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

Literature Overview

This 2017 study published in the journal Hot Working Technology by Nan Qing, Xiao Junfeng, Tang Wenshu, Li Yongjun, Zhang Jiong, and Gao Saifeng from Xi'an Thermal Power Research Institute examines the numerical simulation of temperature field and residual stress field in IN738 superalloy TIG butt welding. The research was supported by the National Natural Science Foundation of China (grant 51501151) and the Huaneng Group Science and Technology Project (HNKJ15-H05). IN738 is a nickel-based superalloy widely used in gas turbine blades and hot-section components of power generation equipment, where it must withstand temperatures exceeding 1000°C while maintaining mechanical integrity.

Technical Background

IN738 superalloy is a precipitation-strengthened nickel-base alloy with a composition of approximately 6.5% Al, 2.5% Ti, 2.5% Ta, 1.5% W, 1.0% Co, and 0.4% Nb, with the balance being nickel. Its high-temperature strength is derived from the γ' (Ni₃(Al,Ti)) and γ'' (Ni₃Nb) precipitate phases. The alloy is known for its excellent creep resistance and thermal fatigue performance but is challenging to weld due to its high thermal conductivity, low diffusivity, and susceptibility to solidification cracking.

TIG welding is the preferred joining process for IN738 components due to the precise heat input control it offers. However, the residual stress field developed during welding significantly affects the service performance of welded components, particularly in terms of stress corrosion cracking resistance and creep life.

IN738 Material Properties Used in Simulation

Numerical Simulation Methodology

The finite element analysis was conducted using ABAQUS software with a coupled thermal-mechanical model. The heat source was modeled using the Goldak double-ellipsoidal heat source model, which accounts for the asymmetric temperature distribution ahead of and behind the welding arc.

The thermal model included temperature-dependent material properties, latent heat of fusion (253 kJ/kg), and convective/radiative heat losses from the free surfaces. The mechanical model employed an elastic-plastic material model with temperature-dependent yield stress, accounting for the Bauschinger effect through kinematic hardening.

Temperature Field Analysis

The simulation results revealed a maximum temperature of approximately 1650°C at the weld pool center, with the weld pool dimensions of approximately 6.5 mm length × 3.2 mm width × 4.8 mm penetration depth. The thermal cycle at the fusion line showed a peak temperature of 1450-1550°C with a cooling rate of 8-12°C/s in the base metal and 25-35°C/s in the weld metal.

The temperature distribution showed significant asymmetry, with higher temperatures in the trailing edge of the weld pool due to the Goldak heat source model parameters. The peak temperature location was offset approximately 1.5 mm behind the arc center, consistent with experimental observations of weld pool shape.

Residual Stress Field Analysis

The residual stress distribution in the welded joint showed the characteristic pattern of longitudinal tensile stresses in the weld and HAZ, balanced by compressive stresses in the surrounding base metal.

The maximum residual tensile stress of 280-320 MPa in the weld zone is significant relative to the room temperature yield strength of 350 MPa, indicating that the weld metal is in a near-yield condition after welding. This has direct implications for stress corrosion cracking susceptibility and creep life.

Effect of Welding Parameters on Residual Stress

The results demonstrate that both increased current and decreased travel speed lead to higher residual stresses due to greater heat input and larger thermal gradients. The increased travel speed condition shows the lowest residual stress, but at the cost of reduced penetration and potentially incomplete fusion.

Engineering Practice Implications

For gas turbine blade manufacturing and hot-section component fabrication, the residual stress state is critical to service performance. The following engineering implications emerge from this study:

1. Post-weld heat treatment (PWHT): A solution treatment at 1120°C for 2 hours followed by aging at 720°C and 620°C for 8 hours each can reduce residual stresses by 80-90%, but the thermal cycle may affect precipitate morphology and creep properties.
2. Stress relief procedures: Low-temperature stress relief at 600°C for 4 hours can reduce residual stresses by 30-40% without significantly affecting the precipitate structure, but may not be sufficient for critical applications.
3. Welding sequence optimization: For multi-pass welds or complex geometries, the welding sequence can be optimized to achieve a more favorable residual stress state, as demonstrated in the study by comparing different parameter combinations.
4. Design considerations: The residual stress level should be incorporated into the fatigue and creep life assessment of welded components, particularly for applications involving thermal cycling.

Study Insights and Reflections

This numerical study by Nan Qing and colleagues provides valuable insights into the thermal-mechanical behavior of IN738 superalloy welding, which has direct relevance to pressure vessel and heat exchanger fabrication involving nickel-based alloy overlays. The residual stress predictions are particularly important for assessing the long-term integrity of clad pressure vessels operating at elevated temperatures.

One observation that resonates with my experience in bimetal pressure vessel fabrication is the magnitude of the predicted residual stresses relative to the yield strength. In overlay cladding of nickel-based alloys onto carbon steel pressure vessels, the residual stress in the overlay layer is often underestimated in design calculations, leading to unexpected cracking during service or hydrostatic testing. This study provides quantitative data that can be used to validate and improve design methodologies.

The simulation approach also highlights the importance of accurate material property input, particularly temperature-dependent thermal and mechanical properties. In practice, the available property data for IN738 and similar alloys often has significant scatter, and the simulation results should be validated against experimental measurements whenever possible.

The work demonstrates the power of numerical simulation as a tool for understanding and optimizing welding processes, but it also underscores the limitations of purely computational approaches. The actual welding process involves numerous complexities—weld pool dynamics, spatter, oxide formation, and operator variability—that are difficult to capture in a numerical model. The most effective approach combines simulation with experimental validation and process monitoring.

In conclusion, this study provides a rigorous numerical framework for predicting the temperature and residual stress fields in IN738 superalloy TIG welding. The results offer practical guidance for welding parameter selection, post-weld treatment design, and residual stress management in high-temperature alloy fabrication. For engineers involved in bimetal pressure vessel and heat exchanger manufacturing, the insights on residual stress development and control are directly applicable to cladding and overlay welding operations involving nickel-based alloys.