Duplex Stainless Steel Strip Electrode Electroslag Weld Overlay: Process Comparison, Selection Criteria, and Engineering Application
1. Definition and Fundamental Principles
Strip electrode electroslag welding (ESW) is a high-deposition-rate, deep-penetration welding process in which a continuous strip of consumable electrode is fed through a molten slag pool to deposit weld metal in successive passes. When applied to duplex stainless steel overlay, the process creates a corrosion-resistant or wear-resistant surface layer on carbon steel or low-alloy steel substrates, leveraging the synergistic properties of the austenite-ferrite microstructure inherent to duplex grades such as UNS S31803 (2205) and UNS S32750 (2507).
The fundamental principle relies on the electroslag pool acting as a thermal reservoir that preheats and melts the electrode strip and the base material interface. The slag composition—typically flux-based with controlled alkalinity and viscosity—stabilizes the arc, protects the weld pool from atmospheric contamination, and ensures uniform heat distribution across wide weld beads. In duplex stainless steel overlay applications, the process parameters must be carefully controlled to maintain the critical ferrite-to-austenite (F/A) phase balance, typically within the 40–60% ferrite range by volume fraction, to ensure optimal pitting resistance, mechanical strength, and resistance to stress corrosion cracking (SCC).
2. Category and Business Positioning
Within the corporate technology framework of Cladding Technology Shanxi Co., Ltd., strip electrode ESW for duplex stainless steel overlay occupies a strategic position bridging the company's three core technology routes:
- TIG/MIG Weld Overlay: ESW serves as a complementary high-volume deposition process for thick-section components where wire-feed processes (TIG/MIG) would be economically impractical due to excessive cycle time.
- Hydraulic Explosive Bonding: ESW provides a post-bonding repair and transition-layer capability when explosive-bonded interfaces require additional thickness or metallurgical refinement.
- Explosion Welding (Clad Plate/Pipe Fabrication): ESW overlay offers an alternative or supplementary route for clad plate production where explosive welding is constrained by geometry, thickness, or material compatibility limitations.
The process is particularly positioned for heavy-wall equipment manufacturing in the oil, gas, chemical, and power generation sectors where large-diameter vessels, heat exchanger tubesheets, and pressure-containing components require corrosion-resistant duplex stainless steel overlays exceeding 6–12 mm in thickness.
3. Technical Purpose and Engineering Value
3.1 Primary Technical Objectives
- Corrosion Protection: Deliver a homogeneous duplex stainless steel surface layer with pitting resistance equivalent number (PREN) ≥ 34 for 2205-grade overlay and ≥ 38 for 2507-grade overlay.
- Thickness Achievement: Economically deposit 6–25 mm of duplex overlay in a single welding sequence with minimal dilution from the base material.
- Microstructural Control: Maintain ferrite content within the specified 40–60% range through controlled cooling rates, heat input, and interpass temperature management.
- Joint Integrity: Ensure full metallurgical bond strength at the overlay-base material interface with no interfacial defects detectable by NDT.
3.2 Economic and Operational Value
Strip electrode ESW achieves deposition rates of 8–15 kg/h, significantly exceeding the 1.5–3 kg/h typical of TIG wire-feed overlay. This translates to a 4–8× reduction in labor hours per unit of overlay thickness, making it the process of choice for large-scale clad plate production, vessel head overlay, and thick tubesheet cladding. The process also offers superior geometric control for wide beads (60–150 mm), reducing the number of passes required for full-width coverage.
4. Key Process and Implementation Points
4.1 Process Parameter Comparison: 2205 vs. 2507 Duplex Overlay
| Parameter | UNS S31803 (2205) Overlay | UNS S32750 (2507) Overlay | Rationale |
|---|---|---|---|
| Electrode Strip Composition | UNS S31803 (22% Cr, 6% Ni, 3% Mo) | UNS S32750 (25% Cr, 7% Ni, 4% Mo, 0.7% Cu) | Match overlay composition to target PREN; 2507 requires higher alloy content |
| Electrode Strip Dimensions | 0.8–1.5 mm × 35–100 mm | 1.0–1.6 mm × 40–120 mm | Thicker strip for higher alloy grades to maintain deposition rate |
| Flux Composition | Low-silica, basic flux (e.g., ESAB ESAB 504 or equivalent) | Low-silica, basic flux with controlled MnO content | Minimize Si and Mn dilution to preserve duplex microstructure |
| Heat Input | 20–45 kJ/mm | 25–55 kJ/mm | Higher heat input for 2507 to promote adequate ferrite dissolution and grain refinement |
| Interpass Temperature | ≤ 200°C | ≤ 180°C | Strict control to prevent σ-phase precipitation and excessive grain growth |
| Travel Speed | 200–400 mm/min | 180–350 mm/min | Adjusted per pass geometry and strip thickness |
| Welding Current | 600–1200 A | 700–1400 A | Higher current for wider beads and thicker sections |
| Welding Voltage | 32–42 V | 35–45 V | Controls slag pool volume and penetration profile |
| Target Ferrite Content | 40–60% (measured by FERRITEST) | 40–60% (measured by FERRITEST) | ASME/ISO requirement for duplex phase balance |
| Post-Weld Heat Treatment | 1050–1100°C, 30–60 min, air cool (solution treatment) | 1050–1100°C, 30–60 min, air cool (solution treatment) | Homogenize microstructure and dissolve any brittle phases |
4.2 Critical Implementation Steps
- Substrate Preparation: The base material surface must be machined to a minimum Ra of 12.5 µm, free from scale, rust, and surface contamination. Preheat to 100–150°C for carbon steel substrates to reduce thermal shock and hydrogen absorption.
