Effect of Lanthanum Oxide on Microstructure and Wear Resistance of Tracked Vehicle Surfacing Deposits

Literature Overview

The paper by Liu Yong, Wang Yajun, Yang Yulin, and Yang Qingxiang, published in Surface Technology (2017, Vol. 46, No. 3, pp. 113–118), presents a systematic investigation into the effect of lanthanum oxide (La₂O₃) addition on the microstructure and wear resistance of surfacing deposits designed for armored vehicle tracks. The research was conducted jointly by China North Industries Group Corporation No. 1 Machinery Group Co., Ltd. and Yanshan University, and was supported by the National Natural Science Foundation of China (Grant No. 51471148). The study develops a novel flux-cored wire containing varying amounts of La₂O₃ and evaluates its effect on grain refinement and wear performance through metallographic examination, hardness testing, and wear testing.

Core Technical Content

The fundamental metallurgical challenge in armored vehicle track surfacing is achieving a microstructure that combines high hardness for wear resistance with adequate toughness to resist spalling under impact loading. The authors address this challenge by introducing La₂O₃ as a grain refiner in the flux-cored wire, leveraging the well-established principle of heterogeneous nucleation to refine the primary austenite grain size.

The key experimental findings are presented below:

The microstructure of the surfacing deposit consists of granular bainite, martensite, and retained austenite. The addition of 2.5 wt% La₂O₃ reduced the average austenite grain size from 42 μm to 36 μm and more than doubled the wear resistance from 6000 min/g to 12300 min/g. Beyond the optimal content of 2.5 wt%, further addition of La₂O₃ led to coarsening of the grain structure and degradation of wear resistance, indicating a clear optimum in the additive content.

Metallurgical Mechanism Analysis

The mechanism by which La₂O₃ refines the austenite grain is explained through the concept of heterogeneous nucleation. During solidification, the La₂O₃ reacts with aluminum in the molten metal to form lanthanum aluminate (LaAlO₃) particles. These LaAlO₃ particles serve as effective heterogeneous nucleation sites for the austenite phase (γ-Fe).

The authors calculate the two-dimensional lattice mismatch between the (001) plane of LaAlO₃ and the (100) plane of austenite to be 3.81%. This low mismatch value indicates a favorable crystallographic relationship for epitaxial nucleation, meaning that the LaAlO₃ particles provide energetically favorable sites for austenite nuclei to form. The result is a significant reduction in the nucleation barrier and a corresponding refinement of the grain structure.

This mechanism is consistent with the well-documented grain refining effect of rare earth elements in steel, where rare earth oxides react with impurities such as sulfur and oxygen to form fine, stable particles that act as nucleation sites. The specific advantage of La₂O₃ in this application is its ability to form LaAlO₃ with a lattice structure that is particularly compatible with the austenite phase.

Engineering Practice Considerations

For engineers designing surfacing consumables for heavy-duty applications such as armored vehicle tracks, this study provides several actionable insights:

1. Optimal additive content — The 2.5 wt% La₂O₃ content represents an optimum. Exceeding this value leads to particle agglomeration and reduced nucleation efficiency, which degrades the grain refining effect. Process control must ensure uniform distribution of La₂O₃ in the flux.
2. Microstructure-property relationship — The refinement of austenite grain size directly translates to improved wear resistance through the Hall-Petch relationship, where smaller grains increase both hardness and toughness. The granular bainite and martensite phases in the microstructure contribute to the overall mechanical performance.
3. Wear testing methodology — The wear resistance is measured in min/g units, which represents the time required for a unit mass of material to be removed under standardized abrasion conditions. This metric is directly relevant to field performance prediction for track applications.

A PDCA cycle applied to this type of consumable development would proceed as follows: Plan the La₂O₃ content based on nucleation theory, Do the welding trials and microstructural characterization, Check the wear resistance and hardness data against design targets, and Act by adjusting the additive content or flux composition for the next iteration.

Key Reflections and Study Insights

The work by Liu Yong and colleagues demonstrates the power of rare earth addition in tailoring the microstructure of surfacing deposits for specific applications. The clear correlation between grain size, additive content, and wear performance provides a rational basis for consumable design that goes beyond empirical trial and error.

However, several questions remain for future investigation. The long-term stability of the refined microstructure under cyclic loading conditions typical of track operation is not addressed. The effect of La₂O₃ on hydrogen embrittlement susceptibility, which is a concern in high-strength martensitic microstructures, also warrants further study. Additionally, the cost of La₂O₃ addition relative to the performance improvement should be evaluated for industrial-scale implementation.

This study represents a significant contribution to the field of rare earth-modified surfacing consumables and provides engineers with a scientifically grounded approach to improving the wear resistance of heavy-duty surfacing deposits.