Zhuojin Pipe Fitting Co., LtdBimetallic Turbine Runner Upper Crown Manufactured by Stainless Steel Strip Electrode Overlay Welding
Literature Overview
This paper by Du Bing and colleagues from the Harbin Welding Research Institute and Harbin Motor Factory (1993) describes the development and application of a stainless steel strip electrode overlay welding technology for manufacturing bimetallic turbine runner upper crowns. The authors conducted experimental research on the overlay welding process and mechanical properties of the deposited metal, and successfully fabricated a bimetallic mixed-flow turbine runner upper crown using the developed technology. This work represents an important contribution to the field of bimetallic construction for hydraulic turbine components.
Core Technical Analysis
Turbine runners are critical components in hydroelectric power generation, and their upper crown is subjected to severe cavitation erosion and wear from high-velocity water flow. The conventional approach of using a single material for the entire runner presents a trade-off between cavitation resistance and mechanical strength. Bimetallic construction, which combines a tough base material with a cavitation-resistant overlay layer, offers an optimal solution. The stainless steel strip electrode overlay welding technology developed in this study provides a practical method for achieving this bimetallic structure.
The stainless steel strip electrode is a solid electrode with a specific alloy composition designed to produce a weld metal with excellent cavitation resistance. The strip form factor provides several advantages over conventional welding consumables: consistent cross-sectional geometry, controlled composition, and predictable deposition characteristics. The overlay welding process using this strip electrode produces a dense, uniform deposit with minimal porosity and excellent mechanical properties.
| Property | Base Material (Carbon Steel) | Overlay Layer (Stainless Steel) | Interface |
|---|---|---|---|
| Hardness | Moderate | High | Gradual transition |
| Cavitation resistance | Low | High | Critical zone |
| Mechanical strength | High | Moderate | Bond strength critical |
| Corrosion resistance | Low | High | Dilution zone |
| Microstructure | Ferrite + Pearlite | Austenitic | Mixed |
The key technical challenge in bimetallic overlay welding is ensuring adequate metallurgical bonding between the base material and the overlay layer while maintaining the composition and properties of both materials. The dilution zone at the interface is critical, as it determines the effective thickness of the functional overlay layer. The authors addressed this challenge through careful control of the welding parameters, including current, voltage, travel speed, and number of passes.
Process Analysis and Standards Considerations
The overlay welding process for turbine runner upper crowns must meet the requirements of relevant standards including JB/T 2658 for welding consumables, GB/T 985 for weld preparation, and industry-specific standards for hydraulic turbine components. The mechanical properties of the overlay layer, including tensile strength, hardness, and impact toughness, must be verified through appropriate testing. The cavitation resistance of the overlay layer is typically evaluated through cavitation erosion testing in accordance with ASTM G134 or equivalent standards.
The strip electrode technology represents an advancement in welding consumable design for overlay applications. Unlike conventional wire electrodes, the strip form provides a larger cross-sectional area, which allows for higher deposition rates while maintaining control over the weld geometry. The composition of the strip electrode can be precisely controlled during manufacturing, ensuring consistency in the deposited metal properties.
Engineering Practice Integration
The successful fabrication of the bimetallic mixed-flow turbine runner upper crown demonstrates the practical viability of the developed technology. The engineering implementation involved several critical steps:
- Surface preparation of the base material to ensure clean, oxide-free surfaces for metallurgical bonding.
- Application of a transition layer with controlled dilution to bridge the composition gap between base and overlay materials.
- Multiple passes of overlay welding with controlled heat input to minimize dilution and maintain overlay composition.
- Post-weld heat treatment to relieve residual stresses and optimize the microstructure of the overlay layer.
- Quality inspection including visual examination, magnetic particle testing, and hardness measurement.
The economic benefits of bimetallic construction are significant. The stainless steel overlay layer provides cavitation resistance equivalent to a fully stainless steel runner, while the carbon steel base material provides the necessary mechanical strength at a fraction of the cost. This approach reduces the overall material cost while maintaining or improving the service life of the turbine runner.
Study Insights and Reflections
This work exemplifies the application of welding technology to solve a practical engineering problem in the hydroelectric power industry. The development of a specialized strip electrode for overlay welding demonstrates the importance of consumable engineering in achieving specific performance requirements. The bimetallic construction approach offers a cost-effective solution that balances the competing requirements of cavitation resistance and mechanical strength. The systematic approach to process development, from experimental research to successful fabrication, provides a model for the introduction of new welding technologies in industrial applications.