CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of WC Particle-Reinforced Nickel-Based Spray Welding Coatings
Literature Overview and Research Context
This literature examines the microstructural evolution and tribological performance of tungsten carbide (WC) particle-reinforced nickel-based coatings applied via spray welding (oxy-fuel flame cladding and arc spray welding) onto carbon steel and alloy steel substrates. The research addresses a persistent engineering challenge in mining, cement, and power generation industries where components subjected to severe sliding and impact wear demand both high hardness and adequate toughness. The study employs a systematic approach varying WC particle size (3–45 μm), coating composition (Ni-Cr-B-Si base with 25–40 wt% WC), and welding parameters (travel speed, torch angle, preheat temperature) to establish structure-property relationships. The relevance to bimetal pressure vessel fabrication is indirect but significant: understanding the carbide dissolution behavior and matrix-carbide interface bonding informs the selection of overlay cladding strategies for pressure vessel components exposed to abrasive slurries.
Core Technical Findings
The microstructural analysis reveals three critical features governing wear performance. First, the WC particles undergo partial dissolution during the high-temperature spray welding process, with dissolution extent dependent on particle size and thermal cycle severity. Particles below 10 μm dissolve nearly completely, while those above 30 μm retain their cubic crystal structure with minimal core dissolution. Second, the dissolution products react with the molten matrix to form secondary carbides, predominantly M6C-type (Co,Fe,Ni)₆W₆C and M₂C-type (Co,Fe,Ni)₂WC, which precipitate at grain boundaries and within the matrix. Third, the bond between the coating and substrate is characterized by a thin diffusion layer (typically 5–20 μm) with martensitic transformation in the heat-affected zone, which critically influences coating adhesion strength.
| Parameter | Low WC Content (25%) | Medium WC Content (32%) | High WC Content (40%) |
|---|---|---|---|
| Coating Hardness (HV30) | 850–950 | 1050–1200 | 1300–1500 |
| WC Retention (%) | 15–25 | 35–50 | 55–70 |
| Adhesive Wear Rate (mm³/N·m) | 1.2×10⁻⁶ | 0.6×10⁻⁶ | 0.3×10⁻⁶ |
| Coating Thickness (mm) | 0.5–1.0 | 0.8–1.5 | 1.0–2.0 |
| Microcrack Density (lines/mm) | 2–4 | 5–8 | 9–14 |
Process-Structure-Property Relationships
The spray welding thermal cycle is the dominant variable controlling carbide integrity. Oxy-fuel flame cladding produces peak temperatures of 1300–1500°C with relatively low heat input per pass, resulting in moderate WC dissolution (20–35%). Electric arc spray welding, operating at 1500–2000°C with higher kinetic energy of deposited particles, causes more severe dissolution (40–60%) but produces denser coatings with fewer porosity defects. The travel speed directly influences the cooling rate: slower travel speeds (50–80 mm/min) promote larger carbide precipitation and coarser microstructures, while faster speeds (120–180 mm/min) produce finer, more uniformly distributed secondary carbides but increase the risk of incomplete fusion between successive layers.
A critical insight from this literature is the optimal balance between WC retention and microcrack control. High WC content coatings exhibit superior abrasive wear resistance but develop extensive microcracking due to thermal stress from coefficient of thermal expansion (CTE) mismatch between the hard carbide phase (CTE ≈ 4.5×10⁻⁶/K) and the nickel matrix (CTE ≈ 13×10⁻⁶/K). The study demonstrates that a two-pass approach—applying a WC-free bonding layer followed by a WC-containing wear layer—reduces microcrack density by 40–60% while maintaining wear performance within 10% of single-pass high-WC coatings.
Engineering Practice Implications
For pressure vessel applications involving abrasive wear, particularly in hydrogenation reactors with catalyst-containing slurry service or slurry pump casings, the following engineering guidelines emerge from this literature:
- Preheat temperature of 200–250°C is recommended to reduce thermal stress during multi-layer application and minimize the risk of coating spallation during hydrostatic testing.
- The final coating thickness should be controlled within 1.5–2.5 mm for pressure vessel components, balancing wear life against the risk of hydrogen-induced cracking (HIC) in the underlying base metal.
- Post-weld stress relief at 500–550°C for 2 hours per 25 mm thickness is essential to relieve residual stresses in the cladding interface, particularly when the base metal is low-alloy steel (e.g., 16Mn, 15CrMo).
- Hardness verification using Vickers indentation (HV30) should be performed at 0.5 mm and 1.0 mm depths to confirm adequate hardness gradient and absence of soft zones at the interface.
Key Questions and Reflections
The literature raises an important question regarding the applicability of laboratory-scale wear data to full-scale pressure vessel components. Laboratory pin-on-disk tests typically employ uniaxial sliding at constant speed, whereas actual pressure vessel wear scenarios involve multi-directional contact, variable loading, and elevated temperatures (150–350°C for hydrogenation service). The study does not address the effect of thermal cycling on carbide-matrix bonding, which is a critical concern for pressure vessels operating in cyclic duty. Furthermore, the hydrogen permeation behavior of WC-reinforced coatings in high-pressure hydrogen environments remains unaddressed, representing a significant gap for hydrogenation reactor applications where hydrogen embrittlement of the overlay layer could compromise structural integrity.
Study Insights and Conclusions
This literature provides valuable quantitative data on the WC content-hardness-wear resistance relationship and establishes that 32–35 wt% WC represents the practical optimum for spray welding applications, offering the best compromise between wear life and coating integrity. The two-pass bonding layer strategy is a particularly useful engineering approach that can be directly applied to pressure vessel overlay fabrication. However, the absence of elevated-temperature and cyclic loading data limits direct extrapolation to pressure vessel service conditions. Engineers should supplement this literature with thermal cycling studies and hydrogen permeation tests before specifying WC-reinforced coatings for critical pressure vessel applications. The fundamental insight that carbide dissolution is a controlled beneficial mechanism—producing secondary carbides that enhance wear resistance without requiring complete WC retention—shifts the design philosophy from maximizing WC content to optimizing the dissolution-precipitation balance.