Zhuojin Pipe Fitting Co., LtdControl of Overlay Welding Deformation on Large Diameter Heat Exchanger Tubesheets
Literature Overview
This paper by Zhou Yinmei from Shanxi Yangmei Chemical Machinery (Group) Co., Ltd. addresses a practical manufacturing challenge encountered in the production of heat exchangers: controlling the deformation of large-diameter tubesheets during the overlay welding process. Published in China Chemical Equipment in 2017 (Volume 19, Issue 4, pages 13–16), the study focuses on the specific conditions of large-diameter, thin-walled tubesheets that require relatively thick corrosion-resistant overlay layers. The research investigates welding methods, pre-set back deformation quantities, rigid fixation techniques, welding sequence optimization, and welding parameter selection to achieve deformation levels that meet machining requirements.
Technical Background and Challenge Analysis
Large-diameter tubesheets in heat exchangers present a unique set of welding challenges. The base plate typically has a large diameter (often exceeding 1000 mm) with relatively thin wall thickness (commonly 20–40 mm), while the corrosion-resistant overlay layer may require a thickness of 3–6 mm or more to ensure adequate service life in aggressive chemical environments. The combination of large surface area, thin base material, and thick overlay requirement creates significant thermal distortion challenges during the overlay welding process.
The primary deformation modes observed in tubesheet overlay welding include:
| Deformation Mode | Description | Typical Magnitude |
|---|---|---|
| Angular distortion | Out-of-plane warping of the tubesheet surface | 0.5–2.0 mm/m |
| Longitudinal shrinkage | Reduction in diameter due to weld shrinkage | 0.1–0.3% |
| Transverse shrinkage | Reduction in width along weld direction | 0.1–0.3% |
| Local buckling | Localized wrinkling in thin-walled sections | Varies |
Methodology and Key Technical Measures
The author systematically investigated multiple approaches to deformation control, which can be categorized using a structured problem-solving framework:
Welding Method Selection
The choice of welding process is the first critical decision. For large-diameter tubesheet overlay welding, the following processes are commonly considered:
- Submerged Arc Welding (SAW): Offers high deposition rates and good penetration, suitable for thick overlay layers, but requires careful control of heat input to minimize distortion.
- Flux-Cored Arc Welding (FCAW): Provides good deposition rates with lower heat input than SAW, offering a balance between productivity and distortion control.
- Magnetic Arc Surfacing: A specialized process that uses magnetic field forces to stabilize the arc, enabling controlled heat input and uniform deposition.
Pre-set Back Deformation (反变形量)
The concept of pre-setting a back deformation amount is a proactive approach to counteracting the expected welding distortion. The author determined that a calculated back deformation should be applied to the tubesheet prior to welding, such that the welding-induced deformation brings the tubesheet back to its intended flat geometry. The magnitude of the back deformation depends on the overlay thickness, base plate thickness, diameter, and welding parameters.
Rigid Fixation
Rigid fixation of the tubesheet during welding is essential to constrain deformation. The study emphasizes the use of robust clamping fixtures that hold the tubesheet firmly against a flat backing plate. The clamping force must be sufficient to resist the welding-induced shrinkage forces without causing local deformation of the thin-walled tubesheet.
Welding Sequence Optimization
The welding sequence is a critical parameter that significantly influences the final deformation pattern. For large-diameter circular tubesheets, the following sequence strategies are employed:
- Symmetric sequence: Starting from the center and welding outward in a symmetric pattern to distribute heat evenly.
- Step-back sequence: Welding in short segments with controlled overlap to limit heat accumulation in any single region.
- Multi-pass sequence: For thick overlay layers, multiple passes are applied with interpass temperature control to minimize thermal gradients.
Welding Parameter Selection
The welding parameters must be optimized to minimize heat input while maintaining adequate penetration and deposition quality. Key parameters include:
| Parameter | Typical Range | Effect on Deformation |
|---|---|---|
| Welding current | 250–400 A | Higher current increases heat input and distortion |
| Welding voltage | 25–35 V | Higher voltage increases arc length and heat spread |
| Welding speed | 200–400 mm/min | Higher speed reduces heat input and distortion |
| Interpass temperature | ≤ 150 °C | Higher temperature increases cumulative distortion |
| Preheat temperature | 50–100 °C | Moderate preheat reduces cracking risk but increases distortion |
Engineering Practice Integration
In practice, the implementation of these deformation control measures requires careful planning and coordination. The author reports that the combined application of all measures—appropriate welding method, calculated back deformation, rigid fixation, optimized welding sequence, and controlled welding parameters—successfully achieved deformation levels within the machining tolerance requirements for the tubesheet.
This case study is particularly relevant for chemical equipment manufacturers that produce heat exchangers with corrosion-resistant overlay welds on large tubesheets. The techniques described can be applied to similar products including:
- Shell-and-tube heat exchanger tubesheets with 309L/310L overlay
- Condenser tubesheets with stainless steel overlay
- Evaporator tubesheets in chemical processing equipment
Key Questions and Reflections
One area that deserves further investigation is the quantitative relationship between the back deformation amount and the final deformation. The paper describes the concept but does not provide detailed calculation methods or finite element analysis results that would allow other engineers to predict the required back deformation for different tubesheet geometries and overlay thicknesses.
Another consideration is the residual stress state after overlay welding. While deformation is a visible and measurable quality attribute, residual stresses can also affect the long-term performance of the tubesheet, particularly in applications subject to cyclic thermal loading or pressure fluctuations. The study does not appear to address residual stress measurement or stress relief requirements.
Study Insights and Implications
The most valuable contribution of this study is the systematic approach to deformation control that combines multiple techniques into an integrated solution. The emphasis on the combination of pre-set back deformation, rigid fixation, and optimized welding sequence reflects a mature understanding of welding distortion management that goes beyond simple parameter adjustment.
For engineers working on similar manufacturing challenges, this paper provides a practical framework that can be adapted to different tubesheet sizes, materials, and overlay thicknesses. The key lesson is that deformation control in large-scale overlay welding requires a holistic approach that addresses both the process parameters and the mechanical constraints simultaneously.