Strip Electrode Electroslag Weld Overlay of Duplex Stainless Steel on Pipe Sheets
1. Definition and Fundamental Principles
Strip electrode electroslag weld overlay (ESW overlay) is a heavy metal-deposition process that leverages the high electrical resistance of a molten slag pool to generate heat, enabling the controlled fusion of strip-shaped consumable electrodes into a thick overlay deposit on base substrates such as pipe sheets, pressure vessel heads, and flanges. Unlike conventional arc welding processes (TIG or MIG), electroslag welding does not rely on an arc to melt the consumable; instead, the electrical current passes through a layer of molten flux (slag), which acts as the primary heat source. The heat generated by resistive heating of the slag melts both the strip electrode and the underlying weld metal, producing a molten pool that solidifies into a dense, homogeneous overlay.
When applied to duplex stainless steel (DSS) overlay on carbon or low-alloy steel pipe sheets, this process enables the construction of thick corrosion-resistant layers—typically ranging from 10 mm to over 50 mm—in a single pass or a small number of passes. The duplex microstructure (a balanced mixture of austenite and ferrite phases, typically 40–60% ferrite) imparts superior resistance to chloride stress corrosion cracking, pitting, and crevice corrosion compared to austenitic stainless steels, while maintaining adequate mechanical strength and toughness.
2. Category and Business Positioning
This technology falls squarely within the company's TIG/MIG weld overlay and heavy overlay technology route, specifically representing the heavy-deposition segment of the welding overlay portfolio. It is positioned as a complementary process to the company's TIG and MIG overlay capabilities, addressing scenarios where:
- Overlay thickness exceeds 8–10 mm, making multi-pass TIG or MIG economically impractical
- Base components are thick-section pipe sheets (commonly 25–150 mm thick) requiring rapid overlay build-up
- Production schedule demands high deposition rates (typically 5–15 kg/h compared to 1–3 kg/h for TIG)
- Large-area coverage is required with minimal travel time and labor cost
The process bridges the gap between conventional weld overlay (TIG/MIG for thin, precision overlays) and explosion welding (for bonded clad plates), providing a versatile solution for mid-to-heavy overlay requirements on complex geometries such as pipe sheets with numerous tube holes.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Corrosion resistance: Provide a continuous, fully fused duplex stainless steel lining that resists aggressive chemical environments (chlorides, sulfuric acid, seawater, wet H₂S)
- Thickness efficiency: Achieve target overlay thickness (typically 12–40 mm) in 2–5 passes, reducing total welding time by 60–80% compared to TIG multi-pass build-up
- Mechanical integrity: Maintain duplex phase balance throughout the overlay thickness to prevent 475°C embrittlement and sigma phase formation
- Weld soundness: Produce overlay with minimal porosity, lack of fusion, and microstructural defects through controlled slag composition and thermal management
3.2 Value to Qualification Building
Mastering strip electrode ESW for duplex stainless steel overlay is critical for:
- Qualifying WPS (Welding Procedure Specifications) for heavy overlay applications per ASME Section IX, Part Q
- Achieving NB/T 20002.2 and GB/T 19542 compliance for nuclear and pressure vessel applications
- Meeting API 579-1/ASME FFS-1 fitness-for-service evaluation requirements for existing pipe sheet refurbishment
- Building competency for NACE SP0169 (corrosion-resistant overlay) and ISO 9073 (welding procedure qualification) certification
3.3 Customer Value
Customers in the oil & gas, chemical processing, and marine engineering sectors benefit from reduced fabrication lead times, lower labor costs, and the assurance of a qualified heavy-overlay process that meets international code requirements. The ability to deliver thick duplex stainless steel linings on large pipe sheets in a single production campaign—rather than multiple TIG overlay campaigns—directly translates to project schedule savings and cost reduction.
