CLADDING TECHNOLOGY SHANXI CO., LTDGrain Refinement Mechanism of La2O3-Containing Austenitic Stainless Steel Cladding Alloy and Its Effect on Corrosion and Wear Performance
Literature Overview
This study investigates the grain refinement mechanism of La2O3 (lanthanum oxide) addition in austenitic stainless steel cladding alloys and its subsequent effects on corrosion resistance and wear performance. Lanthanum, as a rare earth element, is known for its ability to modify inclusion morphology, refine grain structure, and improve the thermodynamic stability of oxide phases in steel. The research examines cladding alloys with 0.05–0.3% La2O3 addition deposited by submerged arc welding (SAW) and evaluates the resulting microstructural, corrosion, and wear properties.
Core Technical Findings
The study demonstrates that the addition of 0.1–0.15% La2O3 to austenitic stainless steel cladding alloys produces a significant grain refinement effect, reducing the average grain size from 35–45 micrometers (without La2O3) to 12–18 micrometers. This refinement is attributed to the formation of La₂O₃-Al₂O₃-MnS composite inclusions that act as heterogeneous nucleation sites during solidification, increasing the nucleation rate and suppressing grain growth during cooling.
Grain Refinement Mechanism
The mechanism of grain refinement by La2O3 operates through three stages:
- Inclusion formation: La2O3 reacts with Al₂O₃ and MnS in the melt to form La₂O₃-Al₂O₃-MnS composite inclusions with a size of 0.2–1.0 micrometers. These inclusions have a low surface energy and are thermodynamically stable throughout the solidification process.
- Heterogeneous nucleation: The composite inclusions serve as effective nucleation sites for austenite grains during solidification. The crystallographic mismatch between La₂O₃-Al₂O₃-MnS and austenite is less than 15%, which is below the critical threshold for effective nucleation according to the Turnbull criterion.
- Grain growth inhibition: During post-solidification cooling, the dispersed inclusions act as Zener pinning obstacles, impeding grain boundary migration. The Zener pinning pressure is calculated as P_z = 3γf/(2r), where γ is the grain boundary energy, f is the volume fraction of inclusions, and r is the inclusion radius. With f = 0.005 and r = 0.5 µm, the calculated pinning pressure is approximately 0.3–0.5 MPa, which is sufficient to limit grain growth during cooling.
Corrosion Performance
The grain refinement induced by La2O3 has a direct and positive effect on corrosion resistance. The following table compares the corrosion performance of cladding alloys with and without La2O3 addition:
| Property | Without La2O3 | With 0.1% La2O3 | With 0.15% La2O3 | With 0.3% La2O3 |
|---|---|---|---|---|
| Average grain size (µm) | 38 | 15 | 12 | 22 |
| Pitting potential (mV vs. SCE) | +180 | +265 | +290 | +240 |
| Corrosion current density (µA/cm²) | 1.20 | 0.45 | 0.38 | 0.62 |
| Intergranular corrosion (ASTM A262 Practice E) | 30 µm attack depth | 8 µm | 5 µm | 12 µm |
| Sensitization temperature (°C) | 850 | 950 | 980 | 920 |
The optimal La2O3 addition of 0.1–0.15% produces the finest grain structure and the best corrosion performance. The pitting potential increases by 110–130 mV compared to the La2O3-free alloy, and the intergranular corrosion attack depth is reduced by 83–87%. The improvement is attributed to the finer grain structure, which promotes more uniform chromium distribution and reduces the driving force for chromium depletion at grain boundaries during sensitization.
However, excessive La2O3 addition (0.3%) leads to the formation of coarse La₂O₃-rich inclusions that serve as initiation sites for pitting corrosion and reduce the overall corrosion performance. This demonstrates the importance of optimizing rare earth addition levels rather than assuming that more is always better.
Wear Performance
The grain refinement also improves wear resistance through multiple mechanisms. The hardness of the cladding alloy increases from 220 HV to 280 HV with 0.15% La2O3 addition, primarily due to grain boundary strengthening. The Hall-Petch relationship predicts a hardness increase of approximately 50 HV for the observed grain refinement from 38 µm to 12 µm, which accounts for about 80% of the total hardness improvement. The remaining improvement is attributed to the increased density of dislocation pile-ups at grain boundaries, which enhances work hardening capacity during abrasive loading.
Engineering Practice Considerations
For engineers incorporating La2O3 into cladding alloy specifications, the following considerations are important:
- Consumable form: La2O3 can be introduced through wire coatings, flux compositions, or as a powder additive in powder-based cladding processes. Wire coatings with 2–5% La2O3 by weight are most practical for SAW applications.
- Addition level: The optimal addition level of 0.1–0.15% La2O3 should be maintained; higher levels risk inclusion-related defects.
- Flux chemistry: The flux should be designed to promote the formation of La₂O₃-Al₂O₃-MnS composite inclusions rather than isolated La₂O₃ inclusions. This can be achieved by ensuring adequate Al and Mn content in the flux composition.
- Heat treatment: A solution treatment at 1050–1100 °C followed by rapid cooling can homogenize the microstructure and dissolve any chromium carbide precipitates at grain boundaries, further improving sensitization resistance.
Key Reflections
The study provides a mechanistic understanding of how rare earth elements can be used to enhance the performance of austenitic stainless steel cladding alloys. The grain refinement mechanism is well-established in the literature, but the specific application to cladding alloys and the quantification of corrosion and wear improvements are valuable contributions.
A practical concern is the cost and availability of La2O3-containing consumables. While La2O3 is relatively inexpensive, the production of wire coatings or fluxes with controlled La2O3 content requires specialized manufacturing processes. For high-value applications such as nuclear reactor components or chemical processing equipment, the performance benefits justify the additional cost. For general industrial applications, the cost-benefit analysis should be conducted on a case-by-case basis.
Summary
The research demonstrates that La2O3 addition at 0.1–0.15% to austenitic stainless steel cladding alloys produces significant grain refinement through heterogeneous nucleation and Zener pinning, resulting in improved corrosion resistance and wear performance. The optimal addition level is critical, as excessive La2O3 can be detrimental, and the study provides clear guidance for engineers specifying rare earth-modified cladding alloys.