Weld Overlay Process Research on Tube Sheets — Technical Analysis and Process Qualification
1. Definition and Technical Principles
Tube-sheet weld overlay (also referred to as tube-sheet cladding or tube-sheet surfacing) is a specialized welding process in which a corrosion-resistant alloy layer is deposited onto the machined surface of a carbon steel or low-alloy steel tube sheet, providing a durable, metallurgically bonded barrier against aggressive process media. The tube sheet is one of the most critical components in heat exchangers, reactors, distillation columns, and other pressure vessels — it simultaneously serves as the structural interface between the shell and the tube bundle, the sealing surface for the channel cover, and the structural attachment point for hundreds of tubes via tube-to-tubesheet welds.
The fundamental principle behind tube-sheet weld overlay is to create a multi-layer weld deposit that achieves:
- Metallurgical compatibility between the base material (typically ASTM A216 WCB, SA-204 Gr. B, or SA-516 Gr. 70) and the overlay alloy (commonly 304L, 316L, 321, Alloy 625, Alloy 825, or Hastelloy C-276)
- Controlled dilution to maintain the required corrosion resistance of the surface layer while ensuring adequate strength and toughness at the weld interface
- Low residual stress to prevent cracking, distortion, and subsequent tube-to-tubesheet weld failure during subsequent tube welding operations
- Uniform surface quality suitable for subsequent precision machining (grinding, turning, or boring) to achieve the required flatness and surface finish for gasket sealing
2. Category and Business Positioning
Within the company's three core technology routes — TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding — tube-sheet weld overlay falls squarely under the TIG/MIG weld overlay category. This is because tube sheets are typically fabricated as flat or slightly domed plates with dimensions ranging from approximately 300 mm to over 2,500 mm in diameter and thicknesses from 20 mm to 120 mm. The geometry is inherently suited to arc welding processes rather than solid-state bonding methods.
Tube-sheet overlay occupies a high-value, high-difficulty niche within the weld overlay business segment. Unlike simple flat-plate cladding, tube sheets present unique challenges:
- The overlay surface contains numerous tube holes (typically 12–25 mm diameter) that must be protected and subsequently cleaned without damaging the overlay
- The overlay must be compatible with subsequent tube-to-tubesheet welding (typically TIG weld plug or TIG weld groove per ASME Section VIII Div. 1 UW-25), meaning residual stresses and microstructural integrity must be preserved
- Flatness and surface finish requirements are stringent (typically within ±0.5 mm over 300 mm for the sealing surface)
- The component often requires overlay on both the channel-side (tube-side) and shell-side surfaces, with different alloy selections for each side
3. Technical Purpose and Value
The primary technical purpose of tube-sheet weld overlay is to extend the service life of pressure-retaining components exposed to corrosive environments. In the petrochemical, offshore oil and gas, power generation, and pulp/paper industries, tube sheets are routinely subjected to:
- Chloride-containing aqueous media (causing pitting and crevice corrosion in carbon steel)
- Sulfuric acid and hydrofluoric acid environments (requiring high-nickel alloys)
- High-temperature hydrogen service (requiring austenitic stainless steel or Alloy 625 overlays)
- Seawater or brackish water cooling (requiring 316L or Alloy C-276)
Without overlay, the tube sheet would need to be fabricated entirely from the corrosion-resistant alloy, which increases material cost by a factor of 5–15×. Weld overlay achieves equivalent corrosion protection at a fraction of the material cost while maintaining the mechanical integrity of the carbon steel base.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
Surface preparation is critical to achieving sound overlay deposits. The tube sheet surface must be:
- Machined to remove mill scale, rust, and contaminants (minimum 3 mm of sound base metal exposed)
