Effect of WC Addition on Microstructure and Properties of FeCrNiSiB Plasma Surfacing Layer

Literature Overview

This 2024 paper by Bing Shaowang and colleagues from the Qingdao Guoshi Technology Group (formerly Qingdao Marine Science and Technology National Laboratory), published in the journal "Materials Development and Application" (Volume 39, Issue 6, pages 44-52), investigates the influence of WC powder addition on the microstructure, microhardness, and friction-wear performance of FeCrNiSiB plasma surfacing layers deposited on Q235 low-carbon steel substrates for marine applications. The research was supported by the Qingdao Marine Science and Technology Pilot National Laboratory "14th Five-Year Plan" major project (20220NLM030001-4).

Experimental Design and Methodology

Process Parameters and Sample Preparation

The study employs plasma powder surfacing to deposit FeCrNiSiB coatings with varying WC additions (by mass fraction) onto Q235 marine-grade low-carbon steel substrates. The welding current is set at 130 A, which represents a moderate energy input suitable for the plasma powder surfacing process. The WC addition levels are varied systematically to establish the relationship between carbide content and coating performance.

Microstructural Characterization

The cross-sectional microstructure of the FeCrNiSiB surfacing layer exhibits a characteristic three-layer structure:

1. Surfacing layer: The deposited coating containing the primary alloying elements and WC carbides.
2. Fusion zone (dilution zone): The transition region where the coating alloy mixes with the base metal, resulting in a composition and microstructure intermediate between the two.
3. Substrate: The Q235 base steel, unaffected by the surfacing process beyond the heat-affected zone.

The primary phases identified in the surfacing layer include Cr₂₃C₆, Cr₃Fe₁₄C, free carbon (C), and retained WC. The microstructure exhibits a distinctive two-phase morphology with black and white phases intermixed:

Key Findings

Microhardness Variation

The microhardness of the surfacing layer increases with WC addition, which is consistent with the well-established relationship between hard carbide volume fraction and coating hardness. The WC particles act as hard reinforcement phases within the softer FeCrNiSiB matrix, and their increasing volume fraction directly elevates the overall hardness of the composite structure.

A notable finding is that the hardness at the bottom of the surfacing layer (near the fusion zone) is higher than at the top surface. This gradient is attributed to several factors:

• Dilution effect: The fusion zone contains additional Cr from the base metal interaction, promoting more Cr₂₃C₆ formation.
• Thermal gradient: The solidification rate near the substrate is higher due to rapid heat extraction into the massive base metal, resulting in finer microstructure and potentially higher hardness.
• Carbide distribution: WC particles may settle preferentially toward the bottom during the deposition process due to gravity and fluid flow dynamics in the molten pool.

Friction and Wear Performance

The friction coefficient of the FeCrNiSiB surfacing layers remains stable in the range of 0.55–0.65 across all WC addition levels, indicating consistent tribological behavior. However, the friction coefficient exhibits a non-monotonic trend with WC addition: it initially decreases and then increases as WC content rises.

This behavior can be explained by the competing effects of carbide content on the friction surface:

• At low WC additions, the increased carbide content provides harder contact asperities that reduce adhesion and ploughing, thereby lowering friction.
• At high WC additions, the excessive carbide volume fraction may lead to increased surface roughness and mechanical interlocking with the counterface, which raises the friction coefficient.

The optimal friction-wear performance is achieved at a WC addition of 30 wt% with a welding current of 130 A. At this composition, the coating achieves a favorable balance between hardness (sufficient to resist abrasive wear) and toughness (sufficient to resist crack propagation and spalling).

Phase Evolution with WC Addition

The volume fraction of the high-melting-point carbide reinforcement phase (black phase) increases with WC addition, which is the primary mechanism for hardness improvement. The white binder phase (low-melting-point alloy phase) decreases correspondingly as the carbide content rises. This shift in phase proportion directly correlates with the observed hardness and wear resistance trends.

The retained WC particles serve as nucleation sites for secondary carbide precipitation during solidification, potentially promoting a finer carbide distribution in the matrix. The decomposition products of WC (including W₂C and Fe₇W₆C) also contribute to the reinforcement phase, although they are generally less stable than intact WC.

Engineering Practice Integration

For marine applications, the FeCrNiSiB/WC system offers several advantages:

• Corrosion resistance: The Cr and Ni content provides resistance to seawater corrosion, which is critical for marine components.
• Wear resistance: The WC carbides provide excellent resistance to abrasive and erosive wear from marine sediments and particulates.
• Weldability: The Fe-based matrix ensures good compatibility with common marine steel substrates such as Q235, AH36, and DH36.

The optimal parameters identified in this study (130 A, 30 wt% WC) provide a practical starting point for process development. However, engineers should note that the actual process window may vary depending on substrate thickness, coating geometry, and production requirements. Scale-up trials are essential to validate the laboratory findings in actual production conditions.

The three-layer structure observation is important for quality control. The fusion zone thickness and dilution level should be monitored during production to ensure consistent coating properties. Ultrasonic testing or cross-sectional metallographic examination can be used for in-process verification.

Key Questions and Reflections

Several aspects of this study merit further consideration. First, the study focuses on dry friction-wear testing, but marine applications often involve wet or corrosive environments. The tribological performance of the coating in seawater or salt spray conditions should be evaluated to ensure practical applicability.

Second, the long-term wear life under cyclic loading conditions is not addressed. Marine components such as propeller shafts, rudder stocks, and hull coating areas experience complex loading patterns that include vibration, impact, and cyclic stress. Fatigue wear behavior would be more representative of actual service conditions.

Third, the study does not address the effect of plasma arc parameters beyond current (such as arc voltage, travel speed, and gas flow rate) on the coating properties. A more comprehensive parameter optimization study would provide a more robust process window for production application.

The identification of the 30 wt% WC addition as optimal is a valuable practical finding, but engineers should recognize that this optimum is specific to the 130 A welding current and the Q235 substrate. Different substrates or process conditions may shift the optimal composition.

Study Insights and Implications

This paper provides a systematic investigation of the WC addition effect on FeCrNiSiB plasma surfacing coatings, with clear practical recommendations for marine applications. The non-monotonic friction coefficient behavior with WC content is a particularly valuable finding, as it highlights the importance of optimizing carbide content rather than simply maximizing it for improved wear resistance.

The three-layer microstructure characterization and the identification of distinct white and black phases provide a clear framework for understanding the coating's mechanical behavior. The hardness gradient from bottom to top is a critical consideration for coating design, as it affects the stress distribution and crack initiation sites during service.

For engineers developing hardfacing coatings for marine applications, this study confirms that plasma powder surfacing with FeCrNiSiB/WC compositions is a viable technology for achieving the dual objectives of corrosion resistance and wear resistance. The optimal parameters identified provide a practical starting point for process development, and the microstructural insights offer a foundation for further alloy design optimization.