Microstructure and Performance of Tungsten Carbide Overlay Layer Prepared by TIG Arc Welding

Literature Overview and Research Background

This study by Yang Lijun, Sun Tao, Wang Yaowei, and Huang Yiming from Tianjin University investigates the microstructure evolution and mechanical properties of tungsten carbide (WC) overlay layers fabricated using gas tungsten arc welding (GTAW/TIG). Funded by the National Natural Science Foundation of China (Grant No. 51875403), the research was published in the Journal of Tianjin University in 2019. The work addresses a critical challenge in wear-resistant overlay technology: achieving a homogeneous, crack-free WC composite layer with high hardness and adequate toughness through a single-pass or multi-pass TIG process.

The motivation behind this research stems from the widespread industrial demand for hardfacing solutions in mining, cement, power generation, and oilfield equipment sectors. Conventional overlay methods such as flame spraying or plasma transfer arc (PTA) often produce layers with porosity, spalling, or excessive dilution. TIG-based WC overlay offers the advantage of precise heat input control, but introduces challenges related to WC phase transformation, intermetallic formation, and residual stress management.

Core Technical Content and Key Findings

Microstructural Characteristics

The study examines the phase composition and microstructural morphology of the WC overlay layer, focusing on how the welding thermal cycle affects the stability of the WC phase. During TIG welding, the extreme thermal gradient causes the WC particles to undergo decomposition into W₂C and free tungsten carbide (W₃C) phases, while simultaneously promoting the formation of intermetallic compounds such as Fe₃W₃C, Fe₂W₄C, and Fe₇W₆C at the interface between the overlay and the substrate.

The researchers utilized scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) mapping to characterize the phase distribution. Key observations include:

• The overlay layer exhibits a cellular dendritic structure with WC-rich zones distributed along dendrite boundaries.
• Partial decomposition of WC into W₂C occurs in regions subjected to peak temperatures exceeding 1200°C.
• The bond layer between the overlay and the carbon steel substrate shows a diffusion zone approximately 20–50 μm in depth, characterized by carbon enrichment and Fe-W intermetallic precipitation.
• Microcracks tend to initiate at the interface between the WC particles and the iron-based matrix, propagating along grain boundaries under high thermal stress.

Mechanical Property Evaluation

The hardness profile across the overlay layer was measured using Vickers microhardness testing at 0.5 N load. The results demonstrate a significant hardness gradient:

The wear resistance was evaluated using a pin-on-disk tribological test against a Si₃N₄ counterface under a load of 5 N. The WC overlay demonstrated a specific wear rate approximately 8–12 times lower than that of the unclad Q235 carbon steel substrate, confirming the effectiveness of the TIG-based WC hardfacing approach.

Process Parameter Analysis

The study systematically varied welding current, travel speed, and filler composition to optimize the overlay quality. The following process window was identified as optimal:

The thermal input (q = UI/v) was maintained within 0.8–1.5 kJ/mm to balance penetration depth against dilution control. Higher thermal inputs led to excessive WC decomposition and increased microcracking, while lower inputs resulted in incomplete wetting and poor bond strength.

Engineering Practice Implications

Defect Analysis and Countermeasures

From a practical standpoint, the most critical defects in TIG-based WC overlay are:

1. Microcracking: Caused by thermal stress concentration at WC-matrix interfaces. Countermeasures include preheating the substrate to 150–200°C, reducing the travel speed to lower thermal gradients, and incorporating ductile alloying elements (Mo, Ni) into the filler composition.
2. Porosity: Arises from gas entrapment during solidification of the high-carbon melt. Mitigation strategies involve ensuring clean filler powder, using a pure argon shielding atmosphere with flow rates above 10 L/min, and maintaining a stable arc length.
3. Spalling: Results from poor interfacial bonding due to excessive dilution or contamination. This can be addressed by using a pure tungsten electrode (WC-2% Th or La), cleaning the substrate surface thoroughly, and employing a back-of-plate argon purge for thin-section applications.

Application Considerations

In pressure vessel and heat exchanger manufacturing, WC overlay layers are increasingly specified for service in slurry erosion environments, such as pump casings, valve seats, and wear plates. The TIG-based approach is particularly suitable for repair welding and localized overlay of small areas where equipment mobility is required. However, for large-area production overlay, PTA or laser cladding methods remain more economical.

Study Insights and Reflections

This research provides valuable quantitative data on the phase transformation behavior of WC under TIG welding conditions, which is essential for process parameter selection in engineering practice. The identification of the critical thermal input range (0.8–1.5 kJ/mm) offers a practical guideline for welders and process engineers. The study also highlights the importance of filler composition optimization — the addition of chromium and molybdenum not only improves corrosion resistance but also enhances the ductility of the intermetallic phase, reducing microcrack susceptibility.

One area for future investigation is the long-term service behavior of TIG-applied WC overlays under cyclic thermal loading, as this condition is common in pressure vessel applications. The combination of thermal fatigue and mechanical wear could accelerate degradation at the overlay-substrate interface, and understanding this mechanism would be critical for service life prediction.