TIG Re-Melting Followed by Rapid Quenching Treatment at Weld Toe

Literature Overview and Research Background

This 2008 study from Dalian Jiaotong University's School of Materials Science and Engineering investigates a post-welding surface treatment technique combining TIG re-melting of the weld toe region followed immediately by water quenching. Published in the Journal of Dalian Jiaotong University, the research addresses fatigue performance improvement in welded joints through microstructural modification at the critical stress concentration zone. The authors—Zhu Ping, Guo Changhong, Shi Chunyuan, Ding Chenggang, and Jiao Jianqiang—developed this technique to enhance fatigue strength without requiring major changes to the welding process itself.

Core Technical Mechanism

The weld toe represents the most critical location in a welded joint from a fatigue perspective, as it contains geometric stress concentrations, residual tensile stresses, and often unfavorable microstructural features resulting from the welding thermal cycle. The TIG re-melting followed by rapid quenching treatment works through several mechanisms simultaneously:

The TIG re-melting process melts the weld toe region with a controlled heat input, creating a smooth, rounded surface profile that eliminates the sharp geometric notch characteristic of as-welded weld toes. The re-melting also redistributes residual stresses in the toe region, converting tensile stresses to compressive stresses through thermal cycling effects. The subsequent water quenching rapidly cools the re-melted zone, producing a refined martensitic or bainitic microstructure in carbon and low-alloy steels, or a refined austenitic grain structure in stainless steels.

Process Development and Microstructural Analysis

The key innovation in this technique is the precise timing between the TIG re-melting and the quenching operation. The re-melted zone must still be above the critical transformation temperature when quenching begins to achieve the desired microstructural refinement. Research indicates that an optimal quenching delay of less than 2 seconds is required for carbon and low-alloy steels to produce a fine martensitic structure in the re-melted toe region.

The microstructural changes produced by this treatment are significant. In as-welded conditions, the weld toe typically exhibits a coarse grain structure with retained austenite and possibly delta ferrite in stainless steels. After TIG re-melting and quenching, the microstructure transforms to:

• In carbon steels: fine martensite with high hardness (400-550 HV) and improved fatigue resistance
• In low-alloy steels: tempered martensite or fine bainite depending on the alloy composition
• In stainless steels: refined austenite grains with reduced grain boundary carbide precipitation

The compressive residual stresses introduced by this treatment can reach values of -200 to -400 MPa at the weld toe surface, which is comparable to or exceeds the compressive stresses achieved by shot peening in many cases.

Fatigue Performance Improvement

The fatigue strength improvement achieved through TIG re-melting with quenching treatment has been documented across multiple steel grades and joint configurations:

The improvement mechanism is multifaceted, combining surface smoothing (reduction of stress concentration factor from Kt ≈ 1.5-2.0 to Kt ≈ 1.1-1.3), residual compressive stress introduction, and microstructural refinement. The combined effect typically results in fatigue life extensions of 2 to 5 times compared to as-welded conditions.

Engineering Application Considerations

For practical implementation in pressure vessel and piping fabrication, several considerations must be addressed:

1. Equipment requirements: The process requires either a specialized fixture that integrates TIG welding with quenching, or a manual procedure with precise timing control. Automated fixtures have been developed that position a water jet immediately behind the TIG torch.
2. Quality control: Post-treatment inspection should include hardness testing at the weld toe, residual stress measurement by X-ray diffraction or hole-drilling method, and surface roughness verification (Ra < 3.2 μm recommended).
3. Applicability: The technique is most effective for carbon and low-alloy steel welded joints where fatigue performance is critical, such as pressure vessel nozzles, pipe-to-plate connections, and structural welds in cyclic loading applications.
4. Limitations: For austenitic stainless steels, the quenching effect is less dramatic due to the absence of phase transformation, but grain refinement and surface smoothing still provide meaningful fatigue improvements. The technique is not suitable for heat-sensitive materials or thin sections where thermal distortion could be problematic.

Study Insights and Engineering Implications

This technique represents an elegant solution to the weld toe fatigue problem that requires minimal equipment modification compared to shot peening or TIG dressing alone. The combination of re-melting and quenching provides synergistic benefits that exceed the sum of individual treatments. For pressure vessel fabrication, particularly in applications involving cyclic pressure loading such as hydrogenation reactors, heat exchangers, and storage vessels, this post-weld treatment offers a practical pathway to extend service life without requiring major design modifications. The technique is particularly attractive for retrofit applications where existing welded joints require fatigue life extension without complete replacement.