CLADDING TECHNOLOGY SHANXI CO., LTDGrain Refinement Mechanism of La2O3-Containing Austenitic Stainless Steel Weld Overlay and Its Effects on Corrosion and Wear Performance
Literature Overview
The research by Wang Zirong and colleagues (2020), funded 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 (2020NMKJ12), investigates the addition of lanthanum oxide (La2O3) to austenitic stainless steel weld overlay alloys. The study was conducted by a collaborative team from the Ordos Special Equipment Inspection Institute, Yanshan University's State Key Laboratory of Metastable Materials Preparation Science and Technology, and Liverpool John Moores University. The work addresses a longstanding challenge in weld overlay technology: achieving simultaneous improvements in corrosion resistance and wear resistance in austenitic stainless steel overlay layers, which typically exhibit a trade-off between these two properties.
Core Technical Viewpoints
The central finding of this research is that the addition of La2O3 as a microalloying agent in the welding consumable promotes significant grain refinement in the weld overlay layer through a multi-mechanism process. La2O3 acts as an effective heterogeneous nucleation site during solidification, reducing the effective undercooling required for nucleation and increasing the nucleation rate. Additionally, La2O3 modifies the interfacial energy between the solidifying austenite and the liquid metal, further promoting fine equiaxed grain formation.
The grain refinement achieved through La2O3 addition produces several beneficial effects. Finer grains increase the density of grain boundaries, which act as barriers to dislocation motion and thus improve wear resistance. Simultaneously, the grain refinement promotes more uniform chromium and nickel distribution, reducing the risk of chromium-depleted zones at grain boundaries that are susceptible to intergranular corrosion. The refined microstructure also reduces the width of the sensitization-prone region adjacent to grain boundaries, thereby improving resistance to intergranular corrosion in chloride-containing environments.
Mechanism of Grain Refinement
The grain refinement mechanism operates through three synergistic pathways. First, La2O3 particles serve as potent heterogeneous nucleation substrates due to their crystal structure compatibility with austenite (FCC). The lattice mismatch between La2O3 and austenite is sufficiently low to reduce the nucleation energy barrier. Second, La2O3 particles modify the local composition of the liquid metal, increasing the local concentration of alloying elements and promoting constitutional undercooling ahead of the solidification front. Third, the rare earth element lanthanum has a strong affinity for sulfur and oxygen, forming stable compounds that clean the grain boundaries and reduce the segregation of impurity elements.
| Parameter | Without La2O3 | With La2O3 (0.05%) | With La2O3 (0.10%) | With La2O3 (0.15%) |
|---|---|---|---|---|
| Average grain size (μm) | 180-220 | 90-110 | 60-80 | 55-70 |
| Grain size reduction (%) | Reference | ~50% | ~65% | ~70% |
| Hardness (HV) | 180-200 | 220-240 | 240-260 | 250-270 |
| Intergranular corrosion resistance | Poor | Good | Excellent | Excellent |
| Wear resistance (wear volume loss, mm³) | 0.8-1.2 | 0.4-0.6 | 0.2-0.4 | 0.2-0.3 |
| Pitting corrosion potential (mV vs. SCE) | -200 to -100 | -50 to +50 | +50 to +150 | +100 to +200 |
The optimal La2O3 addition level appears to be in the range of 0.05 to 0.15 percent by weight. Beyond this range, excessive La2O3 can lead to the formation of brittle intermetallic compounds at grain boundaries, which may compromise the ductility and toughness of the overlay layer. The optimal addition balances grain refinement with the maintenance of adequate mechanical properties.
Corrosion Resistance Performance
The corrosion resistance evaluation employed multiple standard tests including ASTM G48 intergranular corrosion test, potentiodynamic polarization in 3.5 percent NaCl solution, and electrochemical impedance spectroscopy (EIS). The results demonstrate that the La2O3-modified overlay layer exhibits significantly improved resistance to both general corrosion and intergranular corrosion. The pitting corrosion potential shifts positively by 150 to 300 millivolts compared to the unmodified alloy, indicating a substantially increased thermodynamic stability against localized corrosion initiation.
The improvement in intergranular corrosion resistance is attributed to two factors. First, the grain refinement reduces the length of grain boundaries per unit volume, which reduces the total area susceptible to intergranular attack. Second, the rare earth element lanthanum segregates to grain boundaries and forms stable La2O3 or La2S3 phases that block the diffusion of carbon to the boundaries, thereby reducing chromium carbide precipitation and chromium depletion. This dual mechanism provides a robust defense against sensitization-induced intergranular corrosion, which is a critical concern for austenitic stainless steel overlays in aggressive chemical environments.
Wear Resistance Performance
The wear resistance was evaluated using a pin-on-disk tribometer under dry sliding conditions against a counterface of Si3N4 ball. The wear volume loss of the La2O3-modified overlay layer was reduced by 50 to 70 percent compared to the unmodified alloy. The improvement is attributed to the Hall-Petch strengthening effect of grain refinement, which increases the yield strength and thus the resistance to plastic deformation during sliding contact. Additionally, the refined microstructure promotes a more uniform distribution of carbide precipitates, which provide additional resistance to abrasive wear through a composite mechanism of hard phase dispersion.
The wear mechanism analysis through scanning electron microscopy (SEM) revealed that the unmodified alloy exhibits a combination of abrasive wear and adhesive wear, with deep ploughing grooves and material transfer features. In contrast, the La2O3-modified alloy shows predominantly abrasive wear with shallow grooves and minimal material transfer, indicating a more resistant surface layer. The oxide layer formed on the worn surface of the modified alloy is more compact and adherent, providing additional protection against further material loss.
Engineering Application Considerations
The practical application of La2O3-modified austenitic stainless steel weld overlay alloys is particularly promising for components operating in environments where both corrosion and wear are significant degradation mechanisms. Typical applications include pump impellers, valve trim, heat exchanger tubes, and pressure vessel internals in the petrochemical and oil and gas industries. The improved combined performance of corrosion and wear resistance can significantly extend the service life of these components and reduce maintenance costs.
The welding process parameters must be optimized to ensure that the La2O3 particles are uniformly distributed in the weld metal. Powder-based processes such as plasma transferred arc (PTA) welding and laser cladding are particularly suitable because they allow precise control of the powder composition and mixing. For wire-based processes such as gas metal arc welding (GMAW) or submerged arc welding (SAW), the La2O3 must be incorporated into the flux or coating of the consumable, which requires careful process development to ensure uniform distribution.
Study Insights and Implications
This research demonstrates that microalloying with rare earth oxides is a powerful approach to simultaneously enhance multiple properties of weld overlay alloys. The concept of using La2O3 as a grain refiner in austenitic stainless steel overlays opens new possibilities for tailoring the microstructure and properties of weld deposits through compositional engineering. The synergistic improvement in corrosion and wear resistance addresses a critical limitation of conventional austenitic stainless steel overlays, which often require trade-offs between these competing properties.
The broader implication is that the principles of rare earth metallurgy, well established in castings and wrought products, can be effectively applied to weld overlay processes. This opens a pathway for the development of next-generation overlay consumables that combine the corrosion resistance of austenitic stainless steels with the wear resistance of harder alloys, without compromising the toughness and ductility required for reliable service. The engineering community should consider incorporating rare earth modifications into the standard repertoire of overlay alloy design tools.