Strip Electrode Submerged Arc Weld Overlay for Hydroelectric Turbine Bottom Ring
1. Definition and Technical Principles
Strip electrode submerged arc welding (SAW) overlay is a specialized surfacing technology that employs a continuously fed flat strip of filler metal, shielded by a granular flux, to deposit a corrosion-resistant, wear-resistant, or functionally graded layer onto a base substrate. Unlike conventional wire-electrode SAW, the strip electrode provides a wider, flatter arc with higher deposition rates, lower dilution of the base metal, and superior metallurgical control over the weld overlay layer. In the context of the Maogai Power Station turbine bottom ring project, this technology was applied to restore or enhance the hydrodynamic and structural integrity of the turbine bottom ring—a critical component that forms the lower boundary of the runner-volute interface and directly influences hydraulic efficiency, vibration characteristics, and fatigue life.
The fundamental principle relies on the interaction between the strip electrode arc and the granular flux: the flux melts to form a protective slag layer that excludes atmospheric contamination, slows cooling rates, and promotes favorable grain refinement in the deposited metal. The strip geometry ensures a broad, stable arc with minimal spatter, enabling uniform single-pass or multi-pass overlay builds with consistent microstructural properties.
2. Category and Business Positioning
This technology falls squarely within the company's TIG/MIG weld overlay business route, extended into the submerged arc domain for high-productivity applications. While TIG and MIG overlay are preferred for precision transition layers and thin overlay builds, strip electrode SAW overlay is the industry-standard method for achieving substantial overlay thicknesses (typically 3–15 mm) at production-grade deposition rates. This positions the technology as a complementary capability that bridges the gap between precision TIG/MIG overlay and bulk surfacing requirements—particularly relevant for large-diameter hydroelectric components where repair volumes and schedule pressures demand high productivity without compromising quality.
From a business standpoint, this entry demonstrates the company's capability to execute complex, site-specific weld overlay projects on critical power generation infrastructure, reinforcing credibility with hydroelectric operators and OEMs in the energy sector.
3. Technical Purpose and Value
- Restoration of dimensional accuracy: The turbine bottom ring, after years of cavitation erosion, abrasion, or fatigue cracking, may deviate from its original hydrodynamic profile. Overlay welding restores the ring's geometry to design specifications, recovering hydraulic efficiency.
- Material upgrade: The overlay layer can be composed of a duplex stainless steel (e.g., UNS S31803/S32206) or a high-alloy austenitic stainless steel (e.g., UNS S30815/S30916), providing superior resistance to cavitation erosion and chloride-induced stress corrosion cracking compared to the base carbon or low-alloy steel.
- Service life extension: Rather than fabricating and installing an entirely new bottom ring—a process involving months of lead time, transportation logistics, and significant cost—overlay repair extends the component's service life by 10–20 years, delivering substantial capital savings.
- Operational continuity: On-site or near-site overlay execution minimizes turbine downtime, directly translating to preserved revenue generation for the power station.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
- Surface preparation: The overlay area must be ground to a smooth, oxide-free surface (Sa 2½ per ISO 8501-1) with a minimum Ra of 3.2 μm. All existing cracks, porosity, or loose material must be removed via mechanical grinding or gouging.
- Heat input management: Preheating to 100–200°C (depending on base material thickness and carbon equivalent) is critical to prevent hydrogen-induced cracking and control thermal stresses in the thick-section bottom ring.
- Fit-up and backing: A backing strip of compatible material (e.g., low-hydrogen steel or stainless steel) must be installed to ensure full fusion on the back side and prevent slag inclusions.
- Flux conditioning: The granular flux (typically rutile-type, such as HJ431 or equivalent) must be pre-dried at 300°C for 2 hours to minimize hydrogen absorption and porosity risk.
