Domestic Single-Layer Strip Electrode Electroslag Weld Overlay Technology
1. Definition and Fundamental Principles
Single-layer strip electrode electroslag weld overlay is an advanced cladding process in which a continuous strip electrode (typically 300–1000 mm wide) is fed through a slag pool maintained by a consumable or flux-covered backing, producing a thick, uniform overlay deposit in a single pass. The process exploits the high thermal efficiency of electroslag welding (ESW), where the heat source is generated by electrical resistance within a molten slag pool rather than by a direct arc. This fundamental difference from arc-based processes (TIG, MIG, SAW) results in slower cooling rates, reduced hydrogen pickup, and deposit thicknesses of 3–15 mm per pass—far exceeding what conventional arc welding can achieve in a single traversal.
The "domestic" designation in this technology entry signifies the development and qualification of a fully indigenous process using domestically produced strip electrodes, fluxes, and shielding gases, eliminating dependence on imported consumables and enabling full traceability within China's national supply chain. The "single-layer" specification indicates that the overlay achieves the required corrosion-resistant or wear-resistant thickness in one pass without the need for multiple overlapping layers, which simplifies process control and reduces the risk of interpass defects.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s technology portfolio, strip electrode electroslag weld overlay occupies a distinct niche between conventional TIG/MIG weld overlay and explosion welding. The company's three principal technology routes are:
- TIG/MIG Weld Overlay — suited for small-diameter pipes, thin-walled components, and precision cladding with thin deposit thicknesses (0.5–3 mm per pass).
- Hydraulic Explosive Bonding / Explosion Welding — suited for large-area plate cladding with high bond integrity and no dilution, but limited to relatively thin overlay layers (typically 3–25 mm).
- Electroslag Weld Overlay (this entry) — suited for thick single-pass overlay on large-diameter pipes, pressure vessels, and structural components where deposit thickness of 5–15 mm is required efficiently and with excellent metallurgical quality.
This technology bridges the gap where TIG/MIG would require excessive passes (increasing cost and defect probability) and where explosion welding cannot economically produce the required overlay thickness. It is particularly valuable for domestic substitution of imported clad pipes and components in the petrochemical, power generation, and nuclear industries.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Thick single-pass overlay: Achieve 5–15 mm of corrosion-resistant or wear-resistant alloy in one pass, reducing total process time by 40–60% compared to multi-pass TIG/MIG.
- Low dilution control: Maintain base metal dilution below 5–10% through optimized strip electrode composition, travel speed, and heat input management.
- Full-penetration-free bonding: Ensure metallurgical bonding between base metal and overlay without full penetration, preserving base metal mechanical integrity.
- Domestic consumable qualification: Validate domestic strip electrode alloys (e.g., 309, 310, 316L, 625, 825, 2205 compositions) to equivalent or superior performance versus imported equivalents.
3.2 Business Value
- Enables the company to offer large-diameter clad pipe and vessel products that were previously unavailable domestically or required importation.
- Reduces material cost by 20–35% through domestic consumable sourcing while maintaining or exceeding imported product quality.
- Provides a differentiated capability for WPS/PQR qualification packages that customers cannot obtain from competitors limited to TIG/MIG or explosion welding.
- Supports national strategy for import substitution in critical infrastructure components (nuclear, LNG, offshore).
4. Key Process and Implementation Points
4.1 Process Configuration
Single-layer strip electrode electroslag weld overlay typically employs one of two configurations:
- Submerged flux method: Strip electrode is fed through a consumable backing plate (sacrificial) that forms the slag pool; the backing is removed after welding.
- Flux-covered backing method: A permanent backing plate is coated with a specialized electroslag flux; the flux melts to form the slag pool.
For single-layer overlay applications, the consumable backing method is preferred because it eliminates the need for backing plate removal and avoids potential contamination from residual flux on the backing surface.
4.2 Critical Process Parameters
| Parameter | Typical Range | Influence |
|---|---|---|
| Electrode feed speed | 200–600 mm/min | Controls deposit thickness; higher speed = thinner deposit |
| Travel speed | 150–500 mm/min | Controls heat input and bead width |
| Welding current | 800–2000 A | Controls slag pool temperature and fluidity |
| Welding voltage | 25–45 V | Controls arc length and heat distribution |
| Strip electrode width | 300–1000 mm | Determines coverage width per pass |
| Strip electrode thickness | 1.0–3.0 mm | Affects feeding stability and dilution |
| Preheat temperature | 100–250°C (base metal dependent) | Reduces residual stress and cracking risk |
| Shielding gas (if used) | Ar + 5–10% CO₂ or pure Ar | Protects slag pool surface from oxidation |
| Heat input | 25–80 kJ/mm | Controls microstructure and dilution |
4.3 Base Metal Preparation
- Surface preparation to SA 2.5 grade or equivalent (clean, oxide-free, roughened to promote wetting).
- Edge beveling at 15°–30° to create a mechanical key for bonding; bevel depth 2–5 mm.
- Preheating per WPS specification to reduce thermal gradient and prevent cold cracking in high-strength steels.
- For carbon steel substrates, removal of mill scale and rust is critical to prevent slag inclusion and poor bonding.
