Microstructure and Residual Stress Comparison Between TIG and MAG Welding Using Low Transformation Temperature Filler

Literature Overview

The study by Feng Zhong-Yuan and colleagues from Tianjin University, published in Acta Metallurgica Sinica (English Letters) in 2018, investigates the comparative microstructural evolution and residual stress distribution when applying low transformation temperature (LTT) welding fillers using both Gas Tungsten Arc Welding (GTAW/TIG) and Gas Metal Arc Welding (GMAW/MAG) processes. The research is funded under the National Natural Science Foundation of China (Grant No. 51774213) and represents significant progress in understanding how welding process selection influences the performance of LTT overlay deposits, which are critical for residual stress mitigation in high-integrity pressure vessels and cladding applications.

Core Technical Content and Key Findings

Low Transformation Temperature Filler Concept

LTT fillers are designed with a martensite start (Ms) temperature significantly lower than conventional austenitic or ferritic fillers, typically in the range of 100 to 250 degrees Celsius. The underlying principle is that during cooling after welding, the LTT filler undergoes a delayed martensitic transformation accompanied by volume expansion, which generates compressive residual stresses that counteract the tensile residual stresses inherently produced by the welding thermal cycle. This self-relaxation mechanism is particularly valuable in cladding applications where the overlay layer must maintain metallurgical bond integrity while simultaneously managing residual stress in the base metal.

Process Comparison: TIG versus MAG

The study demonstrates that TIG welding produces a narrower weld bead with higher dilution control, allowing the LTT composition to remain more intact in the weld metal. The faster cooling rates associated with TIG welding accelerate the onset of the low-temperature martensitic transformation, resulting in greater compressive stress development. MAG welding, while offering higher deposition rates, tends to produce higher dilution that can shift the actual Ms temperature of the weld metal, potentially diminishing the LTT effect.

Residual Stress Analysis

The residual stress profiles measured by the authors reveal that both processes achieve net compressive residual stresses at the weld surface when LTT fillers are employed. However, the magnitude of compressive stress achieved through TIG welding is approximately 20 to 40 percent higher than that achieved through MAG welding under comparable heat input conditions. This difference is attributed to the superior thermal confinement of the TIG process, which maintains a higher cooling rate gradient through the weld cross-section, promoting more complete martensitic transformation within the LTT temperature window.

Microstructural Characterization

Weld Metal Microstructure

Metallographic examination reveals distinct microstructural differences between the two processes. TIG welds using LTT fillers exhibit predominantly acicular martensite with fine carbide precipitation, consistent with rapid cooling through the transformation range. The grain size in the weld metal is finer, typically 2 to 5 micrometers, which contributes to improved toughness. MAG welds show a coarser microstructure with mixed martensite and retained austenite, where the retained austenite fraction can reach 15 to 25 percent depending on the specific filler chemistry and heat input parameters.

Heat Affected Zone (HAZ) Considerations

The HAZ microstructure is influenced by the peak temperature and cooling rate imposed by each process. TIG welding produces a narrower HAZ with peak temperatures concentrated in a smaller volume, resulting in more localized microstructural changes. MAG welding produces a broader HAZ with more uniform but less intense thermal cycling. For cladding applications on carbon steel or low-alloy steel base metals, the HAZ hardening potential must be carefully evaluated, particularly for base metals with carbon equivalents exceeding 0.40 percent.

Bond Interface Analysis

The metallurgical bond between the overlay layer and the base metal is critical for cladding applications. The study confirms that both processes achieve full metallurgical bonding, but TIG welding produces a slightly more refined transition zone at the bond interface. The reduced heat input of TIG welding minimizes the risk of excessive grain coarsening in the base metal near the bond line, which is essential for maintaining the mechanical integrity of the composite structure.

Engineering Practice Implications

Application to Cladding and Overlay

For bimetal pressure vessel fabrication where residual stress control is paramount, this study provides clear guidance on process selection. When applying LTT fillers for stress-relieving overlays on thick-section pressure vessel components, TIG welding offers superior performance in terms of compressive stress magnitude and microstructural refinement. However, the lower deposition rate of TIG welding (typically 1.5 to 3.0 kg/h for single-pass overlay) must be balanced against the productivity advantages of MAG welding (typically 5.0 to 10.0 kg/h).

Process Selection Matrix for Cladding Applications

Quality Control Considerations

For pressure vessel applications governed by ASME Section VIII Division 1 or GB/T 150, the residual stress state of overlay welds must be documented through appropriate non-destructive testing methods. The compressive stresses achieved through LTT fillers can be verified using X-ray diffraction or neutron diffraction techniques. The study emphasizes that the LTT effect is most effective when the weld metal composition maintains an Ms temperature below 200 degrees Celsius and the cooling rate exceeds 20 degrees Celsius per second through the 600 to 400 degree Celsius range.

Key Questions and Reflections

The research raises important questions regarding the scalability of LTT filler applications from laboratory-scale TIG welding to production-scale MAG welding in pressure vessel fabrication. While the fundamental metallurgical mechanism remains the same, the practical implementation requires careful control of wire feed parameters, shielding gas composition, and travel speed to maintain the desired LTT properties in the deposited metal. The dilution effect in MAG welding can shift the Ms temperature by 50 to 100 degrees Celsius, potentially rendering the LTT concept ineffective if not properly compensated through filler chemistry adjustment.

Another critical consideration is the interaction between LTT-induced compressive stresses and the subsequent post-weld heat treatment (PWHT) required for many pressure vessel applications. The compressive stresses generated by the LTT transformation may be partially or fully relaxed during PWHT, depending on the temperature and duration of the heat treatment cycle. This interaction warrants further investigation for practical implementation in pressure vessel fabrication sequences.

Study Insights and Implications

This study represents a significant contribution to the understanding of how welding process selection interacts with filler metal metallurgy to influence residual stress outcomes in overlay applications. For engineers involved in bimetal pressure vessel fabrication, the key takeaway is that process selection is not merely a productivity decision but a fundamental metallurgical one that directly affects the residual stress state and long-term structural integrity of the component. The LTT concept, when properly implemented through TIG welding with appropriate filler chemistry, offers a practical pathway to achieving beneficial compressive residual stresses without requiring additional stress-relief operations. Future work should focus on multi-layer LTT overlay strategies that combine the productivity of MAG welding in subsequent passes with the precision of TIG welding for the critical bond layer, thereby optimizing both performance and cost for industrial cladding applications.