Grain Refinement Mechanism of La2O3-Added Austenitic Stainless Steel Weld Overlay Alloys and Its Effects on Corrosion and Wear Performance

Literature Overview

This study, published in 2020 by Wang Zirong and colleagues from the Ordos Special Equipment Inspection Institute, Yanshan University, and Liverpool John Moores University, investigates the grain refinement mechanism introduced by lanthanum oxide (La2O3) addition to austenitic stainless steel weld overlay alloys. The research was supported by the National Natural Science Foundation of China (Grant No. 51471148) and the Inner Mongolia Autonomous Region Quality and Technical Supervision Technology Institution Science and Technology Plan Project (2018NMKJ12). The work falls under the domain of surface engineering and addresses a critical challenge in the overlay welding of austenitic stainless steels, where controlling grain structure directly determines the service performance in corrosive and abrasive environments.

Core Technical Content and Grain Refinement Mechanism

The fundamental challenge in weld overlay cladding of austenitic stainless steels lies in the tendency toward coarse columnar grain formation during solidification. Coarse grains degrade both corrosion resistance and wear performance, particularly in environments where intergranular corrosion and abrasive wear are dominant failure modes. The introduction of La2O3 as a micro-alloying addition serves as a heterogeneous nucleation agent during solidification.

Nucleation and Grain Refinement Mechanism

La2O3 particles, with a melting point of approximately 2,470 °C, remain solid throughout the welding solidification process. These particles provide heterogeneous nucleation sites that reduce the critical nucleation undercooling. The grain refinement mechanism can be understood through the following sequence:

1. La2O3 particles dissolve partially or act as interfacial substrates during the melt pool stage.
2. The low interfacial energy between La2O3 and austenite promotes epitaxial nucleation.
3. The increased nucleation rate relative to the growth rate results in equiaxed grain formation, replacing the typical columnar structure.

The refinement effect is further enhanced by the interaction between La2O3 and sulfur in the base material. La has a strong affinity for sulfur, forming La2S3 inclusions that reduce the interfacial energy at the solid-liquid boundary. This dual mechanism — direct nucleation and sulfur scavenging — produces a synergistic grain refinement effect.

Effects on Corrosion Resistance

The grain refinement produced by La2O3 addition significantly improves the corrosion performance of the overlay layer. Finer grains reduce the number of grain boundaries per unit area that are susceptible to intergranular corrosion, while simultaneously reducing the width of the sensitized zone along grain boundaries. In pitting corrosion tests conducted in 3.5% NaCl solution, the La2O3-modified overlay exhibited higher pitting resistance equivalent number (PREN) values compared to the unmodified counterpart. The refined microstructure also reduces the density of chromium-depleted regions at grain boundaries, which are the primary initiation sites for intergranular corrosion in austenitic stainless steels.

Effects on Wear Performance

The wear performance improvement is attributed to multiple factors. The refined grain structure increases hardness through the Hall-Petch relationship, where yield strength increases as grain size decreases. Additionally, the uniform distribution of secondary phases such as Cr7C3 and Cr23C6 carbides within the refined matrix provides better load-bearing capacity and resistance to abrasive wear. In dry sliding wear tests against SiC paper, the La2O3-modified overlay demonstrated a wear rate reduction of approximately 35–45% compared to the baseline material.

Engineering Practice Integration and Standards Considerations

From a practical standpoint, the La2O3 modification technique is most applicable to submerged arc welding (SAW) and gas metal arc welding (GMAW) overlay processes where precise control of filler metal composition is achievable. The addition level of La2O3 is typically in the range of 0.05–0.20 wt%, which is within the practical limits for consumable manufacturing. The technique is particularly relevant for pressure vessel components fabricated in accordance with NB/T 47002 and GB/T 150, where the overlay layer must satisfy both bonding strength requirements and corrosion resistance criteria.

For quality assurance purposes, the refined microstructure must be verified through metallographic examination in accordance with JB/T 4730. The grain size measurement should be conducted using the ASTM E112 linear intercept method, and the overlay layer thickness should be verified by ultrasonic testing (UT) or radiographic testing (RT) to ensure adequate coverage.

Key Reflections and Study Insights

The most significant insight from this research is the demonstration that rare earth element addition, specifically La2O3, offers a cost-effective and process-compatible route to microstructure control in weld overlay alloys. Unlike more aggressive grain refinement strategies that require rapid cooling or mechanical vibration, the La2O3 approach operates within conventional welding parameters, making it readily adoptable in industrial settings. However, engineers should be mindful of the potential for rare earth segregation at grain boundaries, which could affect long-term thermal stability at elevated service temperatures above 600 °C.

This research bridges the gap between fundamental metallurgical understanding and practical surface engineering application. The La2O3 modification technique represents a promising avenue for enhancing the performance of austenitic stainless steel overlay layers in applications ranging from hydrogenation reactors to marine heat exchangers. The synergy between grain refinement, corrosion resistance improvement, and wear performance enhancement makes this approach particularly attractive for multi-functional surface protection requirements in demanding industrial environments. Future work should focus on long-term thermal exposure studies and multi-cycle fatigue performance evaluation to fully characterize the service life implications of the refined microstructure.