Duplex Stainless Steel Weld Overlay on Heat Exchanger Tube Sheets and Tube-to-Tube Sheet Welding

1. Definition and Technical Principles

Duplex stainless steel weld overlay on heat exchanger tube sheets is a specialized surfacing technology applied to the tube-side and shell-side surfaces of carbon steel or low-alloy steel tube sheets to create a corrosion-resistant duplex stainless steel layer. This overlay typically employs grades such as UNS S31803 (2205), UNS S32750 (2507), or UNS S32760 (Zeron 100), providing a ferrite-austenite microstructure with a Ferrite Number (FN) between 35 and 65. The overlay layer serves as a sacrificial barrier against chloride-induced pitting, crevice corrosion, and stress corrosion cracking (SCC) in aggressive process environments.

The subsequent tube-to-tube sheet welding process involves joining heat exchanger tubes (typically made of duplex stainless steel, titanium, or nickel alloys) to the overlaid tube sheet surface using penetration welding or backfill welding techniques. This creates a metallurgically sound, leak-tight joint that maintains the corrosion resistance established by the overlay layer while ensuring mechanical integrity under design pressure and thermal cycling.

The fundamental metallurgical principle relies on controlling dilution between the base carbon steel, the duplex overlay, and the tube material during welding. Excessive dilution from carbon steel into the overlay zone reduces the equivalent chromium (CEq) below the threshold required for duplex microstructure stability, leading to single-phase austenitic or ferritic regions susceptible to corrosion. Conversely, excessive dilution from the overlay into the tube material can destabilize the tube alloy composition. The welding process must therefore be carefully controlled through heat input management, filler metal selection, and multi-pass strategies.

2. Category and Business Positioning

This technology falls squarely within the TIG/MIG weld overlay route of the company's three core technology platforms. It represents a high-value-added, precision welding service that bridges the gap between bulk material cladding and component-level fabrication. In the company's business architecture, this capability serves as:

The technology is positioned at the intersection of weld overlay engineering and pressure equipment fabrication, requiring compliance with both ASME Section IX welding procedure qualification and ASME Section VIII or GB 150 pressure vessel code requirements.

3. Technical Purpose and Value

The primary technical purpose is to provide a durable, corrosion-resistant barrier layer on carbon steel tube sheets that would otherwise be incompatible with the duplex stainless steel or alloy tubes installed therein. Without such overlay, the carbon steel tube sheet would suffer rapid corrosion in chloride-containing environments, leading to tube sheet thinning, tube pull-out failures, and catastrophic heat exchanger leakage.

Key value propositions include:

4. Key Process Implementation Points

4.1 Tube Sheet Preparation

The base tube sheet must be machined to final dimensions before overlay application. The surface to be overlaid requires grinding to a smooth finish (Ra ≤ 6.3 μm) with removal of all scale, oxide, oil, and contaminants. A minimum base material thickness of 1.5 times the total overlay thickness must remain to ensure structural integrity. Preheating temperature depends on base material thickness and carbon equivalent, typically 100-200°C for carbon steel tube sheets exceeding 25 mm thickness.

4.2 Overlay Welding Process Parameters

Parameter First Pass (Transition) Second Pass (Duplex) Final Pass (Duplex)
Process TIG (GTAW) TIG (GTAW) or MIG (GMAW) TIG (GTAW) or MIG (GMAW)
Filler Metal (First Pass) ER309L or ER319L ER2209 or ER2594 ER2209 or ER2594
Filler Metal (Duplex Passes) — ER2209 (S31803) or ER2594 (S32750) ER2209 (S31803) or ER2594 (S32750)
Wire Diameter 1.6 mm (TIG) / 1.2 mm (MIG) 2.0 mm (TIG) / 1.2-1.6 mm (MIG) 2.0 mm (TIG) / 1.2-1.6 mm (MIG)
Current (TIG) 80-120 A 120-180 A 120-180 A
Travel Speed (TIG) 150-200 mm/min 120-180 mm/min 120-180 mm/min
Heat Input 0.5-1.0 kJ/mm 0.8-1.5 kJ/mm 0.8-1.5 kJ/mm
Interpass Temperature ≤ 250°C ≤ 250°C ≤ 250°C
Shielding Gas Ar 99.99% Ar 99.99% or Ar/He 75/25 Ar 99.99% or Ar/He 75/25
Typical Layer Thickness 1.5-2.0 mm 2.0-3.0 mm 2.0-3.0 mm

4.3 Multi-Pass Strategy

The overlay is applied in a minimum of three passes: (1) a transition pass using austenitic 309L filler to manage dilution from the carbon steel base; (2) a second pass using duplex filler to establish the duplex microstructure; (3) a final pass using duplex filler to achieve the required overlay thickness and surface quality. The transition pass dilution from carbon steel is calculated to ensure the subsequent duplex pass achieves a CEq ≥ 22% and FN between 35-65.

Dilution calculation follows the formula:

CEq(duplex pass) = CEq(base × dilution%) + CEq(fill × (100-dilution%))

Typical TIG dilution from base into the first duplex pass is 25-40%, which must be factored into the WPS design to ensure the final microstructure meets duplex requirements.