- Ground to a matte finish (Sa 2½ per ISO 8501-1 if blast cleaning is used)
- Tube holes plugged with PTFE tape, rubber plugs, or ceramic caps to prevent weld spatter and undercut at hole edges
- Preheated to a controlled temperature (typically 150–300°C for carbon steel base materials with stainless steel overlay) to reduce hydrogen-induced cracking susceptibility and thermal gradient
4.2 Process Selection and Parameters
Two primary welding processes are employed for tube-sheet overlay, each with distinct advantages:
| Parameter | TIG (GTAW) Overlay | MIG (GMAW) Overlay |
|---|---|---|
| Typical Wire Diameter | 1.6 – 3.2 mm | 1.2 – 1.6 mm |
| Welding Current | 120 – 250 A (DC) | 180 – 350 A |
| Travel Speed | 100 – 200 mm/min | 200 – 400 mm/min |
| Shielding Gas | Argon (99.99%) | Argon + 2–5% CO₂ or Ar + O₂ |
| Heat Input | 0.5 – 1.5 kJ/mm | 0.8 – 2.5 kJ/mm |
| Number of Layers | 2 – 4 layers | 1 – 3 layers |
| Surface Finish (as-welded) | Excellent (Ra ≤ 12.5 μm) | Good (Ra ≤ 25 μm) |
| Productivity | Low (manual or mechanized) | High (fully automatic) |
| Best Application | Small tube sheets, high-precision surfaces, thin sections | Large tube sheets, thick sections, high-volume production |
4.3 Layer-by-Layer Welding Strategy
A multi-layer approach is mandatory to control dilution and achieve the required surface alloy composition. The typical strategy is as follows:
- First Layer (Binder/Transition Layer): Deposited with a filler alloy that has a composition intermediate between the base metal and the final overlay alloy. For example, when overlaying Alloy 625 on carbon steel, the first layer uses E309L (or ER309L) to prevent chromium carbide precipitation at the fusion line. The first layer is typically 1.5–2.0 mm thick.
- Second Layer (Intermediate Layer): Uses the target overlay alloy (e.g., ERNiCr-3 for Alloy 625) at a slightly higher heat input. This layer further reduces dilution to below 15%.
- Final Layer (Surface Layer): Same alloy as the second layer, deposited with low heat input and short weld beads to minimize further dilution. This layer is subsequently machined to the final dimensions.
4.4 Weld Sequence and Distortion Control
Tube sheets are large, relatively thin plates (thickness-to-diameter ratio typically 0.05–0.15), making them highly susceptible to warping. The welding sequence must follow a symmetrical, spiral-inward or radial pattern to distribute heat input evenly:
- Begin at the outer circumference and weld in a controlled spiral toward the center, or use a radial "clock" pattern starting at 12 o'clock and working in opposite directions
- Weld in segments of 150–300 mm, with interpass temperature monitoring (maximum 250°C for most carbon steel base materials)
- Apply mechanical clamping or backing bars to the tube sheet to restrict through-thickness deformation
- For large tube sheets (>1,200 mm diameter), consider using a water-cooled backing plate to accelerate cooling and reduce distortion
4.5 Post-Weld Treatment
- Stress Relief: Solution heat treatment (1050–1100°C for stainless overlays) or stress relief annealing (550–650°C for carbon steel base) may be required per design specification
- Machining: Final surface grinding or turning to achieve the specified flatness (typically ±0.3 mm over 300 mm per ASME Section VIII Div. 1) and surface roughness (Ra ≤ 6.3 μm for gasket sealing surfaces)
- Tube Hole Cleaning: Remove all plugs, welding residue, and overlay material from tube holes using reaming or honing; verify hole diameter and roundness per ASME Section VIII Div. 1 Table UW-25
5. Applicable Standards and Acceptance Criteria
| Standard | Scope of Application | Key Requirements |
|---|---|---|
| ASME Section VIII Div. 1, UW-25 | Tube-to-tubesheet welds (subsequent operation) | Weld geometry, NDT requirements, minimum weld size |
| ASME Section II Part D | Welding consumables qualification | Filler metal composition, mechanical properties |
| ASME Section IX, QW-11.1 | WPS/PQR qualification | Essential variables for overlay welding |
| ASME Section V, Article 2 | Visual examination | No undercut, porosity, or surface defects exceeding limits |
| ASME Section V, Article 7 | Penetrant testing (PT) | 100% PT of overlay surface for surface-breaking defects |
