Submerged Arc Strip Electrode Overlay Welding of Ma'ergai Power Station Turbine Bottom Ring

Literature Overview

The paper by Li Mingwei, Yao Lijia, Hu Hongwei, and Zhao Ting, published in Welding (2013, No. 7, pp. 67-69), documents the application of submerged arc strip electrode overlay welding for the anti-wear lining of the Ma'ergai Power Station turbine bottom ring. This case study provides valuable practical insights into the overlay welding of large hydraulic turbine components, where resistance to erosion-corrosion is critical for long-term reliability and availability of hydroelectric power generation equipment.

Core Technical Findings

The Ma'ergai Power Station turbine bottom ring, fabricated from Q235B carbon steel, requires an overlay layer that provides resistance to both erosion from high-velocity water flow and corrosion from the water environment. The study describes a two-layer overlay approach:

The results demonstrate that this two-layer approach produces overlay layers with excellent fusion, absence of internal defects, superior surface profile quality, and hardness meeting drawing design requirements. The overlay layer satisfies both anti-wear and anti-corrosion performance requirements for hydraulic turbine service.

Metallurgical Analysis

The selection of materials for the two-layer overlay system reflects careful metallurgical consideration:

The Soudotape 309L intermediate layer provides a transition between the ferritic-pearlitic Q235B base metal and the ferritic Soudotape 430 surface layer. The 309L austenitic composition offers:

• Good ductility and crack resistance during welding
• Compatibility with both ferritic and austenitic materials
• Adequate hardness for load-bearing applications
• Resistance to hydrogen-induced cracking in the weld metal

The Soudotape 430 surface layer provides the primary erosion and corrosion resistance through:

• Ferritic microstructure with high chromium content (approximately 17-19% Cr)
• Excellent resistance to cavitation erosion in water service
• Good corrosion resistance in aerated water environments
• Adequate hardness for wear resistance without excessive brittleness

The use of strip electrode submerged arc welding (SAW) offers several advantages for this application:

The strip electrode geometry provides a wider, flatter weld bead with better surface finish compared to conventional wire electrodes, which is particularly beneficial for overlay applications where surface quality affects both aesthetics and functional performance.

Process Engineering Considerations

The submerged arc welding process for turbine bottom ring overlay requires careful attention to several process parameters:

• Travel speed: Must be optimized to balance deposition rate with heat input and dilution control. Typical speeds for strip electrode SAW range from 200 to 400 mm/min depending on strip width and required bead profile.
• Current: Strip electrode welding typically uses lower current densities than wire electrode welding due to the larger electrode surface area. Currents in the range of 300-500 A are typical for strip widths of 15-25 mm.
• Flux coverage: Adequate flux coverage is critical to prevent oxidation and spatter. The flux should completely cover the arc and molten pool throughout the welding operation.
• Preheating: Q235B steel generally does not require preheating for overlay welding, but the large mass of the turbine bottom ring may require local preheating to reduce thermal stress and prevent cracking.

The use of a positioning machine (rotary table) is essential for achieving consistent weld quality around the circumferential geometry of the bottom ring. The positioning machine ensures uniform travel speed and torch positioning throughout the entire weld circumference.

Quality Control and Inspection

For hydraulic turbine components, overlay weld quality is critical due to the high consequence of failure. The inspection program should include:

• Visual inspection (VT) of the complete overlay surface for uniformity, undercut, and surface defects
• Magnetic particle testing (MT) for surface and near-surface defect detection
• Ultrasonic testing (UT) for subsurface defect detection, particularly incomplete fusion and porosity
• Hardness testing at multiple locations to verify hardness uniformity and compliance with specifications
• Dimensional inspection to verify overlay thickness and surface profile accuracy

The study reports that the overlay layer exhibits good fusion, absence of internal defects, and excellent surface profile quality, indicating that the process parameters and quality control measures were effective in achieving the required weld quality.

Study Insights and Reflections

This case study demonstrates the practical application of strip electrode submerged arc welding for large hydraulic turbine components, where the combination of high deposition rate, excellent surface quality, and controlled dilution makes this process particularly suitable for overlay applications. The two-layer approach with a transition layer and a surface layer provides an effective solution for achieving both metallurgical compatibility and surface performance requirements.

The use of proprietary materials (Soudotape electrodes and Record flux) from a single supplier ensures process compatibility and provides access to technical support for parameter optimization. However, engineers should be aware that proprietary material systems may create supply chain dependencies and limit flexibility in sourcing replacement materials.

The Ma'ergai Power Station application highlights the importance of overlay welding in extending the service life of hydraulic turbine components. Turbine bottom rings are subjected to continuous erosion from high-velocity water flow, and overlay welding provides a cost-effective means of restoring or enhancing their wear resistance without requiring complete component replacement. This approach is consistent with the sustainability goals of the hydropower industry, which seeks to maximize equipment utilization and minimize material consumption.

The successful application of this process at a major power station validates the technology for similar applications in other hydroelectric facilities. Engineers should consider this approach for other turbine components subjected to erosion-corrosion, including runner blades, spiral casing walls, and draft tube surfaces.

Summary

The study by Li Mingwei and colleagues documents the successful application of strip electrode submerged arc welding for the anti-wear overlay of the Ma'ergai Power Station turbine bottom ring, using a two-layer approach with Soudotape 309L transition and Soudotape 430 surface layers. The results demonstrate excellent fusion, absence of internal defects, superior surface quality, and compliance with hardness specifications. This case study provides valuable practical guidance for engineers applying overlay welding technology to large hydraulic turbine components, where the combination of high deposition rate, controlled dilution, and excellent surface quality makes strip electrode SAW an ideal process choice for erosion-corrosion protection.