CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Lanthanum Oxide on Microstructure and Wear Resistance of Track Overlay Layer Alloy
Research Background and Technical Significance
Wear-resistant track overlay alloys are critical components in heavy machinery including bulldozers, excavators, mining trucks, and construction equipment where the undercarriage tracks are subjected to severe abrasion, impact, and adhesive wear conditions. The wear life of track overlay layers directly determines the operational availability and maintenance cost of these machines. This study investigates the effect of lanthanum oxide (La2O3) addition on the microstructure, mechanical properties, and wear resistance of track overlay alloys, exploring the rare earth modification approach to enhance the performance of conventional high-chromium white iron and martensitic steel overlay compositions.
Lanthanum, as a rare earth element, has unique properties including large atomic radius, high melting point, and strong affinity for oxygen and sulfur. When added to weld overlay alloys in the form of La2O3, it acts as a grain refiner, inclusion modifier, and microstructure stabilizer. The study examines how these effects translate into improved wear resistance and extended service life of track overlay layers under actual operating conditions.
Microstructural Effects of La2O3 Addition
The study investigates La2O3 addition levels ranging from 0% to 1.5 wt% in track overlay alloys based on high-carbon martensitic steel and high-chromium white iron compositions. The following table summarizes the microstructural characteristics at different La2O3 addition levels.
| La2O3 Addition (wt%) | Matrix Microstructure | Carbide Size (μm) | Carbide Distribution | Grain Size (μm) | Inclusion Content |
|---|---|---|---|---|---|
| 0.0 | Coarse martensite | 15–25 | Irregular, clustered | 60–80 | High (MnS, Al2O3) |
| 0.3 | Fine martensite + retained austenite | 8–15 | Semi-uniform | 35–50 | Moderate |
| 0.6 | Fine martensite + high retained austenite | 5–10 | Uniform | 20–35 | Low |
| 1.0 | Fine martensite + high retained austenite | 4–8 | Uniform, dispersed | 15–25 | Very low |
| 1.5 | Excess retained austenite | 3–6 | Over-dispersed | 12–20 | Very low |
The results demonstrate that La2O3 addition of 0.6–1.0 wt% provides the optimal microstructural refinement, reducing the average grain size by more than 50% compared to the baseline alloy without rare earth addition. The carbide size reduction from 15–25 μm to 4–8 μm is particularly significant for wear resistance, as finer carbides provide more uniform resistance to abrasive wear without creating stress concentration sites that could initiate crack propagation.
Rare Earth Inclusion Modification
The La2O3 addition fundamentally changes the inclusion morphology in the overlay alloy. Without rare earth addition, the inclusions consist primarily of elongated MnS particles and irregular Al2O3 particles that act as crack initiation sites. With La2O3 addition, the inclusions are modified to spherical La2O2S and La2O3 particles with significantly reduced size and improved distribution.
| Inclusion Type | Without La2O3 | With 0.6% La2O3 | With 1.0% La2O3 |
|---|---|---|---|
| MnS | Elongated, 5–20 μm | Spherical, 1–3 μm | Spherical, 0.5–2 μm |
| Al2O3 | Angular, 3–15 μm | Spherical, 1–5 μm | Spherical, 0.5–3 μm |
| La2O2S | Absent | Spherical, 0.5–2 μm | Spherical, 0.5–2 μm |
| Total Inclusion Area % | 0.8–1.5% | 0.3–0.6% | 0.2–0.4% |
The modification of elongated MnS inclusions to spherical rare earth sulfide inclusions is the most significant metallurgical effect of La2O3 addition. This modification eliminates the stress concentration effect of elongated inclusions and prevents the initiation of transverse cracks that typically limit the fatigue life of martensitic overlay layers.
Wear Resistance Performance
The wear resistance of the track overlay layers was evaluated through dry sliding wear tests, abrasive wear tests (ASTM G65), and field trials on operating equipment. The following table presents the comparative wear resistance results.
| La2O3 Addition (wt%) | Hardness (HRC) | Dry Sliding Wear Rate (mm³/N·m) | Abrasive Wear Rate (mg/1000 cycles) | Field Life Improvement |
|---|---|---|---|---|
| 0.0 | 52–55 | 8.5×10⁻⁷ | 12.5 | Baseline |
| 0.3 | 55–58 | 6.2×10⁻⁷ | 9.8 | +25% |
| 0.6 | 58–61 | 4.5×10⁻⁷ | 7.2 | +55% |
| 1.0 | 60–63 | 3.8×10⁻⁷ | 6.1 | +70% |
| 1.5 | 58–60 | 5.0×10⁻⁷ | 8.0 | +40% |
The results show a clear optimum at 0.6–1.0 wt% La2O3 addition, where the combination of maximum hardness (60–63 HRC), minimum wear rate, and maximum field life improvement is achieved. Beyond 1.0 wt%, the excessive retained austenite content reduces the effective hardness and wear resistance, demonstrating that more rare earth addition is not always beneficial.
The mechanism of wear resistance improvement is attributed to three synergistic effects: (1) the refined grain structure increases the number of grain boundaries that impede dislocation motion and crack propagation; (2) the finer, more uniformly distributed carbides provide more effective resistance to abrasive wear; and (3) the modified inclusions reduce the likelihood of crack initiation and propagation under cyclic loading.
Engineering Application and Process Considerations
For practical implementation of La2O3-modified track overlay alloys, several process considerations must be addressed. The La2O3 powder must be thoroughly mixed with the base alloy powder to ensure uniform distribution, as segregation of rare earth oxide can lead to local compositional variations. The study recommends a high-energy ball milling process for powder preparation, with a milling time of 4–8 hours to achieve homogeneous La2O3 dispersion.
The welding process parameters must also be optimized for La2O3-containing alloys. The study found that slightly higher heat input (10–15% increase in welding current) is required to compensate for the increased melting point of the rare earth-modified alloy, ensuring complete melting and proper bonding of the overlay layer to the substrate.
Conclusions and Practical Implications
The study demonstrates that La2O3 addition of 0.6–1.0 wt% significantly improves the microstructure and wear resistance of track overlay alloys through grain refinement, carbide size reduction, and inclusion modification. The optimal addition level of 0.6–1.0 wt% provides a 55–70% improvement in field service life compared to conventional track overlay alloys, representing a substantial economic benefit for equipment operators. The key engineering insight is that rare earth modification is not a simple additive effect but involves complex interactions between the rare earth oxide, the alloy composition, and the solidification conditions that must be carefully controlled to achieve the desired microstructural and performance outcomes. Engineers specifying track overlay alloys for heavy machinery applications should consider La2O3 modification as a proven and effective approach to extending wear life, with the specific addition level and process parameters validated through qualification testing for each application.