4. Key Process and Implementation Points
4.1 Equipment Configuration
Strip electrode electroslag welding for duplex stainless steel overlay requires a dedicated ESW system comprising:
- Power source: AC or DC (DCEN preferred for DSS to minimize dilution control issues) with a capacity of 800–2000 A, featuring a constant-current (CC) or constant-voltage (CV) control characteristic
- Mechanical travel unit: Precision horizontal travel with adjustable speed (typically 300–800 mm/min for overlay applications)
- Flux delivery system: Automatic flux hopper with even distribution across the welding zone
- Strip electrode feeder: Continuous feeding mechanism for strip electrodes (width 30–60 mm, thickness 2.0–3.0 mm)
- Backing assembly: Copper backing bar or backing ring for single-sided welding on pipe sheets
- Thermal monitoring: Infrared thermometers or thermocouples for interpass temperature control
4.2 Critical Process Parameters
| Parameter | Typical Range | Control Rationale |
|---|---|---|
| Welding Current | 1000–1800 A (DCEN) | Higher current increases deposition rate but risks excessive dilution and ferrite imbalance |
| Travel Speed | 400–800 mm/min | Controls overlay thickness per pass; slower speed = thicker deposit |
| Strip Electrode Width | 30–60 mm | Must match flux channel width; wider strip for thicker deposits |
| Strip Electrode Thickness | 2.0–3.0 mm | Affects feeding stability and heat input distribution |
| Flux Consumption Rate | 4.0–6.0 kg/h | Must maintain stable slag pool depth (12–18 mm) for uniform heat generation |
| Interpass Temperature | ≤ 150°C (first pass); ≤ 250°C (subsequent passes) | Prevents 475°C embrittlement and maintains duplex phase balance |
| Backing Temperature | Controlled to ≤ 300°C | Prevents excessive base metal dilution into the overlay |
| Flux Pre-drying | 250–300°C for 2–4 hours | Eliminates moisture to prevent hydrogen porosity in the overlay |
4.3 Duplex Stainless Steel Strip Electrode Selection
| DSS Grade | Typical Application | Key Alloying Features | Equivalent Strip Electrode |
|---|---|---|---|
| UNS S31803 (2205) | Oil & gas, chemical, marine | 22% Cr, 5% Ni, 3% Mo, 0.15% N | ESAB E31803 / Lincoln E31803 strip |
| UNS S32750 (2507) | High chloride, high temperature service | 25% Cr, 7% Ni, 3% Mo, 0.25% N | ESAB E32750 / Sandvik SX2 strip |
| UNS S32205 (2304) | Cost-effective DSS applications | 23% Cr, 4% Ni, 0.6% Mo, 0.15% N | Custom strip per WPS qualification |
4.4 Pre-Weld Preparation
- Base metal preparation: Pipe sheet surface to be overlaid must be ground to bare metal with a minimum 30 mm transition chamfer at the overlay boundary to ensure full fusion
- Tube hole management: All tube holes must be plugged with copper or steel plugs (with thermal expansion compensation) prior to welding to prevent flux ingress and slag contamination
- Flux channel construction: Build a flux-retaining channel using firebrick or refractory material on both sides of the welding path, with a channel width 20–30% wider than the strip electrode
- Preheating: Apply preheat of 100–150°C to the base metal if thickness exceeds 50 mm, to control cooling rate and prevent cold cracking in the transition zone
- Flux conditioning: Pre-dry the flux in a ventilated oven at 250–300°C for a minimum of 2 hours; store in a dew heater during welding to maintain moisture content below 0.1%
4.5 Welding Execution Sequence
- Strike: Initiate the slag pool by applying current to a pre-heated starter block or using a short-arc starter. The slag pool must reach a stable depth of 12–18 mm before the travel commences.
- Steady-state travel: Once the slag pool is stable, begin horizontal travel at the qualified speed. The strip electrode is fed continuously from above, melting into the slag pool and the underlying weld metal.
- Multi-pass build-up: For overlays exceeding 8 mm, execute multiple passes with controlled interpass temperature. Each subsequent pass deposits on the previously solidified overlay, maintaining phase balance.
- Stop/start management: At each stop/start, allow the slag pool to solidify completely before restarting. Use a starter block for restart to prevent slag inclusions.
- Termination: At the end of the weld, fill the slag pool with flux and allow it to solidify naturally. Remove excess flux and grind the overlay surface flush with the pipe sheet surface.
