gradient: Steep temperature gradients (10³-10⁴ times casting).
3.Directional Solidification: Crystals grow opposite heat flow, stopping at centerline (impurity risk).
4.Epitaxial Growth: Fusion line grains act as nuclei.
Localized heating causes compressive stresses; cooling causes tensile stresses, leading to bending. Post-weld, complex tensile/compressive stresses and strains exist (compressive in weld, tensile nearby). Understanding this is key.
II. Structural Design for Distortion Control
1.Minimize Welds: Fewer welds = less heat/distortion. Use single plates or pre-bend thin sheets (<10mm).
2.Optimize Weld Type/Size: Prefer butt joints. Minimize weld size adequately for strength.
3.Symmetrical Placement: Allows opposing shrinkage to partially cancel. Place near stiff areas.
4.Avoid Concentration/Overlap: Stagger welds (≥100mm apart) to prevent overlapping HAZs, repeated heating, and amplified distortion.
III. Welding Process Design
1.Accurate Cutting: Prevent undersized parts and large gaps (>3mm) requiring excess filler/heat.
2.Precise Fixtures: Essential for large structures. Control dimensions (±0.2%), key diagonals, flatness (≤3mm).
3.Optimal Process:
SMAW: Versatile but inefficient.
SAW: High deposition for long seams.
GMAW: Preferred for high current/deposition, low distortion, wide thickness. Mitigate spatter/porosity (gas purity, flux-cored wires, mixtures).
4.Use Dedicated Fixtures: Prevent distortion accumulation during large component assembly.
Conclusion: Effective distortion control integrates structural design and optimized welding processes.