4.4 Strip Electrode Selection
| Application | Recommended Strip Electrode Alloy | Equivalent Standards |
|---|---|---|
| General corrosion resistance (H₂S, CO₂) | 316L / 321 | ASTM A240, GB/T 24511 | High-temperature oxidation | 310 / 310H | ASTM A240, GB/T 20878 | Chloride stress corrosion | 2205 duplex | ASTM A790, GB/T 24511 | High-temperature strength + corrosion | Inconel 625 | ASTM B166, GB/T 17748 | Acid service (phosphoric, sulfuric) | Alloy 825 / C-276 | ASTM B166, NACE MR0175 |
| Wear resistance (abrasion) | High-carbon martensitic (e.g., 440C) | ASTM A220, GB/T 1222 |
4.5 Process Monitoring and Control
- Real-time monitoring of current, voltage, and travel speed via automated welding control systems.
- Slag pool temperature monitoring (infrared pyrometry) to ensure consistent fluidity.
- Online thickness measurement using ultrasonic gauging at defined intervals.
- Weld seam tracking and alignment via laser or magnetic sensors for large-diameter pipe applications.
5. Applicable Standards and Acceptance Criteria
5.1 Process Qualification Standards
- NB/T 47014 — Welding procedure qualification and testing for pressure vessels (China).
- ASME Section IX — Qualification of welding procedures, welders, and welding operators.
- ASME B31.3 — Process piping requirements (for pipeline overlay applications).
- API 579-1/ASME FFS-1 — Fitness-for-service assessment (for existing component overlay repair).
- GB/T 12467 — Welding consumables — Electrodeposits and weld metal.
- ISO 15614-1 — Qualification testing of welding procedures for metallic materials.
- NB/T 20265 — Welding procedure qualification for nuclear power plant components.
5.2 Product Acceptance Standards
- ASTM A240 — Chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels.
- GB/T 8163 — Seamless steel tubes for fluid transport (base pipe specification).
- NACE MR0175 / ISO 15156 — Materials for H₂S-containing environments in oil and gas.
- ASME BPV Section VIII Div. 1 — Rules for construction of pressure vessels.
- GB/T 150 — Pressure vessels (China national standard).
5.3 NDT Acceptance Criteria
| NDT Method | Acceptance Level | Reference Standard |
|---|---|---|
| RT (Radiographic Testing) | Level II (per ASME V Art. 2) | ASME BPV Section V, GB/T 3323 |
| UT (Ultrasonic Testing) | No lack of bonding; indications ≤ acceptance limit | ASME V Art. 23, NB/T 47013 |
| MT (Magnetic Particle Testing) | No linear indications; round indications ≤ 3 mm | ASME V Art. 7, GB/T 26055 |
| PT (Penetrant Testing) | No linear indications | ASME V Art. 6, GB/T 18851 |
| Hardness Testing | Overlay ≤ specified limit; gradient zone ≤ HRC 35 (for H₂S service) | NACE MR0175, ASTM E10/E18 |
| Chemical Analysis | Dilution ≤ specified limit (typically ≤10%) | ASTM E415, GB/T 20066 |
5.4 Mechanical Property Requirements
- Overlay tensile strength: per base alloy specification (e.g., ≥515 MPa for 316L per ASTM A240).
- Overlay elongation: ≥30% for austenitic alloys; ≥25% for duplex alloys.
- Impact energy (if required): ≥27 J at specified test temperature per ASTM A370.
- Bond strength (peel test): ≥ 25 MPa minimum for overlay-to-base interface.
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Excessive dilution | High heat input, slow travel speed, thick base bevel | Reduce current, increase travel speed, optimize bevel geometry, use higher-alloy electrode | Hot cracking (intergranular) | High sulfur/phosphorus in electrode, high heat input | Use low-S/low-P strip electrode, reduce heat input, increase preheat | Sigma phase formation | Exposure to 600–850°C during slow cooling | Control cooling rate, use Nb-stabilized electrodes, avoid prolonged heat exposure |
| 400-series sensitization | Prolonged exposure to 450–850°C | Use stabilized grades (321, 347), limit interpass temperature, rapid cooling |
| Cracking at overlay-base interface | Thermal mismatch, high residual stress, poor wetting | Appropriate preheat, controlled heat input, surface roughening of base |
| Hardness exceedance in HAZ | High carbon equivalent base metal, high heat input | Preheat and interpass temperature control, post-weld heat treatment (PWHT) |
6.2 Process Risks
- Slag inclusion: Incomplete slag removal between passes (if multi-pass is required for repair); controlled by ensuring single-pass process discipline and adequate slag fluidity.
- Uneven deposit thickness: Travel speed variation, electrode feeding inconsistency; controlled by automated welding systems with closed-loop feedback.
- Backing plate defects: Pores or inclusions in consumable backing transferred to weld; controlled by incoming inspection of backing material and proper backing plate preparation.
- Distortion: High heat input causes angular and longitudinal distortion; controlled by welding sequence planning,拘束夹具 (rigging fixtures), and post-weld straightening if necessary.
