Design and characterisation of new low-alloyed alumina forming ferritic/martensitic steels

Abstract

New efficient energy generation systems require materials capable of withstanding aggressive environments. The Cr-rich oxides formed by commercial stainless steels are not protective enough, so alumina (Al₂O₃) forming steels such as alumina-forming austenitic (AFA) steels and FeCrAl have been proposed as possible materials. However, they are prone to irradiation swelling or exhibit poor creep resistance. This study aims to develop a new kind of alloy, an Alumina Forming Ferritic-Martensitic steel, which combines the superior oxidation resistance from an alumina scale with the creep and irradiation swelling resistance of a martensitic structure. Thermodynamic simulations guided the alloy design of five Fe-Cr-Ni-Al compositions through a powder metallurgy route, ending with Spark Plasma Sintering of solid samples. EBSD and TEM analysis of the samples showed martensite in 4 out of the 5 sintered alloys, with various levels of retained austenite or ferrite in their structures. Tensile and small punch tests from room temperature to 500 °C demonstrated comparable mechanical properties to other AFA steel and to T91 and 316L, candidate materials for nuclear applications. After exposure to air at 800 °C for 500 h, the designed alloys formed protective aluminium oxide scales with corrosion rates similar to 316L and orders of magnitude better than T91. The results show that the developed alloys are promising for components subjected to aggressive environments and elevated temperatures.