Zhuojin Pipe Fitting Co., LtdCavitation and Cavitation Wear Resistance of Overlay Welded Stainless Steel
Literature Overview
This study by Lv Xiaoli and Wei Jianjun, published in Large Electric Machine Technology in 1989, investigates the cavitation erosion and cavitation erosion wear performance of three stainless steel overlay weld metals: 18-8 austenitic stainless steel (304 type), 0Cr13Ni16Mo, and 0Cr16Ni5Mo. The research was conducted at the Zhengzhou Mechanical Research Institute using a rotating disk cavitation test rig and a rotating sand abrasion test apparatus. The work is particularly relevant to hydro turbine runner protection, where cavitation damage is a major failure mode.
Core Technical Approach
Cavitation occurs when local pressure drops below the vapor pressure of the liquid, causing vapor bubble formation followed by violent collapse when the pressure recovers. This collapse generates micro-jets and shock waves with pressures exceeding 1000 MPa, which cause material fatigue and erosion. In hydro turbine runners, cavitation is further exacerbated by the presence of abrasive particles such as sand and silt, leading to a synergistic cavitation-abrasion wear mechanism.
Test Materials and Their Metallurgical Characteristics
| Material Designation | Type | Key Alloying Elements | Expected Microstructure |
|---|---|---|---|
| 18-8 (304 type) | Austenitic SS | 18% Cr, 8% Ni | Fully austenitic |
| 0Cr13Ni16Mo | Austenitic-Martensitic SS | 13% Cr, 16% Ni, Mo | Mixed austenite-martensite |
| 0Cr16Ni5Mo | Precipitation-hardening SS | 16% Cr, 5% Ni, Mo | Martensitic with precipitates |
Test Methods
The rotating disk cavitation test simulates the cyclic pressure fluctuations experienced by turbine runner surfaces. The rotating sand abrasion test evaluates the material's resistance to solid particle erosion. The combined testing approach captures the synergistic effect of cavitation and abrasion, which is more destructive than either mechanism alone.
Key Findings and Metallurgical Analysis
Effect of Hardness on Cavitation Resistance
The study found that hardness plays a significant role in cavitation resistance. Higher hardness generally improves resistance to cavitation erosion because harder materials deform less under the impact of collapsing bubbles. However, there is an optimal hardness range; excessively hard materials may be more susceptible to cracking and spalling under cyclic loading.
Effect of Impact Toughness
Impact toughness is equally important for cavitation resistance. Materials with high toughness can absorb the energy from bubble collapse without fracturing. The 18-8 austenitic stainless steel, with its fully austenitic structure and excellent ductility, demonstrated superior cavitation resistance despite having lower hardness compared to the martensitic variants.
Cavitation Damage Morphology
The observed damage patterns included:
- Pitting: Shallow, randomly distributed pits formed by individual bubble collapse events.
- Grooving: Elongated channels formed by the directional micro-jet action of bubble collapse near a surface.
- Spalling: Large-scale material removal resulting from subsurface crack propagation under cyclic stress.
- Fatigue cracking: Cracks initiating at the surface and propagating inward under repeated cavitation loading.
Technical Insights and Engineering Reflections
The synergistic effect of cavitation and abrasion is a critical consideration in material selection for hydro turbine runners. Abrasive particles can remove the protective oxide film that normally forms on stainless steel surfaces, exposing fresh metal to further cavitation attack. This creates a self-accelerating degradation mechanism.
From a welding metallurgy perspective, the overlay weld metal microstructure is heavily influenced by the welding process parameters. The cooling rate determines the phase transformation sequence, and the resulting microstructure directly affects cavitation resistance. For the 18-8 type overlay, maintaining a fully austenitic structure through proper cooling rate control is essential. For the precipitation-hardening type, post-weld aging treatment may be necessary to achieve the desired hardness and strength.
The study's findings have direct implications for the repair and refurbishment of hydro turbine runners. Overlay welding provides a cost-effective method to restore protective surfaces on damaged runners without requiring complete replacement. The selection of the appropriate overlay material depends on the specific operating conditions, including the degree of cavitation severity, the presence of abrasive particles, and the required service life.
Practical Recommendations
For hydro turbine runner applications, the following material selection guidelines emerge from this research:
- In low-abrasion, high-cavitation environments, the 18-8 austenitic stainless steel overlay offers the best balance of cavitation resistance and toughness.
- In high-abrasion environments, the 0Cr16Ni5Mo overlay with its higher hardness provides better combined cavitation-abrasion resistance, provided that adequate toughness is maintained.
- The 0Cr13Ni16Mo overlay represents an intermediate option, suitable for moderate cavitation and abrasion conditions.
- Overlay weld parameters must be controlled to ensure a fully fused, crack-free deposit with appropriate microstructure.
This research, though published in 1989, remains relevant for modern hydro turbine maintenance practices. The fundamental metallurgical principles governing cavitation resistance have not changed, and the systematic approach to material evaluation through combined cavitation-abrasion testing continues to be a valuable methodology for overlay material selection.