CLADDING TECHNOLOGY SHANXI CO., LTDFinite Element Simulation of Temperature Field in Strip Electrode Submerged Arc Welding of Thick Plates
Literature Overview
This paper, published in the Journal of Welding (焊接学报) in 2009, was authored by Wang Zhifeng, Chen Peiyin, Wu Wei, Chen Yan from the Harbin Welding Research Institute of the Chinese Academy of Mechanical Sciences, together with Chen Jianmin and Bao Hong from China First Heavy Industries Group Co., Ltd. The work addresses the critical challenge of predicting thermal behavior during strip electrode submerged arc welding (SESAW) applied to thick plate cladding operations, a process widely used in the fabrication of large-diameter pressure vessels, hydrogenation reactor shells, and heavy equipment components.
The significance of this research lies in the fact that thick plate cladding—typically involving base plate thicknesses exceeding 50 mm and requiring multiple layers of overlay—presents unique thermal management challenges. Incongruities between the heat input of strip electrode processes (which can exceed 30 kJ/mm) and the thermal mass of thick substrates can lead to excessive residual stresses, distortion, and potential cracking in the cladding layers. Finite element simulation provides a quantitative tool to optimize preheating strategies, interpass temperature control, and multi-pass sequencing before committing to expensive full-scale trials.
Core Technical Approach
The authors employed a three-dimensional finite element model to simulate the transient temperature field during strip electrode submerged arc welding of thick plates. The methodology involves coupling a moving heat source with thermophysical properties that vary with temperature, accounting for the complex thermal gradients inherent in multi-pass cladding on thick substrates.
Key Modeling Parameters
| Parameter | Typical Range | Description |
|---|---|---|
| Base plate thickness | 50–150 mm | Thick plate cladding applications |
| Strip electrode width | 40–80 mm | Standard strip electrode dimensions |
| Current density | 150–250 A/mm | Strip electrode operating range |
| Welding speed | 150–300 mm/min | Linear travel speed |
| Heat input | 25–45 kJ/mm | Total process heat input |
| Preheat temperature | 150–350 °C | Depends on material and thickness |
| Interpass temperature | 150–300 °C | Maximum allowed between passes |
| Thermal conductivity | 25–50 W/(m·K) | Temperature-dependent |
| Specific heat | 450–800 J/(kg·K) | Temperature-dependent |
| Melting temperature | 1450–1550 °C | Base metal / overlay interface |
Heat Source Model
The strip electrode heat source is modeled as a distributed rectangular or elliptical heat source rather than a point source, reflecting the extended contact area between the strip electrode and the workpiece. This is fundamentally different from conventional submerged arc welding with wire electrodes, where a concentrated heat source model is more appropriate. The rectangular distribution accounts for the uniform deposition width characteristic of strip electrode processes.
The governing heat equation solved in the simulation is:
ρ·Cp·(∂T/∂t) = ∇·(k·∇T) + Q(x,y,t)
where ρ is density, Cp is specific heat capacity, k is thermal conductivity (all temperature-dependent), and Q represents the distributed heat source intensity.
Process Analysis and Engineering Implications
Multi-Pass Thermal Accumulation
One of the most critical findings emphasized in the paper is the cumulative thermal effect during multi-pass cladding. As successive layers are deposited, the thermal mass of previously deposited material increases, leading to progressive elevation of the base metal temperature at the cladding interface. This has several engineering consequences:
- Residual stress evolution: The differential cooling rates between the hot overlay layers and the cooler base plate generate significant residual tensile stresses in the cladding, which can approach or exceed the yield strength of austenitic stainless steel overlays (304, 316) at room temperature.
- Microstructural transformation: In the heat-affected zone of carbon steel base plates, excessive interpass temperatures can promote the formation of soft, ductile microstructures (coarse ferrite and pearlite) that reduce hardness and potentially compromise mechanical properties in the base metal.
- Bond strength concerns: While moderate interpass temperatures generally promote good metallurgical bonding between the overlay and base metal, excessively high temperatures can lead to excessive dilution and the formation of martensitic phases at the interface in certain alloy combinations.
Preheating Strategy Optimization
The simulation results provide guidance for preheating temperature selection. For carbon steel base plates with thickness exceeding 80 mm, preheating to 200–250 °C is recommended to reduce thermal gradients and minimize the risk of hydrogen-induced cracking in the base metal HAZ. However, for stainless steel overlays, excessively high preheat temperatures (above 350 °C) can promote sensitization in the overlay layer, particularly for austenitic grades such as 304 or 316, leading to reduced corrosion resistance.
Comparison with Conventional Wire Electrode SAW
| Aspect | Strip Electrode SAW | Wire Electrode SAW |
|---|---|---|
| Deposition rate | 5–10 kg/h | 1–2 kg/h |
| Heat input per pass | 25–45 kJ/mm | 5–15 kJ/mm |
| Dilution control | Lower per-pass dilution | Higher per-pass dilution |
| Layer uniformity | Excellent | Good |
| Equipment complexity | Higher | Lower |
| Suitable plate thickness | 20–200 mm | 5–100 mm |
| Typical overlay thickness per pass | 3–5 mm | 2–4 mm |
| Number of passes (10 mm overlay) | 3–5 | 8–15 |
Key Questions and Reflections
The paper raises important considerations for engineers working on thick plate cladding projects. First, the validity of finite element predictions depends heavily on the accuracy of temperature-dependent thermophysical property data, particularly for multi-layer systems where the properties of the overlay material may differ significantly from the base metal. Second, the simulation typically assumes perfect thermal contact at the interface, which may not reflect reality in cases where interfacial oxide films or porosity exist.
From a practical standpoint, I would emphasize that while finite element simulation provides invaluable guidance for process planning, it must be validated against thermocouple measurements from actual weld trials. The thermal coupling between adjacent passes in a multi-layer cladding sequence is particularly difficult to predict accurately, and the simulation should be used as a decision-support tool rather than a definitive predictor.
Another reflection concerns the applicability of these findings to different overlay materials. The thermal behavior of nickel-based alloy overlays (Inconel 625, Hastelloy C276) differs markedly from austenitic stainless steel overlays due to lower thermal conductivity and higher specific heat, leading to steeper thermal gradients and potentially higher residual stresses. Engineers should not directly extrapolate simulation parameters developed for stainless steel overlays to nickel-based systems without recalibration.
Study Insights and Practical Implications
The most valuable contribution of this work is the systematic demonstration that finite element simulation can effectively predict peak temperatures, thermal gradients, and cooling rates during strip electrode cladding of thick plates. These predictions enable engineers to:
- Select appropriate preheat and interpass temperatures to control residual stresses within acceptable limits
- Determine the minimum number of passes required to achieve target overlay thickness while maintaining metallurgical quality
- Predict the thermal cycle experienced by the base metal and assess the risk of HAZ softening or cracking
- Optimize welding sequence and direction to minimize distortion in large-diameter pressure vessel shells
For pressure vessel fabrication specifically, the ability to predict and control residual stresses in clad plate is essential for meeting the requirements of ASME VIII Div.1 and GB/T 150 regarding stress relief and dimensional tolerances. The simulation approach described here provides a quantitative basis for establishing welding procedure specifications (WPS) that are both technically sound and economically efficient.
In conclusion, this paper represents a mature application of computational methods to a practical welding engineering problem, demonstrating that finite element analysis can significantly reduce the trial-and-error costs associated with thick plate cladding process development while improving the predictability and reliability of the final product.