Microstructure and Wear Performance of Wear-Resistant Electrode Overlay on Hydraulic Gate Underwater SMAW Repair

Literature Overview

This paper by Zhu Sisi, Hu Xing, and colleagues from China Yangtze Power Co., Ltd., the National Engineering Research Center for Efficient Utilization of Water Resources and Engineering Safety, and Hohai University investigates the microstructure and wear performance of overlay layers deposited using a specially designed wear-resistant electrode on Q235 steel hydraulic gates under submerged arc welding conditions. The work is funded by the Open Research Fund of the National Engineering Research Center (GJGCZX-JJ-202408) and was published in the journal "Materials in Mechanical Engineering" (Volume 49, Issue 11, 2025, pages 106–112). The research addresses a critical practical problem in hydroelectric infrastructure maintenance: the need for reliable underwater repair of hydraulic gate surfaces subjected to abrasive wear from sediment-laden water flow.

Electrode Design and Composition Analysis

The wear-resistant electrode is composed of a 0Cr13 stainless steel wire core with a flux coating containing reducing iron powder, attapulgite, potassium feldspar, water glass, rutile, fluorite, manganese powder, cellulose, aluminum-silicon alloy, silicon-iron alloy, and rare-earth silicon-iron alloy. This multi-component flux formulation is carefully designed to achieve several metallurgical objectives simultaneously. The reducing iron powder serves as an active deoxidizer and alloying element supplier, while the aluminum-silicon and silicon-iron alloys contribute silicon and aluminum to the weld pool, promoting the formation of hard carbide phases and adjusting the carbon equivalent. The rare-earth silicon-iron addition plays a dual role: it refines the weld grain structure through heterogeneous nucleation and modifies the morphology of non-metallic inclusions, thereby improving toughness without sacrificing hardness.

The flux composition reflects a classic low-hydrogen, rutile-fluorite hybrid type, which is well-suited for submerged welding because it provides stable arc characteristics, adequate slag coverage, and effective deoxidation even under the adverse conditions of water immersion. The cellulose component contributes to arc stability and helps maintain a protective gas atmosphere around the molten pool. The presence of fluorite (CaF2) reduces the surface tension of the slag, improving wetting and fluidity, while rutile (TiO2) provides good slag viscosity and helps trap hydrogen from the water environment.

Microstructural Characterization

The overlay layer deposited with the wear-resistant electrode exhibits a microstructure consisting of lenticular martensite with a small fraction of delta-ferrite. A notable observation is that the martensite grows along the temperature gradient direction, forming columnar grains. This directional growth pattern is characteristic of rapid solidification under submerged conditions, where the heat extraction rate from the surrounding water is significantly higher than in air welding. The rapid cooling promotes the formation of fine martensitic structures with high dislocation density, which contributes to elevated hardness.

Compared with the overlay layer produced using a conventional electrode, the wear-resistant electrode produces a significantly finer martensite grain size and a higher volume fraction of martensite. The delta-ferrite content is also reduced. This microstructural refinement is attributed to the combined effects of the rare-earth addition in the flux and the higher cooling rate under water. The rare-earth elements act as potent grain refiners, breaking up coarse columnar structures and promoting equiaxed grain formation. The increased martensite fraction is a result of the higher carbon and alloy content delivered by the specialized flux, which raises the martensite start temperature (Ms) and suppresses the formation of softer phases such as bainite or ferrite.

Mechanical Properties and Wear Performance

The microhardness of the wear-resistant electrode overlay layer is substantially higher than that of the conventional electrode overlay. This hardness improvement is directly linked to the finer martensite structure and the presence of hard carbide precipitates formed from the silicon, manganese, and carbon in the weld pool. The friction coefficient and wear rate are both reduced compared with the conventional electrode, indicating superior wear resistance.

The wear surface analysis reveals shallow, uniformly distributed plough grooves with occasional localized spalling, debris accumulation, and microcracks. The primary wear mechanism is identified as mild abrasive wear, which is consistent with the operating conditions of hydraulic gates where solid particles suspended in water abrade the surface. The absence of severe adhesive wear or fatigue wear indicates that the overlay layer maintains good bonding strength with the base metal and does not suffer from catastrophic delamination.

Engineering Practice Implications

From a practical standpoint, this research demonstrates that underwater SMAW overlay repair using a properly designed wear-resistant electrode can produce overlay layers with no cracking defects and significantly improved wear resistance compared with conventional electrodes. This is particularly valuable for hydraulic gate maintenance in hydroelectric dams where dry-docking is expensive and time-consuming. The ability to perform underwater repair with acceptable quality reduces the need for complete gate replacement or extensive dry-dock scheduling.

However, several practical considerations must be addressed in field application. The electrode must be properly dried before use to minimize hydrogen-induced cracking risk, even though the submerged environment provides some inherent protection. The welding parameters—current, voltage, and travel speed—must be carefully controlled to ensure adequate penetration and fusion with the base metal. Multi-pass welding is recommended to achieve sufficient overlay thickness while avoiding excessive dilution and heat input. The three-layer, multi-pass approach used in this study provides a good balance between overlay thickness and residual stress control.

Key Insights and Reflections

The study provides a compelling example of how electrode chemistry can be tailored to address specific service conditions. The inclusion of rare-earth elements in the flux is a sophisticated approach that simultaneously addresses grain refinement, inclusion modification, and microstructure optimization. The fact that no cracks were observed in the overlay layer is particularly significant, as underwater welding is notoriously prone to cracking due to the high hydrogen content in water and the rapid cooling rates.

One area for further investigation would be the long-term durability of the overlay layer under cyclic loading and corrosion-abrasion combined attack. Hydraulic gates are subjected to not only abrasive wear but also cavitation erosion, corrosion, and fatigue. A comprehensive assessment of the overlay layer's performance under these combined damage mechanisms would strengthen the case for widespread adoption of this repair technology.

This research contributes meaningfully to the body of knowledge on underwater repair welding and provides practical guidance for maintenance engineers working on hydroelectric infrastructure. The electrode design philosophy—combining multiple alloying elements and rare-earth additions in a carefully balanced flux—offers a template for developing specialized electrodes for other underwater repair applications in the oil and gas, offshore, and marine industries.