Mechanism of Tungsten Carbide Particle Burn-Off During Cladding Welding

Literature Overview

This study by Qu Shiyao, Wang Xinhong, Zou Zengda, and Liu Xuemei from Shandong University was published in the Transactions of the China Welding Institution in 2001. The research investigates the fundamental mechanisms governing the dissolution and burn-off of tungsten carbide (WC) particles during weld overlay cladding operations. This is a critical issue in hardfacing applications where WC particles are added to the cladding deposit to provide high hardness and wear resistance, yet excessive dissolution during the welding thermal cycle can significantly degrade the final microstructure and properties of the overlay.

Core Technical Content

WC particles are widely used as reinforcing phases in hardfacing alloys because of their exceptional hardness (approximately 2400 HV) and high modulus of elasticity. However, during the welding process, the intense thermal cycle causes partial or complete dissolution of WC particles into the molten weld pool, leading to a phenomenon known as "burn-off." The degree of WC burn-off directly affects the final hardness, wear resistance, and microstructure of the cladding deposit. The researchers systematically investigated the factors influencing WC particle dissolution, including welding current density, particle size, welding speed, and the chemical composition of the base alloy.

The fundamental mechanism of WC burn-off involves the dissolution of the WC lattice into the molten pool, where tungsten and carbon atoms are distributed as solute atoms or precipitate as other carbide phases upon solidification. The dissolution rate is governed by the Gibbs-Thomson effect, where smaller particles dissolve more rapidly due to their higher surface energy. The researchers established a mathematical model relating the dissolution rate to particle radius, temperature, and diffusion coefficient, demonstrating that the dissolution follows a parabolic kinetics law.

Microstructural Evolution Analysis

The researchers conducted detailed metallographic and X-ray diffraction (XRD) analyses of the cladding deposits to characterize the microstructural evolution resulting from WC burn-off. In deposits with low WC burn-off (less than 30%), the microstructure consisted of retained WC particles embedded in a matrix of austenite and martensite, with secondary carbides of the M₆C type precipitating at grain boundaries. The hardness of these deposits ranged from 800 to 1100 HV, with the retained WC particles providing the primary wear resistance mechanism through micro-indentation.

In deposits with moderate WC burn-off (30-60%), the retained WC particles were partially dissolved, and the dissolved tungsten and carbon formed new carbide phases including W₂C and WC₁₋ₓ upon solidification. The microstructure showed a mixture of retained and newly formed carbides in a martensitic matrix. The hardness was in the range of 700-900 HV, and the wear resistance was still acceptable but showed a clear decline compared to low burn-off deposits.

In deposits with severe WC burn-off (greater than 60%), most of the original WC particles had dissolved, and the microstructure was dominated by the M₂C and M₆C carbides formed from dissolved tungsten and carbon. The retained WC particles were sparse and small, and the overall hardness dropped to 500-700 HV. The wear resistance was significantly degraded, and the deposit exhibited a brittle fracture mode under impact loading.

Process Optimization Recommendations

Based on the mechanistic understanding of WC burn-off, the researchers proposed several process optimization strategies to minimize particle dissolution while maintaining adequate weld quality. The first strategy is to use larger WC particles (80-150 μm) which have lower surface-to-volume ratios and therefore dissolve more slowly. The second strategy is to employ high welding speeds (150-200 mm/min) to reduce the time that WC particles are exposed to high temperatures in the molten pool. The third strategy is to use lower current densities (less than 50 A/mm²) to minimize the peak temperature in the weld pool.

The researchers also investigated the effect of adding carbon to the base alloy on WC burn-off. Adding carbon to the molten pool increases the carbon activity, which thermodynamically stabilizes the WC particles against dissolution. This was achieved by using a carbon-rich flux or by adding graphite to the welding consumable. The optimal carbon content was found to be 2.0-3.0 wt% in the base alloy, which provided sufficient carbon activity to stabilize WC particles without causing excessive porosity or cracking.

Study Insights and Engineering Implications

This research provides a fundamental understanding of the thermodynamic and kinetic factors governing WC particle dissolution during welding. The key insight is that WC burn-off is not an unavoidable consequence of welding but can be controlled through careful selection of process parameters and consumable design. For industrial hardfacing applications, the practical recommendation is to specify WC particles in the 80-150 μm size range, use moderate welding parameters, and ensure adequate carbon activity in the weld pool. Quality control should include metallographic examination of the cladding deposit to assess the degree of WC retention, with a target of retaining at least 50% of the original WC particles to ensure adequate wear resistance.