Zhuojin Pipe Fitting Co., LtdEffect of Spherical Cast Tungsten Carbide Particles on Overlay Microstructure and Wear Performance
Literature Overview
This study by Wang Lei and colleagues from the Guangdong Provincial Academy of Science investigates the influence of spherical cast tungsten carbide (WC) particles on the microstructure, hardness, and wear resistance of overlay layers deposited on Q235 steel via oxy-acetylene flame surfacing. The research addresses a critical challenge in surface engineering: how to maximize the wear resistance of hardfacing overlays while controlling particle dissolution and microstructural evolution during the deposition process. The work is published in the journal Materials in Mechanical Engineering (Volume 41, Issue 7, 2017, pp. 13–17) and is supported by multiple national and provincial research programs in China.
Core Technical Content and Key Findings
The authors prepared surfacing electrodes by incorporating spherical cast tungsten carbide particles of varying content and particle sizes into atomized iron powder. The overlay layers were then deposited on Q235 steel using oxy-acetylene flame surfacing. Three key findings emerge from this systematic investigation.
First, particle dissolution is a dominant phenomenon during flame surfacing. The spherical WC particles exhibit pronounced edge dissolution, and this effect intensifies as particle size decreases. Smaller particles present a larger specific surface area, which accelerates the dissolution kinetics of tungsten and carbon into the molten pool. This dissolution behavior directly influences the subsequent microstructural evolution of the overlay.
Second, the ledeburite microstructure (described as "fishbone-shaped ledeburite") is sensitive to both particle content and particle size. As the WC particle content decreases, the amount of fishbone-shaped ledeburite in the overlay reduces. Conversely, as the WC particle size decreases, the ledeburite microstructure tends to coarsen. This observation suggests that fine particles, despite their enhanced dissolution, promote a coarser ledeburite network, possibly due to the formation of a more supersaturated carbon matrix that undergoes sluggish solidification.
Third, hardness and wear resistance follow a clear trend: higher WC particle content and smaller particle size both contribute to increased overlay hardness and improved wear resistance. This is consistent with the classical understanding that finer and more numerous hard carbide particles provide greater resistance to abrasive wear through mechanisms such as micro-ploughing resistance and crack deflection.
Process Analysis and Engineering Interpretation
The choice of oxy-acetylene flame surfacing as the deposition method introduces specific metallurgical considerations. Flame surfacing is characterized by relatively low heat input compared to arc processes but offers excellent portability and simplicity. However, the thermal cycle associated with flame surfacing is less controllable than arc-based methods, which can exacerbate particle dissolution. The following table summarizes the process-structure-property relationships identified in this study.
| Parameter | Effect on Dissolution | Effect on Microstructure | Effect on Hardness and Wear |
|---|---|---|---|
| Higher WC content | More total dissolution | More ledeburite | Higher hardness, better wear |
| Smaller WC size | More pronounced edge dissolution | Coarser ledeburite | Higher hardness, better wear |
| Larger WC size | Less dissolution | Finer ledeburite | Lower hardness, reduced wear |
From a process engineering perspective, the dissolution of WC particles during flame surfacing represents a double-edged sword. On one hand, dissolved tungsten and carbon contribute to the formation of hard carbide networks (ledeburite) within the matrix, which enhances hardness. On the other hand, excessive dissolution depletes the undissolved WC particles that serve as the primary wear-resistant phases. The optimal balance must account for the specific wear mechanism encountered in service.
For abrasive wear applications where undissolved hard particles are critical, a strategy of using moderately sized WC particles (to limit dissolution) at higher content levels (to ensure sufficient hard phase volume fraction) would be advisable. For applications where matrix hardness is the dominant wear resistance factor, finer particles at higher content may be preferred to maximize dissolved carbide formation.
Defect Analysis and Countermeasures
The dissolution of WC particles can lead to several potential defects in the overlay layer. These include uneven hardness distribution, microcracking due to residual stresses from differential thermal expansion between the WC-rich zones and the iron matrix, and reduced spalling resistance if the bonding between the remaining particles and the matrix is weakened.
To mitigate these issues, engineers should consider the following countermeasures:
- Selecting WC particle sizes above a critical threshold (typically above 50 micrometers for flame surfacing) to minimize excessive dissolution while maintaining adequate hard phase volume fraction.
- Controlling the flame speed and travel parameters to reduce the residence time of particles in the molten pool.
- Employing multi-layer surfacing strategies where the first layer uses coarser particles and subsequent layers use finer particles to build a gradient microstructure.
- Conducting post-weld heat treatment to relieve residual stresses and optimize carbide precipitation.
Study Insights and Implications for Practice
This study provides valuable quantitative guidance for engineers designing hardfacing overlays for wear-critical applications such as mining equipment, cement mill liners, and pipe fitting repair. The systematic investigation of both particle content and size as independent variables is methodologically rigorous and directly applicable to production optimization.
The finding that smaller particles cause coarser ledeburite despite enhanced dissolution is particularly noteworthy and warrants further investigation. It suggests that the solidification kinetics of the carbon-supersaturated matrix play a decisive role in determining the ledeburite morphology, and that particle size effects extend beyond simple dissolution kinetics into the realm of nucleation and growth control.
For pipe and fitting repair operations, where flame surfacing is commonly used due to its portability and low equipment cost, this study offers a practical framework for selecting WC particle specifications to achieve target hardness and wear life. Engineers should integrate these findings with service condition analysis to determine the optimal particle content-size combination for each specific application.
The research also highlights the importance of characterizing the undissolved particle population after surfacing, as the effective hard phase volume fraction is always lower than the nominal addition due to dissolution losses. This has direct implications for cost estimation and performance prediction in industrial hardfacing operations.