CLADDING TECHNOLOGY SHANXI CO., LTDComposite Strengthening of High-Strength Steel Welded Joints by Ultrasonic Impact and TIG Remelting
Literature Overview
This paper by Wang Dong-Po, Huo Li-Xing, Zhang Yu-Feng, Jing Hong-Yang, and Yang Xin-Qi from the School of Materials Science and Engineering at Tianjin University was published in the Journal of Mechanical Engineering in 2003. The research presents a composite strengthening technique that combines ultrasonic impact treatment (UIT) with TIG remelting to improve the fatigue performance of high-strength steel welded joints. This approach addresses the well-known fatigue weakness of welded joints in high-strength steels, which is a critical concern for pressure vessel and structural applications.
Core Technical Analysis
Welded joints in high-strength steels typically exhibit fatigue strengths significantly lower than the base metal due to several factors:
- Stress concentrations: Weld toes and surface irregularities create localized stress raisers.
- Microstructural degradation: The heat-affected zone contains coarse, brittle phases that are susceptible to crack initiation.
- Residual stresses: Tensile residual stresses at the weld toe promote fatigue crack initiation and propagation.
- Defects: Porosity, inclusions, and lack of fusion provide preferential crack initiation sites.
The composite strengthening technique addresses these issues through a two-stage process:
Stage 1: Ultrasonic Impact Treatment (UIT)
UIT involves repeatedly impacting the weld toe surface with a hardened tool at ultrasonic frequencies (typically 20-40 kHz). This process produces several beneficial effects:
- Surface compression: Plastic deformation at the weld toe introduces compressive residual stresses that counteract tensile service loads.
- Work hardening: The impacted surface layer is cold-worked, increasing surface hardness and strength.
- Surface refinement: The impacted layer develops a refined microstructure with increased dislocation density.
- Crack deflection: The work-hardened layer can deflect propagating cracks, increasing the fatigue crack growth resistance.
Stage 2: TIG Remelting
After UIT, the weld toe is remelted using TIG welding. This step serves to:
- Smooth the weld toe: The remelting process creates a smoother weld toe geometry, reducing stress concentrations.
- Relax compressive stresses: The controlled thermal cycle partially relaxes the compressive stresses introduced by UIT, optimizing the residual stress state.
- Heal surface defects: Any surface defects introduced during UIT are healed during remelting.
- Maintain work-hardened layer: The remelting is performed with controlled heat input to preserve the beneficial effects of UIT in the subsurface region.
Process Parameters and Fatigue Performance
| Parameter | Typical Value | Effect on Fatigue Performance |
|---|---|---|
| UIT frequency | 20-40 kHz | Higher frequency produces finer surface refinement |
| UIT amplitude | 20-100 μm | Larger amplitude increases compressive stress depth |
| UIT overlap ratio | 0.3-0.5 | Adequate overlap ensures uniform treatment coverage |
| UIT tool angle | 60-90 degrees | Angle affects stress state and surface profile |
| TIG remelting current | 50-150 A | Lower current minimizes thermal effects |
| TIG travel speed | 10-30 mm/s | Higher speed reduces heat input |
| TIG wire feed | None or low rate | Minimizes dilution and maintains surface geometry |
| Shielding gas | Argon or Argon-Helium | Maintains clean weld surface |
Fatigue Performance Improvement
The composite strengthening technique typically achieves significant improvements in fatigue strength:
- Fatigue limit increase: 30-80 percent improvement in fatigue limit compared to as-welded condition.
- Fatigue life extension: 2-5 times increase in fatigue life at high-cycle fatigue regime.
- S-N curve shift: The entire S-N curve shifts upward, indicating improved fatigue performance across all stress levels.
- Crack initiation resistance: Substantially improved due to compressive residual stresses and refined microstructure.
Engineering Practice Applications
For pressure vessel fabrication, the composite strengthening technique has several important applications:
- High-strength steel pressure vessels: Improving fatigue performance of welded joints in vessels made from high-strength low-alloy steels, enabling lighter and more economical designs.
- Fatigue-critical joints: Enhancing the durability of joints in pressure vessels subjected to cyclic loading, such as hydrogenation reactors, autoclaves, and pressure vessels in cyclic service.
- Repair welding: Applying the technique to repaired welds in existing pressure vessels to restore fatigue performance.
- Clad layer interfaces: Improving fatigue resistance at the interface between clad layers and base metal in bimetallic pressure vessels.
Integration with Quality Control
The composite strengthening technique requires careful quality control to ensure consistent results:
- Residual stress measurement: X-ray diffraction or hole-drilling methods to verify compressive stress levels.
- Surface hardness testing: Microhardness profiles to confirm work-hardening effects.
- Surface roughness measurement: Ensuring smooth weld toe geometry after remelting.
- Fatigue testing: Validating the fatigue performance improvement through coupon testing.
Key Questions and Reflections
The composite strengthening technique raises several important considerations for practical implementation:
- How does the technique perform on thick-section welds where thermal effects are more pronounced?
- What is the effect of the technique on the underlying clad layer in bimetallic pressure vessels?
- Can the technique be applied to complex geometries such as nozzle-to-shell joints and branch connections?
- How does the technique interact with other post-weld treatments such as peening or shot peening?
The balance between UIT-induced compressive stresses and TIG remelting-induced stress relaxation requires careful optimization to achieve the best fatigue performance.
Study Insights and Outlook
This research presents an innovative approach to improving the fatigue performance of welded joints in high-strength steels. For the pressure vessel industry, the technique offers a practical solution to a long-standing challenge: the fatigue weakness of welded joints that often governs the design life of pressure vessels. The combination of UIT and TIG remelting leverages the complementary benefits of mechanical surface treatment and thermal processing, creating a synergistic effect that exceeds the benefits of either technique alone.
Future work should focus on developing standardized procedures for the composite strengthening technique, establishing qualification requirements for pressure vessel applications, and investigating the long-term stability of the improved fatigue performance under service conditions. The technique could also be extended to other advanced materials such as nickel-based alloys, titanium alloys, and ODS alloys used in high-temperature and corrosive service pressure vessels. As pressure vessel designs continue to push the boundaries of material performance and service conditions, techniques like this that enhance the fatigue durability of welded joints will become increasingly important for ensuring the safety and reliability of pressure vessel systems.