Experimental Research on Strip Electrode Weld Overlay of Duplex Stainless Steel
1. Definition and Technical Principles
Strip electrode weld overlay of duplex stainless steel refers to a specialized surface engineering process in which a consumable strip (or ribbon) electrode is fed continuously into a TIG or MIG welding arc to deposit one or more layers of duplex stainless steel alloy onto a carbon steel or low-alloy steel substrate. The process leverages the inherently high dilution rate associated with strip electrode geometry—typically 20% to 40% dilution per pass—to achieve a metallurgically sound interface while maintaining the required chromium-to-nickel ratio in the weld metal.
The fundamental metallurgical principle relies on the balanced austenite-ferrite microstructure of duplex stainless steels (typically 40–60% ferrite). During strip electrode overlay, the molten pool composition is governed by the dilution equation:
Cweld = Cbase × D + Cfiller × (1 − D)
where C represents the alloying element concentration, D is the dilution fraction, and subscripts denote base metal and filler material respectively. Because strip electrodes exhibit higher dilution than wire electrodes, the filler alloy must be enriched in chromium, molybdenum, and nitrogen beyond the target duplex composition to compensate for base metal dilution. For example, when overlaying SA508 Gr.3 Cl.1 base metal, a 22Cr-5Ni-3Mo strip electrode may be required to achieve a 2205-equivalent weld metal after dilution.
The experimental research documented in this study systematically investigated the effects of welding parameters, preheat temperature, interpass temperature, and layer sequence on the microstructure, mechanical properties, and corrosion resistance of the resulting overlay. The learnings from this research form the technical foundation for production-scale duplex steel overlay operations.
2. Category and Business Positioning
2.1 Technology Classification
Strip electrode weld overlay of duplex stainless steel falls within the TIG/MIG Weld Overlay technology route of the company's three primary surface engineering capabilities. Within this route, it occupies a premium segment due to the following characteristics:
- High dilution management complexity: Requires precise metallurgical calculations and WPS qualification
- Multi-layer deposition: Typically requires 2–4 layers to achieve full composition homogeneity
- Sensitivity to heat input: Duplex microstructure is highly susceptible to phase transformation outside the optimal window
- Post-weld inspection rigor: Requires metallographic ferrite content verification and corrosion testing
2.2 Business Positioning
This technology serves as a critical bridge between conventional carbon steel fabrication and high-performance corrosion-resistant applications. It positions the company to deliver value-added clad products for the oil and gas, marine, chemical processing, and power generation industries where duplex stainless steel performance is mandated by design codes or operating conditions. The experimental research capability demonstrated through this study is a prerequisite for WPS qualification, customer audits, and entry into qualification-based procurement programs.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Corrosion resistance upgrade: Transform carbon steel or low-alloy steel surfaces into duplex stainless steel-equivalent corrosion performance, achieving pitting resistance equivalent number (PREN) ≥ 34–38
- Metallurgical compatibility: Ensure sound bonding interface without excessive brittle phases (sigma, chi, Laves) at the overlay-substrate boundary
- Mechanical integrity: Achieve overlay hardness of 200–280 HV and tensile strength ≥ 550 MPa while maintaining ductility ≥ 30% elongation
- Dimensional control: Achieve overlay thickness tolerance of ±0.5 mm with acceptable surface profile (Ra ≤ 12.5 μm after grinding)
3.2 Value Proposition
The experimental research on strip electrode duplex overlay provides quantifiable value through:
- Cost optimization: Strip electrode processes offer 30–50% higher deposition rates compared to conventional wire electrode processes, reducing labor hours per unit area
- Material efficiency: Optimized dilution calculations reduce filler material consumption by 15–25% compared to unoptimized processes
