CLADDING TECHNOLOGY SHANXI CO., LTDFlatness Control for Large Tube Sheet Strip Electrode Overlay
Literature Overview
This study note examines the techniques for controlling flatness during strip electrode overlay welding of large tube sheets, as reported by Lv Yanmao and Han Bing from Sinopec Nanjing Chemical Machinery Co., Ltd. in 2018. Large tube sheets are critical components in heat exchangers and reactors, where they are typically overlaid with stainless steel or nickel-based alloy to provide corrosion resistance at the tube-to-tube-sheet joint. The flatness of the overlay surface is paramount because tube holes must be drilled or reamed through the overlay layer, and any deviation from flatness can cause tube misalignment, improper tube-to-tube-sheet fit, and ultimately, leakage at the tube joints. The study addresses the thermal distortion and mechanical deformation that occur during ESW overlay of large-diameter tube sheets and presents practical solutions for maintaining flatness within tight tolerances.
Core Technical Analysis
The thermal distortion during ESW overlay of tube sheets is caused by the high heat input of the process and the inherent asymmetry of the weld deposition. As the strip electrode traverses the tube sheet surface, the deposited weld metal cools and contracts, creating a tensile residual stress field that pulls the tube sheet surface inward (toward the weld side). This results in a concave distortion that is most severe near the weld path. The following table summarizes the key parameters affecting flatness:
| Parameter | Effect on Flatness | Control Strategy |
|---|---|---|
| Heat input (J/mm) | Higher heat input → greater distortion | Reduce current, increase travel speed |
| Weld pass sequence | Unbalanced sequence → asymmetric distortion | Use symmetric, balanced sequence |
| Preheat temperature | Higher preheat → reduced distortion rate | Optimize preheat at 200-300 °C |
| Substrate thickness | Thinner substrate → more distortion | Use thicker backing plate or fixture |
| Backing plate rigidity | More rigid backing → less distortion | Use thick, stiff backing plate |
| Interpass cooling | Uneven cooling → uneven distortion | Control interpass temp uniformly |
The typical flatness tolerance for tube sheet overlay is specified as 0.5 mm per 300 mm of diameter, with a maximum overall deviation of 1.0 mm. For large tube sheets (diameter > 1000 mm), achieving this tolerance is challenging due to the cumulative thermal distortion from multiple passes.
The process parameters for ESW overlay of tube sheets are typically:
| Parameter | Value |
|---|---|
| Strip electrode | 316L or 304L, 1.2 mm × 40 mm |
| Current | 2000–3000 A |
| Voltage | 35–42 V |
| Travel speed | 100–180 mm/min |
| Slag | F5A or equivalent |
| Preheat | 200–250 °C |
| Interpass temperature | ≤ 250 °C |
| Number of passes | 2–4 |
| Overlay thickness | 3–6 mm |
Distortion Control Strategies
Several strategies are employed to control flatness during tube sheet overlay:
- Balanced weld sequence: The weld passes are arranged in a symmetric pattern, starting from the center and progressing outward, or using a spiral pattern that balances thermal input around the circumference. This ensures that the thermal distortion is distributed evenly rather than concentrated in one direction.
- Mechanical clamping and backing: The tube sheet is clamped to a rigid backing plate (typically 50–100 mm thick steel plate) during welding. The clamping force must be sufficient to resist the contraction forces but not so high as to cause plastic deformation. The backing plate should be of sufficient mass to absorb heat and reduce the cooling rate at the substrate.
- Controlled preheat and interpass temperature: Uniform preheat is applied to the entire tube sheet surface using induction heating or gas flames. The interpass temperature is monitored at multiple points across the surface using infrared thermometers or thermocouples, ensuring that no region exceeds the specified limit.
- Post-weld flatness correction: After welding, the tube sheet is inspected for flatness using a straightedge and feeler gauge or a laser flatness scanner. If the deviation exceeds the tolerance, mechanical correction (pressing) or thermal correction (induction heating of the convex areas) may be applied. However, correction should be avoided if possible, as it can introduce new residual stresses and potentially compromise the overlay integrity.
- Multi-directional welding: For very large tube sheets, the overlay is applied in multiple directional passes (radial, circumferential, and diagonal) to distribute the thermal input more evenly. This approach increases the welding time but significantly reduces the net distortion.
Quality Inspection and Acceptance Criteria
The quality of the tube sheet overlay is verified through the following inspection program:
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Surface flatness | Straightedge + feeler gauge | ≤ 0.5 mm per 300 mm |
| Overall flatness | Laser scanner or dial indicator | ≤ 1.0 mm total |
| Overlay thickness | UT thickness gauge | ≥ specified minimum (typically 3 mm) |
| Surface quality | Visual + PT | No cracks, pores, or lack of fusion |
| Bond strength | Peel test (sample coupon) | ≥ 350 MPa |
| Hardness | HV hardness survey | 150–250 HV (304L/316L) |
| Intergranular corrosion | ASTM A923 Method 1B | No intergranular attack |
| Tube hole quality | Visual + dimensional check | No overlay defects at hole edges |
A critical aspect of tube sheet overlay quality is the condition of the overlay at the tube hole locations. After the overlay is completed, the tube holes are drilled or reamed through the overlay layer. The overlay material at the hole edges must be free of defects, as any cracks or lack of fusion at the hole edge can propagate during tube insertion and cause tube-to-tube-sheet joint failure.
Study Insights and Reflections
This 2018 study reflects the mature state of tube sheet overlay technology in China's petrochemical equipment manufacturing industry. The systematic approach to flatness control—combining process parameter optimization, mechanical clamping, balanced weld sequencing, and post-weld inspection—demonstrates the level of engineering sophistication achieved in this field. The practical solutions presented are directly applicable to current fabrication practices and provide a reliable framework for addressing flatness challenges in large tube sheet overlay. The document underscores the importance of process planning and quality control in achieving the tight dimensional tolerances required for reliable tube-to-tube-sheet joints in heat exchangers and reactors.