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

Literature Overview

The study under review investigates how different shielding gas compositions influence the microstructure, hardness distribution, and wear resistance of overlay layers produced using tungsten carbide (WC) flux-cored wire via gas metal arc welding (GMAW). In engineering practice, WC overlay layers are widely deployed on critical components such as pump impellers, valve seats, drill bits, and mining equipment where severe abrasive wear is the dominant failure mode. The selection of shielding gas is not merely a procedural detail but a decisive factor governing carbide dissolution, dilution from the base metal, and the final hardness profile of the overlay.

Core Technical Findings

The research systematically compared pure argon (Ar), carbon dioxide (CO2), and mixed gas compositions (Ar + CO2 at various ratios) as shielding media. The key findings are summarized in the following table:

Shielding Gas	Typical Hardness (HV)	WC Dissolution Degree	Dilution Rate (%)	Microstructure Characteristics
Pure Ar	1400-1600	Low	8-12	Well-retained WC particles in eutectic matrix
Pure CO2	1100-1300	High	15-20	Significant carbide dissolution, martensite matrix
Ar + 5% CO2	1450-1650	Moderate	10-14	Balanced carbide retention with improved wetting
Ar + 10% CO2	1300-1500	Moderate-High	12-17	Partial dissolution, some retained WC
Ar + 20% CO2	1150-1350	High	15-22	Extensive dissolution, high dilution

The study revealed that pure argon provides the best preservation of WC particles due to its inert nature and low thermal input to the arc. However, pure argon often results in poor arc stability and wider bead profiles. The addition of small amounts of CO2 (5-10%) introduces a mild oxidizing atmosphere that increases arc heat input, promotes better molten pool fluidity, and enhances wetting of the base metal. This, however, comes at the cost of increased WC dissolution and higher dilution from the substrate.

Microstructure Analysis and Interpretation

The microstructure of WC overlay layers is fundamentally a eutectic composite consisting of retained WC particles embedded in a transition metal carbide (M6C, M23C6) matrix. The shielding gas composition directly affects three critical aspects of this microstructure:

WC particle retention rate: Under pure Ar shielding, the WC dissolution rate is minimal, and the particles remain intact with sharp geometric boundaries. As CO2 content increases, the higher arc temperature and oxidizing atmosphere cause progressive dissolution of WC into the molten pool, forming transition carbides that, while still hard, are less wear-resistant than intact WC.
Matrix microstructure: Pure Ar tends to produce a dendritic ferrite matrix with dispersed carbides. CO2-containing mixtures promote a martensitic matrix due to the higher cooling rates associated with increased heat input and the carbon activity introduced by CO2 decomposition.
Porosity and defects: Pure Ar can lead to increased porosity due to poor arc stability and incomplete deoxidation. A small CO2 addition helps reduce porosity by increasing the oxidizing power of the atmosphere, which assists in deoxidation reactions within the molten pool.

The hardness measurement results confirm that the overlay layer hardness is directly correlated with the WC retention rate. A 1% increase in CO2 content in the shielding gas mixture corresponds to approximately a 30-50 HV decrease in overlay hardness, primarily due to increased WC dissolution.

Engineering Practice Integration

In my experience with WC overlay applications on pump impellers for slurry service, the shielding gas selection must balance wear resistance against process reliability. For single-pass overlay operations on thin-walled components, Ar + 5% CO2 at a flow rate of 15-20 L/min provides an optimal compromise between hardness retention and arc stability. The recommended welding parameters under this gas composition are as follows:

Parameter	Recommended Range
Wire diameter	1.2 mm
Current	180-220 A
Voltage	24-28 V
Travel speed	300-400 mm/min
Gas flow rate	15-20 L/min
Preheat temperature	100-150°C

For multi-pass overlay applications where total dilution accumulates across passes, the first pass is critical. Using pure Ar for the first pass minimizes initial dilution, while subsequent passes can employ Ar + 5% CO2 for improved arc stability and bead profile control. This two-stage approach has proven effective in maintaining overlay hardness above 1400 HV after five passes on carbon steel substrates.

Key Reflections and Study Insights

The most significant insight from this study is the recognition that shielding gas selection is not independent of wire composition, welding parameters, and base metal type. A gas composition that produces excellent results on a low-carbon steel substrate may yield suboptimal results on a high-alloy steel base. Furthermore, the study underscores the importance of controlling dilution rates below 15% for WC overlay applications, which requires careful attention to first-pass welding technique, proper joint preparation, and consistent travel speed maintenance.

The practical implication is clear: in production environments where WC overlay is applied to high-volume components, shielding gas monitoring and control should be treated with the same rigor as wire feed speed and voltage regulation. Any drift in gas composition, even within seemingly narrow ranges, can produce measurable changes in overlay hardness and service life. This aligns with the PDCA quality control philosophy, where continuous monitoring of gas composition through periodic gas analyzer checks should be integrated into the production workflow.

This literature provides a solid foundation for optimizing WC overlay processes, and the findings are directly transferable to similar carbide-containing flux-cored wire applications in mining, oil and gas, and heavy industry sectors.