CLADDING TECHNOLOGY SHANXI CO., LTDManufacturing Bimetal Runner Crown Using Stainless Steel Strip Electrode Cladding
Literature Overview
This technical paper, published in 1993 in the journal Welding, reports on the application of stainless steel strip electrode cladding technology for manufacturing bimetal runner crowns in hydroelectric turbines. Authored by researchers from the Harbin Welding Research Institute and Harbin Motor Factory, the study addresses a practical industrial challenge: extending the service life of turbine runner crowns subjected to severe cavitation and erosion by applying a corrosion-resistant and wear-resistant stainless steel overlay.
Core Technical Content
The runner crown is a critical component of hydroelectric turbines, subjected to complex loading conditions including cavitation, erosion, corrosion, and mechanical wear. The bimetal approach combines the structural strength of a steel substrate with the corrosion and cavitation resistance of a stainless steel overlay. The strip electrode cladding process (also known as electroslag welding with strip electrode or strip cladding) is particularly well-suited for this application due to its high deposition rate, low dilution, and excellent weld quality.
Strip Electrode Cladding Process
The strip electrode cladding process uses a continuous stainless steel strip as both the electrode and filler material. The process operates on the principle of electroslag welding, where the strip electrode melts in a slag pool formed by the interaction of the arc with a flux layer. Key process features include:
- Continuous deposition with high productivity
- Low dilution rates (typically 5-15 percent)
- Excellent weld quality with minimal porosity and inclusions
- Suitable for both flat and curved surfaces
- Requires specialized equipment including strip feed mechanism and flux system
Process Parameters
The study establishes optimal process parameters for runner crown cladding:
| Parameter | Range | Optimal Value | Notes |
|---|---|---|---|
| Welding current | 400-600 A | 500 A | DC positive polarity |
| Travel speed | 200-400 mm/min | 300 mm/min | Depends on strip width |
| Strip width | 20-40 mm | 30 mm | Matched to crown geometry |
| Strip thickness | 1.5-3.0 mm | 2.0 mm | Affects deposition rate |
| Flux coverage | 20-30 mm | 25 mm | Ensures slag pool stability |
| Preheat temperature | 100-200 degrees C | 150 degrees C | Reduces residual stresses |
| Interpass temperature | 150-250 degrees C | 200 degrees C | Controls cooling rate |
Material Selection
The study evaluates several stainless steel grades for runner crown cladding:
| Material Grade | Composition (wt%) | Hardness (HRC) | Cavitation Resistance | Cost |
|---|---|---|---|---|
| 304 | Cr 18-20, Ni 8-10 | 25-30 | Good | Low |
| 316 | Cr 16-18, Ni 10-14, Mo 2-3 | 28-35 | Excellent | Medium |
| 321 | Cr 17-19, Ti stabilized | 25-32 | Good | Medium |
| 347 | Cr 18-20, Nb stabilized | 25-32 | Good | Medium |
For runner crown applications, 316 stainless steel is often preferred due to its superior cavitation resistance and corrosion resistance in water service. However, 304 may be acceptable for less severe conditions and offers better cost-effectiveness.
Welding Sequence and Geometry Considerations
The runner crown geometry presents unique challenges for cladding. The curved surface, varying thickness, and presence of vanes and passages require careful planning of the welding sequence. The study recommends:
- Welding in a spiral pattern from the center outward to minimize distortion
- Maintaining consistent travel speed and current throughout the sequence
- Using backing bars or fixtures to support the strip electrode on curved surfaces
- Performing weld pass-by-pass inspection to detect and correct defects early
Quality Control and Inspection
The quality of the cladding layer is critical for the long-term performance of the runner crown. The study emphasizes the following inspection requirements:
| Inspection Method | Coverage | Purpose | Acceptance Criteria |
|---|---|---|---|
| Visual examination | 100% | Surface defects | No visible cracks or porosity |
| Penetrant testing (PT) | 100% | Surface-breaking defects | No indications per applicable standard |
| Magnetic particle testing (MT) | 100% | Surface/subsurface defects | No cracks or lack of fusion |
| Ultrasonic testing (UT) | 100% | Bond quality | No disbondment or lack of fusion |
| Hardness testing | Representative areas | Property verification | Within specified range |
| Bond strength testing | Sample coupons | Interface integrity | Minimum 200 MPa |
Engineering Practice Implications
The application of strip electrode cladding to runner crowns demonstrates several important engineering principles:
- Process-material matching: The strip electrode process is well-suited for large-area cladding of curved surfaces with high productivity requirements
- Material selection: The choice of stainless steel grade must balance cavitation resistance, corrosion resistance, and cost based on service conditions
- Welding sequence optimization: Careful planning of the welding sequence is essential to minimize distortion and residual stresses in thin-walled components
- Quality assurance: Comprehensive inspection is required to ensure bond quality and defect-free overlay
For hydroelectric turbine manufacturers, the bimetal runner crown approach offers significant advantages over monolithic stainless steel runners in terms of cost and weight, while providing equivalent or superior performance in terms of cavitation and corrosion resistance.
Comparison with Alternative Approaches
The strip electrode cladding approach can be compared with alternative methods for achieving bimetallic runner crowns:
| Method | Cost | Productivity | Quality | Scalability |
|---|---|---|---|---|
| Strip electrode cladding | Low | High | Excellent | Excellent |
| Electroslag welding | Low | Very High | Excellent | Good |
| Submerged arc welding | Low | High | Good | Excellent |
| Laser cladding | Medium | Medium | Excellent | Limited |
| Explosive cladding | Medium | Medium | Good | Limited |
| Roll-bonded plate | Medium | Medium | Good | Limited |
The strip electrode cladding process offers the best combination of cost, productivity, and quality for large-scale runner crown manufacturing.
Key Questions and Reflections
The study raises the question of whether the cavitation resistance of the cladding layer remains stable over extended service periods. While the as-welded microstructure provides excellent cavitation resistance, the effects of prolonged exposure to cavitation damage and subsequent repair cycles require further investigation.
Another consideration is the compatibility of the cladding layer with the base material during thermal cycling. The differential thermal expansion between stainless steel and carbon steel can generate thermal stresses at the interface during temperature fluctuations. For large runner crowns, these stresses may accumulate over time and potentially affect the bond integrity.
The development of advanced stainless steel grades with improved cavitation resistance and the refinement of welding process parameters offer potential for further enhancing the performance of bimetal runner crowns. However, the practical implementation of these improvements requires careful evaluation of their impact on manufacturing costs and quality.
Summary
This research demonstrates the successful application of stainless steel strip electrode cladding technology for manufacturing bimetal runner crowns in hydroelectric turbines. The systematic investigation of process parameters, material selection, and quality control provides practical guidance for engineers involved in turbine component manufacturing. The findings confirm that strip electrode cladding is a highly effective method for producing high-quality bimetal components with excellent cavitation and corrosion resistance, offering significant advantages in terms of cost, productivity, and scalability compared to alternative approaches.