Stainless Steel One-Strip-One-Flux Double-Layer Electroslag Weld Overlay Process
1. Definition and Fundamental Principles
The Stainless Steel One-Strip-One-Flux Double-Layer Electroslag Weld Overlay Process (不锈钢一带一剂双层电渣堆焊工艺) is an advanced surfacing technique that utilizes a single stainless steel strip electrode fed through a consumable flux layer to deposit a double-layer cladding on a carbon or low-alloy steel substrate. The process leverages electroslag welding (ESW) principles adapted specifically for cladding applications, combining the high deposition rates of electroslag welding with the metallurgical control of a self-shielded strip-and-flux consumable system.
The fundamental principle operates on the following mechanism:
- Electroslag Formation: A direct current (DC) is established between the stainless steel strip electrode and the workpiece. The electrical resistance of the molten slag pool generates sufficient heat to continuously melt the electrode tip and the surface of the preceding weld layer.
- One-Strip-One-Flux Consumable: A pre-formed flux coating is applied to one side of the stainless steel strip electrode, creating a self-contained consumable that simultaneously serves as filler metal and slag source. This eliminates the need for separate flux feeders and simplifies the process setup.
- Double-Layer Deposition: The first layer (transition layer) is deposited to ensure metallurgical compatibility between the base material and the final cladding layer. The second layer (cladding layer) provides the required corrosion resistance and surface integrity.
- Slag Protection: The molten flux creates a protective atmosphere over the weld pool, preventing oxidation and enabling the production of dense, inclusion-free weld metal even in non-inert gas environments.
This technique represents a significant advancement over traditional submerged arc welding (SAW) cladding methods, offering superior deposition rates (typically 8–15 kg/h), consistent weld geometry, and reduced dilution rates compared to manual or mechanized SAW processes.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s capability portfolio, this process belongs to the Weld Overlay Cladding technology route, specifically positioned as a high-productivity mechanized process for large-format plate and pipe cladding applications. The technology bridges the gap between high-deposition-rate automated processes and the metallurgical quality requirements of nuclear-grade, power plant, and chemical equipment components.
Strategic Positioning:
- Productivity Advantage: Deposition rates 2–3 times higher than conventional TIG/MIG overlay processes, making it economically viable for large surface areas.
- Quality Consistency: Mechanized strip feeding and flux delivery ensure uniform weld bead profiles and consistent dilution control across extended weld lengths.
- Nuclear Qualification: The process has been qualified for nuclear-grade applications under relevant standards, supporting the company's capability to deliver nuclear Class 1 and Class 2 cladding products.
- Process Integration: Complements the company's TIG/MIG overlay capabilities for small-diameter pipes and complex geometries, while the electroslag method handles large plates and thick-wall components efficiently.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The process is designed to achieve the following technical objectives:
- Corrosion Resistance: Deposit a continuous, dense stainless steel cladding layer (typically 304, 304L, 316, 316L, 321, or equivalent grades) on carbon steel substrates to provide resistance against aggressive chemical environments.
- Mechanical Compatibility: Ensure adequate metallurgical bonding between the dissimilar materials while maintaining the mechanical properties of both the base material and the cladding layer.
- Dimensional Control: Achieve precise cladding thickness (typically 3–12 mm total) with controlled surface flatness and uniformity.
- Low Dilution: Maintain dilution rates below specified limits (typically ≤30% for single pass, with cumulative dilution controlled through double-layer strategy) to ensure the final cladding surface meets chemical composition requirements.
- Structural Integrity: Produce weld metal free from porosity, cracks, lack of fusion, and other defects that would compromise the cladding's protective function.
3.2 Economic and Operational Value
- Reduction in total production time by 40–60% compared to multi-pass TIG overlay on equivalent surface areas.
- Lower consumable cost per kilogram of deposited metal due to high deposition efficiency.
- Reduced operator skill dependency through mechanized feed systems and automated process control.
- Scalability from laboratory qualification to full production runs with consistent quality metrics.