- Quality assurance: Systematic parameter control reduces rework rates and NDT failure rates to below 3%
- Design flexibility: Enables overlay on geometries (large diameter pipes, thick plate, complex fittings) where explosion welding or hydraulic bonding is impractical
4. Key Process and Implementation Points
4.1 Base Metal Preparation
Proper base metal preparation is the single most critical factor in overlay quality. The following preparation sequence is mandated:
- Machining: Surface must be machined to a minimum depth of 3 mm to remove surface contamination, scale, and prior heat-affected zones
- Beveling: For overlay thickness ≥ 2 mm, a single-V groove with 60° included angle is recommended to ensure adequate fusion at the root
- Cleaning: Solvent degreasing followed by grinding with clean abrasive to expose fresh metal within 4 hours of welding
- Dimensional verification: Base metal thickness must be verified to ensure sufficient backing support and avoid burn-through
4.2 Welding Parameter Matrix
| Parameter | Layer 1 (Bonding) | Layer 2 (Build-up) | Layer 3 (Cap) | Notes |
|---|---|---|---|---|
| Process | Strip Electrode TIG | Strip Electrode TIG | Strip Electrode TIG | Alternative: MIG for thick sections |
| Electrode Alloy | 25Cr-6Ni-3Mo (high Cr) | 22Cr-5Ni-3Mo (target) | 22Cr-5Ni-3Mo (target) | Compensates for dilution gradient |
| Strip Width | 25 mm | 25 mm | 25 mm | 20–30 mm range typical |
| Strip Thickness | 1.6 mm | 2.4 mm | 2.4 mm | Thicker strips for build-up layers |
| Welding Current | 180–220 A | 250–300 A | 250–300 A | DCEN polarity |
| Arc Voltage | 16–18 V | 18–20 V | 18–20 V | Lower voltage = lower dilution |
| Travel Speed | 200–250 mm/min | 300–400 mm/min | 300–400 mm/min | Speed inversely proportional to dilution |
| Heat Input | 0.8–1.2 kJ/mm | 1.0–1.4 kJ/mm | 1.0–1.4 kJ/mm | Critical: must stay below 1.5 kJ/mm |
| Shielding Gas | 100% Ar or 98% Ar + 2% N₂ | 100% Ar or 98% Ar + 2% N₂ | 100% Ar or 98% Ar + 2% N₂ | N₂ addition helps retain nitrogen in weld metal |
| Gas Flow Rate | 15–20 L/min | 15–20 L/min | 15–20 L/min | With back purging 5–10 L/min |
| Preheat Temperature | 50–100°C | — | — | For carbon equivalent > 0.45% substrates |
| Interpass Temperature | ≤ 150°C | ≤ 150°C | — | Critical to prevent sigma phase formation |
4.3 Dilution Management Strategy
The experimental research established a three-layer approach to manage dilution systematically:
- Layer 1 (Bonding Layer): Uses a high-chromium, high-nickel strip electrode with lower heat input to create a transition zone that prevents excessive carbon steel dilution into subsequent layers. Target dilution: 30–35%.
- Layer 2 (Build-up Layer): Uses the target duplex composition strip electrode at higher deposition rate. Dilution from Layer 1 is approximately 15–20%.
- Layer 3 (Cap Layer): Final surface layer using target composition with controlled parameters to achieve final chemistry within specification. Dilution from Layer 2 is approximately 5–10%.
4.4 Metallurgical Control Points
| Control Parameter | Acceptance Range | Measurement Method | Criticality |
|---|---|---|---|
| Ferrite Content (ISO %F) | 35–65% | Ferritscope / Metallographic etch | Critical |
| PREN (Pitting Resistance Equivalent Number) | ≥ 34 | Chemical analysis: %Cr + 3.3×%Mo + 16×%N | Critical |
| Hardness (HV30) | 200–280 HV | Vickers hardness test per ASTM E92 | Important |
| Tensile Strength | ≥ 550 MPa | ASTM E8 tensile test | Important |
| Impact Energy (−40°C) | ≥ 47 J | ASTM E23 Charpy V-notch | Important |
| Chromium Content | 21.5–23.5% | OES / Wet chemical analysis | Critical |
| Nickel Content | 4.5–6.5% | OES / Wet chemical analysis | Critical |
| Molybdenum Content | 2.5–3.5% | OES / Wet chemical analysis | Important |
| Nitrogen Content | 0.14–0.20% | Combustion analysis | Important |
4.5 Experimental Research Key Findings
The systematic experimental program yielded several critical findings that directly inform production WPS development:
- Interpass temperature sensitivity: Increasing interpass temperature from 100°C to 200°C caused ferrite content to drop from 55% to 32%, indicating excessive austenite formation. The maximum allowable interpass temperature was established at 150°C.
