CLADDING TECHNOLOGY SHANXI CO., LTDFlatness Control of Large Tube Sheets During Strip Cladding
Literature Overview
This 2018 study by Lv Yanmao and Han Bing from Sinopec Nanjing Chemical Machinery Co., Ltd. addresses a specific and challenging problem in the fabrication of large tube sheets for heat exchangers: controlling flatness during strip cladding operations. Large tube sheets, typically exceeding 2000 mm in diameter, are critical components in large-scale chemical processing equipment. When these tube sheets require corrosion-resistant cladding, the strip cladding process introduces significant thermal distortions that must be carefully managed to maintain the required flatness tolerance.
Technical Background and Requirements
Tube sheets in heat exchangers serve as the structural and sealing interface between the tube bundle and the shell. The flatness of the tube sheet is critical for achieving proper gasket sealing and preventing leakage. According to GB/T 151 and ASME VIII Div.1, the flatness tolerance for tube sheets is typically specified as a percentage of the tube sheet diameter, with typical values ranging from 0.2% to 0.5% depending on the application and design pressure.
Flatness Tolerance Requirements
| Tube Sheet Diameter (mm) | Typical Flatness Tolerance (mm) | Percentage of Diameter |
|---|---|---|
| 1000 | 2-3 | 0.2-0.3% |
| 2000 | 4-6 | 0.2-0.3% |
| 3000 | 6-9 | 0.2-0.3% |
| 4000 | 8-12 | 0.2-0.3% |
| 5000 | 10-15 | 0.2-0.3% |
For large tube sheets exceeding 3000 mm in diameter, the flatness tolerance becomes increasingly difficult to maintain, especially when strip cladding is involved. The thermal distortion introduced by strip cladding can easily exceed the allowable flatness tolerance if not properly controlled.
Causes of Thermal Distortion During Strip Cladding
The thermal distortion during strip cladding of large tube sheets arises from several mechanisms:
- Differential thermal expansion: The cladding material (typically austenitic stainless steel) has a higher coefficient of thermal expansion than the base carbon steel. During cladding, the overlay expands more than the base metal, creating compressive stresses in the overlay and tensile stresses in the base metal. Upon cooling, these stresses reverse, potentially causing warpage.
- Non-uniform heat input: Strip cladding involves a moving heat source that creates a non-uniform temperature distribution across the tube sheet. The areas being clad experience rapid heating, while the surrounding areas remain cooler. This temperature gradient creates bending moments that distort the tube sheet.
- Residual stress accumulation: Each pass of strip cladding introduces residual stresses that accumulate over multiple passes. The superposition of stresses from successive passes can lead to progressive distortion that exceeds the allowable tolerance.
- Thermal cycling effects: The repeated heating and cooling cycles during multi-pass cladding cause cyclic plastic deformation, which can lead to creep and stress relaxation effects that further contribute to distortion.
Quantitative Analysis of Distortion
The study provides a quantitative analysis of the thermal distortion during strip cladding:
| Distortion Component | Typical Magnitude | Primary Cause |
|---|---|---|
| Edge warpage | 2-5 mm/m | Non-uniform heat input at edges |
| Center bulge | 1-3 mm/m | Compressive stress in overlay |
| Angular distortion | 0.5-2 mm/m | Differential expansion |
| Total flatness deviation | 5-15 mm | Superposition of all components |
The total flatness deviation can easily reach 5-15 mm for large tube sheets, which far exceeds the allowable tolerance. Therefore, active distortion control measures are essential.
Distortion Control Strategies
The study proposes several strategies for controlling flatness during strip cladding of large tube sheets:
Pre-Cladding Measures
- Back-side cooling: Applying cooling water or cryogenic cooling to the back side of the tube sheet during cladding can reduce the temperature gradient and minimize distortion. The cooling should be applied uniformly to avoid creating new thermal gradients.
- Mechanical restraint: Using clamping fixtures or temporary stiffeners to restrain the tube sheet during cladding can limit the magnitude of distortion. However, excessive restraint can increase residual stresses and potentially cause cracking.
