Effect of Y2O3 on Plasma-Surfaced WC-Ni60 Composite Overlay

Literature Overview

This 2024 paper by Wang Zhenyu and colleagues from Shenyang University of Technology, published in Hot Working Technology (Vol. 53, No. 2, pp. 100-104), investigates the influence of yttrium oxide (Y2O3) addition on the microstructure and performance of plasma-surfaced WC/Ni60 composite overlay layers deposited on Q235 carbon steel. The research employs a systematic approach to optimize the Y2O3 content for achieving maximum hardness and wear resistance, providing valuable insights for advanced composite overlay design in industrial applications.

Experimental Methodology and Process Parameters

The plasma surfacing process was employed to deposit nickel-based composite overlay layers containing tungsten carbide (WC) and varying amounts of Y2O3 onto Q235 steel substrates. The plasma transfer arc (PTA) process was selected for its ability to produce dense, low-porosity deposits with controlled dilution rates, making it particularly suitable for high-performance overlay applications.

The characterization methods employed include scanning electron microscopy (SEM) for microstructural analysis, X-ray diffraction (XRD) for phase identification, Vickers hardness testing for mechanical property evaluation, and high-temperature pin-on-disk wear testing for tribological performance assessment.

Microstructural Analysis and Phase Evolution

The XRD analysis reveals a complex phase composition in the WC/Ni60 overlay, including γ-Ni(Fe) matrix, WC, W2C, M23C6, M6C, Cr7C3, and FeNi3 phases. The presence of multiple carbide phases indicates that the WC particles undergo partial decomposition during the plasma surfacing process, with the released tungsten reacting with carbon and alloying elements to form secondary carbides.

The addition of Y2O3 has a profound effect on the carbide morphology and distribution. Without Y2O3, the carbide particles tend to be irregularly shaped and randomly distributed within the matrix. With Y2O3 addition, the carbides become more regular in shape and exhibit a growth pattern radiating outward from WC cores. This morphological improvement is attributed to the nucleation and growth modification effect of Y2O3 particles, which act as heterogeneous nucleation sites for carbide precipitation.

The SEM observations confirm that Y2O3 particles are uniformly dispersed within the overlay matrix, with sizes typically in the 1-5 μm range. These particles do not dissolve during the plasma surfacing process due to the extremely high melting point of Y2O3 (approximately 2480°C), but they significantly influence the solidification microstructure and carbide precipitation behavior.

Mechanical and Tribological Performance

The hardness results demonstrate a clear optimum at 1.2% Y2O3 addition, where the Vickers hardness reaches a maximum of 970 HV0.5, representing an increase of 283 HV0.5 compared to the unmodified WC/Ni60 overlay. This substantial improvement in hardness is attributed to the combined effects of carbide morphology refinement, increased carbide volume fraction, and the dispersion strengthening contribution of the Y2O3 particles themselves.

The high-temperature wear testing results further confirm the beneficial effect of Y2O3 addition. At 1.2% Y2O3, the wear weight loss is minimized at 2.8 mg, with a friction coefficient of 0.32. The wear mechanism is identified as mild abrasive wear, indicating that the overlay surface maintains its integrity under sliding contact conditions without catastrophic failure.

Engineering Practice and Application Considerations

For pipeline and fitting applications, the plasma-surfaced WC/Ni60 composite overlay with optimized Y2O3 addition offers significant advantages in wear-prone service environments. Pump casings, valve seats, and flow control components in oil and gas processing facilities can benefit from this advanced overlay technology. The combination of high hardness, excellent wear resistance, and good thermal stability makes this overlay particularly suitable for high-temperature and high-pressure applications.

The dilution rate of 10-15% achieved by the plasma surfacing process is acceptable for most applications, but engineers must be aware that the dilution introduces iron and carbon from the Q235 substrate into the overlay, which can affect the phase composition and properties. For critical applications where strict compositional control is required, additional pre-cladding layers or substrate preparation techniques may be necessary.

Key Questions and Reflections

The research provides compelling evidence for the beneficial role of Y2O3 in WC/Ni60 composite overlays, but several practical considerations warrant attention. The 1.2% optimum represents a narrow window, and process variations may shift this optimum, requiring careful process control and qualification testing for each production batch. The high-temperature wear testing conditions should be validated against actual service temperatures, as the wear mechanism may change at different temperature regimes.

Furthermore, the paper does not address the long-term creep behavior or thermal fatigue resistance of the modified overlay. In pipeline applications involving cyclic thermal loading, such as in heat exchangers or process piping, the overlay must withstand repeated thermal expansion and contraction without cracking or delamination. The thermal expansion mismatch between the overlay and substrate is a critical design consideration that must be evaluated for each specific application.

This paper represents a valuable contribution to the field of advanced composite overlay technology. The systematic investigation of Y2O3 content provides clear guidelines for optimizing the microstructure and performance of WC/Ni60 overlays. As the demand for high-performance wear-resistant overlays in the oil and gas industry continues to grow, the insights from this research will play an important role in advancing overlay technology for critical industrial applications.