- Flux Conditioning: Flux must be dried at 250–300°C for 2 hours prior to use to eliminate moisture, which is the primary source of hydrogen-induced cracking in duplex overlay welds.
- Transition Layer Application: A 309L or 309CB transition layer (1–2 mm) is typically deposited first via TIG or MIG before commencing the ESW duplex overlay. This mitigates dilution from the carbon steel substrate and prevents the formation of brittle intermetallic phases at the interface.
- First Pass ESW Setup: The first ESW pass is applied directly over the transition layer with a slightly reduced current to ensure adequate fusion without excessive dilution. The slag pool must be stable and uniform before the electrode strip engages the base.
- Multi-Pass Build-Up: Subsequent passes are deposited with full parameters, maintaining strict interpass temperature control. A maximum of 4–6 passes is recommended before interrupting for ferrite measurement and visual inspection.
- Final Pass and Surface Finishing: The top pass may be deposited with a slightly reduced heat input to minimize surface oxidation. The overlay surface is subsequently ground or machined to the required dimensional tolerance and surface finish (Ra ≤ 3.2 µm for gasket faces).
4.3 Dilution Control Strategy
Dilution is the primary metallurgical challenge in duplex ESW overlay. The following controls are implemented:
- Flux Shielding Optimization: A thicker, more viscous flux layer reduces base material melting and limits dilution to 5–10% for the first pass and ≤ 3% for subsequent passes.
- Electrode Positioning: The strip electrode is positioned at a slight angle (5–10°) toward the base material to concentrate heat in the slag pool rather than the substrate.
- Pass Sequence Planning: A staggered pass sequence ensures that each new pass straddles the previous pass, minimizing the cumulative dilution effect across the overlay thickness.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application | Key Requirements |
|---|---|---|
| GB/T 25500-2010 | Welding procedure qualification for ESW | WPS/PQR documentation, essential variables, test coupon dimensions |
| GB/T 8165-2008 | Welding procedure qualification for fusion welding | General qualification requirements, performance qualification |
| NB/T 47014-2011 | Welding procedure qualification for pressure vessels (China) | Essential variables, test methods, acceptance criteria for pressure equipment |
| ASME Section IX, QW-200 Series | Electrode and flux qualification for ESW | Procedure qualification, essential variables, limitations of qualification |
| ASME BPV Code Section VIII, Div. 1, UW-25 | Weld overlay qualification for pressure vessels | Overlay WPS qualification, hardness testing, dilution limits |
| ASTM A240 / A240M | Specification for austenitic and duplex stainless steel plate | Chemical composition, mechanical properties, ferrite content |
| ASTM A928 / A928M | Specification for seamless austenitic stainless steel pipe | Material properties for overlay substrate qualification |
| ISO 15614-1:2017 | Qualification of welding procedures for metallic materials | General requirements for WPS qualification, essential variables |
| ISO 13919-1:2013 | Welding procedure qualification for PTA and ESW | Specific requirements for ESW procedure qualification |
| NACE SP0432 / NACE MR0175 | Sour service requirements for weld overlay materials | Hardness limits (≤ 22 HRC for 2205, ≤ 25 HRC for 2507), chloride SCC resistance |
| API 941 | Welding procedures for sour service | WPS qualification requirements for H2S-containing environments |
| EN ISO 16401:2017 | Welding procedure qualification for ESW | European qualification requirements, essential variables |
5.2 Acceptance Criteria
- Chemical Composition: Overlay metal must conform to the specified duplex grade (e.g., ASTM A240 UNS S31803 or S32750) with dilution-adjusted composition falling within ± 1.0% of the nominal specification.
