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

Literature Overview

The paper by Feng Zhong-Yuan and colleagues from Tianjin University, published in Acta Metallurgica Sinica (English Letters) in 2018, presents a systematic investigation into the microstructural evolution and residual stress (RS) behavior of low transformation temperature (LTT) weld metal deposited by two fundamentally different arc welding processes: tungsten inert gas (TIG) and metal active gas (MAG) welding. The work is supported by the National Natural Science Foundation of China (Grant No. 51774213), which speaks to its significance within the Chinese materials science research community. The study employs electron backscattered diffraction (EBSD) and orientation imaging microscopy (OIM) to characterize the weld microstructure, combined with finite element method (FEM) simulations to quantify tensile residual stress reduction. For engineers working on high-integrity welded structures such as pipeline joints, pressure vessels, and offshore platforms, the findings carry direct practical implications for process selection and filler metal design.

Core Technical Findings

Microstructural Characterization

The authors devised a Cr-Ni type LTT welding filler specifically designed to lower the martensite start (Ms) temperature, thereby reducing the thermal strain during the cooling phase and subsequently mitigating tensile residual stress. The EBSD/OIM analysis revealed a critical difference in retained austenite (RA) content between the two welding processes. The TIG weld metal exhibited approximately 17.5% retained austenite, whereas the MAG weld metal contained only 8.0% retained austenite. This disparity is fundamentally attributable to the different heat input characteristics of the two processes. TIG welding, being a non-consumable electrode process, delivers a more concentrated and lower total heat input, resulting in slower cooling rates that favor the stabilization of retained austenite at the lower transformation temperatures inherent to LTT fillers. In contrast, MAG welding, with its consumable wire electrode and typically higher deposition rates, produces a different thermal cycle that promotes more complete martensitic transformation.

Grain Boundary Analysis and Taylor Factor

One of the most insightful aspects of this study is the application of Taylor factor calculations to assess the mechanical performance of the weld metal. The Taylor factor relates the macroscopic stress to the resolved shear stress on slip systems in crystalline materials. The analysis demonstrated that the MAG weld metal was more competent in repressing fatigue crack initiation compared to the TIG weld metal. This finding is counterintuitive at first glance, as one might expect the higher retained austenite content in the TIG weld to provide superior toughness and crack resistance through the transformation-induced plasticity (TRIP) effect. However, the grain boundary characterization revealed that the MAG weld metal exhibited a higher proportion of high-angle and coincidence site lattice (CSL) grain boundaries. These special grain boundaries, particularly those with high misorientation angles and low-energy CSL relationships, act as effective barriers to dislocation motion and crack propagation. The finer grain structure in the MAG weld metal, combined with the favorable grain boundary character distribution, resulted in superior fatigue resistance despite the lower retained austenite content.

Residual Stress Analysis

The finite element simulation, based on heat input and phase transformation parameters, confirmed that both conventional and LTT welding fillers reduced tensile residual stress compared to the parent material, but the LTT MAG weld metal achieved the most significant tensile RS reduction. The mechanism is well established: LTT fillers with low Ms temperatures undergo martensitic transformation at relatively low temperatures, generating a volumetric expansion (dilatation) of approximately 4% during the transformation. This expansion counteracts the tensile stress generated by the thermal contraction during cooling. The MAG process, with its higher heat input and more complete transformation, produces a more uniform and substantial dilatational strain, leading to a more effective reduction of peak tensile residual stress.

Engineering Practice Implications

Process Selection for LTT Applications

The study provides clear guidance for engineers selecting welding processes for LTT filler applications. When the primary objective is residual stress reduction, MAG welding with LTT filler emerges as the preferred choice. The combination of higher heat input, more complete martensitic transformation, finer grain structure, and favorable grain boundary character distribution collectively contribute to superior residual stress mitigation and fatigue resistance. However, engineers must consider trade-offs. MAG welding typically produces a wider weld bead with greater dilution, which may compromise the LTT characteristics if the dilution is excessive. The TIG process, with its lower dilution rate, may better preserve the intended LTT composition of the filler metal.

Application to Pipeline and Pressure Vessel Welding

In the context of pipeline welding, particularly for subsea pipelines and high-pressure gas transmission systems, residual stress is a critical factor in hydrogen-induced cracking (HIC), stress corrosion cracking (SCC), and fatigue failure. The LTT concept has been widely adopted in the oil and gas industry for welding high-strength steels (HSS) such as X70, X80, and X100 grades. The findings of this study suggest that for applications where fatigue resistance is paramount, such as offshore jacket structures or pipeline joints subjected to cyclic loading, MAG welding with LTT filler may offer a more robust solution. However, for thin-walled components where heat input must be strictly controlled to avoid distortion and microstructural degradation, TIG welding with LTT filler remains a viable option, albeit with somewhat less effective residual stress reduction.

Key Questions and Reflections

A question that arises from this study is the long-term stability of retained austenite in the TIG weld metal. While the higher RA content initially provides beneficial TRIP effects, prolonged service at elevated temperatures or under sustained mechanical loading may lead to the decomposition of retained austenite into martensite, potentially altering the mechanical properties and residual stress state over time. This aspect was not addressed in the study and warrants further investigation, particularly for applications involving thermal cycling or long-term mechanical loading.

Another point of reflection concerns the grain boundary engineering aspect. The dominance of high-angle and CSL grain boundaries in the MAG weld metal suggests that the higher heat input and different solidification morphology associated with MAG welding promote a more favorable grain boundary character distribution. This raises the possibility that grain boundary engineering techniques, such as post-weld heat treatment or controlled cooling rates, could be employed to optimize the grain boundary character distribution in TIG welds, potentially bridging the performance gap between the two processes.

The study also implicitly highlights the importance of the interplay between heat input, cooling rate, and phase transformation in determining the final weld properties. For practitioners, this reinforces the need for careful welding procedure specification (WPS) development, where heat input limits, interpass temperatures, and cooling rate controls are not merely compliance parameters but critical variables that directly influence the microstructural and mechanical performance of the weld.

Study Insights and Reference Value

This paper serves as an excellent example of how microstructural characterization techniques such as EBSD/OIM, combined with computational tools like Taylor factor analysis and FEM simulation, can provide a comprehensive understanding of welding process-performance relationships. The multi-scale approach, linking atomic-level grain boundary character to macroscopic mechanical properties and residual stress, is particularly instructive for engineers seeking to move beyond empirical process selection toward rational, physics-based optimization. For those engaged in the welding of high-strength pipelines, offshore structures, or nuclear components where residual stress control is critical, this study provides both theoretical grounding and practical guidance for selecting between TIG and MAG welding processes when using LTT fillers. The work underscores that process selection is not merely a matter of productivity or cost but a fundamental decision that shapes the microstructure, residual stress state, and ultimately the service life and integrity of the welded joint.