4.2 Welding Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Strip electrode width | 25–40 mm | Wider strips yield flatter profiles; 30 mm is common for bottom ring applications |
| Strip electrode thickness | 3.0–4.0 mm | Thicker strips increase deposition rate but require higher current |
| Welding current | 600–1200 A (DC) | Depends on strip dimensions and base material |
| Welding voltage | 28–35 V | Maintains stable arc; lower voltage improves penetration |
| Welding speed | 250–500 mm/min | Higher speed reduces dilution but may increase undercut risk |
| Flux coverage | Full coverage, 8–12 mm depth | Must completely shield the arc and weld pool |
| Interpass temperature | ≤ 250°C | Monitored via infrared thermometry or temperature-indicating paint |
| Post-weld heat treatment | 550–650°C, 2 h, furnace or induction | Relieves residual stresses and refines microstructure |
4.3 Multi-Pass Overlay Strategy
For overlay builds exceeding 3 mm, a multi-pass strategy is mandatory to ensure metallurgical homogeneity and minimize dilution:
- Transition pass: A single pass of 309L or 309Cb strip electrode to establish a dilution buffer between the carbon/low-alloy base metal and the final overlay alloy.
- Build-up passes: 2–4 passes of the final overlay alloy (e.g., 308L or duplex 2205), each with controlled overlap (15–25%) to ensure full fusion and uniform composition.
- Capping pass: A final pass with reduced current and speed to produce a smooth, well-shaped surface suitable for machining or hydrodynamic profiling.
4.4 Post-Weld Finishing
- Mechanical machining (CNC turning or milling) to restore the bottom ring's hydrodynamic profile to within ±0.1 mm of design tolerance.
- Post-machining polishing to achieve a surface roughness of Ra ≤ 1.6 μm for cavitation-resistant surfaces.
- Non-destructive examination (see Section 5) to verify overlay integrity post-machining.
5. Applicable Standards and Acceptance Criteria
| Category | Standard | Requirement |
|---|---|---|
| Welding procedure qualification | GB/T 9445 / AWS D10.9 | WPS and PQR must be qualified per the applicable standard for surfacing welds on carbon and alloy steels |
| Welder qualification | GB/T 15059 / ISO 9606-1 | Welders must be qualified in SAW with strip electrode on the applicable base material and thickness range |
| Surface preparation | ISO 8501-1 | Visual cleanliness grade Sa 2½ or better |
| Visual examination | GB/T 3323 / AWS D1.1 | No undercut, porosity, or surface cracks; profile within ±0.5 mm |
| Penetrant testing (PT) | GB/T 18851 / ASTM E165 | 100% coverage; no linear indications permitted |
| Magnetic particle testing (MT) | GB/T 26952 / ASTM E1444 | 100% coverage on ferromagnetic base metal; no cracks or linear indications |
| Ultrasonic testing (UT) | GB/T 11345 / ASTM E1650 | 100% coverage; acceptance per AWS D1.1 Level II criteria |
| Hardness verification | GB/T 231.1 / ASTM E182 | Overlay hardness within ±30 HV of nominal alloy specification |
| Corrosion resistance | NACE TM0169 / ASTM G48 | Intergranular corrosion resistance verified by electrolytic etch test (ASTM A262 Practice E) |
| Hydroelectric component acceptance | IEC 60034 / GB/T 8564 | Turbine component dimensional and performance acceptance per hydroelectric generator standards |
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Hydrogen-induced cracking | Moisture in flux, excessive heat input, high carbon equivalent base metal | Flux pre-drying at 300°C; limit interpass temperature; use low-hydrogen flux; post-weld bake at 200°C for 4 h |
| Hot cracking in overlay layer | Excessive sulfur/phosphorus in filler metal; restricted shrinkage; high dilution | Use low-S, low-P filler metal; optimize travel speed to reduce dilution; apply back-stress or compressive pre-stress |
| Porosity | Flux moisture, base metal contamination, inadequate arc shielding | Strict flux handling and storage; thorough surface cleaning; ensure complete flux coverage |
| Undercut | Excessive current, travel speed too fast, improper torch angle | Reduce current by 10–15%; slow travel speed; maintain torch angle at 5–10° trailing |
| Excessive dilution | High heat input, low travel speed, thick base metal | Use multi-pass strategy with transition layer; increase travel speed; reduce current; use higher-alloy transition filler |
| Residual stress and distortion | Thick section, high heat input, constrained geometry | Preheat to 150–200°C; use low-heat-input parameters; apply symmetric welding sequence; post-weld stress relief at 550–650°C |
| Intergranular corrosion | Chromium carbide precipitation at grain boundaries in sensitized austenitic overlay | Use low-carbon (L-grade) filler metal; apply solution heat treatment at 1050–1100°C if feasible; verify via ASTM A262 Practice E |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay
The Maogai turbine bottom ring project highlights the synergy between strip electrode SAW and TIG/MIG overlay within the company's portfolio. In practice, a hybrid approach is often optimal: TIG weld overlay is used for the initial transition layer and for repairing localized defects (cracks, cavitation pits) with precise heat input control, while strip electrode SAW overlay is deployed for bulk build-up of the corrosion-resistant surface. This combination leverages the precision of TIG (low dilution, excellent weld geometry) and the productivity of SAW (high deposition rates, wide coverage), delivering a comprehensive surfacing solution for complex hydroelectric components.
