Dissolution Mechanism of Tungsten Carbide Particles During Weld Overlay

Literature Overview

This 2003 study, published in Functional Materials and supported by the National 863 Program, was conducted by researchers from Huazhong University of Science and Technology and the Institute of High Temperature Materials, Iron and Steel Research Institute. The work investigates the dissolution behavior of WC (tungsten carbide) particles during the cladding process. This is a foundational topic in hardfacing metallurgy, as WC is the most widely used hard phase reinforcement in cobalt-based and nickel-based hardfacing alloys.

Core Technical Content

WC particles are added to cladding alloys to provide extreme wear resistance through their very high hardness (approximately 2300 HV) and chemical stability. However, during the welding process, the WC particles undergo partial or complete dissolution depending on the thermal cycle, alloy composition, and cooling rate. Understanding this dissolution mechanism is essential for predicting the final microstructure and wear performance of the cladding layer.

Dissolution Behavior

The dissolution of WC in a molten weld pool follows the principle of thermodynamic equilibrium and kinetic limitations. The key observations include:

• WC dissolution is a diffusion-controlled process. The carbon atom diffuses out of the WC lattice into the surrounding liquid melt, followed by the dissolution of tungsten atoms.
• The dissolution rate increases with increasing temperature and decreasing cooling rate. At the peak temperature of the weld pool (typically 1600–1800 °C for Co-based alloys), WC dissolution is rapid.
• In Co-based alloys, the solubility of C in liquid Co is relatively low, so WC tends to dissolve more completely, releasing both W and C into the melt. Upon solidification, new carbides (M₇C₃, M₆C) precipitate from the supersaturated matrix.
• In Ni-based alloys, WC dissolution is less complete due to the higher carbon solubility in liquid Ni and the formation of Ni₃W and Ni₇W₆ intermetallics, which act as barriers to further dissolution.

Thermodynamic Considerations

The equilibrium dissolution of WC can be described by the reaction:

WC (solid) → W (liquid) + C (liquid)

The Gibbs free energy of this reaction is temperature-dependent. At temperatures below approximately 1400 °C, WC is thermodynamically stable in a Co-based melt. Above this temperature, dissolution becomes thermodynamically favorable. The kinetics, however, are governed by the diffusion coefficients of W and C in the liquid phase, which are relatively low compared to the short residence time in the weld pool (typically 0.5–5 seconds).

Microstructural Consequences

Process Optimization Strategies

Based on the dissolution mechanism, several process strategies can be employed to control WC dissolution and optimize wear performance:

1. Powder particle size: Larger WC particles (100–200 μm) dissolve more slowly than fine particles (20–50 μm). However, oversized particles may cause incomplete melting and lack of fusion. An optimal particle size of 63–125 μm is commonly used.
2. Cooling rate control: Faster cooling rates (achieved through PTA or laser cladding) limit the time available for WC dissolution, preserving more intact WC particles. Slower cooling (ESW, SAW) promotes more complete dissolution.
3. Alloy composition adjustment: Adding Mo, Cr, or V to the alloy promotes the formation of Mo₂C, Cr₇C₃, or VC carbides that are more thermodynamically stable than WC, effectively reducing the driving force for WC dissolution.
4. Multi-pass deposition: In multi-pass cladding, the first pass experiences the highest thermal input and thus the greatest WC dissolution. Subsequent passes, deposited on a preheated but cooled substrate, experience lower peak temperatures and preserve more WC.

Engineering Practice Integration

In industrial hardfacing applications, such as mining equipment components, coal cutting picks, and valve seats, the control of WC dissolution is critical. The following practical considerations apply:

• Powder formulation: Commercial hardfacing powders (e.g., Stellite 6 with added WC, or proprietary Ni-based WC composites) are designed with specific WC content (typically 5–20 wt%) and particle size distributions to achieve target dissolution levels.
• Process selection: For applications requiring maximum WC retention (e.g., severe abrasion with low impact loading), laser cladding or PTA is preferred. For applications where a tough, carbide-network microstructure is acceptable (e.g., erosion-corrosion environments), ESW or SAW may be more cost-effective.
• Quality verification: Metallographic examination of the cladding cross-section should quantify the volume fraction of retained WC, the morphology of secondary carbides, and the distribution of carbide networks. X-ray diffraction (XRD) can identify the phase composition, while energy-dispersive spectroscopy (EDS) mapping can reveal the distribution of W, C, Co, and Cr.

Study Insights and Implications

This research is notable for its fundamental approach to understanding a process that has been practiced empirically for decades. The dissolution mechanism provides a scientific basis for alloy design and process selection. One important insight is that complete WC dissolution is not always undesirable. In some applications, the M₇C₃ carbides that form upon dissolution and solidification provide adequate wear resistance with better toughness than brittle WC particles. The key is to match the dissolution level to the specific wear mechanism (abrasive, erosive, adhesive, or corrosive).

Another reflection concerns the effect of weld pool convection on WC dissolution. In reality, the weld pool is not a static melt but experiences complex fluid flow driven by surface tension gradients (Marangoni convection), electromagnetic forces, and buoyancy. These flow patterns affect the local residence time and temperature distribution of WC particles, leading to non-uniform dissolution. This aspect is rarely addressed in empirical studies but is critical for accurate prediction of cladding microstructure.

Summary

The study of WC dissolution during cladding provides fundamental insight into the microstructural evolution of hardfacing alloys. Engineers should recognize that WC dissolution is not a simple all-or-nothing phenomenon but a continuous process governed by thermodynamics, kinetics, and process parameters. By understanding the dissolution mechanism, practitioners can select appropriate alloy compositions, powder particle sizes, and welding processes to achieve the desired balance between hardness, toughness, and wear resistance. This knowledge is particularly valuable for designing new hardfacing alloys for emerging applications such as additive manufacturing of wear-resistant components.