CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Lamellar Overlay Materials for Wear-Resistant Applications
Overview of the Study
This 2010 study published in the Journal of Shenyang University of Technology, conducted by Li Deyuan and colleagues at Shenyang University of Technology, addresses the development of lamellar overlay materials — a concept that leverages the synergistic combination of hard and tough phases arranged in a layered (lamellar) structure to achieve superior wear resistance without sacrificing toughness. The research was supported by the Liaoning Provincial Department of Education Science and Technology Fund (Project No. 0024101).
The lamellar approach is inspired by the microstructure of natural materials such as bone and nacre, where alternating hard and soft layers provide exceptional mechanical performance. In the context of overlay welding, this concept involves depositing alternating layers of hard carbide-forming alloy and tough martensitic or austenitic alloy to create a composite overlay with improved fracture resistance and wear resistance compared to a single-phase hardfacing.
Design Principles of Lamellar Overlay Materials
The fundamental principle behind lamellar overlay design is the decoupling of hardness and toughness requirements. A single hardfacing alloy that achieves high hardness (above 600 HV) typically suffers from low fracture toughness, leading to spalling and chipping under impact loading. By introducing a tough interlayer between hard layers, the overlay can absorb impact energy while maintaining the surface hardness needed for wear resistance.
| Layer Type | Composition Strategy | Hardness (HV) | Fracture Toughness |
|---|---|---|---|
| Hard layer | High C + Cr + Mo (carbide-forming) | 600-700 | Low (KIC < 10 MPa·m^0.5) |
| Tough interlayer | Low C + Ni + Mn (austenitic or tempered martensitic) | 250-350 | High (KIC > 20 MPa·m^0.5) |
| Base metal | Standard carbon steel or low-alloy steel | 200-280 | Moderate |
The lamellar structure is achieved through multi-pass welding with alternating filler materials. The hard layer is typically deposited with a high-carbon flux-cored wire or solid wire, while the tough interlayer is deposited with a nickel-based or austenitic stainless steel wire. The layer thickness ratio is typically 2-3 mm for the hard layer and 0.5-1.0 mm for the tough interlayer.
Microstructural Characterization and Wear Mechanisms
The microstructure of the lamellar overlay consists of alternating bands of martensitic matrix with cementite particles (hard layer) and tempered martensite or austenite with fine carbides (tough interlayer). The interface between the hard and tough layers is critical — it must be metallurgically bonded but also act as a crack-arresting barrier.
Under abrasive wear conditions, the wear mechanism differs between the hard and tough layers:
- Hard layer wear: Dominated by micro-ploughing and micro-cutting by hard abrasive particles. The cementite particles in the martensitic matrix resist deformation and are gradually removed through a combination of fracture and plastic deformation.
- Tough interlayer wear: Dominated by adhesive wear and oxidative wear. The austenitic or tempered martensitic structure deforms plastically under load, forming a protective oxide film that reduces further material loss.
The synergistic effect of the lamellar structure is that the hard layers resist initial material removal, while the tough interlayers prevent crack propagation and spalling. When a crack initiates in a hard layer, it is arrested at the interface with the tough interlayer, preventing catastrophic delamination of the overlay.
Welding Process Considerations
The fabrication of lamellar overlays requires careful control of the welding sequence and parameters for each layer type. The following table summarizes the recommended welding parameters:
| Parameter | Hard Layer | Tough Interlayer |
|---|---|---|
| Filler material | High-C flux-cored wire (C > 2.5%) | Ni-based or austenitic wire (C < 0.05%) |
| Welding current (A) | 200-250 | 180-220 |
| Travel speed (mm/min) | 350-450 | 400-500 |
| Interpass temperature (°C) | Below 200 | Below 250 |
| Preheat temperature (°C) | 150-200 | 100-150 |
| Shielding gas | CO2 or Ar+CO2 mix | Ar+CO2 mix (80/20) |
The welding sequence is critical: the first layer deposited on the base metal should be a transition layer (tough type) to minimize dilution-related cracking. Subsequent hard and tough layers are deposited alternately. The final surface layer should be a hard layer to maximize wear resistance.
Defect Analysis and Engineering Challenges
The lamellar overlay concept introduces several unique challenges:
- Interface cracking: The thermal mismatch between hard and tough layers can generate interfacial stresses that lead to cracking. This is mitigated by using compatible thermal expansion coefficients and controlling the interpass temperature.
- Delamination: If the bond strength between layers is insufficient, the overlay may delaminate under cyclic loading. The bond strength must exceed 400 MPa to ensure reliable performance.
- Hardness gradient management: The transition from the hard surface layer to the tough interlayer must be gradual enough to prevent stress concentration. A single-pass hard layer on a tough interlayer is generally acceptable, but multi-pass hard layers should be deposited with decreasing carbon content in the top pass.
- Process complexity: The alternating use of different filler materials increases the fabrication time and cost. However, the improved service life typically justifies the additional expense.
Integration with Engineering Practice
Lamellar overlay materials are particularly valuable in applications where both abrasive wear and impact loading are significant. Examples include:
- Mining conveyors: Chute liners and transfer points where material impact and abrasion are combined.
- Cement kilns: Refractory linings and material handling components subjected to both thermal cycling and mechanical impact.
- Pulp and paper industry: Pump impellers and valve components exposed to abrasive slurry and hydraulic shock.
- Power generation: Coal handling equipment and fly ash handling systems.
The lamellar approach is especially advantageous when the service environment involves periodic impact events — such as material dropping onto a conveyor belt or slurry impact on a pump impeller — because the tough interlayers absorb the impact energy and prevent spalling of the hard surface layer.
Key Reflections and Study Insights
The lamellar overlay concept represents a paradigm shift in wear-resistant cladding design, moving away from the traditional single-phase hardfacing approach toward a composite strategy that explicitly addresses the hardness-toughness trade-off. The key insight is that wear resistance and impact resistance are not inherently contradictory — they can be achieved simultaneously through intelligent microstructural engineering.
The study also highlights an important lesson for engineering practice: the performance of a lamellar overlay is only as good as its weakest interface. Engineers must ensure that every interface between layers is metallurgically sound, free of porosity and inclusions, and properly heat-treated to minimize residual stress. Non-destructive testing (such as ultrasonic testing or magnetic particle testing) should be performed at each interface to verify bond integrity.
Summary
This research demonstrates that lamellar overlay materials — combining hard carbide-forming layers with tough interlayers — offer a superior alternative to single-phase hardfacing for applications involving both abrasive wear and impact loading. The synergistic effect of the layered structure provides wear resistance comparable to high-hardness single-phase overlays while significantly improving fracture toughness and spalling resistance. The key engineering principle is that the interface between layers must be carefully designed and verified to ensure long-term reliability under combined loading conditions.