CLADDING TECHNOLOGY SHANXI CO., LTDWear-Erosion Resistant Cladding and Structural Welding Consumables for Hydraulic Turbine Flow Passage Components
Literature Overview and Technical Context
This study, conducted by the Zhengzhou Institute of Mechanical Research (ZJMI), addresses the development and application of specialized welding electrodes and wires for the wear-erosion resistant cladding and structural welding of hydraulic turbine flow passage components. Hydraulic turbines, particularly reaction turbines such as Francis and Kaplan types, operate under extreme conditions of high-velocity water flow containing solid abrasive particles (sand, silt, and mineral debris). The flow passage components—including the spiral case, guide vane (stay vane), runner, and draft tube—are subjected to a combined degradation mechanism of cavitation erosion, abrasion by solid particles, and chemical corrosion, which can significantly reduce turbine efficiency and service life.
The challenge of protecting these components is multifaceted. The cladding material must withstand the synergistic effects of cavitation (where microbubble collapse generates localized pressures exceeding 1000 MPa), abrasion (where solid particles impact the surface at velocities of 20–40 m/s), and corrosion (from dissolved oxygen and acidic species in the water). Simultaneously, the structural welds connecting these components must maintain high fatigue strength and fracture resistance under cyclic hydraulic loading. The study presents a systematic approach to consumable development that addresses both the cladding and structural welding requirements.
Core Technical Findings
Cladding Material Design and Composition
The study developed a series of welding consumables specifically tailored for hydraulic turbine flow passage components. The cladding materials are based on high-alloy austenitic stainless steel and nickel-based alloy systems with optimized compositions for cavitation-erosion resistance. The key alloying elements include chromium (18–25 wt%), nickel (8–14 wt%), molybdenum (2–4 wt%), and minor additions of tungsten (1–3 wt%) and copper (1–2 wt%) to enhance resistance to the specific degradation mechanisms encountered in hydraulic service.
The microstructure of the deposited cladding layer is designed to be fully austenitic with a fine grain structure and minimal δ-ferrite content (below 3% FN). The austenitic phase provides excellent resistance to cavitation erosion due to its high ductility and capacity for strain-induced martensitic transformation, which absorbs the energy of cavitation bubble collapse. The fine grain structure (grain size below 100 μm) is achieved through controlled cooling rates and the addition of grain refiners such as titanium and zirconium.
| Consumable Type | Base Alloy | Key Additions | Hardness (HV) | Cavitation Erosion Resistance |
|---|---|---|---|---|
| Electrode E-1 | 18Cr-8Ni austenitic SS | Mo 2%, W 2% | 220–260 | Excellent |
| Electrode E-2 | 25Cr-12Ni austenitic SS | Mo 3%, Cu 2% | 200–240 | Excellent |
| Wire W-1 | Ni-20Cr-3Mo (Inconel 600 type) | W 1% | 250–300 | Outstanding |
| Wire W-2 | 18Cr-10Ni austenitic SS | Ti 0.5%, Zr 0.1% | 210–250 | Very Good |
Cavitation-Erosion Performance Evaluation
Cavitation erosion testing was conducted using a standard ultrasonic vibratory apparatus (ASTM G134) with 20 kHz frequency, 100 W power, and 3.5% NaCl solution as the test medium. The mass loss rate of the cladding deposits was measured over exposure durations of 24, 48, 72, and 96 hours. The results demonstrated that the developed consumables produced cladding layers with mass loss rates of 0.05–0.15 mg/cm²/h, which is 5–10 times lower than the unclad carbon steel substrate (0.5–1.2 mg/cm²/h).
The cavitation erosion resistance was found to be strongly correlated with the austenite grain size and the ratio of austenite to δ-ferrite. Deposits with fully austenitic microstructures and grain sizes below 100 μm exhibited the lowest mass loss rates, confirming that the strain-induced martensitic transformation mechanism is the dominant protective mechanism against cavitation erosion. The presence of even small amounts of δ-ferrite (above 5% FN) significantly increased the mass loss rate due to the preferential erosion of the brittle ferrite phase.
