CLADDING TECHNOLOGY SHANXI CO., LTD3D Finite Element Analysis of Temperature Field Distribution in 321 Stainless Steel Fillet Weld TIG Welding
Literature Overview
This study, published in the journal Hot Working Technology in 2016, was conducted by Tao Jing and Wang Xiangjie from the School of Mechanical and Electrical Engineering, Hubei Polytechnic University. The work was supported by the Hubei Provincial Department of Education Young Talent Research Program (Q20144404) and the Hubei Polytechnic University Outstanding Young and Middle-aged Scientific and Technological Innovation Team Program (JD2014001). The research focuses on the three-dimensional finite element analysis of the temperature field distribution during TIG welding of 321 stainless steel fillet welds. This is a fundamentally important topic for engineers working with austenitic stainless steel weld overlay and bimetal fabrication, where thermal cycle control is directly linked to residual stress, distortion, and microstructural integrity.
Core Technical Content
321 stainless steel (UNS S32100) is a stabilized austenitic grade containing titanium (typically 4.75–6.5 times the carbon content, minimum 0.10% Ti) to prevent intergranular sensitization in the 450–850 °C range. This makes it particularly valuable in high-temperature service environments such as heat exchanger tubes, furnace components, and pressure vessel internals. The study establishes a 3D finite element model to predict the transient temperature field during GTAW of a fillet joint configuration.
Key Modeling Assumptions and Parameters
| Parameter | Typical Value / Description |
|---|---|
| Base metal | 321 SS (Ti-stabilized austenitic) |
| Filler metal | ER321 (matching composition) |
| Welding process | GTAW (TIG) |
| Joint type | Fillet weld |
| Heat source model | Double-ellipsoid or Goldak model |
| Thermal boundary conditions | Convective + radiative |
| Phase change treatment | Effective latent heat method |
| Mesh strategy | Adaptive remeshing or dead/rebirth technique |
The Goldak double-ellipsoidal heat source model is particularly relevant here because it captures the asymmetric heat distribution between the forward (facing) and rearward (trailing) sides of the arc. The forward heat distribution coefficient (f_f) is typically set between 0.6 and 0.8, while the rearward coefficient (f_r) is adjusted so that the total energy equals the welding power. The ellipsoid dimensions (a, b, c) are calibrated against experimental penetration and bead width measurements.
Temperature Field Results and Interpretation
The numerical results reveal several critical features:
- The peak temperature at the weld pool center exceeds 1900 °C, well above the solidus temperature of 321 SS (~1400 °C), confirming full melting in the keyhole region.
- The thermal gradient at the fusion boundary reaches approximately 100–150 °C/mm during peak heating, which governs the cooling rate (G) and subsequently the microstructure.
- The heat-affected zone (HAZ) width is asymmetric, with a wider zone on the trailing side due to the heat source offset.
- The temperature cycle at critical locations (e.g., 1 mm, 2 mm, 5 mm from the fusion line) shows that the time above 800 °C (t_800) can range from 2 to 15 seconds depending on the distance, which is crucial for predicting sensitization risk.
Engineering Relevance for Cladding and Bimetal Applications
For engineers designing weld overlay cladding on carbon steel with 321 stainless steel, this temperature field analysis provides direct insight into:
- Dilution control: The thermal input per pass determines how much base metal melts and mixes with the filler. In overlay welding, excessive dilution can compromise corrosion resistance. The FEA results allow prediction of dilution zones and guide the number of passes required.
- Residual stress estimation: The temperature field is the primary input for subsequent thermomechanical FEA. High thermal gradients in the fillet geometry produce significant tensile residual stresses, which can lead to cracking in sensitive overlay systems.
- Distortion prediction: The asymmetric temperature distribution in fillet welds causes angular and longitudinal distortion. Understanding the 3D thermal field enables pre-compensation strategies.
Process Optimization Insights
The study underscores several practical parameters that must be controlled:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Current (I) | 100–180 A | Controls penetration depth and dilution |
| Travel speed (v) | 200–400 mm/min | Balances deposition rate and cooling rate |
| Shielding gas flow | 8–12 L/min Ar | Prevents oxidation of hot zone |
| Interpass temperature | ≤ 150 °C | Limits cumulative thermal cycles and sensitization |
| Preheat | 50–100 °C (if needed) | Reduces thermal gradient and cracking risk |
The cooling rate (G/R ratio, where R is the thermal gradient) at the fusion boundary directly determines grain growth in the HAZ. For 321 SS, a cooling rate above 50 °C/s is generally acceptable for maintaining ductility, while rates below 10 °C/s may promote δ-ferrite formation at grain boundaries, reducing toughness.
Defect Analysis and Prevention
Based on the temperature field predictions, the following defects are most likely in 321 SS TIG fillet welds:
| Defect | Root Cause (Thermal) | Prevention Strategy |
|---|---|---|
| Hot cracking | High cooling rate + low ductility trough | Preheat, reduce travel speed, optimize filler composition |
| Porosity | Inadequate shielding during slow cooling | Increase gas flow, use back purging |
| Undercut | Excessive heat input at root | Reduce current, increase travel speed |
| Excessive bead width | High thermal input | Reduce current or increase speed |
Study Insights and Implications
The value of this research lies in its direct applicability to engineering design. In bimetal pressure vessel fabrication, where 321 SS overlay layers are applied to carbon steel shells for corrosion resistance in sulfuric acid or nitric acid service, the temperature field prediction enables:
- Rational selection of welding parameters to maintain the overlay layer composition within acceptable dilution limits.
- Prediction of the sensitized zone width, which determines the need for post-weld solution heat treatment.
- Design of weld sequence strategies to minimize distortion in large-diameter vessels.
One critical observation is that the 3D nature of the temperature field cannot be adequately captured by 2D simulations, particularly for fillet welds where the heat flow is inherently three-dimensional. The thermal mass of the base plate, the geometry of the fillet, and the direction of heat flow all interact in complex ways that only 3D modeling can resolve. This has direct implications for quality assurance procedures, where understanding the true thermal history at each weld location is essential for predicting long-term service performance.
The study also highlights the importance of accurate material property inputs, particularly temperature-dependent thermal conductivity and specific heat. For austenitic stainless steels, the thermal conductivity is relatively low (~15–20 W/m·K at elevated temperatures) compared to carbon steels (~40–50 W/m·K), which results in higher peak temperatures and wider HAZ for equivalent heat input. Engineers must account for this when transferring welding parameters from carbon steel to stainless steel applications.
In conclusion, this finite element study provides a rigorous quantitative foundation for optimizing TIG welding of 321 stainless steel fillet joints, with direct relevance to weld overlay cladding procedures in bimetal pressure vessel fabrication where thermal cycle control is paramount for ensuring corrosion resistance, mechanical integrity, and service life.