1.How to achieve overall stress relief during annealing?
Process: The entire weldment is placed in a programmable furnace and slowly heated to below the recrystallization temperature and Ac1 point of the steel (typically 580-650°C). It is held at this temperature for a period of time (approximately 1 hour for every 25mm of thickness), and then slowly cooled in the furnace.
Advantages: Provides the most thorough stress relief (over 80%), simultaneously improving the microstructure of the weld and heat-affected zone, and enhancing toughness.
Disadvantages: High equipment investment, high energy consumption, long cycle time, and potential for overall oxidation of the workpiece (requires a protective atmosphere).
Applications: Critical structural components requiring high dimensional stability, fatigue resistance, and stress corrosion resistance, such as pressure vessels, precision machine frames, and critical automotive chassis components.
2.How to perform localized heat treatment?
Process: The weld and surrounding area are heated using a ring-shaped heating element, induction coil, or flame torch, followed by slow cooling and heat preservation.
Advantages: Relatively flexible, suitable for large workpieces or those that cannot be placed in a furnace; low energy consumption.
Disadvantages: Difficult to control temperature uniformity; stress relief is not as thorough as with overall annealing.
Applications: Pipeline circumferential welds, longitudinal/circumferential welds in large storage tanks, and partial repair welds.
3.What are the advantages and disadvantages of vibration aging?
Process: The workpiece (especially large structural components) is supported by elastic pads. A specially designed vibrator is firmly clamped onto the workpiece, and the excitation frequency is adjusted by a controller to induce resonance at its natural frequency (approximately 20-30 minutes). The dynamic stress generated by resonance superimposes with the residual stress, causing localized microscopic plastic deformation, thereby reducing and homogenizing the residual stress.
Advantages: Energy-saving and environmentally friendly (energy consumption is only 1-3% of heat treatment), high efficiency, no oxidation deformation, and can be performed after final machining.
Disadvantages: Requires high operator skill; the resonant frequency must be determined for different components; the effect needs to be verified using parametric curves (such as the a-n curve) or subsequent measurements.
Applications: Ideal for large welded structural components, such as machine tool beds, frames, box girders, and ship sections. It is currently a widely used advanced method.
4.What are the principles, advantages, and disadvantages of shot peening/hammering?
Principle: This method induces compressive plastic deformation and elongation in the surface metal, generating beneficial compressive stress on the surface to counteract internal tensile stress. It also refines grain size and improves fatigue strength.
Advantages: Simple equipment, can be operated on-site, and significantly improves fatigue life in the weld zone.
Disadvantages: Requires high operator skill (improper hammering may cause cracks); the effect is concentrated on the surface, with limited relief of internal stress in thick plates.
Applications: Stress improvement and fatigue strengthening of weld surfaces; commonly used in components subjected to alternating loads.
5.Considering the workpiece size and production conditions, how should this be handled?
For large, batch-welded structures (such as equipment frames), vibration aging is the preferred method due to its high efficiency, economy, and environmental friendliness.
For small to medium-sized, high-value critical components, overall stress-relief annealing is the preferred option if furnace annealing is feasible.
For components with special requirements for weld fatigue life, hammering or shot peening are necessary auxiliary or primary methods.
For large, immovable field welding (such as ships and bridges), localized heat treatment is a viable solution.