Influence of Tungsten Carbide Hard Alloy Particles and Overlay Matrix on Overlay Layer Wear Resistance

Literature Overview

Published in 1996 in the journal Journal of the China Coal Society by researchers from the Beijing Mining Research Institute and Tsinghua University, this paper investigates the wear resistance of tungsten carbide (WC) composite overlay layers. The study employed two-body and three-body wear testing on overlay layers produced from several similar tubular WC wear-resistant welding electrodes. The central finding is that wear resistance improves with increasing WC particle size and volume fraction on the wear surface, while the matrix composition, microstructure, and properties play a critical role in determining overall performance.

Wear Mechanism Analysis

The wear behavior of WC-reinforced overlay layers is governed by the interaction between the hard WC particles and the surrounding matrix during abrasive sliding. The study distinguishes between two primary wear mechanisms:

Two-body abrasion: In this mode, hard asperities on the counterface plow through the overlay surface, causing material removal through micro-cutting and micro-ploughing. The wear resistance is primarily determined by the hardness of the WC particles and their ability to resist deformation. Larger WC particles with a higher area fraction on the wear surface provide more effective resistance to ploughing.

Three-body abrasion: In this mode, loose abrasive particles are trapped between the overlay and counterface, causing material removal through impact and cutting. The wear resistance depends not only on particle hardness but also on the matrix's ability to support the WC particles and prevent their pull-out. The austenitic matrix identified by the authors plays a crucial role here by providing the ductility needed to accommodate deformation without fracturing.

Wear Condition	Primary Mechanism	Key Factor	Optimal Feature
Two-body abrasion	Micro-cutting/ploughing	WC hardness and size	Large WC particles, high area fraction
Three-body abrasion	Impact + cutting	Matrix toughness + WC support	Austenitic matrix, well-bonded WC
Abrasive + impact	Combined	Hardness-toughness balance	Moderate WC size, austenitic matrix

Role of WC Particles

The study found that the size and area fraction of WC particles on the wear surface are the dominant factors controlling wear resistance. Larger WC particles present a greater cross-sectional area to resist abrasive penetration, while a higher area fraction ensures that a greater proportion of the wear surface is protected by hard particles. However, this relationship is not linear—beyond a certain particle size or volume fraction, the WC particles become isolated from the matrix support, leading to pull-out and accelerated wear.

The WC particles themselves are extremely hard (Vickers hardness ~2400 HV) but brittle. Their effectiveness as wear-resistant reinforcements depends on:

Bond strength with the matrix: Weak bonding leads to particle pull-out, which is the primary wear mechanism in poorly designed WC overlays.
Particle size distribution: A graded distribution (fine particles near the surface, coarse particles in the subsurface) provides optimal surface hardness and subsurface toughness.
Particle morphology: Rounded or equiaxed particles distribute stress more evenly than angular particles, reducing crack initiation sites.

Role of the Matrix

The matrix serves as the "glue" that holds the WC particles together and provides the overall structural integrity of the overlay. The authors specifically highlight the importance of the austenitic microstructure in the matrix:

Toughening: Austenite is inherently ductile and can accommodate plastic deformation without fracturing, which prevents crack propagation from WC particle boundaries.
Shock resistance: The austenitic matrix absorbs impact energy through work hardening and transformation-induced plasticity (TRIP effect), protecting the brittle WC particles from spalling.
Thermal stability: Austenitic matrices maintain their mechanical properties at elevated temperatures better than martensitic or ferritic matrices, which is critical for applications involving frictional heating.

The composition of the matrix (typically a high-Cr-Ni austenitic stainless steel or a nickel-based alloy) also influences corrosion resistance, which can be a secondary but important factor in service environments involving moisture or chemical exposure.

Engineering Application Considerations

For practical overlay welding applications involving WC-reinforced consumables, several factors must be considered:

Consumable selection: The choice between tubular, stick, or wire consumables affects WC particle size, distribution, and bonding. Tubular consumables generally allow for better WC incorporation and larger particle sizes.
Welding process: Submerged arc welding (SAW) and plasma arc surfacing are commonly used for WC overlays. SAW provides high deposition rates and good WC retention, while plasma arc offers finer control over dilution and microstructure.
Layer thickness: Multiple thin layers are generally preferred over a single thick layer to ensure uniform WC distribution and minimize residual stresses.
Post-weld treatment: Stress relief annealing can reduce residual stresses but must be performed at temperatures below the WC matrix eutectic to prevent particle degradation.

Key Reflections

This 1996 study was conducted during a period of rapid development in composite overlay welding technology, particularly in the mining and coal industry where wear-resistant surfaces are critical for equipment longevity. The study's emphasis on both WC particles and matrix properties reflects a mature understanding of composite wear mechanisms, moving beyond the simplistic "harder is better" approach to a more nuanced hardness-toughness balance.

One aspect that deserves further attention is the effect of WC particle degradation during wear. Under prolonged abrasive sliding, WC particles can oxidize, fracture, or be chemically attacked, leading to a gradual loss of wear resistance. The study does not extensively address this degradation mechanism, which is particularly relevant for applications involving high-temperature oxidation or chemical exposure. Future research should investigate the long-term stability of WC particles under realistic service conditions.

Additionally, the study's focus on two-body and three-body wear tests, while scientifically rigorous, may not fully capture the complex wear environments encountered in actual industrial applications. Mixed-mode wear, involving abrasion, adhesion, impact, and corrosion simultaneously, is common in mining and coal handling equipment. Engineers should validate overlay performance through field trials or accelerated multi-mode wear testing.

Summary

This paper provides a foundational understanding of how WC hard alloy particles and the overlay matrix interact to determine the wear resistance of composite overlay layers. The key findings—that larger WC particles with higher area fractions improve wear resistance, and that an austenitic matrix is essential for providing toughness and shock resistance—remain valid design principles for WC-reinforced overlays. Engineers working on wear-resistant surface engineering should use these findings as a starting point, supplementing them with application-specific testing and field validation. The study exemplifies the principle that optimal wear performance is achieved not by maximizing hardness alone, but by engineering a balanced composite system where hard particles and tough matrix work synergistically to resist the specific wear mechanisms present in service.