Development of Alumina Forming Alloys for High-Temperature Energy Applications

Abstract: Liquid lead as heat transfer fluid presents attractive features for future power technologies, such as next-generation nuclear reactors, thermal solar power, and thermal storage. Liquid lead has excellent heat transfer properties well suited for operating at high temperatures. While today's water-cooled reactors operate at a temperature of approximately 300°C, the next-generation lead-cooled nuclear power could operate up to 600°C, significantly increasing the energy conversion efficiency. However, it is well known that liquid lead is a corrosive medium for stainless steels, especially at temperatures above 500°C.To address the corrosion issues, aluminium oxide-forming ferritic steels have been thoroughly studied in liquid lead environments and have shown good oxidation properties. Traditionally, these steels are alloyed with 3-6 wt% aluminium and 12-24 wt% chromium, and iron as balance, hence the denotation FeCrAl steel. However, the high addition of aluminium and chromium results in poor weldability and renders them susceptible to embrittlement in the desired temperature range. Therefore, to address the welding and embrittlement issues, a new group of FeCrAl materials was developed in recent years with only 10 wt% chromium and 4 wt% aluminium, which in this work is referred to as Fe-10Cr-4Al. These alloys have shown good ductility and good corrosion properties in liquid lead up to 550°C.In this work, the Fe-10Cr-4Al steels have been further optimised and exposed to liquid lead at temperatures up to 900°C. In addition, detailed studies of the oxidation properties and structure formed on the steel surfaces were conducted using various analytical techniques. The findings showed that the most promising Fe-10Cr-4Al steel, so far, has suitable corrosion properties up to 800°C. However, although these steels have improved mechanical and welding properties, they do not meet the requirements set for the desired high-temperature energy applications.Therefore, another new family of alloys was developed, namely alumina-forming martensitic steels. This development aimed to combine the superior corrosion resistance of the aluminium oxide with the mechanical properties inherent in the martensitic structure. The development was done using thermodynamic modelling, empirical corrosion studies and detailed analytical methods. The results showed that these martensitic steels have a corrosion resistance exceeding even the best1optimised Fe-10Cr4-Al steel in liquid lead at temperatures up to at least 550°C.In parallel, alumina-forming austenitic steels were also developed. The aim was to find a suitable composition for these materials that not only provide good corrosion protection in liquid lead, but also good weldability, good phase stability, and good formability. These austenitic materials have shown, in general, good corrosion properties in liquid lead up to 600°C. However, the combination of elevated Ni, Al and Mn levels resulted in embrittlement after ageing at 600°C. In addition, a series of austenitic alumina-forming welding materials were also developed within this work. The aim was to produce a material that does not suffer from the embrittlement commonly observed in ferritic welds. Bend testing of this welding material has, so far, indicated ductile behaviour with good formability.