CLADDING TECHNOLOGY SHANXI CO., LTDHeat Exchanger Tube Sheet Weld Overlay Operation and Deformation Control
Literature Overview
This study addresses the practical challenges of weld overlaying heat exchanger tube sheets, focusing on welding technique, sequence planning, and deformation control strategies. Tube sheets in large-diameter heat exchangers typically require stainless steel or nickel-based alloy overlays to provide corrosion resistance at the tube-to-tubesheet joint, and the large plate thickness combined with geometric constraints creates significant distortion challenges.
Core Technical Points
The research examines welding sequences, heat input management, and mechanical constraint strategies for overlaying tube sheets with thicknesses ranging from 50 mm to 150 mm. The primary welding processes evaluated include submerged arc welding (SAW) for bulk deposition and gas tungsten arc welding (GTAW) for finishing passes, with electroslag welding (ESW) considered for very thick sections.
Welding Sequence Strategy
The study proposes a segmented welding approach where the tube sheet surface is divided into zones, and welding proceeds from the center outward or from the periphery inward depending on the support configuration. A systematic sequence plan minimizes cumulative distortion by balancing thermal input on opposite sides of the plate.
| Tube Sheet Thickness | Recommended Process | Number of Passes | Interpass Temp | Expected Distortion |
|---|---|---|---|---|
| 50-75 mm | SAW + GTAW | 8-12 | <250°C | <2 mm/m |
| 75-100 mm | SAW + GTAW | 12-18 | <250°C | <3 mm/m |
| 100-150 mm | ESW + SAW + GTAW | 15-25 | <250°C | <4 mm/m |
Deformation Control Methods
Several deformation control techniques were evaluated, including backing bar constraint, chain welding sequences, back-step welding, and mechanical tacking. The most effective approach combines rigid backing with a carefully planned weld sequence that alternates between opposite quadrants of the tube sheet. Preheating to 100-150°C reduces thermal gradients but must be balanced against the risk of sensitization in austenitic stainless steel overlay materials.
Welding Operation Details
The study emphasizes the importance of GTAW finishing passes in achieving a smooth, dense surface suitable for tube drilling and plug welding. The final overlay thickness is typically controlled to 3-5 mm for standard tube-to-tubesheet welds, with surface roughness below Ra 12.5 μm. Edge preparation involves chamfering the tube sheet edge at 30-45° to facilitate fusion between the overlay layer and the tube sheet edge.
Common quality issues include lack of fusion at the overlay-to-base metal interface, surface porosity from insufficient arc shielding, and excessive dilution leading to reduced corrosion resistance. Countermeasures include maintaining strict flux dryness for SAW, using high-purity argon shielding for GTAW, and controlling the number of passes to limit cumulative dilution.
Engineering Practice and Reflections
In practical fabrication, the tube sheet overlay operation requires careful coordination between welding, machining, and assembly departments. The welding sequence must account for the subsequent tube drilling operation, and distortion allowances should be built into the initial tube sheet dimensions. Post-overlay machining is typically required to achieve flatness tolerances of ±0.5 mm over the tube sheet diameter, which means planning for a machining allowance of 1-2 mm in the overlay thickness specification.
This literature provides a comprehensive framework for planning tube sheet overlay operations, emphasizing that deformation control is not merely a welding parameter optimization problem but a system-level engineering challenge requiring integrated process planning.