Comparative Evaluation of Ultrasonic Impact Treatment and TIG Remelting for Fatigue Strength Improvement of Welded Joints

Literature Overview

The paper by Wang Dongpo, Huo Lixing, Jing Hongyang, Zhang Yufeng, and Yang Xinqi (2001), published in Journal of Mechanical Strength (Vol. 23, No. 2, pp. 202–205) and supported by the National Natural Science Foundation of China (Grant No. 59575061) and the Ministry of Education Doctoral Program Fund (Grant No. 1999005605), provides a comprehensive comparison of two widely used methods for improving the fatigue strength of welded joints: ultrasonic impact treatment (UIT) and TIG remelting. The study employed Q235B cruciform joint specimens and evaluated the effects of each method on weld toe geometry, hardness, residual stress, and fatigue performance.

Core Technical Content

Test Specimen and Material

The study used Q235B steel (equivalent to ASTM A36 / EN 10025 S235JR), a mild carbon steel commonly used in structural applications. The cruciform joint configuration was selected because it represents a typical structural detail found in bridges, buildings, and mechanical components, and is well-established in fatigue testing standards (e.g., ISO 15614, AWS D1.1).

Methodology

Both UIT and TIG remelting were applied to the weld toes of cruciform joints, and the following parameters were measured:

• Hardness: Vickers hardness (HV) distribution across the weld toe region.
• Residual stress: X-ray diffraction or strain gauge method to measure surface residual stress.
• Weld toe geometry: Optical or profilometric measurement of toe radius and angle.
• Fatigue strength: Stress range at 2×10⁶ cycles from S-N curve.

Results Comparison

The results reveal an important trade-off: UIT provides a greater fatigue strength improvement (64% vs. 51%) primarily due to the introduction of high-magnitude compressive residual stresses at the weld toe. However, TIG remelting offers superior geometric improvement and eliminates micro-cracks at the fusion boundary, which is beneficial for long-term durability.

Mechanism Analysis

Ultrasonic Impact Treatment (UIT):

• A high-frequency (20 kHz) needle is driven into the weld toe surface, creating plastic deformation.
• The plastic deformation introduces compressive residual stresses (typically -300 to -600 MPa) in the surface layer.
• The compressive stresses counteract the applied tensile stresses during fatigue loading, reducing the effective stress range at the crack initiation site.
• However, UIT does not significantly alter the weld toe geometry or eliminate existing micro-cracks.

TIG Remelting:

• The TIG arc remelts the weld toe region, rounding the toe and eliminating micro-cracks.
• The remelting process introduces a new, fine-grained microstructure at the toe.
• However, the reheating and air cooling cycle introduces tensile residual stresses (typically +50 to +150 MPa), which partially offset the geometric improvement.
• The net fatigue improvement is still substantial (51%) because the elimination of stress concentrators (sharp toe, micro-cracks) is the dominant factor.

Practical Considerations

Engineering Practice Integration

Selection Criteria for Fatigue Improvement Methods

The choice between UIT and TIG remelting should be based on the following considerations:

1. Criticality of the joint: For highly critical joints where fatigue life is paramount, UIT may be preferred due to the higher fatigue improvement.
2. Existing defect condition: If micro-cracks are present at the weld toe, TIG remelting is preferred because it eliminates these defects.
3. Accessibility: UIT requires direct contact with the weld toe, which may be difficult in confined spaces. TIG remelting requires arc access but does not require contact.
4. Cost and availability: TIG remelting is generally more cost-effective and requires less specialized equipment.
5. Regulatory requirements: Some standards (e.g., DNV, NORSOK) specify minimum requirements for fatigue improvement methods, and the selected method must be qualified according to the relevant code.

Combination Approach

In some cases, a combination of both methods may be beneficial: TIG remelting first to improve the geometry and eliminate micro-cracks, followed by UIT to introduce compressive residual stresses. This combined approach could potentially achieve fatigue improvements exceeding 80%, although this would require further experimental validation.

Study Insights and Reflections

This comparative study is valuable because it provides a quantitative basis for selecting between two widely used fatigue improvement methods. The findings highlight the importance of considering not only the fatigue strength improvement but also the underlying mechanisms (residual stress, geometry, microstructure) when evaluating the long-term effectiveness of a treatment.

The study also underscores a fundamental principle in fatigue engineering: the dominant factor in fatigue performance is often the stress concentration at the weld toe, which is influenced by geometry, residual stress, and microstructure. Different improvement methods address these factors to different extents, and the optimal choice depends on the specific application and loading conditions.

A limitation of the study is the use of a relatively low-strength steel (Q235B). The findings may not be directly transferable to high-strength steels (e.g., S690, HSLA) where the fatigue behavior is more sensitive to residual stress and microstructural factors. Further studies on higher-strength materials would be valuable to extend the applicability of these findings.