Microstructure and Wear Resistance of Tungsten Carbide Particle Reinforced Nickel-Based Plasma Transfer Arc Powder Surfacing Overlay

Literature Overview

This research, published by Yu Wei, Li Yuxi, Zhou Wuxi, He Xiangjian, and Song Can in the journal Materials Protection (2022, Vol. 55, No. 10, pp. 112-117), investigates the microstructure and wear resistance of nickel-based plasma transfer arc (PTA) powder surfacing overlays reinforced with tungsten carbide (WC) particles. The study compares three different types of WC particles as the hard phase in combination with a nickel-based alloy powder, examining the effects of WC particle morphology and size on the overlay microstructure and abrasive wear performance. The work was supported by a Sichuan Provincial Science and Technology Program project focused on vanadium and titanium-containing laser cladding powder materials.

Core Findings and Technical Analysis

The study reveals a nuanced and somewhat counterintuitive relationship between WC particle burn-off during the PTA process and the resulting wear resistance of the overlay. Three types of WC particles were evaluated: spherical cast WC, coarse-grain WC, and a third variant (likely fine-grain or agglomerated WC). The key findings are as follows:

1. All three types of WC particles experienced varying degrees of burn-off during the plasma transfer arc powder surfacing process, with spherical cast WC exhibiting the most severe burn-off and coarse-grain WC showing the least.
2. Despite having the most severe WC burn-off, the spherical cast WC reinforced overlay demonstrated the best abrasive wear resistance among the three configurations tested.

This apparent contradiction—where the overlay with the most WC degradation exhibits the best wear performance—warrants careful analysis. The explanation likely lies in the microstructural refinement and the nature of the reaction products formed during the burn-off process. When WC decomposes at high temperatures, it can form complex carbides and intermetallic compounds with the nickel-based matrix, such as Ni₃W, Ni₄W, and other tungsten-rich phases. These reaction products, while not retaining the original WC structure, may still contribute significantly to wear resistance through solid solution strengthening and the formation of hard secondary phases.

The spherical morphology of the cast WC particles likely promotes more uniform melting and mixing with the nickel-based alloy, leading to a more homogeneous distribution of the resulting hard phases throughout the overlay. In contrast, the coarse-grain WC particles, while retaining more of their original structure, may create localized regions of high hardness surrounded by softer matrix, leading to uneven wear behavior and potential initiation of cracks at the particle-matrix interface.

Comparative Analysis of WC Particle Types

The following table presents a comparison of the three WC particle types and their effects on the overlay properties:

Metallurgical Mechanisms

The PTA surfacing process operates at temperatures well above the decomposition temperature of WC (approximately 2720°C for pure WC, but decomposition can occur at lower temperatures in the presence of nickel and other alloying elements). During the PTA process, the plasma arc temperature of 10,000-30,000 K creates conditions where WC can partially or fully decompose, releasing tungsten and carbon atoms that then react with the nickel-based matrix to form new phases.

The extent of burn-off is influenced by several factors, including the melting point and thermal conductivity of the WC particle type, the residence time of the particle in the molten pool, and the local oxygen activity in the arc atmosphere. Spherical cast WC particles, being more dense and having a lower surface-area-to-volume ratio, may experience more complete melting and subsequent reaction with the nickel matrix. Coarse-grain WC particles, with their larger individual grain sizes, may resist complete melting and retain more of their original WC structure.

The formation of fine, uniformly distributed hard phases from the decomposition and re-reaction of WC in the nickel matrix is likely the primary mechanism responsible for the superior wear resistance of the spherical cast WC overlay. This finding has important implications for the design of particle-reinforced composite overlays, suggesting that complete integration of the hard phase with the matrix may be more beneficial for wear resistance than simply retaining the original hard phase particles.

Engineering Applications and Process Considerations

In the context of pipe and fitting engineering, PTA surfacing of nickel-based alloys reinforced with WC particles is employed for the repair and enhancement of components subjected to severe abrasive wear, such as pump impellers, valve components, and pipeline sections in slurry service. The findings of this study suggest that the selection of WC particle type should not be based solely on the amount of WC retained in the overlay, but rather on the overall microstructural homogeneity and the nature of the reaction products formed during the PTA process.

Process control parameters such as arc current, travel speed, powder feed rate, and shielding gas composition must be carefully optimized to achieve the desired balance between WC burn-off and microstructural refinement. The PTA process offers significant advantages over conventional arc surfacing methods, including lower dilution rates, better control over the overlay composition, and the ability to produce thinner, more precisely controlled overlays. These advantages are particularly valuable when repairing expensive components such as alloy pipeline sections or precision valve bodies.

Quality Assessment and Testing Methodology

The authors employed a comprehensive testing approach, including optical microscopy and scanning electron microscopy for microstructural analysis, as well as abrasive wear testing using a standard tribometer. This multi-faceted approach is essential for drawing reliable conclusions about the relationship between microstructure and wear performance. Engineers applying similar overlays in industrial settings should ensure that both microstructural characterization and functional wear testing are performed to validate the overlay performance before full-scale deployment.

Study Insights and Reflections

This study contributes valuable insights to the field of particle-reinforced composite overlays, challenging the conventional assumption that greater retention of the original hard phase particles necessarily leads to better wear resistance. The finding that the overlay with the most severe WC burn-off exhibited the best wear performance suggests that the microstructural refinement and uniform distribution of reaction products play a more critical role than the mere presence of intact WC particles. This insight has broader implications for the design of other particle-reinforced overlay systems, including those based on chromium carbide, titanium carbide, or boron carbide particles.

For practicing engineers, the study underscores the importance of understanding the metallurgical transformations that occur during the surfacing process, rather than relying solely on empirical observations. The selection of reinforcement particles should consider not only their inherent hardness but also their compatibility with the matrix alloy and their behavior during the melting and solidification process. This work by Yu and colleagues represents a meaningful contribution to the optimization of PTA surfacing technology for wear-resistant applications in the oil, gas, and mining industries.