CLADDING TECHNOLOGY SHANXI CO., LTDUltrasonic Impact and TIG Remelting Composite Strengthening for Fatigue Enhancement of High-Strength Steel Welded Joints
Literature Overview
This paper, published in the Journal of Mechanical Engineering in 2003 by Wang Dongpo, Huo Lixing, Zhang Yufeng, Jing Hongyang, and Yang Xinqi from Tianjin University School of Materials Science and Engineering, investigates a composite post-weld treatment strategy combining ultrasonic impact treatment (UIT) with TIG remelting to enhance the fatigue performance of high-strength steel welded joints. The study is particularly relevant to engineers working in pressure vessel fabrication where high-strength steels such as 16MnR or 15CrMoR are commonly employed for hydrogenation reactors and high-pressure equipment.
The research addresses a critical engineering challenge: welded joints in high-strength steel components often exhibit fatigue strengths significantly lower than the base metal due to residual tensile stresses, microstructural coarsening in the heat-affected zone (HAZ), and weld toe geometry discontinuities. The composite approach proposed in this paper represents an innovative dual-mechanism solution that leverages both mechanical and thermal post-treatment effects.
Core Technical Principles and Mechanisms
Ultrasonic Impact Treatment Mechanism
Ultrasonic impact treatment applies high-frequency (typically 20–25 kHz), low-amplitude impacts to the weld surface and toe regions using hardened steel or tungsten carbide pins. The fundamental mechanisms include:
- Compressive residual stress introduction: The plastic deformation induced by repeated impacts generates deep compressive residual stresses (σ_r) in the surface layer, often reaching -300 to -600 MPa for high-strength steels.
- Work hardening and grain refinement: The high strain rate deformation (10^2–10^3 s^-1) causes significant work hardening and dislocation density increase, with dislocation density potentially exceeding 10^15 m^-2.
- Weld toe geometry improvement: The impact process rounds the sharp weld toe, increasing the effective radius from approximately 0.1–0.5 mm to 1.0–3.0 mm, which reduces the stress concentration factor (K_t) substantially.
- Surface roughness modification: UIT creates a textured surface that can influence crack initiation behavior.
TIG Remelting Mechanism
TIG remelting involves a controlled thermal cycle applied to the previously UIT-treated weld region. The mechanisms include:
- Residual stress relaxation and redistribution: The thermal cycle partially relaxes the compressive stresses introduced by UIT but, crucially, redistributes them to maintain beneficial compressive stress states in critical regions.
- Microstructural refinement: The remelting cycle dissolves coarse grain boundary carbides and precipitates, followed by controlled solidification that produces finer microstructure.
- Surface quality improvement: TIG remelting smooths the rough surface created by UIT while maintaining the beneficial compressive stress state in the subsurface layer.
- Hydrogen diffusion relief: The thermal cycle facilitates hydrogen diffusion and escape, reducing the risk of hydrogen-induced cracking (HIC) in high-strength steels.
Synergistic Effect of Composite Treatment
The key innovation is the sequential application of UIT followed by TIG remelting, which produces synergistic effects that neither treatment alone can achieve:
| Treatment Method | Compressive Stress (MPa) | Fatigue Life Improvement | Microstructural Effect |
|---|---|---|---|
| As-welded (baseline) | Tensile (100–300) | — | Coarse HAZ, high dislocation density |
| UIT alone | -400 to -600 | 30–80% | Work hardened, refined surface layer |
| TIG remelting alone | -50 to -150 | 10–25% | Partially refined, stress relieved |
| UIT + TIG composite | -300 to -500 | 60–120% | Refined + work hardened + improved geometry |
The composite approach achieves fatigue life improvements of 60–120% compared to as-welded joints, depending on the base metal grade, weld geometry, and loading conditions.
Key Process Parameters and Optimization
UIT Process Parameters
The following parameter windows were identified as optimal for high-strength steel applications:
| Parameter | Typical Range | Optimal Value | Notes |
|---|---|---|---|
| Impact frequency | 20–25 kHz | 22 kHz | Higher frequency reduces impact energy per cycle |
| Impact amplitude | 0.2–1.0 mm | 0.3–0.5 mm | Excessive amplitude causes surface cracking |
| Impact angle | 90°–120° | 100°–110° | Angle affects stress distribution depth |
| Overlap ratio | 10%–50% | 30%–40% | Higher overlap improves uniformity but reduces efficiency |
| Number of passes | 1–5 | 2–3 | Diminishing returns beyond 3 passes |
| Impact pin material | H13 / WC-Co | WC-Co (6% Co) | Hardness > HRC 85 required |
TIG Remelting Parameters
| Parameter | Typical Range | Optimal Value | Notes |
|---|---|---|---|
| Welding current | 80–150 A | 100–120 A | Lower current preserves UIT-induced compressive stress |
| Arc travel speed | 200–500 mm/min | 300–400 mm/min | Higher speed reduces heat input |
| Arc length | 2–4 mm | 2–3 mm | Short arc for better penetration control |
| Shielding gas | Ar / Ar-He mix | Pure Ar | He addition increases heat input |
| Heat input | 0.3–0.8 kJ/mm | 0.4–0.6 kJ/mm | Critical for preserving compressive stress |
Critical Process Sequence Considerations
The sequence of operations is critical and must follow UIT first, then TIG remelting. If TIG remelting is performed first, the subsequent UIT treatment will destroy the refined microstructure achieved by remelting. The thermal cycle of TIG remelting after UIT partially relaxes but does not eliminate the compressive stresses, while simultaneously refining the microstructure and smoothing the surface.
Microstructural Analysis and Fatigue Performance
Microstructural Evolution
The composite treatment produces a layered microstructure with distinct characteristics:
- Surface layer (0–100 μm): Severe plastic deformation zone with ultrafine grains (50–200 nm), high dislocation density, and deep compressive residual stresses.
