Zhuojin Pipe Fitting Co., LtdExplosion Treatment for Welding Residual Stress Elimination in High-Strength Steel Pressure Pipes of the Three Gorges Project
Literature Overview
The paper by Chen Huaining, Liu Hequan, Lin Quanhong, and Chen Jing (2000), published in Welding (No. 12, pp. 20–22), reports on the application of explosion treatment to eliminate welding residual stresses in thick-section high-strength steel pressure pipes used in the Three Gorges Dam Project. The material studied is NK-HITEN 610U2, a quenched and tempered high-strength steel with a thickness of 60 mm. The study examined both butt welds and fillet welds, and the results demonstrate that explosion treatment not only significantly reduces surface tensile residual stresses but also achieves a uniform and substantially reduced residual stress distribution through the thickness of the weld.
Core Technical Content
Background: Residual Stress in Thick-Section High-Strength Steel Welds
The Three Gorges Dam Project required the fabrication of large-diameter pressure steel pipes with wall thicknesses up to 60 mm, made from NK-HITEN 610U2 quenched and tempered steel. This steel grade, with a minimum yield strength of 610 MPa, presents significant challenges for welding due to:
- High hardenability and susceptibility to hydrogen-induced cracking
- Large residual stress fields due to thick-section welding
- Limited ductility that restricts the effectiveness of conventional stress relief methods
The residual stress field in thick-section welds is particularly problematic because:
- The peak tensile residual stress typically reaches 0.8–1.0 times the yield strength of the base metal.
- The stress distribution through the thickness is highly non-uniform, with peak stresses at the weld surface and significant compressive stresses at the back face.
- Conventional post-weld heat treatment (PWHT) is often impractical for large-diameter pressure pipes due to the enormous thermal mass and the difficulty of achieving uniform heating.
Explosion Treatment Principle
Explosion treatment utilizes the shock wave generated by a controlled detonation of a shaped explosive charge placed on the surface of the weld. The shock wave propagates into the material, inducing plastic deformation that effectively redistributes and reduces the residual stress field. The key advantages of explosion treatment include:
| Advantage | Description |
|---|---|
| Rapid application | Treatment can be completed in seconds to minutes |
| No thermal input | No risk of HAZ embrittlement or microstructural changes |
| Through-thickness effect | Shock wave penetrates the full thickness, achieving uniform stress reduction |
| Field applicability | Can be applied on-site without specialized heating equipment |
| Cost-effective | Lower cost than PWHT for large structures |
Residual Stress Measurement Results
The study measured three-dimensional residual stresses before and after explosion treatment using the incremental hole-drilling method and other techniques.
| Location | Pre-Treatment σ_x (MPa) | Post-Treatment σ_x (MPa) | Reduction (%) |
|---|---|---|---|
| Weld surface center | +520 | -80 to +50 | 85–90 |
| 1/4 thickness | +480 | -50 to +30 | 80–85 |
| 1/2 thickness | +350 | -30 to +20 | 75–80 |
| 3/4 thickness | +200 | -40 to +10 | 70–80 |
| Back face | -150 | -60 to -20 | 30–40 |
The key finding is that explosion treatment not only reduces the magnitude of residual stresses but also achieves a more uniform distribution through the thickness. This is in contrast to conventional methods such as shot peening, which only affect the surface layer, or mechanical vibration stress relief, which has limited effectiveness on thick sections.
Comparison of Residual Stress Reduction Methods
| Method | Through-Thickness Effect | Thermal Impact | Applicable Thickness | Stress Reduction (%) |
|---|---|---|---|---|
| Post-Weld Heat Treatment (PWHT) | Good | Significant (risk of HAZ degradation) | Any | 70–90 |
| Shot Peening | Surface only (0.1–0.5 mm) | None | Any | 50–70 (surface) |
| Mechanical Vibration | Limited (1–3 mm) | None | < 30 mm | 30–50 |
| Explosion Treatment | Full thickness | None | 10–100 mm | 70–90 |
| Thermal Stress Relief (local) | Limited | Moderate | 10–50 mm | 40–60 |
Engineering Practice Implications
Application to Pressure Pipe Fabrication
For the fabrication of large-diameter pressure pipes in hydroelectric projects, explosion treatment offers several practical advantages:
- Field application: Unlike PWHT, which requires a large furnace or specialized heating equipment, explosion treatment can be performed on-site at the assembly location.
- No thermal distortion: The absence of thermal input eliminates the risk of additional thermal distortion in the already-stressed pipe structure.
- Compatibility with high-strength steels: For quenched and tempered steels such as NK-HITEN 610U2, the risk of hydrogen-induced cracking or delayed cracking is eliminated since there is no thermal cycle.
- Time efficiency: The treatment time is minimal compared to PWHT, which can require hours or days for thick sections.
Safety and Regulatory Considerations
Explosion treatment involves the controlled detonation of explosives, which raises significant safety and regulatory concerns:
- Permitting: The use of explosives requires permits from relevant authorities and must comply with local regulations on the storage, transport, and use of explosives.
- Safety zone: A safety exclusion zone must be established around the treatment area. The radius depends on the explosive charge size and the type of explosive used.
- Personnel training: Operators must be trained and certified in the handling and detonation of explosives.
- Environmental considerations: The noise and vibration from the explosion must be assessed for potential impact on surrounding structures and the environment.
Welding Process Optimization
The effectiveness of explosion treatment is influenced by the welding process parameters and the resulting residual stress field:
- Welding sequence: The welding sequence should be optimized to minimize the initial residual stress field, making the explosion treatment more effective.
- Welding parameters: Lower heat input reduces the residual stress magnitude, which in turn reduces the explosive charge size required.
- Preheat and interpass temperature: Controlled preheat and interpass temperatures can reduce the residual stress field and improve the weldability of the high-strength steel.
Key Questions and Reflections
- What is the long-term durability of the residual stress reduction achieved by explosion treatment? Could the stress field relax back toward the original state under service loading?
- How does the explosion-induced plastic deformation affect the fatigue life of the weld? The introduction of plastic deformation could either improve or degrade the fatigue performance depending on the residual stress state.
- Can explosion treatment be combined with other methods (e.g., shot peening, vibration stress relief) to achieve a synergistic effect?
- What are the optimal explosive charge configurations (shaped charge geometry, standoff distance, charge mass) for different weld geometries and thicknesses?
Summary
The study by Chen et al. demonstrates that explosion treatment is an effective and practical method for eliminating welding residual stresses in thick-section high-strength steel pressure pipes. The results from the Three Gorges Project application show that explosion treatment can reduce surface tensile residual stresses by 85–90% and achieve a uniform stress distribution through the full thickness of the weld. This method offers significant advantages over conventional PWHT for large-diameter pressure pipes, particularly in terms of field applicability, absence of thermal input, and time efficiency. However, the safety and regulatory implications of using explosives must be carefully managed. The findings of this study provide a valuable reference for engineers involved in the fabrication of large-scale steel structures where residual stress control is critical for structural integrity and fatigue performance.