- Shielding gas nitrogen effect: Adding 2% N₂ to the shielding gas increased weld metal nitrogen content by 0.03–0.05%, providing a PREN boost of 0.5–0.8 without requiring additional molybdenum.
- Strip feed angle optimization: A feed angle of 10–15° from vertical (leading angle) produced the most uniform dilution profile across the strip width, compared to 5° (trailing) which showed 20% variation.
- Travel speed-dilution relationship: A 10% increase in travel speed reduced dilution by approximately 3–4 percentage points, establishing a practical control mechanism for on-the-fly dilution management.
- Heat input threshold: Heat inputs exceeding 1.5 kJ/mm produced measurable sigma phase at the weld root, confirmed by metallographic examination with Murakami's reagent.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application | Key Requirements |
|---|---|---|
| GB/T 8165 | Welding consumables for stainless steel | Filler metal classification and composition |
| GB/T 20878 | Duplex stainless steel grades | Material specification for 2205, 2507 grades |
| NB/T 47014 | Welding procedure qualification for pressure equipment | WPS qualification requirements, essential variables |
| ASME Section IX | Welding, brazing, and fusing qualifications | WPQ/WPS qualification, P-number classification |
| ASTM A240 / A928 | Duplex stainless steel plate and forgings | Target overlay composition reference |
| NACE MR0175 / ISO 15156 | Materials for H₂S-containing environments | Hardness limits, impact testing requirements |
| ASTM A388 | Clad plate acceptance criteria | Bond test, thickness tolerance, NDT requirements |
| EN ISO 13919 | Welding of duplex stainless steels | Welding procedure recommendations, heat input limits |
| API 5L / API 5CT | Line pipe and tubulars | Overlay thickness requirements for casing |
| GB/T 3323 | RT inspection of welds | Acceptance level for overlay welds (Level B) |
| GB/T 11345 | UT inspection of welds | Thickness measurement and flaw detection |
| GB/T 6060 | PT inspection | Surface defect detection for overlay |
5.2 Acceptance Criteria Summary
- Visual inspection (VT): No cracks, porosity > 1 mm, undercut > 0.5 mm, or surface irregularities. Overlay width must exceed base metal width by minimum 5 mm on each side.
- Penetrant testing (PT): Acceptance per ASTM E165 Level 2. No linear indications exceeding 3 mm in length permitted.
- Ultrasonic testing (UT): Bond integrity verified per ASTM A388. No delamination or lack of fusion indications.
- Radiographic testing (RT): Acceptance per GB/T 3323 Level B or ASME Section V Article 4. No Type I or Type II defects in overlay zones.
- Mechanical testing: Tensile, hardness, and impact tests per NB/T 47014 and ASME Section IX. All results within specified ranges.
- Metallurgical examination: Ferrite content 35–65% ISO %F measured at multiple positions across the overlay cross-section. No sigma phase or other intermetallic compounds permitted.
- Corrosion testing: Salt spray testing per ASTM B117 for minimum 500 hours without pitting. Potentiodynamic polarization per ASTM G5 for PREN verification.
