Zhuojin Pipe Fitting Co., LtdEffect of WC Addition on Microstructure and Properties of Plasma-Surfaced FeCrNiSiB Coatings
Literature Overview
This 2024 paper by Bing Shaowang and colleagues, published in Materials Development and Application (Volume 39, Issue 6, pages 44-52), presents a systematic investigation of the influence of tungsten carbide (WC) powder addition on the microstructure, microhardness, and tribological properties of FeCrNiSiB coatings deposited by plasma surfacing on Q235 low-carbon steel substrates. The research was funded by the Qingdao Pilot National Laboratory for Marine Science and Technology under the "14th Five-Year Plan" major project (20220NLM030001-4), indicating its relevance to marine engineering applications.
Experimental Design and Process Parameters
The study employed a well-structured experimental approach, varying the WC powder mass fraction in the surfacing alloy while maintaining consistent plasma surfacing parameters. The base surfacing alloy was FeCrNiSiB, a nickel-free iron-based hardfacing alloy that has been developed as a cost-effective alternative to cobalt-based hardfacing alloys for marine applications.
| Parameter | Value |
|---|---|
| Substrate | Q235 low-carbon steel |
| Surfacing process | Plasma arc surfacing (PAW) |
| Base alloy | FeCrNiSiB |
| Reinforcement | WC powder |
| WC mass fractions tested | Multiple levels (up to 30%) |
| Surfacing current | 130 A (optimal) |
| Coating structure | Surfacing layer / fusion zone / substrate |
The use of Q235 steel as the substrate is practical and representative of marine structural steel commonly used in shipbuilding and offshore platforms. The selection of FeCrNiSiB as the base alloy reflects the industry trend toward nickel-free hardfacing materials, driven by both cost considerations and supply chain security concerns related to cobalt and nickel.
Microstructural Analysis
The cross-sectional examination of the surfacing coatings revealed a clear three-layer structure: the surfacing layer, the fusion zone, and the unmodified substrate. The surfacing layer exhibited a two-phase microstructure consisting of a white phase and a black phase.
| Phase | Composition | Characteristics |
|---|---|---|
| White phase | Fe, C, with minor Cr, W | Low melting point alloy binder phase |
| Black phase | Lower Fe, higher W, C, Cr | High melting point carbide reinforcement phase |
| Main phases identified | Cr23C6, Cr3Fe14C, C, WC | Carbide-based strengthening phases |
The black phase, rich in tungsten, carbon, and chromium, corresponds to the carbide reinforcement phase that provides the primary wear resistance. The white phase serves as the metallic binder that holds the carbide particles together and provides ductility to the coating. The relative volume fraction of the black phase increases with increasing WC addition, which directly correlates with the observed increase in microhardness.
The microhardness distribution showed a gradient from the surface to the fusion line, with the highest hardness at the bottom of the surfacing layer (near the fusion zone). This is attributed to the higher cooling rate at the fusion line, which promotes the formation of finer and harder carbide phases. The overall hardness increases with WC addition, reaching a peak at the optimal WC content.
Tribological Performance
The friction and wear tests revealed that the friction coefficient of the FeCrNiSiB coatings remained relatively stable in the range of 0.55–0.65 across all WC addition levels. However, the trend was non-monotonic: the friction coefficient first decreased with increasing WC content and then increased beyond a certain threshold. This behavior can be explained by the competition between two effects: the increased surface roughness and carbide protrusion at high WC contents increases friction, while the improved bonding between the coating and the counterface at moderate WC contents reduces friction.
The optimal tribological performance was achieved at a surfacing current of 130 A and a WC addition of 30% by mass. At this composition, the coating exhibited the best balance of hardness, fracture toughness, and surface smoothness, resulting in the lowest wear rate and most stable friction behavior.
Engineering Practice Integration
For marine engineering applications, the findings of this study have several practical implications:
- Coating selection for marine hardware: FeCrNiSiB/WC coatings can be applied to marine valve seats, pump impellers, propeller surfaces, and other wear-critical components operating in seawater environments. The nickel-free composition eliminates concerns about nickel-induced sensitization and reduces material costs.
- WC content optimization: The study demonstrates that increasing WC content does not monotonically improve performance. The optimal content of 30% represents a balance between carbide reinforcement and matrix ductility. Exceeding this level risks coating cracking and delamination due to excessive brittleness.
- Process parameter control: The surfacing current of 130 A was identified as optimal. Deviations from this parameter can lead to incomplete melting of the WC particles (at too low current) or excessive dilution and carbide decomposition (at too high current).
- Quality control protocol: A comprehensive QC protocol should include XRD analysis for phase identification, Vickers hardness profiling across the coating cross-section, and accelerated wear testing under simulated service conditions.
Key Reflections and Study Insights
This paper is a good example of systematic materials research applied to a specific engineering problem. The authors did not merely vary one parameter and report the results; they analyzed the microstructure-property relationships and provided mechanistic explanations for the observed trends. The identification of the two-phase structure (binder phase and carbide reinforcement phase) and the correlation between phase fraction and mechanical properties is particularly valuable for coating designers.
One aspect that I find particularly noteworthy is the stability of the friction coefficient across the tested WC range. In practical marine applications, a stable friction coefficient is essential for predictable component behavior. Coatings with highly variable friction characteristics can lead to unpredictable wear patterns and premature failure.
The choice of FeCrNiSiB as the base alloy is also significant. Traditional hardfacing alloys for marine applications have been dominated by cobalt-based alloys (such as Stellite), which offer excellent wear resistance but are expensive and subject to supply constraints. Iron-based alloys with appropriate alloy additions can achieve competitive performance at significantly lower cost, making them attractive for large-scale marine applications.
However, I would note that the study does not address corrosion resistance, which is a critical property for marine applications. The presence of chromium in the alloy suggests potential for corrosion resistance, but systematic evaluation under simulated seawater conditions is necessary before recommending this coating for submerged or splash-zone applications.
The funding from the Qingdao Marine Science and Technology National Laboratory underscores the strategic importance of developing domestically produced hardfacing materials for China's growing marine engineering sector. As the industry continues to expand into deep-sea operations and offshore wind energy, the demand for reliable, cost-effective hardfacing solutions will only increase.