4.4 Tube-to-Tube Sheet Welding

After overlay completion, the tube sheet is installed into the heat exchanger shell, and tubes are inserted through the overlaid holes. The tube-to-tube sheet joint is welded using one of the following methods:

Welding Method Applicable Tube Material Filler Metal Key Considerations
TIG Penetration Weld (Tube-side) 2205, 2507 Duplex SS ER2209 or ER2594 Low heat input; backfill with backing ring or gas
TIG Backfill Weld (Shell-side) 2205, 2507 Duplex SS ER2209 or ER2594 Full penetration verification required
TIG Penetration + Backfill Ti, Inconel 625, Hastelloy C-276 Material-matched filler Contamination control critical for reactive metals
Resistance Welding (RWB) 316L, 321 SS tubes None (autogenous) Not applicable for duplex overlay tube sheets

4.5 Post-Weld Heat Treatment (PWHT)

For tube sheets with base material thickness exceeding 25 mm (or where required by code), PWHT is performed at 425-450°C for a holding time of 2 hours per 25 mm of thickness (minimum 2 hours), followed by controlled cooling. This relieves welding residual stresses without sensitizing the duplex overlay. The PWHT temperature must not exceed 450°C to avoid sigma phase formation in the duplex microstructure.

5. Applicable Standards and Acceptance Criteria

5.1 Welding Procedure Standards

5.2 Material Standards

5.3 Non-Destructive Testing Standards

5.4 Acceptance Criteria

Inspection Method Scope Acceptance Level Standard Reference
Visual (VT) 100% of overlay surface ASME Section IX QW-191.8 — no cracks, porosity >2 mm, undercut >0.5 mm ASME IX / GB/T 985.1
Penetrant (PT) 100% of overlay surface and tube joints ASME Section V Art. 7 — no linear indications; round indications ≤ 3 mm ASME V Art. 7
Magnetic Particle (MT) 100% of tube-to-tube sheet welds No cracks, incomplete fusion, or porosity clusters ASME V Art. 7 / ASTM E165
Ultrasonic (UT) 100% of tube-to-tube sheet joints Full penetration; no lack of fusion; overlap area ≤ 20% of tube wall thickness GB/T 11345 / ASME V Art. 23
Radiographic (RT) 10% of tube joints (minimum 1 per tube sheet) ASME Section V Art. 2 T-274 — Level II or better ASME V Art. 2
Overlay Thickness Every 100 mm² area (minimum 5 points) ≥ specified minimum thickness (typically 3.0 mm); uniformity ±0.5 mm ASME VIII Div.1 Appendix 2
Hardness 3 points per tube sheet ≤ 350 HV for 2205 overlay; ≤ 380 HV for 2507 overlay ASTM A240 / material spec

5.5 Microstructural Acceptance

The overlay weld metal must demonstrate a duplex microstructure with Ferrite Number (FN) between 35 and 65, verified by magnetic ferrite gauge measurement per ASTM E1538 or ISO 8044. Metallographic examination per ASTM E407 must confirm the presence of both ferrite and austenite phases, with no sigma phase, intermetallic precipitates, or excessive grain growth at the overlay/base metal interface.

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk Cause Consequence Control Measure
Excessive dilution from base steel High heat input; single-pass overlay; poor weld geometry Single-phase austenitic zone; loss of duplex corrosion resistance Multi-pass strategy with transition layer; controlled heat input ≤1.5 kJ/mm; dilution calculation in WPS
Sigma phase formation PWHT temperature >450°C; prolonged exposure in 400-600°C range Brittle intermetallic phase; reduced toughness and corrosion resistance PWHT at 425-450°C maximum; limit holding time; post-PWHT hardness verification
400-series intergranular corrosion Carbon enrichment at grain boundaries during welding Grain boundary attack in overlay layer Low-carbon filler metals (ER2209L); controlled heat input; rapid cooling where possible
Tungsten inclusion in TIG welds Electrode contact with weld pool; improper stick-out Crack initiation site; porosity Proper electrode preparation; consistent stick-out (8-10 mm); visual inspection of each bead

6.2 Process Risks

6.3 Quality Control Measures

7. Application Scenarios Across Three Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

This technology entry is fundamentally a TIG/MIG weld overlay application. The company's TIG/MIG overlay capability is directly exercised in:

7.2 Hydraulic Explosive Bonding Route (Complementary Application)

While hydraulic explosive bonding is primarily used for large-area cladding of bulk components (pipe, plate, sheet), it can complement the weld overlay technology in the following scenarios:

7.3 Explosion Welding Route (Complementary Application)

Explosion welding (air-gap explosion welding) offers additional capabilities for this technology entry:

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

This technology entry represents a critical qualification milestone for the company in the following respects:

8.2 Product Delivery Capabilities

8.3 Customer Value

9. Conclusion

Duplex stainless steel weld overlay on heat exchanger tube sheets and tube-to-tube sheet welding represents a high-value, technically demanding service that requires deep metallurgical understanding, precise process control, and rigorous quality management. By integrating this capability across the company's TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding technology routes, Cladding Technology Shanxi Co., Ltd. can deliver comprehensive cladding solutions for heat exchanger components — from bulk cladding of shells and channel covers to precision overlay of tube sheets and tube joint fabrication. This integrated approach maximizes customer value by providing single-source procurement, unified quality assurance, and optimized cost-performance solutions for demanding corrosion-resistant heat exchanger applications across the oil, gas, chemical, power, and desalination industries.