- Hydrogen-induced cracking: In high-strength base metals; controlled by preheat, low-hydrogen consumables, and post-weld bake-out.
6.3 Quality Assurance Controls
- Full WPS/PQR qualification per NB/T 47014 or ASME Section IX before production.
- First-article inspection with full NDT (RT + UT + MT/PT) on qualification coupons.
- In-process monitoring with documented parameter logs for every production weld.
- Post-production NDT per customer specification with documented acceptance criteria.
- Material traceability from strip electrode mill certificate through to finished product.
- Welder/operator certification and periodic requalification.
7. Application Scenarios
7.1 Petrochemical and Oil & Gas Industry
- Large-diameter process pipes (DN 500–DN 2000) requiring internal corrosion-resistant overlay for sour service (H₂S/CO₂).
- Pressure vessel heads and shells requiring 8–15 mm overlay of 316L, 2205, or Alloy 825.
- Heat exchanger tubesheets and channel covers requiring corrosion-resistant overlay on carbon steel substrates.
- Compliance with NACE MR0175/ISO 15156 for sour service applications.
7.2 Power Generation
- Boiler tubes and headers requiring wear-resistant and corrosion-resistant overlay in high-temperature, high-pressure environments.
- Reactor pressure vessel internals requiring nuclear-grade overlay qualification per NB/T 20265.
- Condenser tube bundles requiring 316L or 904L overlay for cooling water corrosion resistance.
7.3 Liquefied Natural Gas (LNG) and Cryogenic Applications
- Large storage tank inner shells requiring 9% Ni steel or 304L overlay for -162°C service.
- Pump casings and valve bodies requiring overlay for cryogenic thermal shock resistance.
7.4 Shipbuilding and Marine Engineering
- Ballast tank inner surfaces requiring corrosion-resistant overlay (9% Ni steel or duplex stainless).
- Propeller shafts and stern tubes requiring wear-resistant overlay for erosion protection.
7.5 Mining and Heavy Industry
- Crusher housing and conveyor components requiring thick wear-resistant overlay (high-carbon martensitic or Ni-hard alloy).
- Cement kiln components requiring refractory-grade overlay for thermal and chemical resistance.
8. Integration with Company's Three Technology Routes
| Technology Route | Role of Electroslag Overlay | Complementary Relationship |
|---|---|---|
| TIG/MIG Weld Overlay | Electroslag handles thick single-pass overlay; TIG/MIG handles thin precision overlay and transition layers | Combined use: TIG transition layer + electroslag bulk overlay + TIG finish pass for surface quality |
| Hydraulic Explosive Bonding | Explosive bonding provides zero-dilution cladding; electroslag provides thick overlay where dilution is acceptable | Explosive bonding for critical dilution-sensitive applications; electroslag for cost-effective thick overlay |
| Explosion Welding | Explosion welding for large plate cladding; electroslag for pipe and vessel overlay where geometry prevents explosion welding | Explosion welding for flat plates; electroslag for curved surfaces (pipes, vessels, heads) |
9. Contribution to Qualification Building and Customer Value
9.1 Qualification Building
- This technology entry represents a significant capability expansion beyond conventional arc welding, enabling the company to qualify for larger-scale, higher-value contracts requiring thick overlay deposits.
- Domestic consumable qualification creates a defensible intellectual property position and reduces supply chain vulnerability.
- WPS/PQR packages developed under this technology can be leveraged across multiple customer projects, reducing qualification cost per project.
- Alignment with national standards (NB/T 47014, GB/T series) ensures acceptability across Chinese regulatory frameworks.
9.2 Product Delivery Enhancement
- Single-pass overlay reduces production time by 40–60% compared to multi-pass TIG/MIG, enabling faster project delivery.
- Thicker deposits per pass reduce the number of inspection points, lowering NDT cost and schedule.
- Consistent deposit thickness and composition improve product quality uniformity and reduce rejection rates.
- Capability to handle large-diameter pipes and large-area surfaces expands the addressable product portfolio.
9.3 Customer Value Proposition
- Cost reduction: 20–35% reduction in overlay cost versus imported clad products; 30–50% reduction versus multi-pass TIG/MIG for thick deposits.
- Quality assurance: Consistent, repeatable process with full documentation and traceability meeting international standards.
- Schedule reliability: Faster production cycle times with reduced inspection overhead.
- Technical support: Full WPS/PQR development, process consultation, and on-site technical support available.
- Domestic compliance: Full alignment with Chinese regulatory requirements for pressure equipment, nuclear components, and critical infrastructure.
10. Conclusion
Domestic single-layer strip electrode electroslag weld overlay technology represents a strategically important capability for Cladding Technology Shanxi Co., Ltd., filling a critical gap in the domestic cladding technology landscape. By enabling thick, single-pass overlay deposits with controlled dilution and full metallurgical quality, this technology positions the company to serve high-value applications in petrochemical, power generation, LNG, and nuclear industries that were previously dependent on imported products or multi-pass arc welding processes. The combination of domestic consumable qualification, rigorous process control, and alignment with international standards (ASME, ASTM, API, NACE, ISO) ensures that products delivered under this technology meet the most demanding customer and regulatory requirements while offering significant cost and schedule advantages.