Abrasion and Combined Wear-Erosion Performance
Slurry abrasion testing (ASTM G65) using silica sand (60–80 mesh) at concentrations of 10–30% by weight and flow velocities of 3–5 m/s demonstrated that the cladding layers exhibited volumetric wear rates of 10⁻⁶–10⁻⁵ mm³/N·m, compared to 10⁻³–10⁻⁴ mm³/N·m for the unclad substrate. The wear mechanism transitioned from abrasive (at low sand concentrations) to erosive-abrasive (at high concentrations), with the cladding layer maintaining its protective function throughout the test range.
Combined cavitation-abrasion testing, which simulates the actual service conditions in hydraulic turbines, confirmed that the cladding layers maintained their integrity after 500 hours of exposure, with surface roughness increase below 20 μm. The microstructure of the eroded surface showed evidence of work hardening and strain-induced martensite formation, which provided additional resistance to further erosion—a self-reinforcing protective mechanism that is unique to austenitic stainless steel systems.
Structural Welding Consumable Development
The study also addressed the development of structural welding consumables for the main body of hydraulic turbine components. These consumables are designed to produce welds with high fatigue strength and fracture toughness, essential for withstanding the cyclic hydraulic loading that turbine components experience during operation. The structural electrodes and wires are based on low-alloy martensitic or duplex stainless steel systems with controlled carbon content (below 0.08%) to minimize the risk of hydrogen-induced cracking.
The structural welds were evaluated for fatigue performance using rotating bending tests at stress ratios (R) of 0.1 and 0.5, with stress levels ranging from 200–500 MPa. The fatigue limit of the structural welds was found to be 280–350 MPa, which is 70–85% of the fatigue limit of the base material—a ratio that exceeds the typical 60–70% achieved with conventional welding consumables. This improvement is attributed to the optimized weld metal composition that produces a fine-grained acicular ferrite microstructure with low inclusion content.
Engineering Practice Integration and Key Reflections
The development of specialized consumables for hydraulic turbine flow passage components represents a significant advance in the surface engineering of hydroelectric equipment. The study's systematic approach to material design—starting from the understanding of degradation mechanisms and working backward to consumable composition—provides a model for similar challenges in other industries.
From a practical standpoint, the application of these consumables in turbine maintenance and repair programs requires careful consideration of several factors. The cladding thickness must be sufficient to provide the required service life (typically 2–5 mm for Francis turbine runner blades) but not so thick as to alter the hydrodynamic profile of the flow passage. The dimensional accuracy of the cladding deposit must be maintained within ±0.2 mm to ensure proper alignment with adjacent components and to avoid flow separation that would reduce turbine efficiency.
The study's emphasis on combined cavitation-abrasion testing is particularly valuable, as many conventional testing protocols evaluate these degradation mechanisms independently. In actual turbine service, cavitation and abrasion act synergistically, with cavitation removing the protective oxide layer and exposing fresh metal to abrasive attack, while abrasion creates surface roughness that promotes cavitation nucleation. Engineers should insist on combined testing for any consumable intended for hydraulic turbine service.
A key reflection from this study is the importance of consumable selection based on the specific service environment. Turbines operating in water with high sand content (above 50 mg/L) require consumables with enhanced abrasion resistance, while turbines in water with low dissolved oxygen and high chloride content require consumables with superior cavitation-erosion and corrosion resistance. The study's range of consumables (E-1, E-2, W-1, W-2) provides flexibility to match the material to the specific service conditions, but engineers must conduct a thorough environmental assessment before selecting the appropriate consumable.
In conclusion, this literature from the Zhengzhou Institute of Mechanical Research demonstrates a comprehensive approach to the development of specialized welding consumables for hydraulic turbine flow passage components. The combination of optimized cladding materials with high cavitation-erosion and abrasion resistance, and structural welding consumables with superior fatigue performance, addresses the full spectrum of degradation mechanisms encountered in hydraulic turbine service. The practical implementation of these consumables requires careful consideration of cladding thickness, dimensional accuracy, and environmental matching, but the potential for extending turbine service life by 3–5 times justifies the investment in specialized material development and qualified application procedures.