4. Key Process and Implementation Points
4.1 Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Electrode Strip Width | 20–40 mm | Selected based on base material thickness and required cladding width |
| Electrode Strip Thickness | 1.5–3.0 mm | Thinner strips for transition layer, thicker for cladding layer |
| Flux Layer Thickness | 2.0–4.0 mm | Pre-applied flux coating on strip electrode |
| Welding Current (DC) | 400–900 A | Depends on strip dimensions and travel speed |
| Welding Voltage | 28–42 V | Adjusted to maintain stable slag pool |
| Travel Speed | 80–250 mm/min | Higher speed for thinner deposits, lower for thicker layers |
| Interpass Temperature | ≤250°C (1st layer); ≤150°C (2nd layer) | Critical for controlling grain growth and dilution |
| Preheating Temperature | 100–250°C | Based on base material carbon equivalent and thickness |
| Deposition Rate | 8–15 kg/h | Significantly higher than TIG (1–3 kg/h) or MIG (2–5 kg/h) overlay |
4.2 Double-Layer Strategy
| Layer | Function | Typical Material | Target Thickness | Dilution Control |
|---|---|---|---|---|
| First Layer (Transition) | Metallurgical bridge between base and cladding; reduces residual stress | 309L or equivalent (higher Cr-Ni content for crack resistance) | 2–4 mm | Higher dilution acceptable (30–50%) |
| Second Layer (Cladding) | Provides final corrosion-resistant surface | 304L, 316L, 321, or specified grade | 2–8 mm | Low dilution required (≤20–25%) |
4.3 Critical Implementation Steps
- Base Material Preparation: Surface the substrate to remove mill scale, rust, and contaminants to within 50 μm surface roughness. Bevel edges at 15–25° to facilitate initial slag pool formation and ensure adequate penetration at the leading edge.
- Flux Drying and Storage: Flux must be dried at 250–300°C for 2 hours minimum to prevent hydrogen-induced cracking. Store at 100–150°C in heated flux boxes during production runs. Moisture content must not exceed 0.1%.
- Fixture and Tacking: Tack weld the strip electrode to the prepared surface at 100–200 mm intervals using TIG or manual SAW. Ensure tack welds are free from cracks and have adequate fusion with the base material.
- Slag Pool Stabilization: Initiate the process by establishing a stable slag pool (typically 50–80 mm in diameter) before advancing the electrode. Monitor slag pool geometry visually and through current/voltage stability.
- Double-Layer Execution: Complete the first layer across the full width before beginning the second layer. Allow adequate cooling between layers to control interpass temperature. The second layer may be run directly over the first layer or with a slight overlap pattern.
- Post-Weld Heat Treatment: Perform stress relief annealing at 550–650°C for 2–4 hours (depending on base material thickness) to relieve residual stresses and prevent delayed cracking.
4.4 Consumable Selection Guidelines
| Base Material | Recommended Transition Layer | Recommended Cladding Layer | Application |
|---|---|---|---|
| Q235 / A36 | 309L strip | 304L or 304 strip | General chemical equipment |
| 16Mn / A516 Gr.70 | 309L strip | 316L or 316 strip | Pressure vessels, heat exchangers |
| Q345R / SA-516 Gr.70 | 309L strip | 321 or 321H strip | High-temperature service |
| 15CrMo / P91 | 309L strip (with PWHT) | 316L strip | Nuclear and power plant components |
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 12469-2009: Welding consumables — Strip electrodes for electroslag welding
- GB/T 985.1-2008: Tensile testing of metallic materials — Method of test at room temperature
- GB/T 2649-2008: Welding consumables — Flux-cored wire and strip for submerged arc welding
- NB/T 47014-2011: Specification for qualification of welding procedures, welders, and welding procedure technicians (Nuclear)
- NB/T 20002.2-2011: Nuclear surface equipment — Welding procedure qualification
- ASME Section IX: Qualification Rules for Welding, Brazing, and Fusing
- ASME Section II Part D: Specification for welding consumables
- ASTM A240: Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels
- ASTM A388: Standard specification for corrosion-resistant chromium and chromium-nickel stainless steel plate, sheet, and strip
- ISO 13919-1: Welding consumables — Specification for self-shielded flux-cored wire and strip for submerged arc welding
- NACE MR0175/ISO 15156: Materials for use in H2S-containing environments
5.2 Acceptance Criteria
| Test Method | Standard | Acceptance Criteria | Test Frequency |
|---|---|---|---|
| Visual Inspection | GB/T 3375 / ASME Section V | No surface defects (cracks, porosity, undercut, slag inclusion) visible to naked eye | 100% of weld length |
| Magnetic Particle Testing (MT) | GB/T 26952 / ASME Section V Article 7 | No linear indications; round indications ≤1 mm | 100% of cladding surface |
| Penetrant Testing (PT) | GB/T 18851 / ASME Section V Article 6 | No indications exceeding acceptance limits | 100% of cladding surface |
| Hardness Testing | GB/T 231 / ASTM E18 | Cladding surface: ≤250 HV (for NACE service); transition zone gradient verified | 3 points per 100 mm |
| Microstructural Examination | GB/T 19540 / ASTM E3 | No intergranular cracking; sound fusion line; acceptable grain structure | Per WPS qualification; periodic production checks |
| Chemical Composition | GB/T 223 series / ASTM E415 | Final surface layer meets specified grade composition | Per heat lot; periodic production checks |
| Tensile Testing (Transverse) | GB/T 228.1 / ASTM A370 | UTS ≥ specified minimum for cladding grade; no brittle fracture | WPS qualification |
| Impact Testing | GB/T 229 / ASTM E23 | ≥27 J at -29°C (for nuclear service); ≥47 J at -20°C (for pressure vessels) | WPS qualification |
5.3 Nuclear-Specific Requirements
For nuclear applications governed by NB/T 47014-2011, additional requirements include:
- WPS qualification must include full-size coupon testing with actual production consumables and parameters.
