CLADDING TECHNOLOGY SHANXI CO., LTDDouble-Sided Cladding Thick Tube Sheet Manufacturing Technology
Technical Challenges and Process Requirements
The manufacturing of double-sided clad thick tube sheets represents one of the most demanding challenges in pressure vessel fabrication. Thick tube sheets, typically ranging from 50 to 200 mm in thickness, serve as critical pressure-containing components in heat exchangers, reactors, and columns. When both sides of such a thick tube sheet require corrosion-resistant cladding, the manufacturing complexity increases dramatically due to the cumulative effects of welding distortion, residual stress, and the difficulty of achieving adequate fusion on both sides of a thick section.
The primary technical challenges include: achieving full fusion between the cladding and the substrate on both faces of a thick plate, controlling distortion to within tight tolerances that affect tube hole drilling accuracy, managing residual stresses that can lead to delayed cracking or distortion during subsequent machining, and ensuring that the cladding on one side does not compromise the properties of the cladding on the other side. The interaction between the two cladding operations is particularly complex, as the heat input from the second cladding operation can affect the microstructure and properties of the first cladding.
The applicable standards for this application include GB/T 150 for pressure vessel design, NB/T 47014 for welding procedure qualification, and specific standards for tube sheet fabrication such as GB/T 151. The welding procedure qualification must cover the full thickness of the tube sheet, including the cladding layers on both sides, and must demonstrate that the procedure produces acceptable results for the entire component geometry.
Process Development and Welding Sequence Optimization
The development of a manufacturing process for double-sided clad thick tube sheets requires careful consideration of the welding sequence, backing arrangement, and heat input management. The welding sequence must be planned to minimize distortion and residual stress while ensuring that each cladding layer achieves adequate fusion with the substrate.
For a typical tube sheet with 100 mm thickness and 3 mm cladding on each side, the recommended welding sequence is as follows:
- Deposit the first cladding layer on Side A using a multi-pass procedure (typically 3 to 5 passes depending on the cladding alloy and wire diameter).
- Deposit the second cladding layer on Side A to build up the required cladding thickness.
- Allow the component to cool to room temperature or perform a controlled stress relief.
- Flip the component and deposit the first cladding layer on Side B.
- Deposit the second cladding layer on Side B.
- Perform a final stress relief if required by the design specification.
The backing arrangement is critical for achieving full fusion at the root of the first cladding layer. For thick tube sheets, a backing plate of matching material is typically used, with a backing weld deposited on the backing side to ensure full penetration. The backing weld is then removed after the cladding is complete, leaving a clean surface for the second cladding operation.
The heat input for each pass must be carefully controlled to minimize the HAZ width and reduce distortion. For SAW overlay, a heat input of 20 to 35 kJ/mm is typical, while for GMAW overlay, the heat input is lower at 10 to 20 kJ/mm. The lower heat input of GMAW is advantageous for distortion control but may require more passes to build up the required cladding thickness.
| Parameter | Side A Cladding | Side B Cladding | Combined Effect |
|---|---|---|---|
| Plate thickness | 100 mm | 100 mm | N/A |
| Cladding thickness | 3 mm | 3 mm | 6 mm total |
| Number of passes | 4-5 | 4-5 | 8-10 total |
| Heat input per pass | 25-35 kJ/mm | 25-35 kJ/mm | Cumulative |
| Interpass temperature | 150-250°C | 150-250°C | Must be monitored |
| Preheat temperature | 100-150°C | 100-150°C | Reduces cracking risk |
Distortion Control and Residual Stress Management
Distortion control is the single most important factor in the successful fabrication of double-sided clad thick tube sheets. The cumulative heat input from both cladding operations can cause significant warping, bowing, and angular distortion that makes subsequent tube hole drilling inaccurate or impossible. The welding sequence must be designed to counteract the expected distortion, and mechanical constraints such as welding fixtures and clamps must be used to minimize movement during welding.
