Wear Resistance and Wear Mechanism of Metal Overlay Layers Containing Inherent Hard Carbide Particles

Literature Overview

The incorporation of inherent hard carbide particles into metal overlay layers represents a well-established strategy for enhancing wear resistance in industrial cladding applications. The study note under review examines overlay alloys containing ex-solution carbides — primarily WC, Mo₂C, Cr₇C₃, and TiC — that form during solidification and subsequent heat treatment of the weld deposit. The literature investigates the effect of carbide particle size, volume fraction, distribution, and bonding with the matrix on the wear resistance and wear mechanism of the overlay layer. The central finding is that the wear mechanism transitions from abrasive wear to adhesive wear to fatigue wear as the carbide volume fraction increases, and that optimal wear resistance is achieved at an intermediate carbide volume fraction of 25–35% where the particles are uniformly distributed and well-bonded to the matrix.

Carbide Particle Characteristics and Their Effects on Wear Behavior

The inherent carbide particles in metal overlay layers form through the precipitation of refractory carbide-forming elements (W, Mo, Cr, Ti) during solidification. The size of these particles is determined by the cooling rate and the diffusion distance available during solidification. In thick overlay deposits cooled slowly, carbide particles can grow to sizes of 50–200 μm, while in thin deposits cooled rapidly, particle sizes are typically 5–20 μm. The study demonstrates that carbide particles in the 10–30 μm size range provide the best balance of wear resistance and impact toughness, as smaller particles are more resistant to fracture while larger particles are more prone to pull-out under abrasive loading.

The volume fraction of carbides is the most critical parameter governing wear resistance. Below 15% volume fraction, the matrix dominates the wear behavior and the carbides provide limited protection. Between 25% and 35% volume fraction, the carbides provide maximum wear resistance by forming a continuous network that resists material removal. Above 40% volume fraction, the excessive carbide content leads to poor matrix-carbide bonding, carbide clustering, and reduced impact toughness, which can result in catastrophic failure under impact loading. The study recommends targeting a carbide volume fraction of 30% for applications requiring maximum wear resistance and 20% for applications requiring a balance of wear and impact resistance.

The distribution uniformity of carbide particles is equally important. Non-uniform distributions create localized regions of high and low carbide concentration, leading to uneven wear patterns and premature failure at the weak points. The study identifies several factors that promote uniform carbide distribution: appropriate cooling rates (not too slow to allow coarsening, not too fast to cause segregation), adequate stirring during solidification (achieved through multi-pass welding or electromagnetic stirring), and proper composition design (avoiding compositions that promote carbide segregation).

Wear Mechanism Analysis Under Different Loading Conditions

The wear mechanism of overlay layers containing inherent carbide particles varies significantly with the loading conditions. Under low-load sliding conditions, the dominant wear mechanism is abrasive wear, where hard carbide particles resist ploughing by the counterface material. As the load increases, the wear mechanism transitions to adhesive wear, where material transfer between the overlay and counterface occurs at points of contact between the carbide particles. At very high loads, fatigue wear becomes dominant, where cyclic plastic deformation at the carbide-matrix interface leads to microcrack initiation and propagation, ultimately resulting in carbide pull-out and spalling of the overlay surface.

The study presents a detailed analysis of the transition between these wear mechanisms using wear rate versus load plots. For overlay layers with 30% carbide volume fraction, the transition from abrasive to adhesive wear occurs at approximately 50 N of normal load, and the transition from adhesive to fatigue wear occurs at approximately 200 N. The wear rate increases by a factor of 3–5 at each transition point, indicating a fundamental change in the material removal mechanism. Engineers designing overlay systems for specific applications should ensure that the expected service loading does not exceed the transition point for the selected overlay composition.

The role of the matrix in the wear mechanism is often underestimated. The matrix serves as the load-bearing medium that transfers stress from the counterface to the carbide particles. If the matrix is too soft, it deforms plastically under load, causing the carbide particles to lose support and become susceptible to fracture or pull-out. If the matrix is too hard, it becomes brittle and cracks under cyclic loading, leading to fatigue failure. The optimal matrix hardness for supporting WC carbides is approximately 400–500 HV, which provides sufficient strength to transfer load without being so hard as to crack under cyclic stress.

Effect of Heat Treatment on Carbide Stability and Wear Performance

Post-weld heat treatment is a critical process step for overlay layers containing inherent carbide particles. The primary objectives of heat treatment are to relieve residual stresses, stabilize the carbide structure, and optimize the matrix-carbide bonding. Tempering at 500–600°C for 1–2 hours is the most common heat treatment for high-carbon, high-chromium overlay alloys. This treatment converts the brittle as-welded martensite to a tempered martensite structure, reducing hardness by 100–200 HV but improving toughness by 100–200%. The carbide particles remain stable at these temperatures and do not dissolve or coarsen significantly.

However, for overlay alloys containing titanium carbides or vanadium carbides, higher heat treatment temperatures (700–800°C) may cause partial dissolution of the carbides, reducing the effective carbide volume fraction and degrading wear resistance. The study recommends that the heat treatment temperature be carefully selected based on the specific carbide type present in the overlay. For WC-containing alloys, tempering at 550°C is optimal. For TiC-containing alloys, tempering at 450°C is recommended to avoid carbide dissolution. For Cr₇C₃-containing alloys, tempering at 600°C provides the best balance of toughness and wear resistance.

The study also examines the effect of cryogenic treatment on the microstructure and wear properties of overlay layers. Cryogenic treatment at -196°C for 24 hours followed by tempering at 200°C promotes the formation of additional carbide particles from retained austenite, increasing the total carbide volume fraction by 5–10%. This additional carbide formation results in a 15–25% improvement in wear resistance compared to conventionally heat-treated overlays. However, the cryogenic treatment also increases the brittleness of the matrix, which must be balanced against the wear resistance improvement.

Engineering Applications and Design Recommendations

The findings of this study have direct implications for the design and specification of overlay layers in industrial applications. For applications involving severe abrasive wear (mining equipment, cement mill liners, sand handling equipment), overlay alloys containing 30% WC or Mo₂C carbides with a tempered martensite matrix provide the best wear resistance. For applications involving a combination of abrasive and impact wear (conveyor rollers, crusher hammers), overlay alloys containing 20–25% Cr₇C₃ carbides with a tempered martensite matrix provide the best balance of wear and impact resistance. For applications involving erosive wear (boiler tubes, fan blades, pump impellers), overlay alloys containing 10–15% TiC carbides with a nickel-based matrix provide the best erosion resistance.

The study emphasizes that the selection of overlay composition should be based on the dominant wear mechanism in the application, not simply on the hardness of the overlay. A hard overlay with poorly bonded carbides may perform worse than a slightly softer overlay with well-bonded carbides, because the poorly bonded carbides will fracture and pull out under service loading, creating abrasive debris that accelerates wear. Engineers should always evaluate overlay performance through field trials or accelerated wear testing before specifying a particular composition for a critical application.

Study Insights and Future Directions

The most significant insight from this study is that the wear resistance of overlay layers containing inherent carbide particles is a system property that depends on the interaction between the carbide phase and the matrix phase, not simply on the hardness of the individual phases. The carbide particles provide the wear resistance, but the matrix provides the support and toughness that prevent carbide failure. The optimal design is one where both phases are balanced to resist the specific wear mechanism encountered in service. Future research should focus on developing overlay compositions with tailored carbide-matrix interfaces that provide enhanced bonding and load transfer, potentially through the use of nanostructured carbides or functionally graded carbide distributions. The integration of computational modeling with experimental wear testing will be essential for predicting the performance of new overlay compositions under complex loading conditions.