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

Literature Overview

This 2017 study published by researchers from the Yanshan University Extreme Condition Mechanical Structure and Science National Defense Key Discipline Laboratory, in collaboration with the Inner Mongolia First Machinery Group of China Ordnance Industries Corporation, investigates the role of lanthanum oxide (La2O3) as a micro-additive in modifying the microstructure and wear resistance of weld overlay layers applied to crawler tracks. The research was supported by the National Natural Science Foundation of China (Grant No. 51471148). Crawler tracks are subjected to severe abrasive and adhesive wear conditions in mining and construction equipment, making the development of high-performance overlay coatings a critical engineering challenge. The addition of rare earth oxides to welding consumables is a well-established strategy in Chinese welding research, yet systematic studies on La2O3 in the specific context of crawler track overlay remain limited. This work fills that gap by examining the influence of La2O3 content on carbide morphology, matrix microstructure, hardness distribution, and tribological performance.

Core Technical Content

Role of La2O3 in Weld Pool Metallurgy

Lanthanum oxide functions as a multi-purpose modifier in the welding pool. Its primary effects include:

• Deoxidation and slag refinement: La2O3 acts as a potent deoxidizer, reducing the oxygen content in the weld metal and thereby decreasing the number of nucleation sites for coarse carbide formation.
• Grain refinement: Lanthanum ions adsorb preferentially at austenite grain boundaries, inhibiting grain growth during solidification and promoting a finer dendritic structure.
• Carbide modification: La2O3 alters the morphology and distribution of carbides (primarily Cr7C3 and Cr23C6 in high-chromium alloys, or M6C and MC in medium-carbon alloys), promoting more uniform dispersion and reducing the formation of large, brittle carbide clusters that serve as crack initiation sites.
• Inclusion control: The rare earth element modifies the shape and distribution of oxide inclusions, converting elongated, continuous inclusions into discrete, spherical particles that have a less detrimental effect on fatigue and wear properties.

Microstructure Evolution

The study examines how varying La2O3 addition levels (typically ranging from 0.1% to 1.5% by mass in the flux or wire coating) influence the resulting microstructure. At optimal addition levels (commonly around 0.3% to 0.5% La2O3), the following microstructural features are observed:

Wear Mechanism Analysis

The wear resistance improvement is attributed to the combined effects of matrix hardening (due to finer grain size and solid solution strengthening by dissolved rare earth atoms), enhanced carbide dispersion (providing more effective resistance to abrasive particles), and improved interfacial bonding between carbides and the matrix (reducing carbide pull-out during sliding contact). The transition from adhesive-dominant wear to abrasive-dominant wear with increasing La2O3 content is a key finding, as abrasive wear mechanisms generally produce lower volumetric wear rates under the same loading conditions.

Standards and Process Considerations

The research is situated within the framework of Chinese welding standards for overlay welding, particularly NB/T 47014 (qualification of welding procedures) and relevant GB/T standards for surface engineering. The welding process employed is typically shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW), which are the most common processes for field application on crawler tracks. The following process parameters are critical:

• Heat input: Maintained in the range of 8-15 kJ/mm to ensure adequate dilution control while avoiding excessive dilution from the base metal that would reduce the effectiveness of the La2O3 modification.
• Layer thickness: Typically 3-5 mm per pass, with total overlay thickness of 8-12 mm for crawler track applications.
• Interpass temperature: Controlled below 200°C to minimize grain coarsening in previously deposited layers.
• Preheating: Base metal preheated to 150-250°C depending on thickness to reduce hydrogen-induced cracking susceptibility.

Engineering Practice Integration

From a practical standpoint, the findings have direct implications for the maintenance and refurbishment of crawler track assemblies in mining trucks, bulldozers, and excavators. The traditional approach of simply increasing chromium or carbon content to improve wear resistance often leads to excessive brittleness and poor weldability. The La2O3 modification strategy offers a more balanced approach: it achieves significant wear resistance improvement (40-55% over baseline) while maintaining acceptable toughness and weldability. This is particularly important because crawler tracks undergo cyclic loading and impact during operation, and a brittle overlay layer would be prone to spalling and delamination.

The optimal La2O3 content of 0.3-0.5% represents a practical window that can be implemented in flux formulation or wire coating design without requiring specialized rare earth supply chains. The cost increase is minimal compared to the extended service life achieved, making this an economically attractive solution for equipment manufacturers and maintenance contractors.

Key Questions and Reflections

Several questions arise from this research that merit further investigation. First, the long-term stability of the microstructure under thermal cycling conditions (as experienced in high-temperature operating environments) has not been fully characterized. Second, the interaction between La2O3 and other alloying elements (such as molybdenum or vanadium) in multi-alloy systems remains unclear. Third, the transition from laboratory-scale testing to full-scale component qualification requires careful attention to process consistency, as La2O3 additions are sensitive to flux mixing uniformity and arc stability. Engineers implementing this technology should establish rigorous incoming inspection protocols for fluxes containing rare earth additives and perform periodic metallographic verification on production welds.

Study Insights and Implications

This study exemplifies the power of micro-additive metallurgy in surface engineering. The addition of less than 1% La2O3 can produce disproportionate improvements in wear performance, demonstrating that microstructural engineering at the inclusion and grain boundary level is often more effective than bulk composition changes. For practitioners in the cladding and overlay field, this reinforces the principle that rare earth modification should be considered a standard tool in the design of high-performance welding consumables, particularly for applications involving severe abrasive or erosive wear. The methodology of systematic La2O3 content variation, combined with metallographic analysis and standardized wear testing, provides a replicable framework that can be adapted to other alloy systems and overlay applications. Future work should focus on combining La2O3 modification with advanced welding processes such as plasma transferred arc (PTA) or laser cladding to achieve even finer microstructural control and higher dilution ratios.