| ASME Section V, Article 19 | Hardness testing | Overlay hardness within specified range (typically ≤35 HRC for austenitic SS) |
| ASME Section V, Article 23 | Chemical analysis of overlay | Surface layer composition verification (dilution ≤20%) |
| NB/T 47013.2 | PT for welds (Chinese standard) | Acceptance per Level II or higher |
| NB/T 47013.3 | UT for welds (Chinese standard) | Undercut and lack of fusion detection |
| GB/T 12467 | Welding procedure qualification (Chinese standard) | Essential variables, trial plate requirements |
| API 579-1/ASME FFS-1 | Fitness-for-service assessment | Residual stress evaluation, crack susceptibility |
| ASTM A312 / A268 | Stainless steel tube sheet base material (if applicable) | Material composition and mechanical properties |
| ASTM A516 / A216 | Carbon steel tube sheet base material | Base material specification |
5.1 Acceptance Criteria Summary
- Visual: No undercut exceeding 0.5 mm depth, no surface porosity exceeding 2 mm diameter, no cracks or slag inclusions
- PT: No linear indications (cracks) permitted; round indications (porosity) limited to ≤3 mm per 100 mm of weld length
- UT: No lack of fusion or undercut deeper than 10% of overlay thickness at the weld interface
- Hardness: Overlay surface hardness within 10 HRC of the base material's annealed hardness (to prevent cracking during subsequent tube welding)
- Chemical: Surface layer (top 0.5 mm) dilution ≤15% for the target alloy; first-layer dilution ≤35%
- Flatness: ±0.5 mm over any 300 mm diameter after machining (per design specification)
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Undercut at weld interface | Excessive heat input, incorrect travel speed, poor technique | Limit heat input to ≤1.5 kJ/mm; use narrow travel speed range; qualified welder supervision |
| Hydrogen-induced cracking (HIC) | Hydrogen entrapment in high-strength base metal | Preheat to 200–300°C; use low-hydrogen consumables (E71T-8 or equivalent); post-weld bake at 150°C for 2 hours |
| Chromium carbide precipitation (sensitization) | Slow cooling in the 450–850°C range at the fusion line | Use 309L (low-carbon) as first layer; control interpass temperature ≤250°C; rapid cooling with water quench if needed |
| Excessive dilution | Too few layers, high heat input, wide weld bead | Use minimum 3 layers; reduce travel speed to increase deposition rate; use smaller diameter filler wire |
| Tube sheet distortion/warping | Asymmetric heat input, thermal gradient | Symmetrical weld sequence; mechanical clamping; water-cooled backing; monitor flatness after each layer |
| Tube hole damage | Weld spatter, undercut at hole edges, overheating | Use ceramic or PTFE hole plugs; maintain minimum 10 mm distance from hole edge; inspect and ream all holes post-overlay |
| Subsequent tube weld cracking | Hard overlay surface, high residual stress | Limit overlay hardness to ≤35 HRC; stress relief annealing after overlay; verify tube weld WPS is qualified for the overlay material |
7. Application Scenarios Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
Tube-sheet weld overlay is a core application of the TIG/MIG weld overlay technology route. Typical projects include:
- Refinery heat exchanger tube sheets: Overlay of 316L or Alloy 625 on carbon steel tube sheets for sour service (H₂S-containing environments per NACE MR0175/ISO 15156)
- Offshore platform heat exchangers: Overlay of Alloy C-276 or Alloy 625 for seawater cooling applications
- Power plant superheater tube sheets: Overlay of Alloy 800H or Alloy 617 for high-temperature service
- Pulp/paper industry equipment: Overlay of Alloy 254 SMO or Alloy 625 for chloride-containing black liquor environments
For tube sheets with diameters below 800 mm and thicknesses below 40 mm, TIG (GTAW) is preferred for its superior surface finish and precise heat control. For larger tube sheets (800–2,500 mm diameter) with thicknesses above 40 mm, mechanized or semi-automated MIG (GMAW) is employed for productivity, with a final TIG pass for surface finish.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
Hydraulic explosive bonding is not directly applicable to tube-sheet fabrication due to the complex geometry (numerous tube holes, need for precision machining). However, it plays an indirect supporting role:
