CLADDING TECHNOLOGY SHANXI CO., LTDLarge Diameter Tube Sheet Overlay Manufacturing Technology
Literature Overview
The manufacturing of large diameter tube sheets with weld overlay cladding presents one of the most challenging applications in the bimetal pressure vessel fabrication industry. Tube sheets are critical components in heat exchangers, condensers, and reactor internals, where they provide the structural interface between the shell and tube bundles. Large diameter tube sheets, typically exceeding 2000 mm in diameter, require extensive overlay coverage to achieve corrosion resistance in the tube-to-tubesheet weld joints. The study examines the complete manufacturing technology chain for producing large diameter overlaid tube sheets, addressing material selection, welding procedure, quality control, and dimensional accuracy.
Material Selection and Design Considerations
The selection of overlay material for large diameter tube sheets depends on the service environment, design temperature, and applicable code requirements. The following table summarizes typical material combinations for various service conditions.
| Service Environment | Base Material | Overlay Material | Standards |
|---|---|---|---|
| Mild corrosive (water, steam) | SA-516 Gr.70 / SA-537 Gr.1 | 304L / 316L | ASME VIII Div.1 |
| Severe corrosive (acid, chloride) | SA-516 Gr.70 / SA-537 Gr.1 | 316L / 904L / 625 | ASME VIII Div.2 |
| High temperature (400–600°C) | SA-387 Gr.11 / SA-387 Gr.22 | 321 / 347 / 625 | ASME VIII Div.2 |
| Cryogenic (-40°C to -196°C) | SA-350 LF2 / 304L | 304L / 321 | ASME VIII Div.1 |
| Hydrogen service | SA-387 Gr.22 | 321 / 625 | NACE MR0175 |
Manufacturing Process Flow
The manufacturing of a large diameter overlaid tube sheet involves multiple sequential operations, each requiring precise control to ensure the final product meets dimensional and metallurgical specifications.
Step-by-Step Manufacturing Sequence
| Step | Operation | Key Parameter | Quality Check |
|---|---|---|---|
| 1 | Plate selection and inspection | Material certification, UT of plate | Visual, UT |
| 2 | Machining of base plate | Flatness ≤ 1 mm/m, thickness tolerance ±1.0 mm | CMM, thickness gauge |
| 3 | Surface preparation | SA 2.5 grade, degreasing | Visual, solvent test |
| 4 | Tack welding (dab welding) | 10–15 mm spacing, 50–80 A | Visual, MT |
| 5 | Overlay welding | 3–5 passes, 150–250°C IPT | FN measurement, UT |
| 6 | Intermediate grinding | Remove scale, flatten surface | Visual, thickness measurement |
| 7 | Final overlay pass | 1–2 passes, precise thickness control | UT, MT |
| 8 | Final grinding/machining | Surface finish Ra ≤ 1.6 μm, thickness tolerance ±0.5 mm | CMM, profilometry |
| 9 | Final NDE | UT, MT, PT | Code-specific acceptance criteria |
| 10 | Hydrotest and dimensional verification | Pressure test, dimensional inspection | Pressure gauge, CMM |
Welding Procedure Development
The welding procedure specification (WPS) for large diameter tube sheet overlay must address the unique challenges of welding over large flat areas, including thermal distortion control, uniform deposition, and consistent microstructure.
Typical WPS Parameters for SAW Overlay
| Parameter | Range | Notes |
|---|---|---|
| Electrode type | E309L / E316L (SAW) | Low carbon for nuclear service |
| Flux type | F5A309 / F5A316 | Low hydrogen, high basicity |
| Current | 400–600 A | DCEN polarity |
| Voltage | 24–32 V | Stable arc |
| Travel speed | 150–250 mm/min | Depends on plate thickness |
| Wire feed speed | 0.8–1.5 m/min | Matched to current |
| Shielding gas | None (flux-shielded) | SAW process |
| Interpass temperature | 150–250°C | Monitor with IR thermometer |
| Preheat temperature | 100–200°C | For carbon steel base |
| Number of passes | 3–5 | Depending on required thickness |
| Layer thickness per pass | 2–4 mm | Optimal for bonding |
Distortion Control Strategies
| Strategy | Method | Effectiveness |
|---|---|---|
| Symmetric welding sequence | Weld from center outward in concentric rings | Reduces angular distortion by 60–70% |
| Backing plate | Use of copper backing plate with water cooling | Reduces HAZ width by 30–40% |
| Tack welding | Heavy tack welds at 50–100 mm spacing | Reduces overall distortion by 40–50% |
| Interpass cooling | Allow cooling to 150°C between passes | Reduces residual stress by 20–30% |
| Post-weld machining | Machine to final dimensions after cooling | Corrects remaining distortion |
Quality Control and Inspection
The inspection regime for large diameter overlaid tube sheets is extensive and must cover both the overlay layer and the base material.
| Inspection Method | Location | Acceptance Criteria | Frequency |
|---|---|---|---|
| Visual inspection (VT) | Entire surface | No undercut, porosity, or cracks | 100% |
| Magnetic particle testing (MT) | Overlay surface | No linear indications > 6 mm | 100% |
| Penetrant testing (PT) | Overlay surface | No indications per ASME Section V | 100% |
| Ultrasonic testing (UT) | Bond line | No lack of fusion > 3 mm | 100% |
| Radiographic testing (RT) | Selected areas | Per ASME Section V acceptance | 10% of area |
| Ferrite number (FN) | Overlay layer | 5–15 FN (for austenitic) | Every 1000 cm² |
| Hardness testing (HB) | Overlay and HAZ | Within 100 HB of base material | Every 1000 cm² |
| Thickness measurement | Overlay layer | Within ±0.5 mm of nominal | Grid pattern |
| Impact testing | Overlay/HAZ | Per code requirement | Per WPS qualification |
Engineering Case Study
A typical application involves a large diameter tube sheet for a power plant condenser, with a diameter of 3200 mm, base material SA-516 Gr.70 (25 mm thick), and overlay material 316L (6 mm thick). The manufacturing challenge includes achieving uniform overlay thickness across the entire surface while maintaining dimensional accuracy for tube drilling.
Key Manufacturing Challenges and Solutions
| Challenge |
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