Dissolution Mechanism of Tungsten Carbide Particles During Overlay Welding

Literature Overview

This study, published in 2003 under the National 863 Program (Grant 715-005-0010), was conducted by researchers from Huazhong University of Science and Technology and the Central Iron and Steel Research Institute. The work addresses a fundamental metallurgical challenge in hardfacing and wear-resistant overlay welding: the thermodynamic and kinetic behavior of WC (tungsten carbide) particles during the welding thermal cycle. Understanding WC dissolution is critical because the retained carbide phase directly determines the hardness, wear resistance, and service life of the overlay layer. The authors examined the interaction between the molten pool and undissolved WC particles under various welding parameters and filler compositions.

Core Technical Points

The dissolution of WC in the weld pool follows a complex thermodynamic pathway. WC decomposes according to the equilibrium reaction: WC + Fe → Fe₃C + W (in austenitic or ferritic matrices), where the carbon released preferentially forms cementite or retained carbides depending on cooling rate. The key variables governing dissolution extent include:

• Peak temperature and residence time: Higher temperatures and longer dwell times promote greater WC dissolution, reducing the volume fraction of retained primary carbides.
• Matrix composition: Chromium content above 25 wt% stabilizes the austenite phase and suppresses complete WC decomposition; nickel additions further retard dissolution kinetics.
• Particle size: Coarse WC particles (>50 μm) exhibit lower dissolution rates due to their smaller surface-area-to-volume ratio, while fine particles (<10 μm) dissolve more readily.
• Welding process: Submerged arc welding (SAW) produces higher heat input than gas tungsten arc welding (GTAW), resulting in more complete WC dissolution and a softer, more ductile overlay.

Interpretation of Dissolution Kinetics

The study identified that WC dissolution proceeds through a diffusion-controlled mechanism at the particle-matrix interface. During melting, the surrounding liquid enriches in tungsten and carbon, creating a local equilibrium field around each particle. The rate-limiting step is carbon diffusion from the particle surface into the melt, which follows a parabolic kinetics law: Δx = K√t, where K is the dissolution rate constant and t is the time above the liquidus temperature of the local matrix.

A critical insight from this work is the existence of a critical dissolution temperature threshold. Below approximately 1600 °C, WC remains largely inert and retains its hexagonal structure. Between 1600 °C and 1900 °C, partial decomposition occurs with the formation of a W₂C transitional phase. Above 1900 °C, rapid decomposition ensues, and the tungsten solubilizes into the austenite or ferrite matrix, forming solid solution strengthening or secondary carbides upon solidification.

Engineering Practice Implications

From a practical standpoint, this research directly informs the selection of welding processes for WC-based hardfacing applications. For applications requiring maximum surface hardness (e.g., mining tools, coal cutting picks), processes with lower heat input such as PTA powder cladding or hot-wire TIG should be selected to maximize WC retention. Conversely, for applications demanding toughness and resistance to thermal cycling (e.g., thermal spray nozzles, valve seats), a higher heat input process that partially dissolves WC and redistributes tungsten into the matrix may be preferable.

The study also highlights the importance of filler metal design. Composite wires with embedded WC particles offer better control over particle distribution than powder-based systems, but the particle size distribution must be carefully matched to the expected thermal cycle. A bimodal particle size distribution—combining coarse particles (40–75 μm) for wear resistance with fine particles (3–10 μm) for hardness uniformity—represents an optimal design strategy.

Key Questions and Reflections

One question that arises from this literature is whether the dissolution mechanism differs significantly between WC particles in a Ni-based matrix versus a Co-based matrix. Given that cobalt has a higher carbon solubility than nickel, it is plausible that Co-based alloys promote more complete WC dissolution. This has implications for selecting the appropriate binder alloy when formulating hardfacing consumables for specific service environments.

Another consideration is the effect of multi-pass welding. In subsequent passes, previously deposited WC-containing layers are reheated, and partial dissolution in the heat-affected zone may further modify the microstructure. This cumulative effect is not fully captured in single-pass studies and warrants further investigation for thick overlay applications.

Study Insights and Conclusions

This foundational research provides the thermodynamic framework necessary for rational design of WC-based overlay systems. The key takeaway for practicing engineers is that WC dissolution is not an undesirable side reaction but rather a design lever that can be exploited to balance hardness and toughness. By controlling welding parameters and filler composition, one can tune the retained carbide fraction to achieve target mechanical properties. Future work should extend these findings to multi-pass scenarios and explore the interaction between WC dissolution and residual stress development, which together determine the fatigue life of hardfaced components.