- Production of large-format clad plate (e.g., 316L/SA-516) that is subsequently machined into tube sheets, providing a cost-effective alternative to full weld overlay for thick tube sheets
- Supply of pre-clad stock for tube sheets where the overlay thickness exceeds 5 mm (beyond practical weld overlay limits)
- Hybrid approach: hydraulic explosive bonding for the bulk of the overlay thickness, followed by TIG weld overlay for the final surface layer and tube hole area preparation
7.3 Explosion Welding Route (Complementary Application)
Similar to hydraulic explosive bonding, explosion welding serves as a material supply technology rather than a direct tube-sheet fabrication method:
- Manufacture of large-diameter clad pipes that are used as tube sheet blanks in certain pressure vessel designs
- Production of clad plate stock for tube sheets where the base material is a high-strength alloy (e.g., 9Cr-1Mo) and the overlay is a corrosion-resistant alloy (e.g., 321 or Alloy 625)
- Explosion-welded clad plate can be machined into tube sheets with the overlay surface pre-formed, reducing the number of weld overlay layers required
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The tube-sheet weld overlay process research directly contributes to the company's qualification portfolio in the following ways:
- WPS/PQR Development: Qualified welding procedures covering a range of base materials (SA-516 Gr. 60/70, A216 WCB, A387 Gr. 11), overlay alloys (304L, 316L, 321, Alloy 625, Alloy 825, C-276), and thickness ranges (20–120 mm)
- ASME Section IX QW-11.1 Qualification: Overlay welding qualification covering essential variables including filler metal type, heat input range, preheat temperature, and interpass temperature
- NB/T 47014 Qualification (Chinese National Standard): WPS qualification for pressure vessel weld overlay in accordance with Chinese regulatory requirements
- Welder Qualification: Certified welders with demonstrated competence in tube-sheet overlay welding, including TIG and MIG techniques
8.2 Product Delivery
- Complete tube sheet packages: Delivery of overlay-applied, stress-relieved, machined, and NDT-inspected tube sheets ready for tube welding and assembly
- Custom alloy selection: Ability to supply tube sheets with different overlay alloys on each side (e.g., 316L on channel side, Alloy 625 on shell side)
- Dimensional accuracy: Post-overlay machining to achieve ±0.3 mm flatness and ±0.1 mm tube hole concentricity
- Traceability: Full material traceability from base plate through overlay consumables, with heat number tracking and NDT records
8.3 Customer Value
- Cost reduction: 40–70% material cost savings compared to solid alloy tube sheets, while achieving equivalent corrosion protection
- Service life extension: 10–20× life extension in aggressive chemical environments compared to unclad carbon steel
- Design flexibility: Ability to use carbon steel for structural strength and corrosion-resistant alloys for surface protection, optimizing the overall design
- Regulatory compliance: Full ASME, NB, and API compliance documentation, reducing customer qualification burden
- Reduced downtime: Overlay repair of existing tube sheets in service, avoiding full heat exchanger replacement
9. Conclusion
The tube-sheet weld overlay process research represents a critical technical competency for Cladding Technology Shanxi Co., Ltd. It bridges the gap between base material fabrication and final pressure vessel assembly, delivering a high-value-added service that directly impacts the reliability, safety, and economic performance of process equipment. Through rigorous WPS development, qualified welder certification, and comprehensive NDT protocols, the company ensures that every tube sheet delivered meets the most demanding specifications of the petrochemical, power, and offshore industries. The integration of this capability with the company's hydraulic explosive bonding and explosion welding routes creates a comprehensive cladding technology platform capable of serving the full spectrum of corrosion-resistant overlay requirements, from thin-walled precision components to large-scale pressure vessel fabrication.