- Ferrite Content: Measured by FERRITEST or equivalent, ferrite content must be within 40–60% of the weld metal cross-section, measured at a minimum of 3 locations per coupon.
- Hardness: Surface hardness must not exceed 22 HRC for 2205 overlay or 25 HRC for 2507 overlay in accordance with NACE MR0175/ISO 15156. Hardness is measured at 5 locations per 100 mm of overlay surface.
- Microstructure: No brittle phases (σ-phase, χ-phase, Laves phase) shall be present. Ferrite grains shall be equiaxed with no abnormal grain growth exceeding ASTM E112 grain size No. 3.
- NDT Inspection:
- Visual inspection (VT) per ASTM E94: No surface cracks, porosity > 1.5 mm, or undercut > 1 mm.
- Magnetic particle inspection (MT) per ASTM E709: No linear indications ≥ 3 mm in length at the overlay interface.
- Ultrasonic testing (UT) per ASTM E164 or GB/T 11345: No interfacial delamination, lack of fusion, or internal defects exceeding the acceptance threshold of Level II.
- Positive material identification (PMI) per ASTM E1686: Overlay composition verified at minimum 3 locations per coupon.
- Mechanical Properties: Tensile strength ≥ 550 MPa for 2205 overlay and ≥ 620 MPa for 2507 overlay. Elongation ≥ 25% for 2205 and ≥ 20% for 2507.
- Corrosion Resistance: Pitting resistance verified by ASTM G48 (potentiodynamic polarization in 3.5% NaCl solution) with no pitting initiation within the test duration.
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| σ-phase precipitation | Excessive interpass temperature (> 250°C), prolonged heat exposure in the 600–800°C range | Enforce interpass temperature ≤ 200°C; use thermal imaging for real-time monitoring; limit total welding time per section |
| Excessive ferrite (> 60%) | High cooling rate, excessive Cr/Ni dilution from base material | Optimize heat input; apply transition layer; perform post-weld solution treatment |
| Insufficient ferrite (< 40%) | Excessive heat input, high Ni dilution, prolonged thermal cycling | Reduce heat input; increase travel speed; verify flux composition; apply post-weld heat treatment |
| Hydrogen-induced cracking | Moisture in flux, inadequate preheat, rapid cooling | Dry flux at 250–300°C; preheat base material to 100–150°C; apply post-weld bake-out at 250°C for 2 hours |
| Hot cracking (solidification cracking) | Excessive sulfur/phosphorus in electrode, low dilution, rapid solidification | Use low-S (< 0.015%) and low-P (< 0.030%) electrode strip; optimize heat input to promote grain refinement |
6.2 Process Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Slag inclusion | Inadequate slag removal between passes, excessive slag thickness | Mandate slag removal between every pass; verify slag thickness at 50–80 mm; use mechanical slag removal with inspection |
| Weld undercut | Excessive travel speed, inadequate current, poor electrode alignment | Optimize travel speed-current ratio; use electrode guide alignment fixtures; inspect each pass with VT |
| Porosity | Air entrainment, flux degradation, surface contamination | Ensure flux coverage over the entire weld area; shield the process from wind; clean base material surface prior to each pass |
| Dilution exceeding limits | Inadequate transition layer, excessive heat input on first pass | Apply 1–2 mm 309L transition layer; reduce first-pass current by 10–15%; measure dilution via PMI |
6.3 Inspection and Documentation Risks
- Incomplete WPS/PQR documentation: Ensure all essential variables per NB/T 47014-2011 and ASME Section IX are recorded, including electrode composition, flux type, heat input range, preheat/interpass temperature, and post-weld heat treatment parameters.
- Inadequate NDT coverage: Implement 100% UT for interfacial bond integrity and 100% MT for surface and near-surface defects on all production welds, not merely on qualification coupons.
- Traceability gaps: Maintain a complete welding log for each production weld including operator ID, electrode batch number, flux lot number, thermal charts, and NDT reports. This is critical for customer audits and regulatory compliance.
7. Application Scenarios Across Technology Routes
7.1 Integration with TIG/MIG Weld Overlay
Strip electrode ESW is most effectively deployed in a hybrid workflow with TIG/MIG overlay processes. The recommended sequence for thick overlay applications (≥ 6 mm) is:
- TIG/MIG Transition Layer: A 1–2 mm 309L or 309CB layer is deposited by TIG or MIG to establish a metallurgically compatible interface between the carbon steel substrate and the duplex overlay.
- ESW Bulk Build-Up: The bulk of the overlay (4–20 mm) is deposited by strip electrode ESW, achieving high deposition rates and economic efficiency.
- TIG/MIG Final Cap: A 1–2 mm duplex stainless steel cap is deposited by TIG or MIG to achieve the final surface finish, refine the microstructure, and ensure the top layer meets the most stringent ferrite and hardness requirements.
