Mechanism of Burning Loss of WC Hard Alloy Particles During Overlay Welding

Literature Overview

This study by Qu Shiyao, Wang Xinhong, Zou Zengda, and Liu Xuemei from the School of Materials Science and Engineering at Shandong University, published in the Transactions of the China Welding Institute in 2001 (Vol. 22, No. 2, pp. 85-88), addresses a fundamental metallurgical challenge in the overlay welding of tungsten carbide (WC) hard alloy particles onto steel substrates. The research employs scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) to characterize the melting and burning loss behavior of WC particles during the overlay welding process. The classification number TG455 places this work squarely within the domain of overlay welding and surfacing technology, which is directly relevant to engineers working on wear-resistant components in piping systems, valve internals, and pump impellers.

Core Technical Findings

The study establishes that burning loss of WC hard alloy particles during overlay welding is an unavoidable phenomenon that cannot be entirely eliminated through process optimization alone. The fundamental characteristics of this burning loss include three observable features: melting of the binder phase at the edges of hard alloy particles, coarsening of carbide particles, and increased spacing between single-crystal WC grains. These observations are critical because they indicate that even when the bulk of the WC particle remains intact, the microstructural integrity at the particle boundaries is compromised, leading to a degradation in the mechanical and tribological properties of the overlay.

Types of Burning Loss

The researchers identified three primary modes of WC particle burning loss:

• Edge melting: The binder phase (typically Co, Ni, or Fe-based) surrounding the WC particles has a significantly lower melting point than the carbide itself. During the welding thermal cycle, the binder melts first, creating a liquid pool that wets and partially dissolves the WC particle surfaces.
• Dissolution burning loss: Carbon and tungsten atoms diffuse from the WC particle into the molten binder and weld pool. This diffusion-driven dissolution reduces the effective volume of intact carbide and alters the local carbon-to-tungsten ratio at the particle interface.
• Elemental diffusion and reaction burning loss: Elements from the base metal and the welding consumable diffuse into and out of the WC particles, forming new intermetallic compounds and carbide phases that differ from the original WC structure. This reaction-based degradation is often the most detrimental to the final properties of the overlay.

Mechanism of Burning Loss

The proposed mechanism centers on the decomposition of carbides at the edges of the hard alloy particles. As the thermal cycle progresses, the binder phase melts and creates a concentration gradient between the WC particle interior and the surrounding liquid. At the outer boundary of this diffusion zone, where atomic concentrations are highest, phase transformations occur, forming new compounds that are mechanically inferior to the original WC. The degree of burning loss is governed by three key variables: the type of hard alloy used, the particle size distribution, and the thermal input of the welding process.

Engineering Practice Implications

For engineers specifying overlay welding of WC-containing consumables on piping components, pump impellers, or valve trim, this literature provides several actionable insights. First, the use of larger WC particles (ideally 80-150 μm) is preferred over fine particles, as the higher surface-to-volume ratio of smaller particles accelerates dissolution. Second, low-heat-input processes such as gas tungsten arc welding (GTAW) or plasma arc welding (PAW) should be prioritized over high-deposition-rate processes like submerged arc welding (SAW) when WC preservation is critical. Third, the choice of binder alloy matters: Ni-based binders generally exhibit better WC retention than Co-based binders under equivalent thermal conditions.

Process Recommendations

1. Preheating strategy: Moderate preheating (150-200°C) can reduce thermal gradients and slow the diffusion rate at the particle edges, though excessive preheating extends the time above the critical temperature and must be avoided.
2. Interpass temperature control: Maintaining interpass temperatures below 200°C minimizes cumulative thermal exposure to previously deposited WC layers.
3. Post-weld heat treatment: A controlled tempering cycle (typically 500-600°C for 2-4 hours) can relieve residual stresses without causing significant additional WC degradation.
4. Multi-pass deposition: Building up the overlay in multiple thin passes, rather than a single thick pass, limits the peak temperature experienced by each individual layer.

Key Questions and Reflections

This 2001 study, while foundational, raises questions that remain relevant to contemporary practice. The research characterizes burning loss in terms of microstructural observations but does not quantify the relationship between the degree of particle degradation and the resulting tribological performance. In practical terms, an engineer must determine the acceptable threshold of WC degradation before the overlay fails to meet its wear resistance specification. Furthermore, the study does not address the effect of welding position (flat, vertical, overhead) on the thermal cycle and consequently on WC preservation, which is a significant practical consideration in field repair applications.

The diffusion-based mechanism proposed in this paper aligns with modern understanding of solute transport in multicomponent systems. However, contemporary computational tools such as finite element thermal-metallurgical coupling models and phase field simulations could provide more precise predictions of the concentration gradients and phase transformation kinetics described here. The engineering value of this literature lies not in its advanced methodology but in its clear identification of the fundamental mechanisms that govern WC degradation, which serves as a foundation for process optimization and consumable development.

In summary, this study provides essential mechanistic understanding of why WC hard alloy particles degrade during overlay welding and identifies the controllable parameters that influence the severity of this degradation. For any engineer involved in the design or specification of hardfacing overlays for wear-critical piping and valve components, understanding these mechanisms is prerequisite to achieving reliable long-term performance.