Numerical Simulation of Temperature and Residual Stress Fields in TIG Butt Welding of IN738 Superalloy

Literature Overview

The paper by Nan Qing and colleagues from Xi'an Thermal Power Research Institute, published in 2017 in the journal Hot Working Technology, presents a finite element analysis (FEA) study of the temperature field and residual stress field during tungsten inert gas (TIG) butt welding of IN738 nickel-based superalloy. The research was supported by the National Natural Science Foundation of China (Project No. 51501151) and the Huaneng Group headquarters science and technology project (HNKJ15-H05). The authors employed MSC.Marc software to simulate the welding thermal-mechanical process under varying welding current conditions, providing quantitative insight into the thermal cycles and residual stress distributions that govern weld integrity in this critical aerospace-grade material.

Core Technical Findings

The study systematically investigated how welding current influences the peak temperature, weld pool dimensions, and residual stress magnitudes. The key results can be summarized as follows:

The residual stress distribution along the weld centerline surface shows that both longitudinal and transverse residual stresses are tensile in nature. In the mid-section surface of the weldment, the longitudinal residual stress is tensile in the weld zone, reaches its maximum in the HAZ, and transitions to compressive stress in regions far from the weld. The transverse residual stress remains relatively small and tensile throughout.

Interpretation of Technical Points

IN738 is a precipitation-strengthened nickel-based superalloy containing approximately 20% Cr, 8% Mo, and 8% Co, designed for applications at temperatures up to 1093°C in turbine discs, blades, and hot-section structural components. The material's high strength at elevated temperatures is achieved through γ' (Ni3Al) and δ (Ni3Nb) precipitate phases, making it extremely sensitive to thermal cycling during welding.

The finding that peak longitudinal residual stress occurs in the HAZ rather than the weld metal is particularly significant. This is because the HAZ experiences the most severe thermal gradients during cooling, leading to differential plastic strain accumulation. In IN738, the HAZ is also the region where grain growth and precipitate coarsening are most pronounced, creating a zone of reduced ductility and elevated crack susceptibility. The tensile stress concentration in this weakened zone constitutes a serious metallurgical concern for long-term creep-fatigue performance.

The transition from tensile to compressive residual stress at distances far from the weld is a manifestation of elastic equilibrium requirements. The total residual stress field must self-equilibrate, meaning that the integrated longitudinal stress over any cross-section perpendicular to the weld axis must sum to zero. The compressive stresses at the edges serve as a balancing reaction to the tensile stresses near the weld and HAZ.

Process and Standards Analysis

For TIG welding of IN738, typical process parameters include welding currents in the range of 150–300 A for plate thicknesses of 6–12 mm, with argon shielding gas flow rates of 15–25 L/min. The heat input for such welds typically ranges from 1.5 to 4.0 kJ/mm. According to ASTM A396, IN738 forgings and castings are specified for turbine components, and welding procedures must comply with ASME Section IX, Qualification of Welding, Brazing, and Filler Metal Procedures, particularly regarding preheating and interpass temperature control.

The simulation approach using MSC.Marc involves coupled thermo-mechanical analysis, where the temperature field is solved first using the heat equation with moving heat source models (typically Goldak's double-ellipsoidal model or a conical model), and the residual stress field is subsequently calculated using an elastic-plastic constitutive law that accounts for thermal expansion, plastic deformation, and phase transformation effects. The material properties, including temperature-dependent yield strength, thermal conductivity, and specific heat, must be accurately defined to ensure simulation fidelity.

Integration with Engineering Practice

In practical welding of IN738 components, such as turbine disc repair or hot-section structural fabrication, the residual stress field predicted by this study has direct implications for post-weld treatment strategies. Stress relief annealing at 870°C for 4 hours is commonly specified to reduce residual stresses to acceptable levels, but the distribution pattern revealed by the simulation helps optimize the annealing parameters. The maximum longitudinal stress in the HAZ suggests that this region requires particular attention during non-destructive examination, especially when using ultrasonic testing (UT) or radiographic testing (RT) to detect potential cracks.

From a quality control perspective, the welding procedure specification (WPS) for IN738 must carefully control the welding current to balance weld pool adequacy against excessive thermal input. Excessive current not only increases residual stresses but also promotes grain coarsening in the HAZ, reducing creep life. Conversely, insufficient current may result in incomplete fusion and lack of penetration defects. The FEA results provide a quantitative basis for establishing acceptable current ranges within the WPS.

Key Questions and Reflections

Several important questions arise from this study. First, the simulation assumes a single-pass butt weld, but in practical applications, multi-pass welding is often required for thicker sections. The interaction between passes and the resulting residual stress superposition is not captured in this study. Second, the constitutive model used may not fully account for the complex precipitation kinetics of IN738 during the rapid thermal cycles of welding, which can lead to discrepancies between predicted and measured residual stresses. Third, the study does not address the effects of welding sequence on multi-bead welds, which is a critical consideration in production welding of large IN738 components.

The finding that transverse residual stresses are relatively small compared to longitudinal stresses is consistent with general welding residual stress theory, but it raises questions about the dominant crack initiation mode. In creep-fatigue applications, the interaction between longitudinal tensile stresses and cyclic loading perpendicular to the weld axis may be more detrimental than transverse stresses alone.

Study Insights and Implications

This study provides valuable quantitative data for welding procedure development and post-weld quality assurance of IN738 components. The FEA methodology demonstrated here can be extended to more complex geometries and multi-pass welding sequences, offering a powerful tool for predicting residual stress distributions before physical welding trials. For engineers involved in superalloy welding, the key takeaway is that the HAZ, not the weld metal, represents the most critical region for residual stress management, and welding current must be carefully controlled to minimize the magnitude of peak stresses while ensuring adequate weld pool geometry. The integration of numerical simulation with experimental validation is essential for reliable prediction of residual stress fields in critical aerospace applications.