Effect of Vanadium on Microstructure and Wear Properties of Tungsten Carbide-Based Wear-Resistant Overlay Layers

Literature Overview

This paper by Wei Wei, Huang Zhiquan, Zhang Haiyan, and Yang Wei from Zhengzhou Research Institute of Mechanical Engineering, published in 2019 in the Transactions of the China Welding Institution (Vol. 40, No. 6, pp. 131-136), investigates the influence of vanadium addition (0%-3 wt%) on the microstructure and wear performance of tungsten carbide (WC)-based self-shielded flux-cored wire overlay welds. The research addresses a well-known problem in WC-based overlay welding: the dissolution of WC particles during the welding thermal cycle, which leads to a loss of the hard phase and degradation of wear resistance.

Technical Background

WC-based overlay alloys are widely used in mining, petroleum, and cement industries for components subjected to severe abrasive wear. The hardness of WC (HV ~2000-2300) provides exceptional resistance to abrasive material removal. However, WC is thermodynamically unstable in the presence of iron and carbon at welding temperatures. During the welding process, WC particles undergo the following reaction:

$$WC + 6Fe \rightarrow Fe_3W_3C + 3Fe$$ (simplified)

This dissolution consumes the WC hard phase and releases carbon, which may form other carbides but generally results in a net loss of wear resistance. The challenge is to find alloying elements that can suppress WC dissolution without introducing other detrimental effects.

Experimental Design and Methodology

The researchers developed self-shielded flux-cored welding wires with varying vanadium content (0%, 1%, 2%, 3 wt%) and deposited overlay welds on steel substrates. The microstructure was characterized using SEM, XRD, and EDS, and wear performance was evaluated through standardized abrasive wear testing.

Key Findings

Vanadium-Carbide Interaction

The central finding is that vanadium preferentially fixes carbon atoms released from WC decomposition in the form of vanadium carbides (VC or V₂C). This mechanism operates as follows:

1. During welding, WC particles begin to dissolve, releasing W and C atoms into the molten pool.
2. Vanadium, being a strong carbide former, combines with the released carbon to form stable vanadium carbides.
3. By consuming the released carbon, vanadium shifts the equilibrium, reducing the thermodynamic driving force for further WC dissolution.
4. The net effect is a preservation of the WC particle population in the final microstructure.

Optimal Vanadium Content

The study identifies 2 wt% vanadium as the optimal addition level:

The diminishing returns at 3 wt% vanadium may be attributed to the formation of coarse vanadium carbide clusters that act as crack initiation sites, partially offsetting the benefit of reduced WC dissolution.

Matrix Strengthening

Beyond the WC preservation effect, vanadium also strengthens the matrix through:

• Solid solution strengthening of the ferritic/martensitic matrix
• Formation of fine VC precipitates that impede dislocation motion
• Refinement of the matrix microstructure through nucleation effects

Reduced Particle Pullout Risk

A particularly important finding is that vanadium addition reduces the risk of WC particle pullout during abrasive wear. The strengthened matrix provides better mechanical support for the hard WC particles, preventing their premature detachment from the overlay surface.

Engineering Practice Considerations

For engineers selecting overlay welding consumables for WC-based applications, this research suggests:

1. Consumable selection: Wires with 2 wt% vanadium addition should be preferred for applications where WC dissolution is a concern, such as thick multi-pass overlays or high-heat-input processes.
2. Process parameter control: Even with vanadium addition, excessive heat input should be avoided. The protective effect of vanadium is most effective at moderate heat input levels.
3. Microstructural verification: XRD analysis should be performed on production welds to confirm that the expected carbide phases (WC, VC, Fe₃C) are present in the anticipated proportions.
4. Wear testing: Bench-scale wear testing should be conducted under conditions representative of the actual service environment, as laboratory wear results may not fully capture the complex tribological interactions in field applications.

Key Questions and Reflections

An interesting question that arises is whether other strong carbide formers—such as titanium, niobium, or tantalum—could provide similar or superior protection against WC dissolution. Vanadium is chosen for its favorable combination of carbide stability, cost-effectiveness, and availability. However, titanium carbide (TiC) is even more stable than VC and might offer comparable protection at lower alloy content levels.

Another consideration is the interaction between vanadium and other alloying elements commonly present in WC-based overlays, such as chromium, molybdenum, and tungsten. Multi-component interactions could either synergistically enhance or antagonistically reduce the protective effect of vanadium.

Study Insights and Implications

The work by Wei et al. provides a clear mechanistic understanding of how vanadium suppresses WC dissolution in overlay welds. The 2 wt% vanadium optimum represents a practical, implementable improvement to existing WC-based overlay welding consumables. For manufacturers of welding consumables, this research provides a roadmap for developing next-generation WC-based wires with improved wear performance. For end-users, it offers guidance on consumable selection for critical wear applications where overlay performance directly impacts equipment availability and maintenance costs.