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Detection Method | Control Measure |
|---|---|---|---|
| Sigma phase formation | Heat input > 1.5 kJ/mm; interpass > 200°C; slow cooling | Metallographic examination with Murakami's reagent | Limit heat input; control interpass temperature; use water quench if needed |
| Excessive austenite (>65%) | Low dilution; high nickel from base metal; excessive heat input | Ferritscope measurement; metallographic examination | Increase Cr in filler; reduce heat input; increase travel speed |
| Excessive ferrite (<35%) | High dilution from high-Cr base; insufficient Ni in filler | Ferritscope measurement | Use higher-Ni strip electrode; reduce dilution by increasing travel speed |
| Chromium carbide precipitation | Weld metal in sensitization range (450–850°C) for extended time | Intergranular corrosion test per ASTM A262 Practice E | Minimize time in sensitization range; use low-carbon filler |
| Hot cracking | Excessive sulfur/phosphorus; low dilution; high restraint | Visual/RT inspection | Control base metal impurities; ensure adequate dilution; reduce restraint |
6.2 Process Risks
| Risk | Cause | Detection Method | Control Measure |
|---|---|---|---|
| Lack of fusion at root | Insufficient preheat; low current; excessive travel speed | UT per ASTM A388; destructive cross-section | Verify preheat temperature; increase current; reduce travel speed for Layer 1 |
| Porosity | Inadequate shielding; moisture in electrode; contamination | RT or PT inspection | Verify gas flow; use dry electrodes; clean base metal thoroughly |
| Uncontrolled dilution | Parameter drift; inconsistent base metal chemistry; operator error | Chemical analysis of overlay cross-section | Implement parameter monitoring; verify base metal chemistry pre-weld; certified operators |
| Surface irregularities | Inconsistent strip feed; arc wander; vibration | Visual inspection; surface profilometry | Use automated feed system; stabilize workpiece; maintain consistent torch angle |
6.3 Risk Mitigation Protocol
- Pre-production verification: All new WPS must undergo minimum three-coupon qualification with full metallurgical and mechanical testing before production release
- In-process monitoring: Real-time heat input calculation using current, voltage, and travel speed data; automated interpass temperature logging
- Statistical process control: Ferrite content monitoring on 10% of production lots; hardness mapping on 5% of lots
- Non-conformance management: Any ferrite content outside 35–65% range triggers immediate process review and additional testing on adjacent areas
- Operator qualification: Minimum 500 hours of documented duplex overlay experience; annual re-qualification testing per ASME Section IX
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
Strip electrode duplex overlay is the flagship application of the TIG/MIG weld overlay route. Specific deployment scenarios include:
- Large diameter pipe overlay: Internal overlay of API 5L X65/X70 line pipe for sour service (H₂S > 0.25% partial pressure). Strip electrode provides uniform coverage on large internal diameters (DN400–DN1200) where wire electrode processes are impractical.
- Thick plate overlay: Surface overlay of heat exchanger channel plates, reactor shells, and pressure vessel heads. Multi-pass strip electrode achieves 3–6 mm overlay thickness in fewer passes than wire processes.
- Fitting and flange overlay: Complex geometry overlay on elbows, tees, reducers, and flanges where geometric constraints limit wire electrode accessibility.
- Repair overlay: Restoration of duplex surface on previously clad equipment that has suffered erosion, corrosion, or mechanical damage.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (hydroforming with explosive assist) primarily produces bulk clad plate, the strip electrode overlay technology serves as a complementary process in the following scenarios:
- Edge sealing: After hydraulic bonding produces a clad plate, the exposed edges (where the duplex layer terminates) require overlay to prevent corrosion initiation at the clad edge. Strip electrode overlay extends the duplex protection 5–10 mm beyond the bonded area.
- Post-forming repair: Clad plate that has been formed, cut, or drilled requires overlay repair at damage sites where the duplex layer has been removed. Strip electrode overlay restores the protective surface locally.
- Transition zone creation: When joining clad plate to unclad steel, a graded overlay transition zone is created using strip electrode to prevent galvanic coupling at the joint.
7.3 Explosion Welding Route (Supporting Application)
In explosion welding operations, strip electrode overlay serves as a supporting technology for:
- Pre-weld surface preparation: A thin pre-overlay of compatible material on the base plate before explosion welding can improve bond quality by providing a controlled interface composition.
- Post-explosion repair: Localized defects at the explosion weld interface (typically affecting < 2% of the bond area) are repaired by removing the defective zone and applying strip electrode overlay to restore duplex integrity.
- Overlay on explosion-welded products: After explosion welding produces a base clad product, additional duplex overlay can be applied to specific zones requiring enhanced corrosion protection (e.g., crevice areas, weld zones).
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The experimental research on strip electrode duplex overlay directly supports the company's qualification portfolio in the following ways:
- WPS Database Development: The parameter matrix established through experimental research forms the basis for qualified welding procedure specifications covering multiple base metals (SA508 Gr.3, A516 Gr.70, A333 Gr.6), multiple duplex grades (2205, 2507, 254 SMO), and multiple geometries (flat plate, pipe internal, pipe external, curved surfaces).
- Operator Qualification: The research establishes the competency framework for operator training programs. Operators must demonstrate ability to maintain heat input within ±10% of WPS parameters, control interpass temperature, and produce ferrite content within specification on qualification coupons.
