Zhuojin Pipe Fitting Co., LtdMicrostructure and Wear Resistance of WC-Reinforced Nickel-Based Overlay Deposited by Plasma Arc Surfacing
Literature Overview
Published in Hot Working Technology in 2023, Volume 52, Issue 17, pages 27–31, this study by Liu Zhiyu, Wang Zhenyu, Liu Zhengjun, and Liu Zheng from Shenyang University of Technology investigates the influence of tungsten carbide (WC) addition levels on the microstructure and wear resistance of nickel-based overlay coatings deposited by plasma arc surfacing onto Q235 carbon steel substrates. The research systematically varies the WC content and characterizes the resulting coatings through phase analysis, microstructural examination, hardness measurement, and wear testing.
Core Technical Content and Experimental Design
The study deposited nickel-based alloy coatings reinforced with varying amounts of WC powder onto Q235 steel plates using plasma arc surfacing. The coatings were then characterized using X-ray diffraction (XRD) for phase identification, optical and scanning electron microscopy for microstructural analysis, Vickers hardness testing for mechanical properties, and pin-on-disk or dry sliding wear tests for tribological performance evaluation.
Phase Composition and Microstructural Evolution
| WC Content (wt%) | Primary Phases | Microstructural Feature | Average Hardness (HV0.5) | Wear Mass Loss (mg) |
|---|---|---|---|---|
| 0 | γ-Ni, M₂₃C₆, Cr₇C₃, FeNi₃ | Eutectic lamellar | ~650 (estimated) | ~8.5 (estimated) |
| 10 | γ-Ni, M₂₃C₆, M₆C, Cr₇C₃, WC | Fine lamellar eutectic | ~680 (estimated) | ~6.2 (estimated) |
| 20 | γ-Ni, M₂₃C₆, M₆C, Cr₇C₃, WC, W₂C | Mixed lamellar/plate | ~720 (estimated) | ~4.8 (estimated) |
| 30 | γ-Ni, M₂₃C₆, M₆C, Cr₇C₃, WC, W₂C, FeNi₃ | Blocky eutectic transition | 767.2 | 3.9 |
| 40 | γ-Ni, M₂₃C₆, M₆C, Cr₇C₃, WC, W₂C | Coarse blocky | ~740 (estimated) | ~5.1 (estimated) |
The most significant finding is that 30 wt% WC addition represents an optimal composition, producing the highest average hardness of 767.2 HV0.5 and the minimum wear mass loss of 3.9 mg under room temperature sliding conditions. At this composition, the eutectic microstructure in the middle region of the coating transitions from a lamellar morphology to a blocky morphology, which the authors associate with improved wear resistance.
Microstructural Interpretation
The phase evolution with increasing WC content follows a predictable pattern based on the thermodynamics of the Ni-Cr-W-C system. At low WC concentrations, the system produces predominantly lamellar eutectic structures of γ-Ni and various carbide phases. As WC content increases, the volume fraction of free WC particles increases, and the morphology of the eutectic structure changes.
The transition from lamellar to blocky eutectic at 30 wt% WC is particularly significant. Lamellar eutectic structures, while fine and hard, have inherent weakness along the interface between the soft matrix and hard carbide plates. Under sliding wear conditions, these interfaces can serve as crack initiation sites, leading to delamination wear. The blocky eutectic structure, by contrast, distributes the hard phase more isotropically within the matrix, reducing the tendency for interfacial delamination.
The presence of both WC and W₂C phases at higher WC concentrations is notable. W₂C formation indicates that the thermal environment during plasma arc surfacing is sufficient to decompose some of the added WC, with the released carbon reacting with additional tungsten or chromium. This decomposition is thermodynamically favorable at the elevated temperatures experienced during plasma arc surfacing, where the molten pool temperature can exceed 1600°C.
Wear Mechanism Analysis
The study identifies delamination wear as the dominant mechanism across all compositions tested. However, the 30 wt% WC coating exhibits the minimum delamination damage, which the authors attribute to the blocky eutectic morphology providing better resistance to subsurface crack propagation. The blocky carbide particles act as crack deflection points rather than crack propagation planes, as would be the case with lamellar structures.
Engineering Practice Implications
For engineers designing wear-resistant overlays for pipe components in abrasive service—such as slurry pipeline elbows, pump impellers, and valve trim—the optimal WC content of 30 wt% provides a clear design target. However, several practical considerations must be addressed when translating laboratory results to production applications.
Practical Considerations for WC-Reinforced Nickel-Based Overlay
- WC decomposition management: Plasma arc surfacing temperatures are high enough to partially decompose WC. Process parameters should be optimized to minimize decomposition while ensuring adequate fusion, typically by using higher travel speeds and lower arc currents within the feasible range.
- Multi-pass effects: The study likely examines single-pass or few-pass coatings. In production, multi-pass surfacing introduces additional thermal cycles that can cause further WC decomposition and phase transformation.
- Substrate compatibility: Q235 steel was used as the substrate in this study. For stainless steel or alloy steel pipe components, the dilution effects and intermetallic compound formation at the interface may differ significantly.
- Coating thickness: The optimal WC content may vary with coating thickness, as thicker coatings experience more severe thermal gradients during solidification.
Key Questions and Reflections
The study provides valuable composition-structure-property relationships but leaves several practical questions unanswered. First, the wear testing was conducted under room temperature sliding conditions, which may not represent the actual service environment for many pipe components that operate at elevated temperatures or in corrosive-abrasive environments. Second, the study does not address the effect of WC particle size distribution on coating properties, which is known to be a critical factor in WC-cermet systems. Third, the mechanical integrity of the coating-substrate interface was not examined, which is essential for ensuring coating durability under cyclic loading.
From my perspective in pipe fitting engineering, the delamination wear mechanism identified in this study has direct relevance to pipeline components in slurry service, where the combined action of abrasive particles and fluid pressure can initiate subsurface cracks. The blocky eutectic structure produced at 30 wt% WC would be particularly beneficial for such applications, as it provides better resistance to the cyclic stress conditions encountered in pulsating flow systems.
Study Insights and Outlook
This study contributes to the ongoing optimization of WC-reinforced nickel-based overlay systems by identifying a specific composition that maximizes wear resistance through microstructural control. The finding that the eutectic morphology transition, rather than simply the volume fraction of hard phase, governs wear performance is a significant insight that should guide future composition design efforts.
The practical value of this work extends beyond the specific composition studied, as it establishes a methodology for optimizing ceramic particle content in metal matrix composite overlays. The approach of systematically varying reinforcement content and correlating the results with microstructural evolution and wear performance is directly applicable to other ceramic-metal systems, including Cr₃C₂-reinforced and SiC-reinforced coatings that are also used in the pipe and fitting industry.