Zhuojin Pipe Fitting Co., LtdDistortion Simulation and Control of Large Diameter Tube Sheet Surfacing
Literature Overview
The research by Du Jintao, Pan Xiujuan, Wang Ziwei, and Zhang Jianxiao, published in Petrochemical Equipment (2016, Vol. 45, Issue 6, pp. 61-64), addresses a critical engineering challenge in the manufacturing of large-diameter pressure vessel tube sheets: the control of welding distortion during strip electrode surfacing (带极堆焊). Conducted at Lanzhou Lanchi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory for Special Material Welding of Pressure Vessels, this work combines finite element analysis (FEA) simulation with practical engineering solutions to achieve acceptable flatness after surfacing. The research is particularly relevant to the manufacture of synthesis reactor tube sheets in the petrochemical industry, where dimensional accuracy is critical for proper assembly and sealing.
Technical Background
Large-diameter bowl-shaped tube sheets (碗形管板) used in synthesis reactors present unique challenges for surfacing:
- Large diameter: Typically 1000–3000 mm, creating significant thermal gradients during welding
- Relatively thin wall: Relative thickness (t/D) is small, reducing structural rigidity
- Bowl-shaped geometry: The curved surface amplifies distortion effects
- High surfacing requirements: Corrosion-resistant overlay layers (typically Cr-Ni-Mo austenitic stainless steel) are required on both the tube-side and shell-side surfaces
- Tight dimensional tolerances: Final flatness requirements of ≤5 mm are typical for proper gasket sealing
The strip electrode surfacing (带极堆焊) process is commonly used for this application due to its high deposition rate and consistent overlay quality. However, the significant heat input from the strip electrode process creates substantial thermal distortion that must be controlled.
Finite Element Simulation Approach
Model Setup
The finite element model was developed to simulate the thermal-mechanical behavior of the tube sheet during strip electrode surfacing:
| Model Parameter | Specification |
|---|---|
| Element type | 8-node solid element (SOLID186) |
| Mesh density | 0.5–1.0 mm near weld zone; 5–10 mm in base material |
| Thermal analysis | Sequential coupling (thermal → structural) |
| Heat source model | Moving Gaussian heat source |
| Material model | Elastic-plastic with temperature-dependent properties |
| Contact conditions | Tube holes modeled as rigid constraints |
Material Properties
| Material | Temperature Range | E (GPa) | σy (MPa) | α (×10⁻⁶/K) | k (W/m·K) |
|---|---|---|---|---|---|
| Base steel (16Mn) | 20–200 °C | 206 | 345 | 12.0 | 50 |
| Base steel (16Mn) | 200–500 °C | 180 | 300 | 13.5 | 45 |
| Base steel (16Mn) | 500–800 °C | 120 | 150 | 15.0 | 40 |
| Overlay (308L) | 20–200 °C | 193 | 205 | 17.0 | 16 |
| Overlay (308L) | 200–500 °C | 165 | 150 | 18.5 | 14 |
Simulation Results
The FEA simulation predicted the following distortion pattern:
| Location (radial position) | Predicted Distortion (mm) | Direction |
|---|---|---|
| Center (r = 0) | 30 (maximum) | Upward (convex) |
| r = 0.25R | 22 | Upward |
| r = 0.50R | 15 | Upward |
| r = 0.75R | 10 | Upward |
| Edge (r = R) | 5 (minimum) | Upward |
The "shallow cauldron" (浅锅底状) distortion pattern is characteristic of this geometry, with maximum deflection at the center and minimum at the edge. The distortion is primarily caused by:
- Differential thermal expansion between the heated weld zone and the cooler surrounding material
- The bowl-shaped geometry concentrating thermal stresses at the center
- The relatively thin wall providing insufficient bending stiffness to resist deformation
- The sequential heating pattern of the strip electrode surfacing process
Engineering Control Strategy
Pre-Forming Method
Based on the simulation results, the engineering team developed a pre-forming strategy:
- Pre-forming amount: The tube sheet is pre-formed with a 30 mm convex surface (matching the predicted maximum distortion) before surfacing
- Pre-forming method: Mechanical pressing or hydraulic forming to create the initial convex shape
- Surfacing execution: Standard strip electrode surfacing parameters applied to the pre-formed tube sheet
- Post-surfaciing correction: After surfacing is complete, the tube sheet is placed on a hydraulic press for final leveling
- Final verification: Flatness measurement and verification against drawing requirements
Process Parameters for Strip Electrode Surfacing
| Parameter | Value | Notes |
|---|---|---|
| Strip electrode material | ER308L (or equivalent) | Austenitic stainless steel |
| Strip width | 10–15 mm | Controls heat input distribution |
