Effects of Cobalt Addition on the Precipitation Evolution and Tensile/creep Behavior of Fe-15Cr-25Ni-2.5Al-NbWCu Alumina-forming Austenitic Steel at 700°C

Abstract

The effects of Co on the precipitation evolution and tensile/creep properties of the alumina-forming austenitic (AFA) stainless steel at 700°C were systematically investigated. Two investigated steels based on the Fe-15Cr-25Ni-2.5Al (in mass%) AFA steel were prepared; one is added 2.5 mass% W sorely, and the other is added 2.5 mass% W and 5 mass% Co together. After aging treatment, secondary NbC was precipitated in both steels and dispersed within the matrix. However, after adding Co to AFA steels, nanoscale L12 Ni–Al–Cu phase was not observed, and the amount of Laves and the δ-ferrite phase decreased with the same aging time, indicating that addition of Co improved microstructure stability. The tensile strength of AFA specimens with added Co did not significantly decrease, especially with a long-term aging treatment, compared to samples without added Co. However, introducing Co greatly improved the toughness of AFA steels. Also, the results of the creep fracture tests show that adding Co improved the creep life of AFA steels. The reason for this enhancement is that the formation of detrimental secondary phases at the grain boundary was greatly suppressed, after adding Co to AFA steels.