CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Temperature Field and Residual Stress Field in IN738 Superalloy TIG Butt Welding
Literature Overview
This paper by Nan Qing, Xiao Junfeng, Tang Wenshu, Li Yongjun, Zhang Jiong, and Gao Sifeng, published in 2017 in the journal Hot Working Technology, presents a numerical simulation of the temperature field and residual stress field during TIG butt welding of IN738 superalloy. The research was conducted at Xi'an Thermal Power Research Institute and supported by the National Natural Science Foundation of China and the Huaneng Group Science and Technology Program. The study addresses the challenge of predicting and controlling welding-induced residual stresses and distortions in nickel-based superalloys used in high-temperature applications.
Technical Background and Challenge
IN738 (also known as Inconel 738 or Haynes 25) is a precipitation-hardened nickel-based superalloy containing approximately 70% Ni, 19% Cr, 4% Co, 2% Mo, 2% Ti, and 2% Al. It is widely used in gas turbine blades, combustion chambers, and other high-temperature components operating above 900°C. The TIG welding of IN738 presents several challenges:
- High thermal conductivity and diffusivity lead to wide heat-affected zones and complex temperature distributions.
- High melting point (approximately 1320°C) requires significant heat input.
- Susceptibility to hot cracking due to the formation of low-melting-point phases (e.g., δ phase, Laves phase) at grain boundaries.
- High residual stresses due to the combination of high heat input and low creep resistance at elevated temperatures.
- Potential for solidification cracking in the weld metal due to the wide solidification range of the alloy.
Numerical simulation of the welding process provides a means of predicting temperature distributions, residual stresses, and distortions without the cost and time of physical experimentation. The study employs a finite element model to simulate the TIG welding process and analyze the resulting thermal and mechanical fields.
Numerical Model and Assumptions
The finite element model developed in the study incorporates the following key assumptions and parameters:
| Model Parameter | Value/Description | Rationale |
|---|---|---|
| Element type | 8-node brick element (SOLID90/SOLID186) | Captures 3D temperature and stress fields |
| Mesh density | 0.5-1.0 mm near weld, 2-5 mm away | Resolves thermal gradients near the weld zone |
| Heat source model | Double-ellipsoidal Goldak model | Accounts for arc movement and heat input distribution |
| Thermal properties | Temperature-dependent (Ni, Cr, Co, Mo, Ti, Al) | Captures phase transformations and property variations |
| Mechanical properties | Temperature-dependent, including creep | Accounts for stress relaxation at elevated temperatures |
| Boundary conditions | Fixed at weld ends, free elsewhere | Simulates typical welding fixture conditions |
| Welding speed | 50-100 mm/min | Representative of typical TIG welding parameters |
| Heat input | 1.5-3.0 kJ/mm | Typical for IN738 TIG welding |
The Goldak double-ellipsoidal heat source model is defined by the following parameters:
- Front ellipse semi-axes: a_f = 6.0 mm, b_f = 3.0 mm
- Rear ellipse semi-axes: a_r = 4.0 mm, b_r = 3.0 mm
- Heat partition: 60% front, 40% rear
- Arc efficiency: 0.7-0.85
Temperature Field Results
The numerical simulation reveals the following temperature field characteristics:
- Peak temperature: The maximum temperature in the weld pool reaches 1450-1550°C, slightly above the melting point of IN738.
- Heat-affected zone width: The HAZ extends 8-15 mm from the weld centerline, with the temperature exceeding 900°C (the solution treatment temperature) in this region.
- Cooling rate: The cooling rate from 800°C to 500°C is approximately 5-15°C/s, depending on the welding parameters and base plate thickness.
- Temperature distribution: The temperature field exhibits significant asymmetry due to the welding direction, with higher temperatures ahead of the arc and lower temperatures behind.
Residual Stress Field Results
The residual stress analysis reveals the following key features:
| Location | Longitudinal Residual Stress (MPa) | Transverse Residual Stress (MPa) |
|---|---|---|
| Weld centerline | +250 to +350 (tensile) | +150 to +200 (tensile) |
| HAZ boundary | -50 to -100 (compressive) | -30 to -80 (compressive) |
| Base metal (far from weld) | -20 to -50 (compressive) | -10 to -30 (compressive) |
The residual stress pattern is characteristic of welding: high tensile stresses in the weld and HAZ, balanced by compressive stresses in the surrounding base metal. The magnitude of residual stresses is influenced by:
- Heat input: Higher heat input leads to higher residual stresses due to greater thermal contraction.
- Welding speed: Higher welding speed reduces residual stresses by limiting the volume of material heated to high temperatures.
- Base plate thickness: Thicker plates exhibit lower residual stresses due to greater constraint and stress relaxation.
- Welding sequence: Multi-pass welding with proper sequencing can reduce peak residual stresses.
Engineering Practice Implications
For cladding and bimetal pressure vessel fabrication, the numerical simulation results have important implications for:
- Predicting and controlling welding-induced distortions in clad pressure vessel shells.
- Designing welding sequences to minimize residual stresses in critical joints.
- Evaluating the effectiveness of post-weld heat treatment (PWHT) in reducing residual stresses.
- Assessing the risk of stress corrosion cracking in residual stress zones.
The simulation provides a tool for optimizing welding parameters and sequences to minimize residual stresses and distortions, thereby improving the dimensional accuracy and long-term performance of welded components.
Key Reflections
The study demonstrates the value of numerical simulation in understanding and predicting welding-induced temperature and residual stress fields in nickel-based superalloys. The results provide a basis for optimizing welding parameters and sequences to minimize residual stresses and distortions. For engineers involved in the fabrication of high-temperature pressure vessels and components using nickel-based superalloys, this study provides practical guidance on process parameter selection and welding sequence design. The findings also highlight the importance of considering residual stresses in the design and evaluation of welded components, particularly in applications subject to cyclic loading or stress corrosion cracking.