1.Why is residual stress more severe after welding?
Residual stress in cold-rolled coils:
Cold rolling is a violent plastic deformation process that creates inherent, distributed residual stresses within the material (typically compressive stress on the surface and tensile stress in the core).
If post-cold rolling annealing is insufficient, these stresses will persist.
New residual stresses introduced by welding (this is the primary issue):
Locally intense heating: During welding, the weld area is rapidly heated to melting point, while the surrounding base material (cold-rolled coil) remains at a lower temperature.
Uneven thermal expansion and contraction: During heating, the weld area's expansion is restricted, resulting in plastic compression; during cooling, the weld's contraction is restricted, leading to high tensile residual stresses in the weld and heat-affected zone, often approaching the material's yield strength.
Microstructure transformation: Some steels undergo microstructure transformations (such as martensitic transformation) under welding thermal cycling, and these volume changes further exacerbate stress.
2.What are the dangers of not eliminating residual welding stress?
Deformation and Instability: Stress release leads to component twisting and warping, affecting dimensional accuracy and assembly.
Stress Corrosion Cracking: In corrosive environments, tensile stress greatly accelerates crack initiation and propagation.
Reduced Fatigue Strength: Residual tensile stress superimposed on the tensile stress in the working load reduces the fatigue life of the component and easily induces fatigue cracks at welds.
Reduced Load-Bearing Capacity: High residual stress may cause the component to approach its yield point before bearing external loads.
Impact on Subsequent Machining: During machining, stress rebalancing can lead to new deformations, causing loss of control over machining accuracy.
3.How to eliminate residual stress in welding?
Heat Treatment Methods (Most Classic, Most Effective, and Widely Used)
Principle: Utilizing the characteristic that the yield strength of materials decreases at high temperatures, stress is relieved through plastic deformation. Simultaneously, it can improve the coarse microstructure of the weld heat-affected zone.
Main Processes:
Stress-Relief Annealing: Also known as "stress-reducing annealing." The workpiece is uniformly heated to a temperature below Ac₁ (typically 550-650℃ for carbon steel), held at that temperature for a period of time (calculated based on thickness, e.g., 1 hour per 25mm), and then slowly cooled. This is the most thorough and reliable method.
Overall High-Temperature Tempering: For components requiring post-weld tempering, the tempering process also eliminates stress.
Vibration Aging Technology (Highly Efficient, Energy-Saving, Increasingly Popular)
Principle: A portable vibrator applies a periodic load to the workpiece, causing microscopic plastic deformation at the resonant frequency, thereby achieving stress homogenization and reduction.
4.What are the principles, advantages, and disadvantages of the mechanical stretching method?
Principle: Applying a tensile load exceeding the yield strength to the welded component, either as a whole or partially, causes plastic deformation across the entire cross-section, thereby releasing residual stress. Commonly used for plate materials and welded containers.
Implementation Methods: Hydraulic jacks, explosive loading, hydrostatic testing (for pressure vessels, hydrostatic testing serves as both inspection and overload stress relief).
Advantages: Direct effect, especially suitable for plate structures.
Disadvantages: Requires powerful loading equipment; suitable for workpieces of specific shapes and sizes.
5.What are some recommendations for material selection and process coordination for welded cold-rolled coils?
Source control: For critical welded structures, fully processed cold-rolled steel sheets (i.e., annealed and leveled, such as SPCC-SD) should be prioritized due to their lower initial residual stress levels.
Design optimization: Reduce the number, length, and thickness of welds; avoid weld intersections and dense welds; optimize bevel design.
Welding process: Employ low-heat-input welding methods (e.g., MAG welding instead of manual arc welding), multi-layer multi-pass welding, symmetrical welding, and pre-deformation methods to reduce welding stress and deformation at the source.
Post-weld treatment selection:
Generally important structural components: Vibration aging is the preferred option (economical and efficient).
Critical load-bearing components, pressure vessels, and structures requiring resistance to stress corrosion: Stress-relief annealing is mandatory.
Large structures that cannot be furnace-tested: Localized heat treatment or vibration aging should be used.