- Welder qualification requires demonstration on a test coupon with 100% NDT coverage and all tests passing.
- Traceability records must be maintained for all consumables (flux lots, strip electrode heats) throughout the production cycle.
- Non-destructive examination must follow the specific acceptance criteria defined in the applicable reactor code (e.g., RBP-G00001 for Chinese nuclear codes).
6. Common Risks and Controls
| Risk | Cause | Control Measure | Severity |
|---|---|---|---|
| Hot Cracking in Transition Layer | Excessive sulfur/phosphorus in base material; high dilution; slow cooling rate | Use 309L strip (high Mn, Si for crack resistance); control interpass temperature; apply preheat per CEV | Critical |
| Porosity in Cladding Layer | Wet flux; contaminated base surface; excessive travel speed | Dry flux at 300°C/2h; clean base to SA 2.5 minimum; maintain optimal travel speed | High |
| Excessive Dilution | Too deep slag pool; excessive current; thin first layer | Control current density; ensure adequate first layer thickness; verify slag pool geometry | High |
| Lack of Fusion at Leading Edge | Insufficient heat input at start; poor bevel preparation | Establish stable slag pool before advancing; verify bevel angle and surface preparation | Medium |
| Hydrogen-Induced Cracking (HIC) | Moisture in flux; high hydrogen diffusibility in low-alloy steel | Strict flux drying; post-weld bake at 150–200°C for hydrogen diffusion | Critical (NACE service) |
| Intergranular Corrosion | Sensitization of cladding layer due to excessive heat input | Use L-grade strips (304L, 316L); control interpass temperature; avoid excessive preheat | High |
| Slag Inclusion at Fusion Line | Insufficient slag removal between passes; inadequate slag fluidity | Remove slag between layers using wire brush and grinder; verify flux chemistry | Medium |
| Weld Geometry Irregularities | Unstable electrode feed; operator error; fixture misalignment | Use automated feed system; implement real-time monitoring of current/voltage; regular fixture calibration | Medium |
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
The electroslag overlay process complements TIG and MIG overlay within the company's manufacturing portfolio:
- Large Plate Cladding: Electroslag is the primary process for plates exceeding 2000 mm × 1000 mm where deposition rate is critical. TIG overlay handles edges, corners, and areas inaccessible to the electroslag torch.
- Small-Diameter Pipes: For pipes with OD < 100 mm, TIG overlay remains the preferred method. Electroslag is applied to pipe ends and flanges after fabrication.
- Repair and Rework: MIG overlay is used for local repair of defects identified in electroslag-clad components, particularly for small area touch-ups where full electroslag rework is impractical.
- Multi-Layer Complex Cladings: For thick cladding requirements (>12 mm), the company may combine electroslag (for bulk deposition) with TIG (for final surface finishing) to achieve optimal surface quality.
7.2 Hydraulic Explosive Bonding Complementarity
While electroslag overlay is a weld-based process, it serves a different niche compared to hydraulic explosive bonding:
- Thickness Range: Electroslag overlay is most economical for cladding thicknesses of 3–12 mm. Hydraulic explosive bonding is preferred for thinner claddings (0.5–3 mm) where dilution-free bonding is required.
- Material Compatibility: Electroslag overlay is limited by metallurgical compatibility (weldability of both materials). Hydraulic explosive bonding can join otherwise non-weldable material combinations (e.g., aluminum to steel).
- Surface Integrity: For applications requiring zero dilution and perfect metallurgical bond (e.g., certain nuclear primary circuit components), hydraulic explosive bonding is specified. Electroslag overlay is used where some dilution is acceptable and the cladding thickness provides adequate corrosion resistance.
- Cost Efficiency: For standard carbon steel to stainless steel cladding in thickness ranges >3 mm, electroslag overlay offers superior cost efficiency compared to hydraulic explosive bonding.
7.3 Explosion Welding Complementarity
Explosion welding and electroslag overlay occupy complementary positions in the company's capability matrix:
- Scale and Geometry: Explosion welding produces large cladding plates (up to 6000 mm × 2000 mm) with uniform thickness in a single operation. Electroslag overlay is applied to fabricated components (vessels, headers, nozzles) where the geometry prevents explosion welding.
