Effect of Lanthanum Oxide on Microstructure and Wear Resistance of Track Cladding Layer Alloy

Literature Overview

The research by Liu Yong and colleagues from the Inner Mongolia First Machinery Group and Yanshan University (2017) examines the influence of lanthanum oxide (La2O3) addition on the microstructure and wear resistance of alloy cladding layers applied to track surfaces. This work was supported by the National Natural Science Foundation of China (Grant No. 51471148) and published in the journal "Surface Technology." Track cladding is a critical application in heavy machinery and mining equipment where abrasive wear is the dominant failure mechanism.

Core Technical Content

Track surfaces in mining and construction equipment are subjected to severe abrasive wear from soil, rock, and debris. Conventional hardfacing alloys often suffer from insufficient toughness or inadequate wear resistance under high-impact conditions. The addition of rare earth elements, particularly lanthanum oxide, has been explored as a microstructure-refining agent that can enhance both hardness and fracture resistance simultaneously.

Chemical Composition of Cladding Alloys

The base alloy composition is typical of high-carbon, high-chromium martensitic hardfacing alloys. The La2O3 was added in varying amounts (0.3%, 0.5%, and 0.8%) to investigate the optimal addition level.

Microstructural Evolution

Metallographic analysis revealed that La2O3 addition significantly refined the grain structure of the cladding layer. In the base alloy without La2O3, coarse carbide networks and relatively large martensitic packets were observed. With the addition of 0.5% La2O3, the microstructure exhibited:

• Finer martensitic laths with reduced packet size
• More uniformly distributed carbide particles
• Reduced carbide network continuity at grain boundaries
• Increased volume fraction of retained austenite (approximately 8–12%)

The mechanism by which La2O3 achieves this refinement is attributed to two primary effects. First, lanthanum oxide acts as a heterogeneous nucleation site during solidification, promoting the formation of more nuclei and thereby reducing grain size. Second, La3+ ions adsorb at the grain boundaries, inhibiting grain growth during the subsequent cooling and tempering stages.

Wear Testing Results

The wear resistance index is defined as the ratio of base alloy wear volume loss to modified alloy wear volume loss. The optimal La2O3 addition was found to be 0.5%, which provided the highest hardness (785 HV) and the lowest wear volume loss. Beyond 0.5%, excessive La2O3 addition led to the formation of large La2O3 particles and rare earth-rich phases that acted as stress concentrators and reduced the overall toughness of the cladding layer.

Engineering Practice Integration

The findings have direct implications for the manufacturing of track cladding layers in mining equipment, earthmoving machinery, and heavy-duty conveyor systems. Several practical considerations emerge:

1. Powder preparation: La2O3 must be thoroughly homogenized in the cladding powder. Inconsistent powder mixing can lead to localized segregation and non-uniform properties.
2. Welding process selection: The study employed submerged arc welding (SAW) overlay, which provides excellent dilution control and high deposition rates. For repair applications where access is limited, GTAW or PTA may be more suitable alternatives.
3. Heat treatment: Post-weld tempering at 550–600 °C for 2 hours was found to be optimal for balancing hardness and toughness. Over-tempering reduces hardness significantly, while under-tempering leaves residual stresses that promote cracking.
4. Layer thickness: For track applications, a minimum cladding thickness of 4–6 mm is recommended to ensure adequate wear life while maintaining structural integrity of the base material.

Key Questions and Reflections

One important question that arises from this study is the long-term stability of La2O3-modified cladding layers under thermal cycling conditions. Mining equipment often experiences repeated heating and cooling cycles that can cause microstructural changes over time. Whether the beneficial effects of La2O3 addition persist after extended service remains an open question.

Another consideration is the cost-benefit analysis. La2O3 is a relatively inexpensive rare earth oxide, and the 0.5% addition represents a minimal material cost increase. However, the powder preparation process must be carefully controlled to ensure uniform distribution, which may require additional process steps and quality assurance measures.

From a standards perspective, the addition of rare earth elements to hardfacing alloys is not yet covered by existing international standards such as AWS A5.23 or ISO 3677. Engineers must therefore rely on internal specifications and qualified welding procedure specifications (WPS) for production applications.

Summary

This research demonstrates that the addition of 0.5% La2O3 to high-carbon, high-chromium hardfacing alloys produces a significant improvement in both microstructure refinement and abrasive wear resistance. The optimal addition level represents a balance between grain refinement benefits and the risk of rare earth phase segregation. For engineers designing cladding solutions for track surfaces and similar high-wear applications, this study provides valuable guidance on alloy design and process optimization. The practical implementation of La2O3-modified cladding alloys should be accompanied by rigorous qualification testing and adherence to established welding quality assurance practices to ensure consistent performance in field service.