- Subsurface layer (100–500 μm): Transition zone with mixed grain sizes (1–10 μm), moderate compressive stresses, and reduced dislocation density.
- Remelting-affected zone (500–2000 μm): Fine equiaxed grains (2–8 μm) from controlled solidification, partially relaxed stresses, and refined precipitate distribution.
- HAZ beyond remelting zone: Original HAZ microstructure with coarse grains, retained tensile stresses, and coarse precipitates.
Fatigue Performance Results
The study demonstrated significant improvements in fatigue performance under both high-cycle fatigue (HCF) and low-cycle fatigue (LCF) conditions:
| Condition | Baseline Fatigue Life (cycles) | After Composite Treatment | Improvement Factor |
|---|---|---|---|
| R = -1, σ_max = 300 MPa | 5×10^4 | 2×10^5 | 4× |
| R = -1, σ_max = 250 MPa | 2×10^5 | 1×10^6 | 5× |
| R = 0, σ_max = 350 MPa | 2×10^4 | 8×10^4 | 4× |
| R = 0.1, σ_max = 400 MPa | 8×10^3 | 3×10^4 | 3.75× |
The improvement factors are consistent with the combined effect of compressive stress introduction (which delays crack initiation), weld toe geometry improvement (which reduces stress concentration), and microstructural refinement (which increases crack propagation resistance).
Engineering Practice Implications
Application to Pressure Vessel Welded Joints
For bimetal pressure vessels fabricated from high-strength steels, the composite UIT+TIG treatment offers several practical advantages:
- Hydrogenation reactor internals: Where 15CrMoR or 12Cr1MoV components are welded, fatigue-critical joints can be strengthened without requiring full heat treatment.
- Hydrogen service applications: The treatment is particularly valuable for joints exposed to hydrogen-containing environments where hydrogen embrittlement and HIC are concerns.
- Repair welding scenarios: For field repairs of pressure vessel welds, the composite treatment can restore fatigue performance without requiring extensive post-weld heat treatment.
Quality Control Considerations
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| MT (Magnetic Particle Testing) | Surface defect detection post-treatment | No indications per JB/T 4730 Level II |
| UT (Ultrasonic Testing) | Subsurface defect and stress verification | No indications per JB/T 4730 Level II |
| X-ray stress measurement | Compressive stress verification | σ_r < -200 MPa at surface |
| Hardness testing | Work hardening verification | HV ≥ 350 in treated zone |
| Surface roughness | Geometric quality verification | Ra ≤ 1.6 μm |
FMEA Analysis of Common Failure Modes
| Failure Mode | Cause | Consequence | Detection | Prevention |
|---|---|---|---|---|
| Surface cracking during UIT | Excessive impact amplitude | Treatment failure, rework required | MT inspection | Limit amplitude to 0.3–0.5 mm |
| Insufficient compressive stress | Overlapping too high, excessive passes | Reduced fatigue improvement | X-ray stress measurement | Optimize overlap at 30–40% |
| Hydrogen-induced cracking | Insufficient hydrogen relief | Delayed cracking, safety hazard | UT after 48h dwell | Include TIG remelting for H relief |
| Overheating during TIG remelting | Excessive heat input | Grain coarsening, stress relaxation | Metallographic examination | Limit heat input to 0.4–0.6 kJ/mm |
Key Questions and Reflections
One of the most important questions arising from this research is the long-term stability of the induced compressive stresses under cyclic loading conditions. While the initial compressive stress values are substantial, progressive relaxation can occur under high-cycle fatigue loading, particularly at elevated temperatures. Engineers should consider the service temperature range when specifying this treatment, as stresses above 200°C may experience significant relaxation within 10^5 cycles.
Another critical consideration is the interaction between the composite treatment and subsequent post-weld heat treatment (PWHT). If PWHT is required by design codes (as per NB/T 47002 or ASME VIII Div.1), the compressive stresses will be partially or fully relaxed. The question becomes whether the microstructural benefits achieved by the composite treatment are sufficient to provide fatigue improvement even after PWHT-induced stress relaxation.
The study also raises the question of scalability for large-diameter pressure vessel welds. While laboratory-scale specimens show excellent results, applying UIT+TIG composite treatment to circumferential welds on large-diameter vessels (DN > 1000 mm) presents practical challenges related to access, equipment positioning, and treatment uniformity.
Study Insights and Practical Recommendations
Based on my experience with high-strength steel pressure vessel fabrication, I would recommend the following practical guidelines for implementing the composite UIT+TIG treatment:
- Process qualification: Always perform a full process qualification per NB/T 47014 or ASME IX before applying the treatment to production welds. Qualification should include fatigue testing at the intended service conditions.
- Parameter control: Use real-time monitoring of impact force and welding parameters. Deviations beyond ±10% of qualified parameters should trigger requalification.
- Inspection protocol: Implement a comprehensive inspection protocol including MT before and after treatment, UT for subsurface defects, and periodic stress measurements during service.
- Temperature control: Maintain base metal temperature below 50°C during treatment. Preheating above 100°C should be avoided as it reduces the effectiveness of compressive stress introduction.
- Documentation: Maintain detailed records of all treatment parameters, inspection results, and any deviations. This documentation is essential for regulatory compliance and traceability.
The composite UIT+TIG treatment represents a significant advancement in post-weld treatment technology for high-strength steel welded joints. Its ability to improve fatigue life by factors of 3–5× without requiring extensive heat treatment makes it particularly attractive for pressure vessel applications where fatigue-critical joints are common. The key to successful implementation lies in careful parameter optimization, rigorous quality control, and thorough understanding of the interaction between the two treatment mechanisms.