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

Literature Overview

This 2024 study by Bing Shaowang et al., published in "Materials Development and Application" (Vol. 39, No. 6, pp. 44-52), investigates the influence of WC powder addition (varying mass fractions) on the microstructure, microhardness, and tribological performance of FeCrNiSiB overlay layers deposited by plasma overlay welding on Q235 low-carbon steel substrates. The research was funded by the Qingdao Marine Science and Technology Pilot National Laboratory "14th Five-Year Plan" major project (20220NLM030001-4), indicating its relevance to marine engineering applications.

Research Methodology and Key Parameters

The study employs a systematic approach to optimize WC content in the FeCrNiSiB matrix, testing multiple addition levels to establish the relationship between carbide content and performance metrics.

Test Variable	Range Tested	Measurement Method
WC addition (mass %)	0, 10, 20, 30, 40, 50	Powder blending ratio
Overlay current	100-160 A	Ammeter monitoring
Layer thickness	1.5-3.0 mm	Caliper and sectioning
Microhardness	200-1200 HV	Vickers hardness tester
Friction coefficient	0.55-0.65	Pin-on-disk tribometer
Wear rate	Variable	Weight loss method

Microstructural Analysis

Phase Composition

The overlay layers exhibit a three-layer cross-sectional structure: overlay layer, fusion zone, and base metal. The primary phases identified in the overlay include:

Phase	Type	Composition Characteristics	Role
Cr23C6	M7C3 carbide	High Cr, moderate C	Hardness contribution
Cr3Fe14C	Complex carbide	Mixed Cr-Fe-C	Matrix reinforcement
C (graphite)	Carbon phase	Pure carbon	Lubrication effect
WC	Tungsten carbide	W-C compound	Primary reinforcement
Fe-based binder	Matrix	Fe, C, small amounts of Cr, W	Phase connectivity

Two-Phase System Characterization

The overlay microstructure is characterized by a distinctive two-phase system:

White phase (binder phase): Low melting point alloy matrix composed primarily of Fe, C, and small amounts of Cr and W. This phase provides ductility and toughness to the composite overlay.
Black phase (reinforcing carbide phase): High melting point carbides with lower Fe content but higher W, C, and Cr concentrations. This phase provides hardness and wear resistance.

As WC addition increases, the volume fraction of the reinforcing carbide phase increases proportionally, creating a denser carbide network that enhances wear resistance but may reduce toughness.

Performance Results and Optimization

Hardness Distribution

The microhardness profile shows two important characteristics:

Hardness increases monotonically with WC addition, rising from approximately 350 HV at 0% WC to over 1000 HV at 50% WC.
The hardness at the bottom of the overlay (near the fusion zone) is consistently higher than at the top surface, attributed to higher dilution with the base metal and slower cooling rates at the interface, which promote carbide coarsening and precipitation hardening.

Tribological Performance

WC Addition (%)	Friction Coefficient	Relative Wear Rate	Optimal?
0	0.63	Baseline	No
10	0.58	Reduced	No
20	0.56	Further reduced	No
30	0.55	Minimum	Yes
40	0.57	Slight increase	No
50	0.62	Significant increase	No

The optimal friction coefficient of 0.55 at 30% WC addition corresponds to the best overall wear performance, achieved with an overlay current of 130 A. Beyond 30%, the increased carbide volume fraction leads to higher inter-particle stress, promoting micro-cracking and spalling under frictional loading.

Engineering Practice Integration

For marine applications (the primary focus of this research), the FeCrNiSiB/WC overlay system addresses specific challenges:

Cavitation erosion resistance: The carbide-reinforced microstructure resists micro-jet attack from cavitation bubbles in propeller shaft bearings and pump impellers.
Abrasive wear in marine sediments: The high-hardness carbide network provides resistance to sand and mineral particle abrasion in underwater equipment.
Corrosion-wear synergy: The Cr-Ni content provides base corrosion resistance in seawater, while the WC carbides resist mechanical degradation, creating a synergistic protection mechanism.

Study Insights and Implications

The finding that the friction coefficient exhibits a non-monotonic behavior with WC addition (decreasing then increasing) is particularly instructive. It demonstrates that maximum hardness does not necessarily correlate with optimal tribological performance. The 30% WC optimum represents a balance between carbide reinforcement and matrix ductility, where the binder phase maintains sufficient connectivity to prevent catastrophic carbide spalling.

The hardness gradient from bottom to top of the overlay layer suggests that multi-pass deposition with varying parameters could create a graded structure optimized for different loading conditions. Engineers should consider this gradient effect when designing overlay specifications for components with specific surface stress distributions.