- Pre-distortion: Intentionally pre-distorting the tube sheet in the opposite direction to the expected cladding distortion can compensate for the thermal distortion. This requires accurate prediction of the distortion magnitude and direction.
Cladding Process Optimization
| Parameter | Recommended Approach | Rationale |
|---|---|---|
| Travel speed | Moderate to high | Reduces heat input per unit length |
| Electrode current | Moderate | Balances penetration and heat input |
| Travel pattern | Spiral from center outward | Uniforms heat distribution |
| Pass sequence | Symmetric passes | Minimizes angular distortion |
| Interpass temperature | Low (100-150 C) | Reduces thermal cycling effects |
| Number of passes | Minimum required | Reduces cumulative distortion |
The study specifically recommends a spiral travel pattern starting from the center and moving outward. This pattern ensures that the heat input is distributed more uniformly across the tube sheet, reducing the edge warpage that is typical of conventional linear travel patterns.
Post-Cladding Correction
Even with optimal process control, some degree of distortion is inevitable. The study discusses post-cladding correction methods:
- Mechanical straightening: Using hydraulic presses or mechanical straighteners to flatten the tube sheet. This is the most common method but can introduce work hardening and residual stresses.
- Thermal straightening: Applying localized heating to the convex areas to cause differential expansion and straightening. This method requires careful control to avoid overheating and material degradation.
- Shot peening: Applying compressive residual stresses to the surface through shot peening can partially compensate for the tensile residual stresses that cause distortion.
Quality Inspection and Acceptance Criteria
After cladding and distortion correction, the tube sheet must be inspected for flatness. The study recommends using a precision straightedge and feeler gauges, or a laser flatness measurement system for large tube sheets. The measurement should be taken at multiple points across the tube sheet diameter, including the center, mid-radius, and edge.
Acceptance Criteria for Clad Tube Sheets
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Flatness | Straightedge + feeler gauge | <= 0.2% of diameter |
| Overlay thickness | UT measurement | >= specified minimum |
| Bond strength | Peel test or shear test | >= 90% of base metal strength |
| Surface quality | Visual + PT | No cracks, porosity, or lack of fusion |
| Hardness | Vickers hardness test | Within specified range |
Engineering Practice Cases
The study includes practical cases from Sinopec Nanjing Chemical Machinery's production experience. One notable case involved a 3500 mm diameter tube sheet for a large heat exchanger in a petrochemical plant. The tube sheet was clad with 316L stainless steel using a 50 mm wide strip cladding process. Without distortion control measures, the initial flatness deviation was 18 mm, far exceeding the allowable 7 mm tolerance.
By implementing a combination of back-side cooling, spiral travel pattern, and post-cladding mechanical straightening, the final flatness was achieved within 5 mm, meeting the acceptance criteria. The total cycle time increased by approximately 15% due to the additional distortion control measures, but this was considered acceptable given the importance of flatness for the application.
Key Questions and Reflections
The study raises an important question about the trade-off between distortion control and production efficiency. The additional measures required for distortion control increase cycle time and cost. In some cases, it may be more economical to use alternative cladding methods, such as weld overlay with a wire electrode, which introduces less distortion. However, strip cladding offers superior productivity and consistency for large areas, making it the preferred method despite the distortion challenges.
Another important consideration is the effect of distortion control measures on the metallurgical quality of the overlay. For example, back-side cooling can increase the cooling rate of the overlay, potentially leading to a harder and more brittle microstructure. The study does not extensively address this trade-off, which should be investigated in future work.
Study Insights and Implications
The most significant contribution of this study is the systematic approach to distortion control that integrates pre-cladding, in-process, and post-cladding measures. The emphasis on process optimization, particularly the spiral travel pattern, provides a practical and effective solution that can be implemented without significant equipment modifications. The quantitative analysis of distortion components and their magnitudes provides a valuable reference for predicting and controlling distortion in similar applications.
For modern practitioners, this study highlights the importance of considering distortion as an integral part of the cladding process design, rather than as a problem to be solved after the fact. Early involvement of fabrication engineers in the design phase can identify potential distortion issues and implement preventive measures, resulting in better quality and lower costs.