Zhuojin Pipe Fitting Co., LtdEffect of Spherical Cast Tungsten Carbide Particles on Overlay Weld Microstructure and Wear Resistance
Literature Overview
This paper, published in 2017 in the journal "Materials in Mechanical Engineering" (Vol. 41, No. 7, pp. 13-17), was authored by Wang Lei and colleagues from the Guangdong Provincial Institute of Materials and Processing Technology, Chinese Academy of Sciences. The study investigates how the addition of spherical cast tungsten carbide (WC) particles—varying in content and particle size—into atomized iron powder affects the microstructure, hardness, and wear resistance of overlay welds produced by oxy-acetylene flame surfacing on Q235 carbon steel. The research was supported by multiple national and provincial science and technology programs in China.
Core Technical Findings
The authors prepared overlay welding electrodes by incorporating spherical cast WC particles of different contents and particle sizes into atomized iron powder. The overlay welds were then deposited on Q235 steel substrate using oxy-acetylene flame surfacing. The key findings can be summarized as follows:
- Dissolution behavior: Spherical cast WC particles at the edges exhibited obvious dissolution during the surfacing process, and the dissolution became more pronounced with decreasing particle size. This indicates that finer particles have a larger specific surface area, promoting interfacial reactions with the molten pool.
- Microstructure evolution: With decreasing WC content, the fishbone-shaped ledeburite structure in the overlay weld decreased. With decreasing WC particle size, the ledeburite structure gradually coarsened. This suggests a complex interplay between particle size, interfacial reaction kinetics, and solidification morphology.
- Hardness and wear resistance: Higher WC content and smaller particle size both contributed to increased overlay hardness and improved wear resistance. The combination of high content and fine particle size yielded the best wear performance.
Microstructure and Property Correlation
| Parameter Variation | Microstructural Change | Hardness Trend | Wear Resistance Trend |
|---|---|---|---|
| Increasing WC content | More ledeburite structure | Increasing | Improving |
| Decreasing WC content | Less ledeburite structure | Decreasing | Degrading |
| Decreasing WC particle size | Coarsened ledeburite | Increasing | Improving |
| Increasing WC particle size | Finer ledeburite | Decreasing | Degrading |
Interpretation of Technical Points
The dissolution of WC particles at their edges is a critical metallurgical phenomenon. During flame surfacing, the molten pool temperature can reach 1400–1700°C, which is sufficient to partially dissolve WC and promote the formation of iron carbides (Fe3C, Fe7W6C, etc.) at the particle-matrix interface. The dissolution of finer particles is more severe because the surface-to-volume ratio is higher, leading to greater interfacial area available for reaction. This partial dissolution does not necessarily degrade performance; in fact, the dissolved carbon and tungsten contribute to the formation of hard carbide phases in the matrix, which can enhance overall hardness.
The fishbone-shaped ledeburite structure is a characteristic feature of high-carbon, high-iron-carbide welds. Its presence indicates that the weld composition exceeds the eutectic carbon content, promoting the formation of austenite-carbide eutectic upon solidification. The coarsening of ledeburite with decreasing particle size is counterintuitive at first glance but can be explained by the fact that finer WC particles dissolve more rapidly, releasing carbon into the liquid in a more concentrated manner near the particle location, which promotes localized carbide nucleation and growth.
The improvement in wear resistance with increasing WC content and decreasing particle size is consistent with the well-established principle that hard ceramic particles act as abrasion-resistant reinforcements. However, the practical engineering implication is that there exists an optimal balance: excessively fine particles may over-dissolve and lose their reinforcing effect, while excessively coarse particles may create stress concentration points leading to spalling under impact loading.
Engineering Practice Implications
For engineers working on overlay welding applications—such as hardfacing of pipeline components, valve seats, and wear parts in oil and gas production—the following practical guidelines emerge from this study:
- Particle selection: Spherical cast WC particles are preferred over irregularly shaped ones because their geometry promotes better wetting and more uniform distribution in the molten pool.
- Content optimization: The WC content should be selected based on the specific wear mechanism. For abrasive wear against hard particles, higher WC content is beneficial; for impact-abrasion combined wear, a moderate content with good toughness is preferable.
- Process considerations: Oxy-acetylene flame surfacing provides relatively low heat input compared to arc processes, which helps minimize dilution of the substrate into the overlay. However, the lower heat input also means less opportunity for particle dissolution, which may be advantageous for preserving the integrity of WC particles.
- Multi-pass strategies: In industrial practice, multi-pass surfacing is often employed. The first pass establishes a metallurgical bond with the substrate, while subsequent passes refine the microstructure and increase the volume fraction of reinforcing carbides.
Key Questions and Reflections
A critical question that arises from this study is whether the dissolution of WC particles represents a loss of reinforcing material or a beneficial redistribution of carbide-forming elements. The authors note that hardness and wear resistance improve with finer particles despite greater dissolution, suggesting that the dissolved carbon and tungsten contribute positively to the matrix hardness. However, the long-term wear performance under severe conditions—such as those encountered in drilling tools or slurry pumps—may be compromised if the original WC particles are significantly degraded.
Another important consideration is the effect of particle size distribution. The study appears to use monodisperse particles, whereas commercial WC powders typically have a distribution of sizes. A bimodal or trimodal distribution might provide synergistic benefits: coarse particles maintain their integrity and provide primary reinforcement, while fine particles dissolve and enrich the matrix.
Study Insights and Implications
This paper contributes valuable data on the relationship between WC particle characteristics and overlay weld performance. For engineers designing hardfacing systems for pipeline and equipment components, the findings underscore the importance of particle size selection as a critical process parameter. The observation that ledeburite coarsens with decreasing particle size is particularly noteworthy, as it suggests that the solidification kinetics are significantly influenced by the dissolution behavior of the reinforcing particles. In practical applications, such as the hardfacing of casing and tubing components in sour service, the selection of WC particle size and content should be guided by both the required hardness and the need for adequate fracture toughness to resist chipping and spalling under cyclic loading.