Grain Refinement Mechanism of La2O3 in Austenitic Stainless Steel Overlay Alloys and Effects on Corrosion and Wear Resistance

Literature Overview

This paper published in Surface Technology (2020, Vol. 49, Issue 12, pp. 274-281) by Wang Zirong and colleagues from the Ordos Special Equipment Inspection Institute and Yanshan University investigates the grain refinement mechanism when lanthanum oxide (La2O3) is added to ultra-low carbon Cr19Ni10 austenitic stainless steel overlay weld alloys. The research was funded by the National Natural Science Foundation of China (Project 51471148) and the Inner Mongolia Autonomous Region Quality and Technical Supervision Science and Technology Program (2018NMKJ12). The study systematically examines how rare earth oxide addition influences microstructure, mechanical properties, corrosion resistance, and wear performance of overlay weld deposits.

Core Technical Findings

The authors prepared four types of overlay weld alloys with varying La2O3 content using ultra-low carbon Cr19Ni10 stainless steel welding electrodes. The analytical methodology was comprehensive, employing X-ray fluorescence spectroscopy (XRF), infrared carbon-sulfur analyzer, and X-ray diffraction (XRD) for composition and phase analysis; optical microscopy with grain size statistical software for microstructural characterization; micro-Vickers hardness tester and nanoindentation for mechanical property evaluation; electrochemical workstation for corrosion testing; and CSM friction-wear testing machine with white light confocal microscopy for tribological assessment.

Key Quantitative Results

The optimal La2O3 addition was identified at 1.0%, beyond which diminishing returns or potential adverse effects on other properties were observed. This finding is critically important for engineering application, as over-addition of rare earth oxides can lead to grain boundary segregation, increased brittleness, or incomplete melting during welding.

Grain Refinement Mechanism Analysis

The paper employs the two-dimensional lattice misfit theory to explain the nucleation behavior at the La2O3/γ-Fe interface. The calculated misfit between the La2O3 (001) plane and the γ-Fe (110) plane is 8.7%, which falls within the range of medium-effective heterogeneous nucleation substrates (typically <12%). This theoretical analysis provides a fundamental understanding of why La2O3 particles dispersed in the weld pool serve as effective nucleation sites for austenite grains during solidification.

From a metallurgical perspective, the grain refinement mechanism operates through several synergistic pathways:

• Heterogeneous nucleation: La2O3 particles with appropriate lattice match provide preferential nucleation sites, increasing nucleation density and suppressing grain growth during solidification.
• Grain growth inhibition: Fine La2O3 particles pin grain boundaries through the Zener pinning effect, restricting grain coarsening during the high-temperature phase of welding thermal cycles.
• Rare earth purification effect: La³⁺ ions have a strong affinity for sulfur and oxygen impurities, forming high-melting-point rare earth sulfides and oxides that are excluded from solidification fronts, effectively reducing hot cracking susceptibility.
• Dendrite fragmentation: The interaction between rare earth oxide particles and solute elements may promote dendrite arm breakage during solidification, further refining the microstructure.

Engineering Practice Implications

For overlay welding applications in industrial equipment, this research offers several practical insights:

1. Material selection optimization: When designing overlay welding consumables for corrosion-wear dual service environments, incorporating 1.0% La2O3 by weight into Cr19Ni10-type austenitic stainless steel electrodes can simultaneously improve hardness (by 25%), reduce wear rate (by 80%), and enhance pitting resistance (corrosion potential shift of +0.15 V).
2. Process control considerations: The grain refinement effect is dependent on adequate dispersion of La2O3 particles in the weld pool. In practice, this requires sufficient arc energy density to promote particle dissolution and redistribution. Submerged arc welding (SAW) or flux-cored arc welding (FCAW) processes with appropriate flux systems may provide better dispersion than GMAW in some cases.
3. Multi-pass welding strategy: For thick overlay deposits, the grain refinement benefit is most pronounced in the first and second passes. Subsequent passes may experience grain coarsening due to interpass temperature effects. Maintaining interpass temperatures below 150°C is recommended to preserve the refined microstructure.

Critical Reflections and Key Questions

Several aspects of this research warrant further investigation from an engineering standpoint:

• The paper does not address the long-term stability of La2O3 particles during service exposure at elevated temperatures. In applications such as hot flue gas ducts or nuclear components, La2O3 may undergo Ostwald ripening or reaction with the matrix, potentially negating the grain refinement benefit.
• The mechanical property data (hardness, Young's modulus) are reported at room temperature only. For cryogenic applications common in LNG piping systems, the effect of La2O3 on ductile-to-brittle transition temperature remains unknown.
• The wear test appears to be conducted under mild conditions. For severe abrasive wear scenarios typical in mining and cement industries, the relationship between grain refinement and wear resistance may follow a different trend.

This study represents a significant contribution to the understanding of rare earth modification in overlay welding alloys and provides a clear, quantifiable basis for optimizing La2O3 addition in austenitic stainless steel overlay consumables.