4.6 Post-Weld Heat Treatment (PWHT)
For duplex stainless steel overlays, post-weld heat treatment is generally not required and is in fact detrimental, as prolonged exposure to the 450–600°C range promotes 475°C embrittlement and sigma phase precipitation. However, if the base metal (carbon or low-alloy steel) requires PWHT for residual stress relief per ASME Section VIII Div. 2 or NB/T 47015, the following protocol must be followed:
- Limit PWHT temperature to ≤ 425°C (below the 475°C embrittlement threshold)
- Hold time should not exceed 2 hours per 25 mm of base metal thickness
- Alternatively, apply local PWHT to the base metal only, with the overlay region protected by insulation to maintain temperature below 300°C
- Post-PWHT, perform ferrite number measurement on the overlay to confirm the phase balance remains within the 35–65% ferrite range
5. Applicable Standards and Acceptance Criteria
5.1 Procedure Qualification Standards
- ASME Section IX, Part Q: Qualification of welding procedures for heavy overlay; establishes essential variables including current range, travel speed, flux type, and preheat temperature
- ISO 15614-1 / ISO 15614-16: Qualification testing of welding procedures for steel; Part 16 specifically addresses electroslag welding
- GB/T 19542: Chinese national standard for welding procedure qualification of steel
- NB/T 20002.2: Nuclear industry standard for welding procedure specification qualification
- ISO 9073-1: Welding procedure specification qualification for weld overlay
5.2 Material Standards
- ASTM A240: Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip (for base pipe sheet material)
- ASTM A182 / A269: Standards for duplex stainless steel forgings and tubing
- EN 10204: Technical delivery inspection documents for steel products
- NACE MR0175 / ISO 15156: Materials for H₂S-containing environments (critical for oil & gas applications)
5.3 Acceptance Criteria
| Inspection Method | Standard Reference | Acceptance Criteria |
|---|---|---|
| Visual Inspection (VT) | ASME Section V, Art. 2 / ISO 17637 | No cracks, no undercut > 0.5 mm, no slag inclusions visible, smooth surface with uniform transition |
| Magnetic Particle Testing (MT) | ASME Section V, Art. 7 / ASTM E709 | No linear indications; round indications ≤ 3 mm in any direction |
| Ultrasonic Testing (UT) | ASME Section V, Art. 4 / ISO 17640 | No indications exceeding Level II per ASME acceptance; full penetration confirmed |
| Radiographic Testing (RT) | ASME Section V, Art. 2 / ISO 17636 | No cracks, no slag inclusions > 1.5 mm, no porosity exceeding Group 2 per ISO 10675-1 |
| Ferrite Number (FN) | ASTM E490 / ISO 8044 | 35–65 FN (target 40–60 FN) for balanced duplex microstructure |
| Hardness Testing | ASTM E10 / ISO 6507 | ≤ 350 HV for S31803; ≤ 380 HV for S32750 (per NACE MR0175 requirements) |
| Macrographical Examination | ASTM E3 / ISO 15190 | Full fusion to base metal, no cracks, no unmelted strip, uniform grain structure |
| Chemical Analysis | ASTM E415 / ISO 3525 | Overlay composition within ±0.5% of nominal DSS grade; dilution ≤ 5% from base metal |
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Preventive/Corrective Control |
|---|---|---|
| Excessive base metal dilution | High current, slow travel speed, thick base metal without adequate preheat control | Reduce current by 10–15%; increase travel speed; apply back-gas shielding; use a dilution-reducing first pass with dilution-resistant filler |
| Ferrite imbalance (>65 FN or <35 FN) | Excessive dilution from carbon steel base; improper interpass temperature; PWHT in 450–600°C range | Monitor dilution via chemical analysis of first pass; control interpass ≤ 150°C; avoid PWHT above 425°C; adjust strip electrode alloy composition |
| Slag inclusions | Inadequate slag pool depth; excessive travel speed; poor flux distribution; stop/start without proper slag pool management | Maintain slag depth at 12–18 mm; reduce travel speed; ensure uniform flux distribution; use starter blocks for restarts |
| Hydrogen porosity | Moisture-contaminated flux; insufficient flux pre-drying; wet environment | Pre-dry flux at 250–300°C for 2+ hours; store in dew heater; control ambient humidity below 70% |
| Cracking in transition zone | High cooling rate; hydrogen pickup; low ductility of weld metal | Apply preheat to base metal; use low-hydrogen flux; control interpass temperature; consider a transition layer of 309L between base and DSS |
| Tube hole damage | Flux ingress into unsealed tube holes; thermal distortion from excessive heat input | Plug all tube holes with copper plugs; use thermal barriers; monitor distortion with dial indicators; limit heat input per pass |
| Undercut at overlay boundary | Inadequate transition chamfer; excessive current; poor travel control at start/end | Machine a 30° × 5 mm chamfer at overlay boundary; reduce current at boundaries; use run-on/run-off tabs |
6.2 Quality Management Controls
- WPS/PQR traceability: Each production weld must reference a qualified WPS with a corresponding PQR (Procedure Qualification Record) per ASME Section IX
- Welder qualification: Operators must hold current certifications per ASME Section IX, Part QW-100 through QW-300, with qualification tests performed on the specific DSS strip electrode and flux combination
- Flux traceability: Each batch of flux must have a certificate of analysis (CoA) confirming moisture content, chemical composition, and melting point
- In-process monitoring: Record current, voltage, travel speed, and interpass temperature for every pass; deviations from WPS parameters must be documented and reviewed
- NDT coverage: 100% VT and MT on all overlay surfaces; RT or UT on 10% of weld length (or 100% for critical applications); ferrite number measurement on 3+ locations per overlay
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