- Production Flow: The company may use explosion welding to produce clad plates, which are then fabricated into components. Electroslag overlay is applied directly to fabricated components for final cladding or as an alternative to explosion-welded plate fabrication.
- Thick Cladding: For cladding thicknesses exceeding 10 mm, electroslag overlay may be more practical than explosion welding, which typically produces 3–8 mm clad layers.
- Material Grade Flexibility: Electroslag overlay allows on-demand selection of cladding grade for specific service conditions without requiring inventory of pre-made explosion-welded clad plates.
8. Qualification Building and Certification
8.1 WPS Qualification Requirements
Establishing a qualified Welding Procedure Specification (WPS) for this process requires:
- Essential Variables Documentation: Define all essential variables including base material P-number, filler metal F-number, electrode strip dimensions, flux type, current range, voltage range, travel speed, preheat, and PWHT.
- Coupon Testing: Weld qualification coupons in the configuration matching production geometry (plate-to-plate, pipe-to-plate, pipe-to-pipe as applicable).
- Mechanical Testing: Perform tensile, impact, and hardness testing on transverse and longitudinal specimens per NB/T 47014-2011 or ASME Section IX.
- NDT: Subject qualification coupons to MT/PT for surface defects and radiographic testing for volumetric defects.
- Metallographic Examination: Verify fusion line integrity, microstructure, and absence of intergranular cracking or other metallurgical defects.
8.2 Welder Qualification
- Each operator must demonstrate proficiency on a test coupon welded with the qualified WPS.
- Qualification includes visual inspection and NDT (MT/PT) of the test weld with zero defects.
- Qualification is valid for a specified period (typically 6–12 months) and must be renewed upon expiry.
- For nuclear applications, welder qualifications must be maintained in accordance with NB/T 47014-2011 and the specific project quality plan.
8.3 Factory Acceptance Testing (FAT) Support
This qualified process enables the company to:
- Provide qualified WPS documentation as part of FAT packages for nuclear and power plant customers.
- Demonstrate process capability through statistical process control data collected during production runs.
- Support customer audits with traceable records of consumable certification, operator qualification, and NDT results.
- Qualify for additional projects by expanding the WPS envelope (base material thickness, cladding grade, geometry) through supplementary testing.
9. Process Optimization and Continuous Improvement
9.1 Parameter Optimization
Based on production experience, the following optimization strategies have been established:
- Current-Voltage Matching: Maintain a current density of 15–25 A/mm² (based on strip cross-section) for optimal slag pool stability and penetration control.
- Slag Pool Monitoring: Implement visual and thermal monitoring of the slag pool to detect instability before defects form. Slag pool diameter should be maintained at 1.5–2.0 times the electrode strip width.
- Travel Speed Control: Adjust travel speed based on real-time current feedback to compensate for variations in base material thickness or surface condition.
- Flux Chemistry Optimization: Select flux compositions with appropriate basicity (1.5–2.5) for the specific cladding grade to ensure adequate deoxidation and slag fluidity.
9.2 Quality Assurance Integration
- In-Process Monitoring: Continuous recording of welding current, voltage, travel speed, and electrode feed rate for each production weld.
- Interim Inspection: Visual and MT inspection after each layer completion before proceeding to the next layer.
- Final Inspection: 100% MT/PT of the final cladding surface, supplemented by random UT thickness measurements.
- Documentation: Complete weld maps, NDT reports, and material traceability records for each production lot.
10. Conclusion
The Stainless Steel One-Strip-One-Flux Double-Layer Electroslag Weld Overlay Process represents a critical capability within Cladding Technology Shanxi Co., Ltd.'s manufacturing portfolio. Its high deposition rates, metallurgical reliability, and qualification readiness for nuclear-grade applications position it as the preferred process for large-format cladding of carbon steel substrates with stainless steel corrosion-resistant overlays.
Through systematic WPS qualification, operator certification, and quality management system integration, this process directly contributes to:
- Product Delivery: Reliable production of clad plates, pipes, and components meeting nuclear, power plant, and chemical industry specifications.
- Qualification Building: Expansion of the company's qualified WPS library, enabling acceptance of increasingly complex and demanding projects.
- Customer Value: Delivery of certified, traceable cladding products with documented process capability, reducing customer risk and accelerating project approval cycles.
The continuous refinement of process parameters, consumable selection, and quality control methodologies ensures that this technology remains competitive and compliant with evolving industry standards and customer requirements across the nuclear, power generation, and petrochemical sectors.