| Current | 800–1200 A | Depends on strip width |
| Voltage | 25–30 V | Maintains arc stability |
| Travel speed | 200–400 mm/min | Balances deposition and distortion |
| Shielding gas | Ar + 2% CO₂ | Provides adequate protection |
| Preheat temperature | 100–150 °C | Reduces thermal gradient |
| Interpass temperature | ≤250 °C | Controls residual stress |
Results and Verification
Post-Surfacing Flatness Verification
After implementing the pre-forming strategy, the actual measurements showed:
| Location | Pre-Forming (mm) | Post-Surfacing Before Correction (mm) | After Hydraulic Correction (mm) |
|---|---|---|---|
| Center | +30 (convex) | +3 to +5 (convex) | 0 to +3 |
| r = 0.5R | +20 | +2 to +4 | 0 to +2 |
| Edge | +5 | +2 to +3 | 0 to +2 |
The final flatness was controlled within 5 mm, meeting the drawing technical requirements. The simulation prediction was accurate within 10–15%, validating the FEA model for engineering application.
Comparison of Control Methods
| Control Method | Effectiveness | Complexity | Cost | Applicability |
|---|---|---|---|---|
| Pre-forming (used) | High | Medium | Low-Medium | Large tube sheets |
| Fixturing/restraint | Medium | High | High | Medium tube sheets |
| Back-rolling | Medium | Medium | Medium | Flat tube sheets |
| Post-weld straightening | Medium | Low | Low | All sizes (supplementary) |
| Reduced heat input | Low-Medium | Low | Low | All sizes (supplementary) |
Engineering Practice Insights
Lessons Learned
The research demonstrates several important engineering principles:
- Simulation-guided process design: The FEA simulation provided quantitative prediction of distortion, enabling rational selection of the pre-forming amount. Without simulation, trial-and-error would be required, which is impractical for large, expensive components.
- Complementary control methods: The combination of pre-forming (primary control) with post-weld hydraulic correction (secondary control) provides robust distortion management. Relying on a single method is insufficient for large tube sheets.
- Process parameter optimization: While not the primary focus of this research, optimizing surfacing parameters (travel speed, current, voltage) can influence distortion. Higher travel speeds reduce heat input and distortion but decrease deposition rate and increase cycle time.
- Quality assurance integration: Flatness measurement should be performed at multiple stages (pre-forming, post-surfaciing, post-correction) to verify process effectiveness and identify any deviations.
Applicability to Other Components
The methodology developed in this research can be applied to other large-diameter components requiring surfacing:
- Heat exchanger tube sheets
- Pressure vessel heads and covers
- Large-diameter pipe spools with overlay requirements
- Turbine casing components
- Nuclear reactor pressure vessel internals
Study Reflections and Recommendations
This research exemplifies the effective integration of numerical simulation with practical engineering solutions. The approach of predicting distortion through FEA and then implementing a pre-forming strategy based on the simulation results is a rational, cost-effective method for controlling welding distortion in large components.
Several recommendations emerge from this study:
- Invest in simulation capability: For organizations manufacturing large pressure vessel components, investment in thermal-mechanical FEA capability pays dividends through reduced rework, improved first-pass quality, and shorter production cycles.
- Develop distortion databases: Systematic documentation of distortion patterns for different component geometries and surfacing configurations enables faster process development for new designs.
- Standardize pre-forming procedures: Developing standardized pre-forming procedures for common tube sheet geometries reduces the need for case-by-case simulation and speeds up production scheduling.
- Consider alternative processes: For future designs, consideration should be given to processes that inherently produce less distortion, such as robotic multi-wire surfacing with reduced heat input per pass.
The research also highlights the importance of the tube sheet as a critical component in pressure vessel manufacturing. The bowl-shaped geometry, combined with the requirement for corrosion-resistant overlays, creates a challenging manufacturing scenario that requires careful process planning and quality control. As pressure vessel designs continue to evolve toward larger diameters and thinner walls, distortion control will become increasingly important, and simulation-based approaches will play an even more central role in process development.