New AFA steel production routes for energy and nuclear components

The energy sector increasingly faces challenging operational environments marked by extreme temperatures, corrosive agents and severe mechanical stresses that often exceed the limits of conventional materials. Alumina-Forming Austenitic (AFA) steels are engineered to develop a stable Al2O3 protective layer while maintaining an austenitic matrix, providing exceptional oxidation and corrosion resistance in high-temperature steam and liquid lead, two of the most demanding environments in advanced energy systems. AFA steels combine excellent corrosion resistance with superior mechanical properties compared to conventional materials like 316LN, and they offer a cost-effective alternative to Ni-based alloys.

Currently, no steel mills produce AFA alloys as standard products. The production of high-aluminum AFA steels presents a series of metallurgical challenges that must be carefully addressed throughout the entire manufacturing route. The project’s innovation lies in assessing the replacement of high-purity ferroalloys with high-quality austenitic scrap, typically used in electric arc furnace (EAF). This represents a first step for the substitution of the Vacuum Induction Melting with EAF, paving the path for the industrialization of these steels. The project aims to evaluate also the weldability challenges, providing a preliminary assessment of welded products.

At the same time, the project targets the production of seamless tubes by extrusion, assessing process parameters to hot-extrude AFA alloy. Achieving defect-free large plates and long tubes, the project will demonstrate that AFA steels can be fabricated with methods analogous to conventional stainless steels. Another innovative aspect of the project is to achieve a reduction of at least 30% in CO2 emissions considering material fabrication and operating service conditions compared to traditional steel 316LN by increasing the scrap utilization rate during the manufacturing process, extending components’ lifetime.

Objectives and ambition

The energy sector is increasingly confronted with demanding operational environments characterized by extreme temperatures, corrosive media such as molten lead and steam, and intense mechanical stress conditions that often exceed the capabilities of conventional materials.

For instance, nuclear power generation and Concentrated Solar Power (CSP) systems use molten lead and salts as heat transfer media, while chemical reactors and fuel pipelines expose structural materials to highly corrosive liquid or gaseous atmospheres. Moreover, in advanced nuclear plants and CSP systems, supercritical CO₂ (sCO₂) power cycles are being actively considered because they can deliver higher thermal efficiency, enable much more compact turbomachinery and heat exchangers, and reduce water consumption. At the same time, sCO₂ cycles impose simultaneous high pressure, elevated temperature, and CO₂-rich environments that can accelerate the degradation of materials. In these scenarios, standard stainless steels, particularly at elevated temperatures, may exhibit insufficient corrosion resistance. This limitation underscores the need for advanced steel grades with enhanced passivation properties to ensure reliable performance and longevity in such harsh conditions.

To meet these challenges, the development of critical components that combine high mechanical strength, long-term durability, and superior corrosion resistance is essential—not only for operational efficiency but also to support broader goals of environmental sustainability.

A promising solution lies in the adoption of Alumina-Forming Austenitic (AFA) steels, which offer a unique combination of properties: excellent corrosion resistance and robust mechanical performance. Compared to conventional materials like 316LN, AFA steels demonstrate significantly improved behaviour in aggressive environments, particularly those involving molten lead and high-pressure steam.

AFA steels are designed to form a stable Al₂O₃ protective scale while retaining an austenitic matrix, enabling superior oxidation/corrosion resistance in high-temperature steam and liquid lead, two of the most demanding environments in advanced energy systems. These characteristics establish a credible pathway to industrialization if the remaining gaps in scale-up, product forms, welding procedures, and environment-specific aspect are addressed.

The production of high-aluminum AFA steels presents a series of metallurgical challenges that must be carefully addressed throughout the entire manufacturing route. The initial melting step, performed via Vacuum Induction Melting (VIM), is crucial to ensure accurate chemical composition and to minimize aluminum oxidation. However, this remains susceptible to segregation phenomena and the formation of brittle intermetallic phases if cooling rates and stirring dynamics are not properly controlled.

To mitigate these issues, a secondary melting step using Vacuum Arc Remelting (VAR) has to be implemented. VAR significantly enhances chemical homogeneity, reduces macro-segregation, and eliminates shrinkage porosity typically observed in as-cast ingots. Additionally, it promotes the refinement of non-metallic inclusions, thereby improving toughness and preparing the material for subsequent thermo-mechanical processing.

The hot working phase—whether by forging, rolling or extrusion—remains critical due to the increased risk of hot cracking and reduced ductility associated with aluminum enrichment. Nevertheless, the superior cleanliness and microstructural uniformity achieved through the VIM+VAR sequence substantially widen the processing window. To fully exploit these advantages, thermo-mechanical schedules must be carefully designed to stabilize the distribution of strengthening precipitates and to ensure the optimal balance between mechanical integrity and corrosion resistance of the AFA steel.

Addressing these issues requires advanced melting and refining protocols, combined with carefully designed thermo-mechanical processing strategies to stabilize the microstructure and achieve the desired performance. The scope of RENEW project is to tackle key challenges in the production and transformation processes involved in the development of new Alumina Forming Austenitic steel (AFA) for the energy sector. The focus is placed on optimizing manufacturing routes while improving performance, processability, cost-effectiveness, long-term reliability, and reducing CO₂ emissions.

Key components will be selected to define target material requirements and specifications to face the challenges of nuclear, chemical, and broader energy applications. These components will be identified among those subjected to extreme operating conditions—such as exposure to molten lead, molten salts, and high-pressure steam—where conventional materials often fail due to inadequate corrosion resistance and mechanical integrity.

Through casting trials employing different technological route, processability tests, welding trials, mechanical and corrosion testing and a comprehensive Life Cycle Assessment (LCA), the project will validate the performances, reliability, and sustainability of the proposed material under realistic operating conditions. These results will be benchmarked against current industry-standard materials and solutions to demonstrate clear technical and environmental improvements.