Cladding Welding Process for Heat Exchanger Tube Sheets

Literature Overview

This 2009 technical paper by Shi Liang from Tianjin Guanjie Petrochemical Engineering Co., Ltd. addresses the cladding welding process specifically applied to heat exchanger tube sheets. Published in the journal "Welding Technology," the study tackles one of the most technically demanding applications of weld overlay in pressure vessel fabrication — the creation of corrosion-resistant surfaces on tube sheets that interface with aggressive process fluids.

Technical Context and Challenges

Heat exchanger tube sheets serve a dual function: they provide mechanical support for the tube bundle while creating a pressure boundary between two process fluids. In petrochemical service, tube sheets are frequently subjected to:

• Corrosive process fluids (acidic, chlorinated, or reducing environments)
• High temperatures (150–400°C depending on service)
• Cyclic thermal loading
• Potential for erosion-corrosion at tube entries

The base material is typically carbon steel or low-alloy steel (SA-266 Gr.1B / SA-516 Gr.70) for strength and cost efficiency, while the cladding material must provide corrosion resistance (typically 316L, 321, or Hastelloy C-276 depending on the service).

Process Selection and Parameters

Cladding Process Comparison for Tube Sheet Application

Recommended SAW Overlay Parameters

For tube sheet cladding, the submerged arc welding process is typically selected for its high deposition rate and consistent quality:

• Base material: SA-266 Gr.1B, thickness 80–200 mm
• Cladding material: 316L or 321 stainless steel
• Electrode: E309LT (first pass) transitioning to E316LT (subsequent passes)
• Flux: Low-hydrogen rutile type (e.g., AS-FB-222 equivalent)
• Current: 450–650 A
• Voltage: 28–36 V
• Travel speed: 150–250 mm/min
• Number of passes: 3–5 depending on required cladding thickness
• Required cladding thickness: 3–6 mm minimum (per ASTM A264 / EN 10028-7)
• Preheat: 100–150°C to reduce HAZ hardness and hydrogen cracking risk

Welding Sequence Strategy

The welding sequence is critical for tube sheet cladding due to the presence of tube holes and the need to maintain dimensional accuracy:

1. Surface preparation: Machining or grinding to remove surface imperfections, ensuring Ra < 12.5 μm
2. First pass (transition layer): E309LT deposited in a zigzag pattern to minimize dilution from the carbon steel base
3. Second pass (buffer layer): E316LT deposited parallel to first pass to further reduce dilution below 30%
4. Subsequent passes (face layers): E316LT deposited to achieve full cladding thickness with uniform coverage
5. Post-weld machining: Precision machining to final dimensional tolerances (typically ±0.1 mm flatness)

Critical Quality Considerations

• Dilution control: The final cladding layer must have less than 30% dilution to maintain corrosion resistance per ASTM A264 requirements. Multi-layer approaches with transition layers reduce cumulative dilution.
• Intergranular corrosion sensitivity: The heat input must be controlled to avoid sensitization of the austenitic overlay. Heat input should be limited to 25 kJ/cm maximum for the final passes.
• Chloride stress corrosion cracking (SCC): Residual stresses from cladding welding can promote SCC in the overlay. Stress relief at 425–450°C for 2 hours per inch of thickness is recommended when chloride exposure is anticipated.
• Tube hole integrity: Welding sequence must avoid excessive heat input near tube holes to prevent distortion that would compromise tube-to-tube-sheet joint quality.

Inspection Requirements

Per NB/T 47014 and relevant pressure vessel codes, the following inspections are required:

Engineering Practice Reflection

The tube sheet cladding application exemplifies the complexity of weld overlay in pressure vessel fabrication. The challenge lies in balancing multiple competing requirements: corrosion resistance demands low dilution and controlled microstructure; mechanical strength requires adequate bond strength; dimensional accuracy constrains post-weld machining allowances; and economic considerations favor high-productivity processes.

The most significant practical lesson is that tube sheet cladding cannot be treated as a simple surface treatment. The interaction between the cladding weld, the base metal HAZ, and the subsequent tube-to-tube-sheet weld creates a complex metallurgical environment. Engineers must ensure that the tube-to-tube-sheet joint (typically GTAW) is compatible with the pre-existing cladding HAZ to avoid cracking during tube welding operations.