Strip electrode ESW serves as the heavy overlay complement to the company's TIG and MIG overlay capabilities. The typical workflow involves:
- TIG/MIG for thin overlays (0.5–8 mm): Precision TIG or MIG overlay for thin duplex stainless steel linings on small-diameter pipe sheets, flanges, and repair applications where geometric access is limited
- ESW for heavy overlays (8–50+ mm): Strip electrode ESW for thick overlays on large pipe sheets (> DN500), pressure vessel heads, and bulkhead plates where deposition rate is the primary constraint
- Hybrid approach: TIG for the first pass (to establish a controlled transition zone with minimal dilution) followed by ESW for subsequent heavy build-up passes
This layered approach allows the company to offer a complete overlay solution from 0.5 mm precision linings to 50+ mm heavy corrosion-resistant cladding, all under a single qualified procedure framework.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding produces solid-state bonded clad plates, strip electrode ESW provides a complementary finishing and repair capability:
- Post-bonding repair: ESW overlay to repair defects in the bonded interface or to add additional corrosion-resistant material on the bonded clad plate surface
- Edge cladding: ESW overlay on the edges of explosion-bonded plates to prevent corrosion initiation at the free edges where the base metal is exposed
- Transition layer: ESW to apply a compatible transition layer between the explosion-bonded interface and a subsequent TIG weld overlay
- Thickening: When explosion-bonded clad plate thickness is insufficient for the application, ESW overlay can add additional DSS material to the cladding face
7.3 Explosion Welding Route
In the explosion welding route, strip electrode ESW integrates as follows:
- Pre-weld preparation: ESW overlay on the base plate surface to create a compatible metallurgical transition before explosion welding (rare but used for specific material combinations)
- Post-weld overlay: After explosion welding produces a clad plate, ESW overlay can be applied to the clad face for additional thickness or to repair surface defects
- Component fabrication: For pipe sheets that require both explosion-welded clad sections and heavy overlay sections, ESW provides the overlay component of a hybrid fabrication strategy
- Repair and refurbishment: ESW overlay to restore corrosion-thinned pipe sheets that were originally manufactured using explosion welding, extending service life without full replacement
8. Learning Insights and Best Practices
8.1 Key Takeaways from Process Development
- Dilution control is paramount: The single greatest challenge in ESW overlay of DSS on carbon steel is controlling base metal dilution. Even 5% dilution can shift the ferrite content outside the acceptable duplex range. A dilution-resistant first pass (using a high-nickel strip electrode or a 309L transition layer) followed by standard DSS strip electrode is the most reliable strategy.
- Flux quality determines weld quality: The slag pool is the heart of the ESW process. Flux moisture, composition, and consistency directly affect slag pool stability, heat input distribution, and weld metal cleanliness. Only certified, batch-traceable flux should be used.
- Travel speed is the primary thickness control: Unlike TIG or MIG where wire feed speed and torch speed are adjusted independently, in ESW the travel speed is the dominant parameter controlling deposit thickness per pass. Precise speed control (±5%) is essential for dimensional consistency.
- Interpass temperature discipline: Maintaining interpass temperature below 150°C for the first pass and below 250°C for subsequent passes is non-negotiable for preserving duplex phase balance. Temperature monitoring must be continuous and documented.
- Geometric constraints require planning: Pipe sheets with tube holes present unique challenges. The welding path must be planned to avoid tube holes, and plugs must be designed to withstand thermal cycling without distortion or failure.
8.2 Process Optimization Recommendations
- Develop a parameter matrix correlating current, travel speed, strip electrode dimensions, and flux type to overlay thickness and dilution rate for each DSS grade
- Implement real-time monitoring of welding current and voltage with automated deviation alerts to prevent parameter drift
- Establish a flux conditioning protocol with documented drying cycles and moisture testing before each production run
- Conduct periodic macrographical verification of overlay cross-sections to confirm full fusion, phase balance, and absence of defects
- Develop standard operating procedures (SOPs) for tube hole plugging, flux channel construction, and stop/start management to ensure consistency across operators
9. Conclusion
Strip electrode electroslag weld overlay of duplex stainless steel on pipe sheets represents a high-value, technically demanding capability that positions Cladding Technology Shanxi Co., Ltd. as a comprehensive overlay solution provider. By mastering this process, the company extends its overlay portfolio from thin, precision TIG/MIG linings to heavy, industrial-scale DSS cladding, enabling the delivery of complete corrosion protection solutions for the most demanding applications in the oil & gas, chemical, marine, and power generation industries.
The successful qualification and deployment of this process requires rigorous adherence to ASME Section IX, ISO 15614-16, NB/T 20002.2, and applicable material standards (ASTM A240, NACE MR0175, ISO 15156), combined with disciplined process control, thorough NDT, and continuous improvement. The learning insights documented in this analysis provide a foundation for ongoing WPS optimization, operator training, and customer-specific procedure development.