Zhuojin Pipe Fitting Co., LtdEffect of Shielding Gas on Microstructure and Wear Resistance of Tungsten Carbide Flux-Cored Wire Overlay Layers
Literature Overview
This study, published in the Welding Journal (2017, Vol. 38, No. 11, pp. 71–76) by Yuan Xiaobo, Li Feng, Wang Juan, Niu Ben, Yi Jianglong, and Zheng Kaihong from Shenyang University of Technology and the Guangdong Provincial Research Institutes for Materials Processing and Welding Technology, investigates how shielding gas composition influences the dissolution behavior of tungsten carbide (WC) particles, the resulting microstructure, hardness, and wear resistance of WC/iron-based overlay layers deposited via flux-cored wire welding. The work was supported by multiple Guangdong Provincial International Science and Technology Cooperation Projects and Industry-University-Research collaboration programs, reflecting a strong emphasis on practical industrial application.
Core Technical Findings
The researchers prepared WC/iron-based overlay layers using a self-developed flux-cored wire under three different shielding gas conditions: pure argon (Ar), a mixed gas of 80% Ar + 20% CO₂, and pure CO₂. The key results are summarized in the following table:
| Parameter | Pure Ar | 80% Ar + 20% CO₂ | Pure CO₂ |
|---|---|---|---|
| WC dissolution/diffusion layer width | ~3 μm | Intermediate | ~5 μm |
| Eutectic morphology at WC edge | Vermicular (finger-like) | Transitional | Chrysanthemum, fishbone, or flocculent |
| Microhardness | 790 HV ± 20 HV | Intermediate | 590 HV ± 15 HV |
| Wear loss | 11.4 mg | Intermediate | 4.2 mg |
| Relative wear resistance improvement | Baseline | Moderate | 63% reduction in wear vs. Ar |
The most striking finding is that pure CO₂ shielding, despite yielding lower microhardness (590 HV versus 790 HV for pure Ar), produced a wear loss that was 63% lower than that of the pure argon condition. This counterintuitive result warrants careful interpretation and is the central technical insight of this paper.
Interpretation of the Hardness-Wear Resistance Paradox
The apparent contradiction between hardness and wear resistance requires a metallurgical explanation rooted in the microstructural morphology. Under pure argon shielding, the WC particles experience minimal chemical interaction with the molten pool, resulting in a thin diffusion layer of approximately 3 μm and a predominantly vermicular eutectic structure at the particle boundaries. While the overall microhardness is high (790 HV), the morphology of the eutectic phase around the WC particles is not optimal for resisting abrasive attack. The vermicular structures may create stress concentration points and facilitate microcrack initiation during sliding wear.
Under pure CO₂ shielding, the active carbon in the CO₂ atmosphere promotes greater dissolution and diffusion of WC particles into the molten pool, widening the diffusion layer to approximately 5 μm. The CO₂ atmosphere also introduces additional carbon into the weld metal, promoting the formation of complex eutectic morphologies including chrysanthemum, fishbone, and flocculent patterns. These morphologies, while associated with a lower measured microhardness, create a more uniform and interconnected hard phase network that effectively resists abrasive material removal. The chrysanthemum and fishbone structures distribute the abrasive load more evenly across the overlay surface, reducing localized stress concentrations and preventing the catastrophic detachment of WC particles that can occur with isolated vermicular structures.
Welding Process Considerations
The choice of shielding gas in flux-cored wire overlay welding is not merely a matter of arc stability or penetration control; it fundamentally alters the thermodynamic and kinetic conditions governing particle dissolution, carbon activity, and phase formation in the weld metal. The following process parameters are critical:
- Arc voltage and current: Must be maintained within a narrow window to ensure adequate melting of the flux-cored wire while preventing excessive burn-off of the active flux constituents.
- Travel speed: Affects the thermal cycle and cooling rate, which in turn influences the morphology of the eutectic phases around WC particles.
- Preheating: Typically not required for WC overlay on mild steel substrates, but excessive preheating can increase dilution and reduce the volume fraction of retained WC particles.
- Interpass temperature: Should be controlled to prevent excessive softening of previously deposited layers during multi-pass builds.
The CO₂ atmosphere, being an active gas, also contributes to a slightly deeper penetration and a wider melt pool compared to pure argon, which may affect dilution rates and the overall geometry of the overlay layer. Engineers must balance these competing effects when selecting the optimal shielding gas for a given application.
Engineering Practice Implications
In industrial applications where WC overlay layers are used for wear protection—such as on mining equipment, cement mill components, and material handling machinery—the selection of shielding gas should be guided by the dominant wear mechanism. For abrasive wear conditions where the counterface contains hard particles (e.g., sand, gravel, or mineral ore), the pure CO₂ shielding condition demonstrated in this study offers superior wear resistance despite lower microhardness. This suggests that microstructural morphology and phase connectivity are more critical than peak hardness in determining practical wear life.
For applications involving adhesive wear or erosion-corrosion, the high-hardness microstructure produced under pure argon may be more advantageous. Engineers should not rely solely on hardness measurements as a proxy for wear performance; rather, tribological testing under representative conditions should be used to validate the shielding gas selection.
Key Questions and Reflections
Several questions emerge from this study that merit further investigation. First, the intermediate condition of 80% Ar + 20% CO₂ is mentioned but not fully characterized in the abstract; understanding the transition behavior between pure Ar and pure CO₂ would help engineers optimize the gas mixture for specific applications. Second, the wear testing methodology is not described in detail in the abstract—knowing whether the tests were conducted under dry sliding, lubricated, or abrasive conditions is essential for interpreting the results. Third, the long-term stability of the WC particles under thermal cycling (as would occur in service) is not addressed; diffusion and coarsening of the WC phase at elevated temperatures could degrade the wear performance over time.
Study Insights and Conclusions
This study provides a valuable demonstration that shielding gas selection in overlay welding is a metallurgical lever, not merely a process parameter. The CO₂ atmosphere's ability to promote WC dissolution and create complex eutectic morphologies that enhance wear resistance, despite reducing microhardness, challenges the conventional assumption that higher hardness always translates to better wear performance. For engineers specifying overlay welding procedures for wear-critical components, this paper underscores the importance of tailoring the shielding atmosphere to the desired microstructural outcome rather than defaulting to inert gas shielding. The findings directly inform welding procedure specifications (WPS) for hardfacing operations and highlight the need for tribological validation rather than relying exclusively on hardness measurements in acceptance criteria.