Microstructure and Wear Resistance of WC-Reinforced Nickel-Based Overlay Layers Deposited by Plasma Arc Welding

Literature Overview and Research Background

This study investigates the microstructural evolution and tribological behavior of tungsten carbide (WC)-reinforced nickel-based overlay layers deposited via plasma transferred arc (PTA) welding. The research addresses a critical engineering challenge in high-wear applications such as mining equipment, hydrocarbon processing valves, and oilfield downhole tools, where conventional hardfacing alloys often fail prematurely under severe abrasive and erosive conditions. The addition of WC particles to nickel-based matrices creates a composite overlay system that leverages the extreme hardness of tungsten carbide (Vickers hardness exceeding 2300 HV) while retaining the corrosion resistance and ductility inherent to nickel-based superalloys.

The literature focuses on how plasma arc deposition parameters influence the retention of WC particles, the formation of intermetallic compounds at the particle-matrix interface, and the resulting wear resistance in sliding and abrasive tests. This is particularly relevant for engineers working on pressure vessel internals and heat exchanger tubes that require both corrosion resistance and erosion resistance in aggressive service environments.

Core Technical Content and Microstructural Analysis

Deposition Parameters and Their Influence

The plasma arc welding process offers superior thermal control compared to conventional arc methods such as GTAW or GMAW overlay. The study examines typical process windows including arc current ranging from 120 to 220 A, arc voltage from 18 to 30 V, travel speed from 150 to 400 mm/min, and powder feed rate from 1.5 to 3.5 kg/h. The plasma gas flow rate typically falls between 3 and 6 L/min of argon, with a transfer gas flow of 2 to 4 L/min.

A critical finding is that excessive heat input leads to the decomposition of WC particles into W₂C and WCₓ phases, accompanied by the formation of Fe₃W₃C and Fe₇W₆C intermetallics in steel substrates. The decomposition reaction follows the thermodynamic pathway: WC + 2Fe → Fe₇W₆C + W₂C, which becomes thermodynamically favorable above approximately 1200 °C in the local melt pool.

Microstructural Characterization

The deposited overlay exhibits a columnar dendritic microstructure with WC particles distributed throughout the interdendritic regions. At optimal deposition parameters, the WC particle retention rate reaches 75 to 85 percent, with particle sizes in the range of 5 to 40 micrometers. The nickel-based matrix shows a solid solution of carbon and tungsten in the FCC structure, with precipitates of Ni₃W and Ni₃W₂C observed at grain boundaries under prolonged cooling conditions.

Wear Resistance Results

The study reports that the WC-reinforced overlay achieves a wear rate reduction of 60 to 80 percent compared to the unmodified nickel-based matrix alloy under dry sliding conditions. The coefficient of friction decreases from approximately 0.45 to 0.28 when WC retention exceeds 70 percent. In three-body abrasive wear tests using 600-grit SiC abrasive paper, the specific wear rate drops from 2.5 × 10⁻⁶ mm³/(N·m) for the base alloy to 0.5 × 10⁻⁶ mm³/(N·m) for the optimally deposited WC-reinforced overlay.

Engineering Practice Integration

Process Optimization Recommendations

Based on the literature findings, the following process optimization strategy can be applied in engineering practice:

1. Maintain heat input below 12 kJ/mm to minimize WC decomposition while ensuring adequate bond strength with the substrate.
2. Employ multi-pass deposition with the first pass using pure nickel-based powder to establish a compatible metallurgical bond, followed by WC-containing passes.
3. Use a backing plate or backing rod with high thermal conductivity to extract excess heat from the substrate side.
4. Implement post-deposition stress relief at 600 to 700 °C for 2 hours to relieve residual stresses without triggering further WC decomposition.

Application Considerations for Pressure Vessel Internals

For engineers involved in bimetal pressure vessel fabrication, this research has direct implications for the design of erosion-resistant internals in hydrogenation reactors, amine treating units, and slurry service equipment. The WC-reinforced nickel overlay can be applied to tube sheets, baffle plates, and impeller surfaces where combined corrosion-erosion service conditions exist. The overlay thickness typically ranges from 1.5 to 3.0 mm per pass, with a total build-up of 2 to 6 mm being practical for most applications.

Key Questions and Reflections

A significant question that emerges from this study is the long-term stability of WC particles under thermal cycling conditions. In pressure vessel applications where temperature fluctuations of ±200 °C may occur during start-up and shutdown, the repeated thermal expansion mismatch between WC (thermal expansion coefficient of 5.5 × 10⁻⁶ /°C) and the nickel matrix (13.5 × 10⁻⁶ /°C) could generate interfacial stresses leading to particle debonding over extended service life. This suggests that fatigue wear performance, rather than steady-state sliding wear, may be the limiting factor in cyclic thermal service.

Another critical consideration is the inspection methodology for verifying WC retention in production environments. Standard metallographic examination requires careful preparation to avoid grinding-induced decomposition of WC particles. The use of focused ion beam (FIB) tomography or micro-CT scanning for quantitative particle characterization represents a promising but costly approach for production verification.

Study Insights and Implications

This literature provides valuable guidance for engineers specifying hardfacing overlays in severe wear applications. The fundamental insight is that the synergy between WC hardness and nickel matrix toughness is maximized not by maximizing WC content, but by optimizing the thermal cycle to retain a critical fraction of intact WC particles while ensuring good particle-matrix bonding. For bimetal pressure vessel applications, this research supports the feasibility of localized overlay repairs on critical components without compromising the pressure boundary integrity, provided that appropriate heat input control and post-weld treatment protocols are implemented.