CLADDING TECHNOLOGY SHANXI CO., LTDControl of Flatness in Overlay-Welded Tube Sheets
Literature Overview
This technical paper by Lv Yanmao from the Chemical Machinery Factory of Nanjing Chemical Company, Sinopec Group, was published in 2013 in the journal "Chemical Equipment Technology" (化工装备技术). The study addresses a critical manufacturing challenge in the fabrication of heat exchanger tube sheets that require corrosion-resistant overlay layers. Tube sheet flatness directly affects the sealing integrity of tube-to-tubesheet joints and the overall structural reliability of the pressure boundary.
Technical Background and Challenge
Overlay-welded tube sheets are commonly used in heat exchangers fabricated to GB/T 151 or ASME VIII Div.1 where the tube sheet must resist corrosion from process fluids while maintaining mechanical strength. The overlay layer (typically 304, 316L, or 321 stainless steel, 3-6 mm thick) introduces significant welding distortion due to:
- Thermal expansion mismatch between the base carbon steel and stainless overlay
- Residual stresses from multiple overlay weld passes
- Differential cooling rates between the overlay and base metal
- Geometric constraints of the tube sheet (typically 20-80 mm thick, 500-3000 mm diameter)
The allowable flatness tolerance for tube sheets is typically 1:1000 of diameter (not exceeding 3 mm for typical sizes), as specified in GB/T 151 and ASME VIII Div.1. Exceeding this tolerance leads to tube joint leakage, gasket failure, and premature fatigue damage.
Process Control Strategies
The paper discusses several methods for controlling flatness during and after overlay welding:
| Control Method | Principle | Typical Effectiveness | Applicable Size Range |
|---|---|---|---|
| Backing plate constraint | Mechanical restraint during welding | High for small distortions | All sizes |
| Symmetric welding sequence | Balanced thermal input | Moderate to high | Large diameters |
| Backing weld pre-compensation | Intentional pre-distortion | High | Thick tube sheets |
| Post-weld machining | Removal of distorted material | High (material waste) | All sizes |
| Induction heating correction | Local thermal correction | Moderate | Localized distortion |
| Mechanical hammering | Plastic deformation correction | Low to moderate | Minor distortions |
The welding sequence design is critical. For a circular tube sheet, the overlay should be applied in a spiral or radial pattern starting from the center or edge (depending on the specific distortion tendency observed in trial welds). The interpass temperature should be controlled to 150-250°C to minimize thermal gradients.
Distortion Mechanism Analysis
The primary distortion mechanism in overlay-welded tube sheets is the contraction of the overlay layer upon cooling. Since the stainless steel overlay has a thermal expansion coefficient (17.3 × 10⁻⁶/°C) close to but slightly higher than carbon steel (12 × 10⁻⁶/°C), the cooling contraction of the overlay creates a compressive stress in the base metal and tensile stress in the overlay. This stress couple causes the tube sheet to warp.
The magnitude of distortion depends on:
- Overlay thickness to tube sheet thickness ratio (higher ratio = more distortion)
- Number of overlay passes (more passes = more cumulative distortion)
- Welding heat input (higher heat input = more distortion)
- Constraint conditions during welding
Engineering Practice Recommendations
Based on the analysis in this paper and practical experience, the following recommendations are made for tube sheet overlay fabrication:
- Pre-weld preparation: Ensure the base tube sheet is within half the final flatness tolerance before overlaying, as welding will always introduce additional distortion.
- Welding procedure qualification: Perform a full-scale trial weld to characterize the distortion pattern before production welding. This allows optimization of the welding sequence.
- Backing plate design: Use a rigid backing plate with matching contour to the tube sheet, secured by clamps or welding at regular intervals (typically every 100-200 mm).
- Post-weld correction: Plan for a machining allowance of 0.5-1.0 mm on the overlay surface to correct residual distortion after welding.
- Inspection: Perform flatness measurement using a straightedge and feeler gauge or coordinate measurement machine after machining, per the requirements of GB/T 151.
Study Insights and Reflections
This paper highlights a practical engineering challenge that is often underestimated during the design phase. The interaction between overlay welding distortion and tube sheet flatness requirements creates a significant manufacturing constraint that must be addressed in the fabrication procedure specification. The key lesson is that flatness control must be considered as an integrated part of the welding procedure, not as a post-weld correction step. Process planning should begin with the final flatness requirement and work backwards to determine the acceptable starting condition, welding sequence, and correction strategy. This systematic approach, consistent with PDCA methodology, ensures that quality is built into the process rather than inspected in afterward.