CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of WC-Reinforced Nickel-Based Overlay Deposited by Plasma Transferred Arc Cladding
Research Background and Significance
Wear-resistant overlay coatings are essential for components operating in abrasive and erosive environments, such as mining equipment, cement mill liners, and chemical pump impellers. The combination of tungsten carbide (WC) particles with nickel-based alloy matrices offers exceptional wear resistance through a synergistic mechanism where hard WC particles provide abrasion resistance while the ductile nickel matrix ensures toughness and prevents catastrophic failure. Plasma transferred arc (PTA) cladding is one of the most effective methods for depositing such coatings due to its high energy density, precise thermal control, and ability to produce dense, well-bonded overlays.
Microstructural Characteristics
The study examined the microstructure of WC-reinforced nickel-based overlays deposited by PTA cladding on carbon steel substrates. Key microstructural features observed include:
| Microstructural Feature | Description | Influence on Wear Resistance |
|---|---|---|
| WC particles | 20–100 μm size, uniformly distributed | Primary abrasion resistance mechanism |
| M₇C₃ carbides | Formed at WC/matrix interface | Secondary hard phase contributing to hardness |
| γ-Ni matrix | Austenitic structure with dissolved alloying elements | Provides toughness and crack resistance |
| M₆C carbides | Found in interdendritic regions | Moderate hardness contribution |
The PTA process produces a columnar grain structure growing perpendicular to the substrate surface, which is typical of rapid solidification welding processes. The dilution rate was controlled to 20–35%, which is significantly lower than conventional arc welding methods, ensuring that the WC particles retain their chemical integrity and do not decompose excessively into W₂C and free carbon.
Wear Resistance Performance
Wear testing was conducted using pin-on-disk and dry sliding wear tests under various loads. The results demonstrated that the WC-reinforced overlay exhibited wear rates 5–8 times lower than the base carbon steel and 2–3 times lower than conventional hardfacing alloys. The specific wear rate was approximately 10⁻⁴ to 10⁻³ mm³/N·m depending on the applied load and sliding speed.
The enhanced wear resistance is attributed to several mechanisms:
- WC particles act as hard abrasion-resistant phases that resist material removal through ploughing and micro-cutting mechanisms.
- The nickel matrix provides a ductile binder that absorbs energy and prevents crack propagation through the overlay.
- The fine microstructure produced by rapid solidification in PTA cladding increases the overall hardness and resistance to deformation.
Process Parameters and Their Influence
The PTA cladding process parameters were optimized to achieve the best combination of wear resistance, bond strength, and overlay integrity:
- Plasma current: 150–250 A
- Arc voltage: 25–35 V
- Travel speed: 200–400 mm/min
- Powder feed rate: 0.5–1.5 kg/h
- Gas flow rate: 10–20 L/min (argon)
- Powder composition: 70% Ni-20Cr + 30% WC (by weight)
A critical finding was that the WC particle size distribution significantly affects the final overlay performance. Particles larger than 100 μm tend to create stress concentration points and increase the risk of particle pull-out during wear. Conversely, particles smaller than 20 μm may decompose more readily during the melting process, reducing their effectiveness as hard phases. The optimal particle size range of 40–80 μm provided the best balance between particle retention and mechanical performance.
Defect Analysis and Quality Control
Common defects observed in PTA-clad WC overlays include:
- Particle pull-out: Caused by excessive heat input or insufficient bond strength between WC and matrix.
- Cracking: Resulting from thermal stresses during cooling, particularly in thick overlay deposits.
- Porosity: Due to gas entrapment from powder moisture or inadequate shielding gas coverage.
- Incomplete fusion: Occurring at the substrate-overlay interface when preheating is insufficient.
Quality control measures include visual inspection, magnetic particle testing for surface cracks, ultrasonic testing for subsurface defects, and metallographic examination of cross-sections. Hardness profiling across the overlay thickness helps verify uniform composition and detect areas of excessive dilution or incomplete melting.
Study Conclusions and Engineering Recommendations
This research confirms that PTA cladding of WC-reinforced nickel-based alloys produces overlays with exceptional wear resistance suitable for demanding industrial applications. The key to success lies in controlling the dilution rate, maintaining appropriate WC particle integrity, and optimizing the process parameters to balance deposition rate with overlay quality. Engineers should note that while the wear resistance is outstanding, the overlay thickness is typically limited to 2–5 mm due to cracking susceptibility in thicker deposits. For applications requiring thicker overlays, multiple PTA passes with intermediate grinding may be necessary, or alternative processes such as laser cladding with pre-alloyed powder should be considered.