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

Literature Overview

This 2024 study by Bing Shaowang and colleagues, published in Materials Development and Application, investigates the influence of tungsten carbide (WC) addition on the microstructure and mechanical properties of FeCrNiSiB overlay layers produced by plasma transferred arc (PTA) cladding. Funded by the Qingdao Pilot National Laboratory for Marine Science and Technology under the "14th Five-Year Plan" major project, this research addresses the needs of marine and offshore engineering, where corrosion-resistant and wear-resistant overlay layers are critical for equipment longevity in harsh marine environments.

Technical Background and Application Context

The marine environment presents unique challenges for metallic components: high salinity, biological fouling, abrasive marine sediments, and cyclic loading. Traditional stainless steel overlays may provide adequate corrosion resistance but insufficient wear resistance for components such as propeller shafts, pump impellers, and valve seats. The FeCrNiSiB alloy system was selected as the base matrix because it combines iron-based strength with chromium and nickel for corrosion resistance and silicon and boron for hardening through the formation of borides and silicides. The addition of WC particles is intended to further enhance wear resistance while maintaining or improving corrosion resistance.

Process Parameters and Experimental Design

The study used PTA cladding with a systematic variation of WC content in the powder blend. The base powder was an FeCrNiSiB alloy with the following approximate composition: Fe balance, Cr 20–25 wt%, Ni 10–15 wt%, Si 3–5 wt%, B 0.5–1.0 wt%. WC particles were added at varying weight percentages to create a series of test coatings.

Microstructural Analysis

The study performed comprehensive microstructural characterization including optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). The key findings were:

Phase Composition Evolution

• 0 wt% WC: The overlay layer consisted primarily of austenite (γ-Fe), Cr23C6, Fe2B, and FeSi phases. The microstructure showed a typical cast structure with dendritic grains.
• 5–10 wt% WC: Residual WC particles were observed in the matrix, indicating partial survival of WC through the thermal cycle. Additional Cr carbides and some tungsten carbides formed. The microstructure became more complex with a higher density of carbide phases.
• 15–20 wt% WC: Significant WC decomposition occurred, with tungsten dissolving into the matrix and forming W2C and mixed carbides. The austenite content decreased as the matrix became more ferritic. The overall carbide volume fraction increased substantially.
• 25–30 wt% WC: Excessive WC decomposition led to carbon enrichment and the formation of brittle phases including Fe3C. Microcracks were observed at the overlay-base interface and within the overlay layer. The microstructure became increasingly heterogeneous.

Hardness and Mechanical Properties

The data clearly shows that hardness increases monotonically with WC addition, but the rate of increase diminishes beyond 20 wt%. More importantly, the compressive strength peaks at approximately 15–20 wt% WC and then declines, while microcracking begins to appear at higher WC contents. This indicates that 15–20 wt% WC is the optimal range for balancing hardness, strength, and integrity.

Corrosion Resistance Evaluation

The study evaluated corrosion resistance through potentiodynamic polarization tests and salt spray testing in simulated marine conditions (3.5 wt% NaCl solution). The findings were:

• The base FeCrNiSiB alloy (0 wt% WC) showed good corrosion resistance with a corrosion potential of approximately -0.2 V vs. Ag/AgCl and a low corrosion current density.
• WC addition up to 15 wt% did not significantly degrade corrosion resistance, likely because the residual WC particles were embedded in the passive austenitic matrix.
• WC addition above 20 wt% led to a slight decrease in corrosion resistance due to the formation of a more ferritic matrix and the presence of carbon-rich phases that could act as local corrosion initiation sites.
• Salt spray testing confirmed that coatings with 15 wt% or less WC content showed no significant corrosion after 720 hours, while coatings with 30 wt% WC showed localized pitting at the WC-matrix interfaces.

Wear Performance

Pin-on-disk wear tests were conducted under dry sliding conditions against a steel counterface. The wear rates showed a clear trend:

• Wear rate decreased with increasing WC content up to 20 wt%, indicating improved wear resistance.
• Beyond 20 wt%, the wear rate increased slightly due to microcracking and the embrittlement of the overlay layer.
• The optimal wear performance was observed at 15–20 wt% WC, where the combination of hard carbide phases and a ductile matrix provided the best balance of wear resistance and integrity.

Engineering Practice Implications

This study has direct relevance to marine and offshore engineering applications where overlay welding is used to protect critical components from combined wear and corrosion. The key recommendations for engineering practice are:

1. WC content optimization: For marine applications, a WC addition of 15–20 wt% is recommended to balance hardness, wear resistance, corrosion resistance, and structural integrity.
2. Process parameter control: Maintaining a low dilution ratio (below 10%) is critical for preserving the alloy composition of the overlay layer and ensuring the desired phase balance.
3. Multi-layer deposition: For thick overlay layers, a graded approach is recommended, with lower WC content in the first layer (to reduce dilution effects) and higher WC content in subsequent layers.
4. Post-deposition treatment: A stress-relief heat treatment at 400–500°C for 1–2 hours can reduce residual stresses and minimize the risk of cracking, particularly for coatings with higher WC content.

Integration with Standards and Quality Control

For engineering applications, the overlay layers must meet relevant standards for mechanical properties, corrosion resistance, and integrity. The study's findings can be mapped to standard requirements as follows:

Reflections and Study Insights

This 2024 study represents the current state of the art in PTA cladding technology for marine applications. Several aspects are particularly noteworthy:

• The systematic approach to varying WC content across a wide range (0–30 wt%) provides comprehensive data that can guide practical design decisions.
• The correlation between microstructure, phase composition, and mechanical properties is clearly established, demonstrating the importance of microstructural control in overlay welding.
• The identification of an optimal WC content range (15–20 wt%) that balances competing requirements is directly actionable for engineers.
• The study's focus on marine applications is timely, given the growing demand for offshore energy and marine infrastructure.

For engineers working in the cladding field, this study reinforces several fundamental principles: the importance of systematic experimentation, the value of comprehensive characterization, and the need to balance multiple performance criteria when designing overlay solutions. The study also highlights the ongoing relevance of PTA cladding as a versatile process capable of producing high-performance overlay layers with controlled properties. As marine and offshore engineering continues to expand, the demand for reliable overlay solutions will only increase, and studies like this one provide the technical foundation for meeting that demand.