CLADDING TECHNOLOGY SHANXI CO., LTDAbrasion Resistance and Wear Mechanism of Metal Cladding Layers Containing Intrinsic Hard Carbide Particles
Introduction
The present study investigates the wear behavior and underlying mechanisms of metal overlay layers that contain inherently formed hard carbide particles, as distinguished from externally added ceramic reinforcement. In these systems, carbides such as Cr7C3, Cr23C6, Fe3C, or M7C3 (where M = Mo, W, V) precipitate during solidification and subsequent heat treatment of the overlay alloy. The unique advantage of intrinsic carbides is their strong metallurgical bonding with the matrix, superior to the mechanical bonding of externally added particles, which reduces the risk of particle pull-out during wear.
Carbide Morphology and Distribution
The study characterizes the carbide phases formed in typical Fe-Cr-C and Fe-Cr-Mo-C overlay systems:
| Overlay Alloy System | Primary Carbide Phase | Carbide Hardness (HV) | Typical Size | Distribution |
|---|---|---|---|---|
| Fe-25Cr-2C | Cr7C3, Cr23C6 | 1400–1800 | 5–20 μm | Dendritic inter-dendritic |
| Fe-20Cr-1Mo-1.5C | M7C3 (Cr,Mo) | 1600–2000 | 3–15 μm | Inter-dendritic, eutectic |
| Fe-15Cr-0.8C-0.5V | (Fe,Cr)7C3, VC | 1500–2200 | 2–10 μm | Fine dispersed |
| Ni-20Cr-2C | Cr7C3, Cr3C2 | 1200–1600 | 10–30 μm | Eutectic network |
The morphology of these carbides—whether they appear as discrete particles, network structures, or eutectic colonies—has profound effects on wear performance. Discrete, well-distributed carbides generally provide superior wear resistance compared to continuous networks, which can act as crack initiation sites.
Wear Mechanism Analysis
The study identifies three primary wear mechanisms operating in these overlay systems:
- Abrasive wear: Hard carbide particles resist material removal by ploughing and micro-cutting. The carbide phase bears the majority of the contact load, protecting the softer matrix.
- Adhesive wear: At elevated temperatures or under heavy loads, the metallic matrix may undergo adhesive transfer to the counterface, while carbide particles remain intact.
- Fatigue wear: Under cyclic loading, micro-cracks initiate at carbide-matrix interfaces or at carbide networks, leading to progressive material loss through micro-pitting and spalling.
The dominant wear mechanism shifts with operating conditions: abrasive wear prevails at low loads and room temperature, while fatigue wear becomes dominant at higher loads and elevated temperatures. The transition load is typically in the range of 50–200 N for laboratory ball-on-disc testing.
Microstructure-Wear Performance Correlation
The study establishes several key relationships between microstructural features and wear resistance:
- Carbide volume fraction: Wear resistance increases with carbide volume fraction up to approximately 30–40%, beyond which the matrix becomes insufficiently connected and catastrophic failure occurs.
- Carbide size: Finer carbides (2–10 μm) generally provide better wear resistance than coarse carbides (>20 μm) because they distribute the contact stress more uniformly and resist fracture more effectively.
- Carbide morphology: Rounded or polyhedral carbides outperform acicular or network-type carbides in terms of fatigue resistance, as they do not provide preferential crack paths.
- Matrix hardness: A harder matrix (achieved through martensitic transformation or solid solution strengthening) delays the onset of matrix wear and extends the service life of the overlay.
Process Optimization for Carbide Control
The study provides guidance on controlling carbide formation through alloy design and heat treatment:
- Carbon content: Higher carbon promotes more carbide precipitation but also increases matrix brittleness. An optimal carbon content of 1.0–2.0 wt% is recommended for most Fe-Cr-C systems.
- Chromium content: Chromium above 15% promotes stable chromium carbides while below 10% favors iron carbides. The Cr/C ratio significantly influences carbide type and stability.
- Mo and W additions: These elements form very hard M6C and M23C6 carbides and enhance high-temperature wear resistance through solid solution strengthening of the matrix.
- Post-weld heat treatment: Tempering at 500–600°C transforms retained austenite to tempered martensite and spheroidizes carbide networks, improving toughness without significantly reducing hardness.
Study Conclusions
Metal overlay layers containing intrinsic hard carbide particles offer excellent wear resistance through a synergistic mechanism where the hard carbide phase resists material removal and the metallic matrix provides structural support and toughness. The key to optimizing performance lies in controlling carbide volume fraction, size, and morphology through careful alloy design and appropriate heat treatment. Engineers should target a carbide volume fraction of 20–35%, with individual particle sizes below 15 μm and discrete (non-network) morphology for the best combination of abrasive resistance and thermal fatigue durability.