Effect of Shielding Gas on Microstructure and Properties of Tungsten Carbide Flux-Cored Wire Overlay Layers

Literature Overview

The paper by Yuan Xiaobo et al. (Welding Journal, 2017, Vol. 38, No. 11, pp. 71-76) investigates how three different shielding gases—pure argon (Ar), mixed gas (80% Ar + 20% CO₂), and pure carbon dioxide (CO₂)—affect the dissolution-diffusion behavior of tungsten carbide (WC) particles, the resulting microstructure, hardness, and wear resistance of WC/iron-based overlay layers produced via self-developed flux-cored wires. This work is significant because shielding gas selection in overlay welding is often treated as a secondary parameter, yet this study demonstrates that it fundamentally governs the morphology of the hard phase distribution and, consequently, the tribological performance of the overlay.

Core Technical Findings

The authors systematically varied the shielding atmosphere while keeping other welding parameters constant, enabling a clear isolation of gas composition effects. The key quantitative results are summarized below:

The counterintuitive finding here is that the lower-hardness CO₂-shielded overlay exhibits substantially superior wear resistance compared to the higher-hardness Ar-shielded overlay. This challenges the simplistic engineering assumption that higher hardness always correlates with better abrasion resistance.

Interpretation of Microstructural Mechanisms

Under pure argon shielding, the inert atmosphere results in minimal chemical interaction with the molten pool. WC particles undergo limited dissolution and diffusion, producing a narrow diffusion layer (~3 μm). The eutectic structure at WC particle boundaries is predominantly tendril-like, which creates relatively weak interfacial bonding between the hard carbide phase and the iron-based matrix. During wear testing, these poorly bonded regions become preferential sites for particle pull-out and matrix fatigue, leading to accelerated material removal despite the nominally higher bulk hardness.

Under pure CO₂ shielding, the oxidizing atmosphere promotes more vigorous dissolution and diffusion of WC particles into the molten pool. The wider diffusion layer (~5 μm) indicates greater chemical interaction between the carbide and the iron matrix. The resulting eutectic morphologies—chrysanthemum-shaped, fishbone-shaped, and flocculent-like—represent more interconnected and mechanically interlocked structures. These complex morphologies provide superior load-bearing capacity and crack-deflection mechanisms during sliding wear.

The mixed gas (80% Ar + 20% CO₂) condition presumably produced intermediate results, though the paper emphasizes the contrast between the two extremes. The CO₂ component introduces carbon and oxygen activity into the weld pool, which modifies both the thermodynamic driving force for carbide dissolution and the kinetic pathway for eutectic solidification.

Engineering Practice Implications

For industrial overlay welding applications involving WC-hardened surfaces—such as drill bits, valve seats, pump impellers, and mining equipment—the shielding gas selection should be optimized based on the dominant wear mechanism rather than simply maximizing hardness. In abrasive wear scenarios where the hard phase must remain embedded in a ductile matrix, the CO₂-rich atmosphere produces superior results by creating better mechanical interlocking of the WC particles.

However, several practical considerations must be weighed:

• Porosity risk: CO₂ shielding increases the risk of gas porosity due to carbon-oxygen reactions in the molten pool. Preheating and controlled travel speed become more critical.
• Spatter: Pure CO₂ produces higher spatter rates compared to argon, which affects process efficiency and surface quality.
• Heat input: The oxidizing nature of CO₂ can increase the effective heat input at the workpiece, potentially affecting dilution and residual stress.
• Multi-pass overlay: In multi-pass applications, the interpass temperature combined with CO₂ shielding may promote excessive grain growth in the matrix, requiring careful interpass temperature control (typically 150-250°C).

Key Questions and Reflections

The study raises an important question about the role of the mixed gas (80% Ar + 20% CO₂), which is the most commonly used shielding gas in industrial GMAW operations. If pure CO₂ yields the best wear resistance, why is the mixed gas not investigated in greater depth? In practice, the mixed gas represents a compromise between weld quality and process stability, and its intermediate performance may actually be optimal for most industrial applications where surface finish and porosity control are also important criteria.

Another reflection concerns the wear test methodology. The reported wear loss values (11.4 mg and 4.2 mg) suggest a dry sliding or pin-on-disk configuration. Real-world overlay applications often involve three-body abrasion, erosion, or fretting, where the relationship between microstructure and wear resistance may differ significantly. Engineers should validate gas selection decisions through wear testing that replicates the actual service conditions.

Study Insights and Implications

This paper provides a compelling case study in how process parameters that are often considered "secondary" can dominate the functional performance of overlay welds. For engineers specifying overlay welding procedures for wear-critical components, the shielding gas composition should be elevated to a primary process variable requiring systematic optimization. The finding that a 63% improvement in wear resistance can be achieved solely by changing from Ar to CO₂ shielding—without altering the wire composition or welding current—represents a significant cost-saving opportunity in industrial applications where shield gas switching is trivial compared to material changes.