Mechanism of Thermal Insulating Agent in Tungsten Carbide Arc Overlay Welding

Literature Overview

This 2001 study published in the Journal of the Chinese Society for Metals by Chu Shaojun, Diao Shusheng, Li Yonglin, and Liang Dongtu from the University of Science and Technology Beijing and the Central Iron and Steel Research Institute investigates the fundamental metallurgical mechanisms by which thermal insulating agents (fluxes) influence the microstructure and performance of tungsten carbide (WC) arc overlay welds. Funded by the National Natural Science Foundation of China (Grant No. 59874022), this research addresses one of the most critical yet poorly understood aspects of WC hardfacing technology.

Core Technical Content

The Fundamental Challenge of WC Overlay Welding

Tungsten carbide arc overlay welding is one of the most widely used hardfacing techniques in the mining, cement, and oil and gas industries due to the exceptional hardness (2,300 HV for pure WC) and wear resistance of tungsten carbide. However, WC is thermodynamically unstable during welding: at the high temperatures of the arc (6,000–10,000°C), WC decomposes according to the following reactions:

• WC + C → W2C (carbide transformation)
• WC → W + C (decomposition, particularly at temperatures above 1,400°C)
• W + O → WO2 (oxidation of liberated tungsten)

The decomposition of WC leads to loss of hardness, reduced wear resistance, and the formation of brittle iron-tungsten intermetallics (Fe2W, Fe3W, Fe7W6) that are detrimental to the mechanical properties of the overlay. The thermal insulating agent (flux) is employed to mitigate these problems.

Mechanism of Thermal Insulating Agent Action

The study identifies four primary mechanisms by which the thermal insulating flux functions:

1. Thermal insulation and heat distribution: The flux, typically applied as a powder coating on the base metal surface before welding, acts as a thermal barrier that reduces the peak temperature in the base metal and moderates the cooling rate. This reduces the dilution rate from typical values of 40–60% (without flux) to 20–35% (with flux). The lower dilution preserves more WC in the final overlay composition.
2. Carbide stabilization: The flux contains carbon sources (graphite, calcium carbide, or other carbonaceous materials) that maintain a carbon-rich atmosphere at the weld pool surface. This carbon-rich environment suppresses the decomposition of WC by shifting the equilibrium toward carbide stability. The flux also provides a protective slag that shields the molten pool from atmospheric oxygen.
3. Microstructural modification: The flux promotes the formation of a more favorable microstructure by controlling the solidification conditions. The moderated cooling rate allows for the formation of a more uniform distribution of WC particles in a nickel-iron matrix, rather than the coarse, segregated structure that results from rapid cooling without flux.
4. Reduction of residual stresses: By reducing the thermal gradient in the weld zone, the flux decreases the magnitude of residual stresses in the overlay layer. This is critical for preventing cracking in the overlay, particularly in thick deposit applications.

Quantitative Results

The study demonstrates that the use of an optimized thermal insulating flux can:

• Reduce the dilution rate by 40–50% compared to unfluxed welding
• Increase the WC retention rate from approximately 30–40% to 60–75%
• Improve overlay hardness from 800–1,000 HV (without flux) to 1,200–1,600 HV (with flux)
• Reduce the specific wear rate by 30–50% in pin-on-disk wear tests
• Decrease the likelihood of microcracking in the overlay by 60% or more

Microstructural Evolution

Without flux, the overlay microstructure typically shows large, irregularly shaped carbide clusters with extensive Fe-W intermetallic phases at the carbide-matrix interface. These intermetallics are extremely brittle and serve as crack initiation sites. With flux, the microstructure exhibits more uniformly distributed, smaller WC particles with a cleaner interface between the carbide and the nickel-iron matrix. The reduction in Fe-W intermetallics is attributed to the lower dilution and the carbon-rich environment that favors WC stability over iron-tungsten compound formation.

Process Optimization Insights

The study provides valuable guidance on the application of thermal insulating fluxes:

• Application method: The flux should be applied as a uniform layer of 1–2 mm thickness on the base metal surface, with the overlay wire fed through the flux layer.
• Welding parameters: The flux application requires slightly lower current and voltage settings compared to unfluxed welding, as the flux provides additional heat input and moderates the cooling rate.
• Multi-pass welding: For thick overlays, the flux should be reapplied between passes to maintain the protective and stabilizing effects throughout the entire deposit.
• Flux compatibility: The flux composition must be matched to the specific overlay wire composition; a flux optimized for a high-WC-content wire may not be optimal for a lower-WC-content wire.

Reflections and Practical Implications

This study exemplifies the importance of understanding fundamental metallurgical mechanisms in hardfacing technology. The thermal insulating flux is not merely an auxiliary consumable but a critical process variable that fundamentally alters the microstructure and performance of the overlay. Engineers who treat flux application as a secondary consideration often encounter disappointing results in terms of hardness, wear resistance, and crack resistance.

The research methodology employed here—combining thermodynamic analysis, microstructural characterization, and mechanical property testing—provides a template for evaluating other process variables in hardfacing technology. The approach of systematically varying the flux composition and correlating the results with microstructural and performance outcomes is a rigorous scientific methodology that should be applied more broadly in hardfacing research.

For practical engineering applications, this study reinforces the recommendation that WC overlay welding should always employ a properly formulated thermal insulating flux. The cost of the flux is negligible compared to the performance improvement and the reduction in premature failure that results from proper flux application.