- Customer Audit Readiness: The systematic experimental data provides traceable evidence for customer qualification audits. Documentation includes raw data sheets, test reports, parameter logs, and acceptance/rejection records that demonstrate consistent process control.
- Third-Party Certification Support: The experimental findings support applications for NB (National Supervision Bureau) certification of overlay procedures, ASME U stamp qualification for pressure equipment overlay, and API Q1 quality system certification for overlay operations.
8.2 Product Delivery Enhancement
- Production scalability: The strip electrode process achieves deposition rates of 15–25 kg/h compared to 8–12 kg/h for wire electrode processes, enabling delivery of large-volume overlay orders within tight project schedules.
- First-pass quality: Systematic parameter control based on experimental research reduces first-pass acceptance rates from industry average of 75–85% to > 95%, significantly reducing rework costs and schedule risk.
- Multi-grade capability: The dilution management methodology developed through research enables rapid adaptation to different duplex grades (2205, 2507, 329, S32750) by adjusting strip electrode chemistry while maintaining the same process parameters.
- Documentation completeness: Each production overlay package includes: WPS, WPQ, parameter logs, NDT reports, metallurgical test reports, and material traceability records—all traceable to the qualified experimental procedures.
8.3 Customer Value Delivery
- Extended asset life: Duplex overlay extends equipment service life from 5–10 years (bare carbon steel) to 25–40 years (duplex overlay) in aggressive chloride environments, providing 3–4× life extension value.
- Cost avoidance: Overlay on existing carbon steel equipment costs 40–60% less than replacement with solid duplex stainless steel, while achieving equivalent corrosion performance at the surface.
- Design flexibility: Customers can specify duplex overlay on standard carbon steel products, avoiding the cost premium and supply limitations of solid duplex materials while achieving required corrosion performance.
- Compliance assurance: The qualified overlay procedures meet NACE MR0175/ISO 15156 requirements for sour service, ASME Section VIII requirements for pressure vessels, and API requirements for oil and gas equipment—providing regulatory compliance assurance.
- Performance verification: Each delivered product includes full metallurgical and corrosion test data, providing customers with verifiable evidence of overlay performance rather than relying solely on process certification.
9. Continuous Improvement and Future Development
9.1 Technology Development Priorities
- Automated strip electrode systems: Development of CNC-controlled strip electrode overlay robots for consistent parameter control on complex geometries
- Advanced monitoring: Integration of real-time arc voltage/current monitoring with automatic travel speed adjustment to maintain constant heat input
- Multi-layer optimization: Research into two-layer systems (eliminating bonding layer) for cost reduction where base metal dilution is manageable
- Super-duplex extension: Extension of qualified procedures to 2507 and 254 SMO grades for ultra-aggressive environments
- Hybrid process development: Combination of strip electrode overlay with post-weld heat treatment for optimized microstructure control
9.2 Knowledge Management
The experimental research findings are systematically captured in the company's technical knowledge base, including:
- Complete parameter-dilution correlation databases for all qualified base metal/filler combinations
- Microstructure atlas documenting acceptable and unacceptable weld metal morphologies
- Failure mode library with root cause analysis and corrective action protocols
- Customer-specific procedure adaptations with traceable qualification data
- Operator training materials derived from experimental findings and production experience
10. Conclusion
The experimental research on strip electrode weld overlay of duplex stainless steel represents a foundational technical capability that underpins the company's premium surface engineering services. The systematic investigation of dilution control, microstructure management, and process parameter optimization has produced qualified procedures, trained operators, and documented knowledge that directly enable reliable product delivery across the oil and gas, chemical processing, marine, and power generation industries.
This capability is not merely a welding technique but a comprehensive quality system encompassing metallurgical science, process engineering, non-destructive evaluation, and documentation management. The experimental research methodology demonstrated in this study establishes the technical rigor required for qualification-based procurement environments where every parameter, every test result, and every acceptance decision must be traceable to qualified procedures and documented evidence.
As the company continues to expand its technology portfolio across all three surface engineering routes, the strip electrode duplex overlay capability serves as the connective tissue—enabling repair, transition, and enhancement of products produced by both hydraulic explosive bonding and explosion welding processes. This integrated approach positions the company as a comprehensive surface engineering solutions provider capable of delivering qualified, traceable, and performance-verified duplex overlay products for the most demanding industrial applications.