7.2 Hydraulic Explosive Bonding
While hydraulic explosive bonding is primarily employed for clad plate and pipe fabrication—producing metallurgical bonds between dissimilar metals at the interface without melting—the Maogai project underscores the complementary role of weld overlay in the post-bonding finishing stage. For turbine components where a clad substrate (e.g., duplex steel clad on carbon steel) may be used, the hydraulic explosive bonding route provides the base material, and subsequent TIG or SAW overlay restores surface dimensions and applies the final functional layer. This integrated approach allows the company to offer a complete value chain: from clad material production to component-level surfacing and repair.
7.3 Explosion Welding
Explosion welding (explosive cladding) is the company's flagship technology for producing large-format clad plates and forgings. The turbine bottom ring application demonstrates how the knowledge and metallurgical expertise gained from explosion welding—particularly in understanding interfacial bonding, dilution control, and multi-layer metallurgical design—directly inform the design of weld overlay processes. The same alloy combinations validated through explosion welding qualification (e.g., 2205/C12A, 304L/Q345R) can be replicated in weld overlay configurations, ensuring metallurgical consistency across the company's product portfolio.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR portfolio expansion: The Maogai project generates qualified welding procedures for strip electrode SAW overlay on thick-section carbon and low-alloy steel, filling a critical gap in the company's WPS library. These procedures can be leveraged for future hydroelectric, nuclear, and petrochemical projects.
- Welder certification: The project certifies welders in a specialized SAW strip electrode technique, enhancing the company's skilled workforce depth and reducing dependence on external contractors.
- Customer-specific qualification: Execution at the Maogai Power Station establishes a track record with the specific hydroelectric operator, facilitating qualification for subsequent maintenance and repair contracts across their fleet of power stations.
8.2 Product Delivery
- On-site execution capability: The project demonstrates the company's ability to mobilize equipment, personnel, and quality control resources to remote hydroelectric sites, reducing component transport costs and downtime.
- Multi-process integration: The project validates the company's capability to combine TIG, MIG, and SAW overlay in a single project, offering customers a one-stop surfacing solution.
- Quality assurance documentation: Comprehensive NDT records, hardness reports, and metallurgical analyses from the project serve as reference documentation for future bids and technical proposals.
8.3 Customer Value
- Capital cost savings: Overlay repair costs typically represent 15–30% of new component fabrication costs, delivering immediate ROI for the power station.
- Schedule optimization: On-site overlay reduces turbine outage duration from months (for new fabrication and shipping) to weeks, preserving power generation revenue.
- Performance restoration: Restoring the bottom ring's hydrodynamic profile recovers turbine efficiency by 0.5–2.0%, directly translating to additional energy output over the component's extended service life.
- Technical credibility: Successful execution at a major hydroelectric facility positions the company as a trusted technical partner in the power generation sector, opening doors to nuclear, offshore wind, and marine applications with similar overlay requirements.
9. Conclusion
The Maogai Power Station turbine bottom ring strip electrode submerged arc weld overlay project exemplifies the company's technical depth in weld overlay manufacturing. It bridges the precision demands of hydroelectric component repair with the productivity requirements of large-scale surfacing, while demonstrating the company's ability to integrate multiple welding processes into a cohesive, quality-controlled solution. The project contributes materially to the company's WPS qualification portfolio, welder certification depth, and customer trust in the power generation sector. As the global hydroelectric fleet ages and maintenance budgets tighten, this capability positions the company at the forefront of the turbine component repair market, offering a sustainable, cost-effective alternative to full component replacement.