Zhuojin Pipe Fitting Co., LtdFlatness Control in Strip Electrode Overlay Welding of Large Tube Sheets
Literature Overview
This paper by Lü Yanmao and Han Bing from Sinopec Nanjing Chemical Machinery Co., Ltd., published in Chemical Equipment Technology in 2018 (Vol. 39, No. 1, pp. 44-46), addresses a critical manufacturing challenge in the production of large tube sheets for heat exchangers and pressure vessels. The study focuses on the control of flatness during strip electrode overlay welding, a process used to deposit corrosion-resistant or wear-resistant alloy layers onto carbon steel tube sheets. The paper presents an engineering methodology based on deformation calculation formulas and practical experience to achieve acceptable flatness through pre-deformation techniques.
Technical Challenge and Background
Large tube sheets are critical components in heat exchangers and pressure vessels, serving as the interface between tube bundles and shell. These components are often fabricated from carbon steel for structural strength but require corrosion-resistant overlay layers to withstand aggressive process media. Strip electrode overlay welding (also known as submerged arc strip welding or electrode strip welding) is an efficient method for depositing thick overlay layers, but it introduces significant thermal distortion due to the high heat input and asymmetric heating pattern.
The flatness requirements for tube sheets are stringent because:
- Tube holes must maintain precise alignment and perpendicularity to the tube sheet surface
- Gasket sealing surfaces must be flat to ensure pressure boundary integrity
- Subsequent machining operations require a reasonably flat starting surface
- Assembly tolerances for tube-to-tube-sheet welding depend on tube sheet flatness
Typical flatness specifications for large tube sheets range from 1-3 mm per meter of diameter, depending on the application and governing standards such as ASME Section VIII or GB/T 151.
Deformation Analysis and Pre-Deformation Methodology
The paper employs a systematic approach to flatness control based on understanding the deformation mechanisms during overlay welding:
- Thermal expansion mismatch: The overlay layer and base metal have different coefficients of thermal expansion, leading to differential contraction upon cooling.
- Residual stress distribution: The asymmetric heating pattern of strip electrode welding creates a non-uniform residual stress field that drives angular and warpage deformation.
- Plastic deformation accumulation: Multiple welding passes accumulate plastic strain, particularly in the overlay layer and near-surface regions of the base metal.
The pre-deformation method involves intentionally introducing controlled deformation before welding to counteract the expected post-weld distortion. The methodology includes:
- Calculation of expected deformation: Using established formulas for circular plate deformation under thermal loading, the expected warpage is predicted based on tube sheet dimensions, overlay thickness, and welding parameters.
- Determination of pre-deformation amount: The calculated deformation is used to determine the magnitude and direction of intentional pre-deformation to be applied to the tube sheet before welding.
- Application of pre-deformation: Mechanical or thermal methods are used to introduce the calculated pre-deformation, typically through controlled rolling or localized heating.
- Post-weld verification: After welding and cooling, the flatness is measured and compared to the predicted value to validate the methodology.
Process Parameters and Control Strategies
| Parameter | Typical Range | Effect on Flatness |
|---|---|---|
| Strip electrode width | 100-200 mm | Wider strips reduce angular distortion but increase heat input |
| Welding current | 800-1200 A | Higher current increases heat input and distortion |
| Travel speed | 1-3 m/min | Higher speed reduces heat input per unit length |
| Number of passes | 1-5 | Multiple passes distribute heat more uniformly |
| Preheat temperature | 100-200°C | Reduces thermal gradient and distortion |
| Interpass temperature | ≤300°C | Controls residual stress accumulation |
| Backing plate thickness | 20-50 mm | Provides thermal mass to reduce warpage |
The paper emphasizes that flatness control requires a holistic approach that considers not only welding parameters but also fixture design, welding sequence, and post-weld treatment. The welding sequence is particularly critical for large tube sheets, where a spiral or symmetric pattern can minimize net distortion by balancing thermal inputs around the circumference.
Engineering Practice and Case Studies
In practice, the pre-deformation method has been successfully applied to tube sheets with diameters exceeding 2000 mm, achieving flatness within specification after overlay welding. The key success factors include:
- Accurate prediction of deformation using validated calculation models
- Precise application of pre-deformation through controlled mechanical means
- Systematic welding sequence planning to minimize residual stresses
- Post-weld stress relief to stabilize dimensions and reduce residual stresses
The methodology aligns with modern quality management principles, particularly the PDCA (Plan-Do-Check-Act) cycle. The prediction phase corresponds to Plan, the welding execution to Do, the flatness measurement to Check, and the refinement of pre-deformation parameters to Act. This systematic approach enables continuous improvement and consistent quality across production batches.
Summary and Reflections
This paper presents a practical engineering solution to a significant manufacturing challenge in pressure vessel fabrication. The pre-deformation methodology, grounded in deformation theory and validated through engineering experience, provides a reliable approach to achieving flatness specifications for large tube sheets after overlay welding. The integration of calculation, process control, and quality verification forms a comprehensive framework that can be adapted to similar distortion control problems in welding operations. For engineers in the piping and pressure vessel industry, this work demonstrates that systematic analysis and proactive control strategies are more effective than reactive correction methods for managing welding distortion in large components.