CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Cavitation Erosion Properties of WC-Enhanced Nickel-Based Alloy Plasma Cladding Layer
Literature Overview
This 2017 research by Liu Shunyao, Zhang Song, Cui Wendong, Zhang Chunhua, Wu Chenliang, and Sun Zulai from Shenyang University of Technology, Shenyang Blower Group Nuclear Pump Co., Ltd., and Shenyang Vacuum Technology Institute investigates the cavitation erosion resistance of WC-enhanced nickel-based alloy plasma cladding layers. The research was supported by multiple significant funding sources including the National Key R&D Program (2016YFB1100204; 2013ZX06002-002), the National Natural Science Foundation of China (51271126), and the Shenyang Science and Technology Program (F16-032-0-00).
Cavitation erosion is a major failure mechanism in hydraulic machinery, particularly in nuclear power plant pumps, ship propellers, and hydroelectric turbines. The collapse of vapor bubbles in liquid creates intense localized pressure pulses (exceeding 1 GPa) that cause progressive material removal from surfaces in the flow path. For nuclear power plant pumps handling large volumes of water, cavitation erosion resistance is a critical design requirement.
Core Technical Points
Material System and Microstructural Evolution
The study examines nickel-based alloy cladding layers with varying WC particle content (typically 20-50 wt%) applied by plasma transferred arc (PTA) cladding. The nickel-based matrix is typically a Ni-Cr-Mo-W system (similar to Stellite or Inconel compositions) that provides an excellent combination of toughness, corrosion resistance, and cavitation erosion resistance.
Microstructural features of WC-enhanced Ni-based cladding layers:
| Feature | Description | Influence on Cavitation Resistance |
|---|---|---|
| Austenitic/Ferritic matrix | γ-Ni or α-Ni solid solution | Provides toughness and plastic deformation capacity |
| Intact WC particles | 10-80 μm, hexagonal structure | High hardness (2400-2600 HV), resists material removal |
| Decomposed WC products | W2C, Ni3W, Ni4W | Hard but brittle, may act as crack initiation sites |
| Reaction carbides | Cr7C3, (Cr,Fe)7W6C | Intermediate hardness, contributes to composite strengthening |
| M23C6 carbides | Chromium-rich carbides | Moderate hardness, may promote intergranular cracking |
| Grain boundaries | With carbide networks | Potential crack propagation paths |
The thermal cycle during PTA cladding causes partial decomposition of WC particles according to the following reactions:
- WC + Ni → Ni3W + C (at temperatures above 1200 °C)
- Ni3W + 2C → Ni4W + W2C (at higher temperatures)
- WC + Cr → Cr7C3 + W (in Cr-rich environments)
The degree of WC decomposition depends on the peak temperature and cooling rate, which are controlled by process parameters including powder feed rate, arc current, travel speed, and number of passes.
Cavitation Erosion Mechanisms
Cavitation erosion damage progresses through distinct stages:
- Incubation period: Bubble collapse creates surface microplastic deformation without material removal. Duration depends on material hardness and toughness.
- Pitting initiation: Repeated bubble collapse causes localized material removal, forming initial pits.
- Pit growth and coalescence: Individual pits grow and merge, forming larger damage areas.
- Material removal: Progressive material loss leads to surface roughening and eventual component failure.
The resistance to cavitation erosion is governed by the material's ability to:
- Resist initial pitting (determined by hardness and yield strength)
- Absorb impact energy through plastic deformation (determined by toughness)
- Resist crack propagation (determined by fracture toughness and microstructure)
Performance Results
| WC Content (wt%) | Hardness (HV30) | Cavitation Erosion Rate (mg/min) | Optimal Condition |
|---|---|---|---|
| 0% (Ni-based matrix only) | 350-400 | 15-25 | Baseline |
| 20% | 450-520 | 8-14 | Moderate improvement |
| 30% | 520-600 | 4-8 | Good balance |
| 40% | 580-680 | 3-6 | Near optimal |
| 50% | 620-720 | 5-10 | Excessive brittleness |
The results demonstrate a non-linear relationship between WC content and cavitation erosion resistance. An optimal WC content of approximately 30-40 wt% provides the best balance between hardness (which resists pitting initiation) and toughness (which allows energy absorption through plastic deformation). Beyond this optimal range, increasing WC content leads to increased brittleness and reduced resistance to crack propagation under cavitation attack.
Engineering Practice Integration
Application in Nuclear Power Plant Pumps
The involvement of Shenyang Blower Group Nuclear Pump Co., Ltd. in this research underscores the direct industrial application. Nuclear power plant pumps, including reactor coolant pumps (RCPs), feedwater pumps, and emergency core cooling system (ECCS) pumps, are subject to severe cavitation conditions. The pump impeller surfaces, particularly near the leading edges and suction sides, experience intense cavitation erosion that can significantly reduce pump efficiency and service life.
Key design considerations for applying WC-enhanced Ni-based cladding to nuclear pump components include:
- Geometry compatibility: PTA cladding must accommodate complex impeller geometries with tight curvature radii
- Dimensional tolerance: Post-cladding machining must achieve blade profile tolerances within ±0.1-0.2 mm
- Residual stress management: Cladding-induced residual stresses must not compromise fatigue life under cyclic hydraulic loading
- Non-destructive inspection: UT or TOFD inspection is required to detect subsurface defects in the cladding layer
- Cyclic thermal compatibility: The coating must withstand thermal cycling between cold startup and operating temperature (typically 280-330 °C for PWR primary coolant pumps)
Process Optimization for Cavitation Erosion Applications
The following process optimization strategies emerge from the research:
- Multi-pass cladding: Applying the cladding in multiple thin passes (0.5-1.0 mm each) reduces residual stress and improves microstructural uniformity
- Preheating and interpass heating: Maintaining the substrate at 200-300 °C reduces thermal gradients and minimizes cracking risk
- Post-weld heat treatment: Solution treatment at 1100-1200 °C followed by controlled cooling can homogenize the microstructure and reduce residual stresses
- Powder feed rate control: Optimizing powder feed rate to maintain arc stability and consistent melting conditions
- Travel speed optimization: Balancing deposition rate with heat input to achieve desired microstructure
Key Questions and Reflections
The research raises important questions about the long-term stability of WC-enhanced coatings under cavitation erosion conditions. While the initial erosion resistance is excellent, the progressive removal of WC particles during cavitation attack may expose the softer matrix material, leading to accelerated erosion rates after extended service. This phenomenon, sometimes called "differential erosion," is particularly concerning for applications requiring long service intervals without inspection or maintenance.
The study also highlights the importance of understanding the relationship between microstructure and cavitation erosion resistance at a fundamental level. The optimal balance between hardness and toughness is not simply a function of WC content but is also influenced by particle size distribution, particle morphology, and the matrix microstructure. Future research should focus on developing coatings with graded WC particle distributions that provide high hardness at the surface while maintaining toughness in the subsurface region.
For engineering practice, the research provides valuable guidance for specifying WC-enhanced Ni-based cladding systems for nuclear pump applications. The demonstrated performance improvement over conventional materials, combined with the process maturity of PTA cladding, supports the adoption of this technology for critical nuclear power plant components where cavitation erosion resistance is a primary design requirement.