CLADDING TECHNOLOGY SHANXI CO., LTDThree-Dimensional Dynamic Simulation of Temperature Field in Submerged Arc Flat Plate Overlay Welding
Literature Overview
This 2009 study published in Welding Technology by Shi Baoshan, He Kuanfang, and He Hezhi from Beijing Institute of Technology Zhuhai College and South China University of Technology presents a three-dimensional dynamic finite element simulation of the temperature field during submerged arc welding (SAW) overlay on flat plate substrates. Supported by the National Natural Science Foundation of China (projects 10472034 and 10590351), this work represents an important advancement in the computational modeling of overlay welding processes, providing engineers with a powerful tool for predicting thermal cycles, residual stresses, and microstructural evolution in overlay welds.
Core Technical Points
Finite Element Model Development
The 3D dynamic thermal model was developed using the moving heat source approach, where the welding arc is represented as a Gaussian heat source that moves along the weld path at the welding speed. The key model parameters include:
| Parameter | Value / Description |
|---|---|
| Heat source type | Double-ellipsoidal Gaussian |
| Heat source efficiency | 0.7 – 0.85 |
| Thermal conductivity | Temperature-dependent, k(T) |
| Specific heat | Temperature-dependent, c(T) |
| Density | Temperature-dependent, ρ(T) |
| Convection coefficient | 10 – 25 W/(m²·K) |
| Radiation coefficient | ε = 0.7 – 0.9 |
| Mesh size | 1 – 3 mm near weld, 5 – 10 mm far field |
The temperature-dependent material properties are critical for accurate simulation, as the thermal conductivity and specific heat of both the substrate and the deposited material change significantly with temperature. The model must account for the phase change from solid to liquid and the latent heat of fusion, which is typically handled using an effective specific heat method.
Thermal Cycle Prediction
The simulation provides detailed predictions of the thermal cycles at various locations within the overlay layer and the substrate. Key thermal cycle parameters include:
| Parameter | Definition | Typical Range |
|---|---|---|
| Peak temperature (Tp) | Maximum temperature reached | 1200 – 1600°C |
| Dwell time (t800) | Time above 800°C | 5 – 50 s |
| Cooling rate (t8/5) | Time from 800°C to 500°C | 2 – 20 s |
| Cooling rate (t8/3) | Time from 800°C to 300°C | 10 – 100 s |
| Number of thermal cycles | For multi-pass overlay | 1 – 10 |
These thermal cycle parameters are directly related to the microstructure and mechanical properties of the overlay layer. For example, a fast cooling rate (short t8/5) promotes the formation of fine grain structures and can increase hardness, while a slow cooling rate allows for grain growth and potentially the formation of undesirable phases.
Multi-Pass Thermal Accumulation
One of the most significant contributions of this study is the analysis of thermal accumulation during multi-pass overlay welding. As successive passes are deposited, the base metal temperature increases, leading to:
- Reduced cooling rates in subsequent passes, which can promote grain growth and sensitization
- Increased residual stresses due to thermal expansion mismatch between hot and cold regions
- Altered microstructural evolution as the thermal history of each pass is modified by the heat from previous passes
The simulation results show that for overlay welds with 5 or more passes, the thermal accumulation can increase the peak temperature of the first pass by 100 – 200°C and reduce the cooling rate by 30 – 50%. This has important implications for the design of multi-pass overlay sequences.
Simulation Results and Validation
Temperature Distribution
The 3D simulation reveals a complex three-dimensional temperature distribution that cannot be captured by 1D or 2D models. The temperature field extends significantly into the substrate, with regions exceeding 500°C at depths of 10 – 20 mm below the surface. This deep thermal influence zone is critical for predicting distortion and residual stresses in thick overlay welds.
Comparison with Experimental Data
The simulation results were validated against thermocouple measurements taken during actual SAW overlay welding. The comparison showed good agreement between predicted and measured temperatures, with deviations typically within ±50°C. This level of accuracy is sufficient for predicting thermal cycles and their effects on microstructure and properties.
Engineering Applications
Process Optimization
The thermal simulation model can be used to optimize welding parameters for specific applications:
- Minimizing dilution: By adjusting the heat input and travel speed, the dilution of the overlay layer with base metal can be controlled. The simulation can predict the dilution ratio as a function of welding parameters.
- Controlling cooling rates: For applications requiring specific microstructures (e.g., fine-grained for high toughness), the simulation can identify parameter combinations that achieve the desired cooling rates.
- Predicting residual stresses: The thermal model can be coupled with a mechanical model to predict residual stresses, which is essential for assessing the risk of cracking and distortion.
- Optimizing multi-pass sequences: The simulation can evaluate different pass sequences and interpass temperatures to minimize thermal accumulation and ensure uniform properties throughout the overlay.
Design of Bimetal Pressure Vessels
For the fabrication of bimetal pressure vessels, the thermal simulation provides valuable information for:
- Determining the number of overlay passes required for a given cladding thickness
- Selecting appropriate interpass temperatures to avoid excessive heat input
- Predicting the heat-affected zone (HAZ) in the base metal and assessing its susceptibility to cracking
- Evaluating the effect of overlay welding on the mechanical properties of the base metal
Key Questions and Reflections
One limitation of the current simulation approach is the assumption of temperature-dependent but isotropic material properties. In reality, the deposited material may exhibit anisotropic properties due to the columnar grain structure typical of weld deposits. This anisotropy can affect heat flow and, consequently, the predicted temperature field. Future work should incorporate anisotropic thermal conductivity to improve the accuracy of the simulation.
Another important consideration is the effect of slag on the heat transfer. The slag layer in SAW acts as an insulating medium, reducing heat loss from the top surface of the weld. The simulation should include a model for the slag layer to accurately capture the heat flow conditions. Current models often simplify this by applying a reduced convection coefficient, but a more detailed treatment of the slag would improve the accuracy of the predictions.
Study Insights
This research demonstrates the power of 3D dynamic thermal simulation as a tool for understanding and optimizing the SAW overlay welding process. The detailed temperature field predictions provide insights into the thermal history of the overlay layer and the substrate, which are essential for predicting microstructure, mechanical properties, and residual stresses. For engineers involved in the design and fabrication of bimetal pressure vessels, this type of simulation can significantly reduce the need for expensive trial-and-error process development and provide a rational basis for selecting welding parameters. The work also highlights the importance of considering thermal accumulation in multi-pass overlay welds, a factor that is often overlooked in practice but can have a significant impact on the final properties of the overlay layer.