Microstructure and Wear Resistance of WC-Reinforced Nickel-Based Plasma Surfacing Layer

Literature Overview

The paper by Liu Zhiyu et al., published in Hot Working Technology (Vol. 52, No. 17, 2023, pp. 27–31), investigates the effect of tungsten carbide (WC) content on the microstructure and wear resistance of nickel-based plasma surfacing layers deposited on Q235 steel substrates. The authors, from Shenyang Institute of Technology and the Shenyang Inspection and Testing Center, systematically varied the WC addition level and characterized the resulting coatings through XRD, metallography, microhardness testing, and pin-on-disk wear testing.

Core Technical Content

Nickel-based alloys are widely used in surfacing applications due to their excellent corrosion resistance, good weldability, and thermal stability. However, their relatively low hardness limits their wear resistance in severe abrasion conditions. The addition of WC, a hard ceramic phase with a hardness exceeding 2000 HV, is a common strategy to enhance wear resistance. The challenge lies in optimizing the WC content to maximize hardness and wear resistance without introducing excessive brittleness or cracking.

The study deposited nickel-based surfacing layers with varying WC contents on Q235 steel plates using plasma arc surfacing. The coatings were characterized for phase composition, microstructure, microhardness distribution, and wear performance under dry sliding conditions.

Phase Composition and Microstructure

The optimal WC content was found to be 30 wt%. At this level, the eutectic microstructure in the middle of the surfacing layer transitions from a lamellar to a blocky morphology, resulting in a more stable hardness distribution with an average of 767.2 HV0.5. The wear loss at room temperature was minimized at 3.9 mg.

Microstructural Evolution with WC Content

The transition from lamellar to blocky eutectic microstructure at 30 wt% WC is a critical finding. Lamellar eutectic structures, while hard, are prone to crack initiation at the interface between the hard carbide phase and the soft matrix. Blocky eutectic structures provide more isotropic mechanical properties and better resistance to delamination. This transition explains why the wear resistance peaks at 30 wt% rather than at higher WC contents.

At WC contents above 30 wt%, the excessive amount of hard WC particles leads to increased brittleness and reduced ductility of the coating. The hardness may remain high, but the wear resistance decreases due to increased delamination. This is a classic example of the hardness-toughness trade-off in hardfacing alloys.

Wear Mechanism Analysis

The wear mechanism in all coatings was identified as delamination wear, which is characteristic of hard coatings on relatively soft substrates. In delamination wear, cracks initiate at the coating surface or near-surface region, propagate laterally, and eventually cause flakes or chunks to detach from the coating. The severity of delamination depends on the coating's fracture toughness and the stress state at the coating-substrate interface.

At 30 wt% WC, the blocky eutectic structure provides the best resistance to delamination because:

• The blocky carbides distribute stress more uniformly than lamellar structures.
• The matrix retains sufficient ductility to accommodate plastic deformation.
• The interface between WC particles and the Ni-based matrix is well bonded.

Process Parameters for Plasma Surfacing

The plasma arc surfacing process offers precise control over heat input, which is essential for maintaining the desired microstructure. The high energy density of the plasma arc allows for low dilution rates, preserving the alloying elements and WC particles in the coating.

Engineering Application Considerations

For applications involving severe sliding wear, such as pump shafts, valve seats, and hydraulic cylinder liners, the 30 wt% WC nickel-based plasma surfacing layer represents an optimal balance between hardness and toughness. The hardness of 767 HV0.5 provides excellent resistance to abrasive particles, while the blocky eutectic structure ensures adequate fracture toughness to prevent catastrophic delamination.

However, engineers must be aware that the wear performance is measured under dry sliding conditions at room temperature. In lubricated or high-temperature environments, the wear mechanism may shift to adhesive or oxidative wear, and the optimal WC content may differ.

Key Questions and Reflections

A significant question is whether the 30 wt% WC optimum is universal or specific to the particular nickel-based alloy composition used in this study. Different nickel-based alloys (e.g., Stellite 6, Alloy 6, or custom formulations) may have different optimal WC contents due to variations in matrix composition, carbide formation tendencies, and thermal expansion coefficients.

Another consideration is the effect of heat treatment on the WC-containing surfacing layers. Post-weld heat treatment could potentially dissolve some of the WC particles, altering the phase composition and wear properties. This was not investigated in the present study but represents an important direction for future research.

Summary

This study demonstrates that the addition of 30 wt% WC to a nickel-based plasma surfacing layer yields the optimal combination of hardness (767 HV0.5) and wear resistance (3.9 mg loss). The transition from lamellar to blocky eutectic microstructure at this WC content is the key microstructural feature responsible for the improved wear performance. Engineers selecting WC-reinforced nickel-based coatings for wear-critical applications should target this composition window while considering the specific service conditions. The work provides a clear guideline for optimizing hardfacing alloy design through controlled WC addition.