A key strategy for distortion control is the use of a symmetric welding pattern. By welding in a balanced sequence that alternates between different regions of the plate, the thermal expansion and contraction can be made to cancel out, reducing net distortion. The welding pattern should also avoid long continuous welds, which generate large thermal gradients and high distortion. Instead, the welds should be segmented and welded in short sections, with the welding direction reversed periodically.
Residual stress management is equally important. The residual stresses from the first cladding operation interact with those from the second cladding operation, potentially creating stress concentrations that exceed the yield strength of the material. Post-weld stress relief is typically required, but the temperature and duration must be carefully controlled to avoid sensitization of the cladding alloy. For austenitic stainless steel cladding, a stress relief temperature of 425 to 475 degrees Celsius for 1 hour per 25 mm of thickness is recommended, while for nickel-based alloy cladding, higher temperatures may be required.
The effect of the second cladding operation on the first cladding layer must be carefully evaluated. The heat input from the second cladding can cause the first cladding layer to undergo tempering or phase transformation, potentially reducing its hardness or changing its corrosion resistance. The interpass temperature during the second cladding operation should be monitored to ensure that it does not exceed the maximum temperature that would adversely affect the first cladding layer. For austenitic stainless steel cladding, the maximum interpass temperature should be limited to 250 degrees Celsius to avoid sensitization.
Quality Control and Inspection Strategy
The quality control strategy for double-sided clad thick tube sheets must address the unique challenges of the application. Non-destructive testing (NDT) is required for both cladding layers, with particular attention to the interface between the cladding and the substrate. The following NDT methods are typically employed:
- Visual inspection (VT) of all cladding surfaces for surface defects, porosity, and undercut.
- Magnetic particle testing (MT) or dye penetrant testing (PT) of the cladding surface for surface-breaking defects.
- Ultrasonic testing (UT) of the cladding/substrate interface for lack of fusion, porosity, and cracks.
- Radiographic testing (RT) of the cladding for volumetric defects such as porosity and inclusions.
- Hardness testing of the cladding surface and interface to verify that the microstructure is within acceptable limits.
The acceptance criteria for NDT must be defined in the applicable specification. For critical applications such as hydrogenation reactors, the acceptance criteria may be more stringent than those specified in general standards, requiring zero defects at the interface and limiting the size and number of porosity in the cladding.
Mechanical property testing includes tensile testing of the cladding/substrate system, bend testing of the interface, and impact testing of the HAZ in the substrate. The tensile test coupon must be oriented so that the fracture path crosses the cladding/substrate interface, providing a direct measure of the bond strength. The bend test is performed with the cladding facing inward (testing the interface) and outward (testing the cladding surface), with acceptance criteria requiring no cracks exceeding 1 mm in length.
Engineering Lessons and Practical Recommendations
The fabrication of double-sided clad thick tube sheets requires a holistic approach that integrates welding process design, distortion control, residual stress management, and quality assurance into a unified manufacturing strategy. The key lessons from engineering practice are as follows:
First, the welding procedure must be qualified for the full thickness of the tube sheet, including both cladding layers, and the qualification must demonstrate that the procedure produces acceptable results for the entire component geometry. Second, the welding sequence must be carefully planned to minimize distortion and residual stress, with the use of symmetric welding patterns and mechanical constraints to control movement. Third, the interaction between the two cladding operations must be carefully managed, with the interpass temperature during the second cladding operation limited to avoid adverse effects on the first cladding layer. Fourth, the quality control strategy must include NDT of both cladding layers and the interfaces, with acceptance criteria defined in the applicable specification.
In conclusion, the manufacturing of double-sided clad thick tube sheets is a challenging but achievable task that requires careful process planning, rigorous quality control, and a deep understanding of the metallurgical interactions between the substrate, the cladding layers, and the interfaces. The successful implementation of this technology depends on the integration of welding expertise, materials knowledge, and manufacturing discipline into a cohesive manufacturing approach.