This hybrid approach leverages the high deposition rate of ESW for the bulk layer while utilizing the precision and microstructural control of TIG/MIG for the critical transition and cap layers. It is the preferred method for clad plate production, vessel head overlay, and thick tubesheet cladding where overlay thickness exceeds 6 mm.
7.2 Complementarity with Hydraulic Explosive Bonding
In hydraulic explosive bonding applications, the primary bonding mechanism is the high-velocity collision of the cladding material with the substrate. However, when the bonded interface requires additional thickness or when the explosive bonding process is limited by geometry (e.g., large-diameter cylindrical components), ESW overlay can be applied as a post-bonding thickening process. The duplex stainless steel ESW overlay is applied over the explosively bonded interface to achieve the required total cladding thickness while maintaining the metallurgical bond established by the explosive process.
7.3 Alternative to Explosion Welding for Clad Plate/Pipe Fabrication
For clad plate and pipe fabrication, explosion welding is the preferred process for achieving a metallurgical bond with minimal dilution. However, strip electrode ESW offers several advantages in specific scenarios:
- Thick Overlay Requirements: When overlay thickness exceeds 12–15 mm, ESW is more economical than explosion welding, which is typically limited to 3–10 mm of bonded cladding.
- Large Format Production: ESW can be applied to plates up to 3000 mm × 12000 mm with automated multi-gun configurations, offering flexibility for large-scale clad plate production.
- Material Flexibility: ESW can accommodate a wider range of substrate materials (including high-strength steels, low-alloy steels, and some nickel alloys) that may be challenging for explosion welding due to material compatibility constraints.
- Repair and Retrofit: ESW is the preferred process for repairing or retrofitting existing equipment with duplex stainless steel overlay, where explosion welding is not feasible due to the existing geometry and operational constraints.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The development and validation of strip electrode ESW for duplex stainless steel overlay directly contribute to the company's qualification portfolio in the following ways:
- WPS/PQR Expansion: Each qualified WPS for 2205 or 2507 ESW overlay covers a range of essential variables (heat input, electrode dimensions, flux type, interpass temperature), enabling qualification coverage for a broad spectrum of production welds with minimal additional testing.
- Multi-Standard Compliance: Qualification under NB/T 47014-2011, ASME Section IX, and ISO 15614-1:2017 simultaneously satisfies Chinese, American, and international regulatory requirements, expanding the company's market access.
- Operator Certification: The process knowledge gained from ESW qualification programs supports the certification of welding operators under GB/T 15169 (Welder Qualification), enhancing the company's workforce qualification depth.
8.2 Product Delivery
Strip electrode ESW enables the company to deliver products that would otherwise be economically or technically infeasible:
- Large-Format Clad Plate: Production of clad plates up to 3000 mm × 12000 mm with 6–25 mm duplex overlay, meeting the requirements of major EPC contractors for refinery, chemical, and LNG projects.
- Thick-Section Vessel Components: Overlay of large-diameter vessel heads, heat exchanger tubesheets, and pressure-containing components with duplex stainless steel, extending service life in corrosive environments.
- Retrofit and Repair: Application of duplex overlay to existing equipment in service, providing a cost-effective alternative to full component replacement and reducing customer downtime.
8.3 Customer Value
- Extended Service Life: Duplex stainless steel overlay provides 3–5× the corrosion resistance of conventional austenitic stainless steel in chloride-containing environments, directly extending equipment service life and reducing replacement frequency.
- Reduced Lifecycle Cost: The economic efficiency of ESW (high deposition rate, low labor cost) translates to lower overlay application costs, reducing the total lifecycle cost of corrosion-resistant equipment.
- Regulatory Compliance: Fully qualified WPS/PQR documentation and comprehensive NDT reporting provide customers with the documentation required for regulatory submissions, insurance claims, and operational audits.
- Technical Differentiation: The company's capability in duplex ESW overlay positions it as a preferred supplier for high-specification projects where corrosion resistance and regulatory compliance are critical selection criteria.
9. Conclusion
Strip electrode electroslag welding for duplex stainless steel overlay represents a high-value, technically demanding process that bridges the gap between conventional TIG/MIG overlay and explosive bonding/explosion welding in the company's technology portfolio. Its successful implementation requires rigorous control of process parameters, strict adherence to metallurgical requirements, and comprehensive NDT and documentation practices. By investing in the qualification and optimization of this process, Cladding Technology Shanxi Co., Ltd. strengthens its position in the heavy-wall, large-format, and high-specification overlay market, delivering superior corrosion-resistant solutions to demanding industrial customers while maintaining full regulatory compliance and traceability.