Zhuojin Pipe Fitting Co., LtdMechanism of WC Carbide Hard Alloy Burnout During Overlay Welding
Literature Overview
The paper by Qu Shiyao, Wang Xinhong, Zou Zengda, and Liu Xuemei from Shandong University, published in the Transactions of the China Welding Institute in 2001 (Vol. 22, No. 2, pp. 85–88), investigates the burnout mechanism of tungsten carbide (WC) hard alloy particles during the overlay welding process. This work is particularly significant for engineers dealing with surface hardening applications on piping components, valve seats, and wear-resistant fittings where WC-reinforced overlays are commonly specified. The study employed scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) to characterize the microstructural evolution during the welding thermal cycle.
Core Technical Findings
The authors established that WC particle burnout during overlay welding is an unavoidable phenomenon, regardless of process parameters employed. The fundamental characteristics of burnout include melting of the binder phase at carbide particle edges, coarsening of carbide particles, and increased inter-particle spacing between single-crystal WC grains. These observations are critical for understanding why hardness retention in WC-based overlays is inherently limited compared to the base hard alloy.
Types of Burnout Identified
| Burnout Type | Description | Microstructural Evidence |
|---|---|---|
| Edge melting | Localized melting of carbide particle boundaries | Rounded particle edges, liquid-phase traces |
| Dissolution burnout | Dissolution of WC into the molten matrix | Reduced particle size, W enrichment in matrix |
| Diffusion and reaction burnout | Elemental diffusion forming new compounds | Concentration gradient layers, new intermetallic phases |
Burnout Mechanism Interpretation
The proposed mechanism involves decomposition of carbides at the hard alloy edges during the high-temperature phase of the thermal cycle. Elemental diffusion occurs between the decomposed carbide and the binder metal, forming a diffusion layer with a defined concentration gradient. At the outer boundary of this diffusion layer, regions of high atomic concentration undergo phase transformation, producing new compounds that degrade the overall hardness and wear resistance of the overlay.
Engineering Practice Implications
For piping and fitting applications, this research has direct consequences for the selection and application of WC-based overlay materials. The degree of burnout is governed by three factors: the type of hard alloy, particle size, and welding process parameters. In practice, this means that:
- Larger WC particles exhibit greater resistance to burnout but provide lower hardness retention
- Faster welding speeds and lower heat inputs reduce the extent of burnout
- Multi-pass overlay strategies can partially mitigate burnout by allowing the first pass to act as a sacrificial layer
When specifying overlay welding for critical components such as pump impellers, valve trim, or wear plates in slurry service, engineers should account for a hardness reduction of 15–25% from the nominal WC alloy hardness. The remaining carbide particles, though degraded at their edges, still provide meaningful wear resistance due to the residual WC core.
Key Questions and Reflections
This 2001 study remains foundational in understanding the limitations of WC-based overlay systems. The question that arises for modern practice is how to minimize burnout while maintaining adequate bond strength. Contemporary approaches include using flux-cored wires with controlled WC content, applying plasma arc transfer (PAT) processes with reduced dilution, and employing cold spray technologies that avoid melting altogether. However, for conventional arc overlay processes still widely used in field repair of piping components, the insights from this paper remain directly applicable for quality assessment and process optimization.
The study underscores the principle that in overlay welding of cemented carbides, the welding thermal cycle inevitably modifies the reinforcement phase. Engineers must therefore design overlay specifications with realistic expectations of performance, rather than assuming the as-manufactured hardness of WC alloys will be fully retained in the deposited layer.