Zhuojin Pipe Fitting Co., LtdEffect of Lanthanum Oxide on Microstructure and Wear Resistance of Track Surfacing Layer
Literature Overview
Liu Yong, Wang Yajun, Yang Yulin, and Yang Qingxiang published this research in Surface Technology in 2017, investigating the effect of lanthanum oxide (La2O3) addition on the microstructure and wear resistance of surfacing layers for armored vehicle tracks. Funded by the National Natural Science Foundation of China (51471148), this work was conducted jointly by the Inner Mongolia First Machinery Group (China Ordnance Industry Corporation) and the Key Laboratory of Mechanical Structures and Science under Extreme Conditions at Yanshan University.
Research Objective and Material Design
The primary objective was to develop a novel flux-cored wire containing La2O3 for surfacing armored vehicle tracks, aiming to refine the microstructure of the surfacing layer and improve wear resistance. Armored vehicle tracks operate under extreme conditions — high impact loading, abrasive contact with terrain, and cyclic stress — demanding surfacing layers with exceptional wear resistance and fracture toughness.
Microstructural Analysis
The surfacing layer microstructure consists of a complex mixture of phases:
| Phase | Morphology | Function |
|---|---|---|
| Granular bainite | Fine acicular/cellular | Provides strength and toughness balance |
| Martensite | Lath/carbon martensite | High hardness, wear resistance |
| Retained austenite | Matrix surrounding martensite | Toughness, transformation toughening |
| Carbides (Cr7C3, Fe3C) | Dispersed particles | Primary wear resistance mechanism |
The critical finding concerns austenite grain refinement through La2O3 addition:
| Condition | Austenite Grain Size | Wear Resistance (min/g) | Hardness (HRC) |
|---|---|---|---|
| Without La2O3 | 42 μm (average) | 6,000 | ~55-58 |
| With 2.5 wt% La2O3 | 36 μm (minimum) | 12,300 | ~58-62 |
| Improvement | 14.3% reduction | 105% increase | ~5% increase |
Mechanism of Grain Refinement
The study provides a crystallographic explanation for the grain refinement mechanism. The key insight is that La2O3 reacts with aluminum in the weld metal to form lanthanum aluminate (LaAlO3), which serves as a heterogeneous nucleation site for austenite grains.
The two-dimensional lattice misfit between LaAlO3 (001) plane and austenite γ-Fe (100) plane is calculated as 3.81%, which is sufficiently low to provide effective heterogeneous nucleation. This low misfit angle means that the atomic arrangement at the interface closely matches, reducing the energy barrier for nucleation and promoting the formation of numerous small austenite grains rather than fewer large ones.
The refinement mechanism follows this sequence:
- La2O3 dissolves in the molten weld pool
- La2O3 reacts with Al to form LaAlO3 particles
- LaAlO3 particles serve as nucleation sites for austenite
- Increased nucleation density → finer austenite grains
- Finer grains → finer subsequent transformation products (bainite, martensite)
- Finer microstructure → improved wear resistance through Hall-Petch strengthening
Wear Resistance Enhancement
The dramatic improvement in wear resistance (from 6,000 to 12,300 min/g — a 105% increase) can be attributed to multiple mechanisms:
- Hall-Petch effect: Finer grains increase yield strength (σy = σ0 + k·d^(-1/2)), making dislocation movement more difficult and reducing plastic deformation during abrasive contact.
- Higher volume fraction of hard phases: Grain refinement promotes more uniform carbide distribution, increasing the effective volume of hard second-phase particles.
- Reduced microcracking: Smaller grains accommodate stress more uniformly, reducing crack initiation sites.
- Improved deformation resistance: The combination of fine martensite/bainite matrix with dispersed carbides provides superior resistance to both adhesive and abrasive wear mechanisms.
Process Considerations for La2O3 Addition
Practical implementation of La2O3-containing flux-cored wire requires attention to:
| Process Parameter | Consideration |
|---|---|
| Powder metallurgy | La2O3 must be uniformly distributed in the flux core |
| Wire stability | High-density La2O3 may affect wire geometry during drawing |
| Arc stability | Rare earth oxides can affect arc characteristics and spatter |
| Deposition efficiency | La2O3 inclusions may slightly reduce deposition efficiency |
| Welding parameters | May require slight adjustment for optimal arc performance |
The optimal La2O3 content of 2.5 wt% represents a balance between grain refinement benefit and potential adverse effects on weldability and mechanical properties. Excessive rare earth addition can promote:
- Brittle intermetallic formation
- Excessive grain boundary segregation
- Degraded impact toughness
- Increased susceptibility to hot cracking
Engineering Application and Study Insights
This research has direct relevance to military and heavy industrial applications where track surfacing life is critical. The 105% improvement in wear resistance translates to approximately double the service life between track replacement intervals, with significant implications for operational readiness and lifecycle cost.
The study exemplifies the power of rare earth addition as a microstructure refinement tool in welding applications. The crystallographic analysis providing the mechanistic basis for grain refinement — specifically the 3.81% lattice misfit between LaAlO3 and austenite — offers a scientific foundation that can guide further optimization. Future work could explore:
- Optimal La2O3 particle size distribution for maximum nucleation efficiency
- Combined addition of multiple rare earth elements for synergistic effects
- Long-term wear testing under actual track service conditions
- Effect of heat treatment on the refined microstructure
- Fracture toughness characterization to ensure adequate toughness is maintained
The engineering lesson from this work is that microstructure control through thermodynamic and crystallographic understanding — rather than empirical trial and error — provides a reliable pathway to achieving targeted property improvements in surfacing applications. The quantification of lattice misfit as a predictor of nucleation effectiveness offers a methodology applicable to other grain-refining additions in welding processes.