Effect of Lanthanum Oxide on Microstructure and Wear Resistance of Crawler Overlay Alloy

Research Background and Motivation

This study examines the influence of lanthanum oxide (La₂O₃) addition on the microstructure and wear resistance of overlay alloys deposited for crawler track applications. Crawler tracks and similar heavy-duty earthmoving components are subjected to severe abrasive wear, impact loading, and cyclic fatigue, requiring overlay materials that combine high hardness, adequate toughness, and resistance to spalling. The addition of rare earth elements, particularly lanthanum oxide, has emerged as a promising approach to modify the solidification behavior, refine the microstructure, and enhance the mechanical performance of iron-based overlay alloys. The study provides valuable data on the optimal La₂O₃ addition level and the resulting metallurgical changes that govern wear behavior.

Alloy Design and Processing Parameters

The overlay alloy system investigated is based on a high-carbon, chromium-bearing iron matrix, which is typical for wear-resistant overlay applications. Lanthanum oxide is introduced as a deoxidizer and microstructure modifier through the addition of La₂O₃-containing flux or as a direct powder addition to the wire or electrode coating. The processing involves open-arc submerged arc welding or flux-cored arc welding with varying La₂O₃ concentrations.

The study employs a systematic variation of La₂O₃ content to identify the optimal addition level that maximizes wear resistance without introducing detrimental effects such as excessive brittleness or reduced ductility. The dilution ratio between the overlay and the substrate is controlled to maintain consistent metallurgical conditions across all test specimens.

Microstructural Evolution with La₂O₃ Addition

The baseline overlay alloy without La₂O₃ exhibits a typical hypereutectic cast iron microstructure consisting of primary carbides (predominantly MC type, where M represents Cr and Fe) embedded in a martensitic matrix with retained austenite. The primary carbides are relatively coarse, ranging from 15 to 30 micrometers in size, and are distributed in a non-uniform pattern with local clustering.

The introduction of La₂O₃ at 0.05 to 0.20 wt% produces a progressive refinement of the carbide morphology and distribution. At the optimal addition level of 0.10 wt%, the primary carbide size is reduced to 6–12 micrometers, and the distribution becomes significantly more uniform. The La₂O₃ acts as a heterogeneous nucleation site during solidification, providing additional nucleation surfaces for both carbide and ferrite formation. This mechanism is consistent with the well-established role of rare earth oxides as grain refiners in iron and steel systems.

The matrix microstructure also undergoes significant changes. The volume fraction of retained austenite increases from approximately 15% in the unmodified alloy to 22–25% at the optimal La₂O₃ level. This increase in retained austenite is attributed to the microalloying effect of lanthanum, which stabilizes the austenite phase by reducing the equilibrium temperature for austenite-to-ferrite transformation. The retained austenite contributes to wear resistance through the transformation-induced plasticity (TRIP) mechanism, where mechanical stress during wear contact causes localized austenite-to-martensite transformation, generating work hardening and crack deflection.

The data clearly demonstrate that 0.10 wt% La₂O₃ represents the optimal addition level, providing the finest carbide distribution, highest retained austenite content, maximum hardness, and lowest wear volume loss. Beyond this level, the benefits diminish due to potential La₂O₃ clustering and reduced effectiveness as a nucleation agent.

Wear Mechanism Analysis

The wear behavior of the overlay is governed by a combination of abrasive wear, adhesive wear, and microploughing mechanisms. In the unmodified alloy, the coarse carbides provide initial resistance to abrasive penetration, but the large inter-carbide spacing allows plastic deformation of the softer matrix between carbides, leading to material removal through microploughing. The non-uniform carbide distribution also creates stress concentration points that promote microcracking and spalling.

With La₂O₃ modification, the refined and uniformly distributed carbides create a more effective barrier against abrasive particle penetration. The increased retained austenite provides additional wear resistance through the TRIP mechanism, as discussed above. The combined effect results in a transition from matrix-dominated wear to carbide-dominated wear, where the hard carbide phase bears the majority of the abrasive load while the tough matrix provides support and crack resistance.

Energy-dispersive X-ray spectroscopy (EDS) mapping of the worn surfaces confirms that the wear-resistant phase in the La₂O₃-modified overlay is predominantly Cr-rich carbide, with minimal matrix material removal. This is in contrast to the unmodified alloy, where significant matrix material is removed between the carbide particles.

Engineering Application and Quality Control

For practical application in crawler track overlay, the La₂O₃-modified alloy offers a wear life improvement of approximately 2.0 to 2.5 times compared to the unmodified alloy under equivalent service conditions. This translates to reduced maintenance intervals, lower total cost of ownership, and improved equipment availability for mining and construction operations.

Quality control for La₂O₃-modified overlay fabrication requires attention to several factors. The La₂O₃ addition must be precisely controlled during wire or electrode manufacturing to ensure consistent composition. Post-overlay hardness testing should be performed at regular intervals to verify that the hardness exceeds 750 HV0.3, which is the target specification for optimal wear performance. Dilution testing should be conducted to ensure that the dilution ratio does not exceed 30%, as excessive dilution with the carbon steel substrate will reduce the carbide volume fraction and compromise wear resistance.

Study Insights and Conclusions

The research demonstrates that rare earth modification through La₂O₃ addition is a highly effective approach for enhancing the wear resistance of iron-based overlay alloys without requiring changes to the welding process or substrate preparation. The mechanism is well understood: La₂O₃ acts as a nucleation site for carbide formation, refines the carbide size and distribution, and stabilizes retained austenite through microalloying effects. The optimal addition level of 0.10 wt% represents a practical and achievable target for industrial production. This finding has broad implications for the design of wear-resistant overlay materials, suggesting that systematic rare earth modification could be applied to other iron-based overlay systems, including those used in mining equipment, cement kiln linings, and power plant coal handling systems. The key challenge for industrial implementation is ensuring consistent La₂O₃ distribution in the filler metal, which requires careful control of the wire or electrode manufacturing process.