Investigation Of Microstructural Stability And Mechanical Properties Of Alumina-Forming Austenitic Steels

Motivated by the energy crisis and environmental protection, the advanced ultra super-critical power plants are designed to operate under conditions of 700℃/30 MPa. However this proposes more challenging requirements on structural materials for key components. Due to the combination of good high-temperature mechanical properties and oxidation resistance, alumina-forming austenitic steels （AFA） are considered as a most promising candidate structural material. Until present, many studies have focused on alloy development and oxidation mechanism. However, the long term serving stability, especially the evolution of precipitates and mechanical properties, was only studied in a limited way. In this paper, Fe-18Ni-12Cr based AFA steels with different additions of alloying elements were fabricated. The evolution of microstructure and mechanical properties during long term aging at 700℃ were investigated. Also their creep properties at 700℃ were studied.The Fe-18Ni-12Cr based AFA steels were prepared by vacuum induction melting, followed by forging and hot rolling. Dynamic recrystallization occurred during hot rolling at around 1150℃, resulting in equiaxial structures.For 18-12-AlNbC based AFA steel, the tensile strength at RT increased with aging time. Also tensile ductility at RT did not get deteriorated during aging up to 1000 h. The softening mechanism of dynamic recovery dominated during the tensile test at 700℃. NbC precipitates with different morphologies were observed after different aging periods. The spherical particles with size around 5 nm were very stable during aging, while the plate-like NbC grew to 89 nm after aging for 1000 h. These NbC precipitates were supposed to be the main strengthening phase, which resulted in the increasing work hardening rate during tensile test at RT. The Fe₂（Nb,Mo） Laves phase exhibited relatively high coarsening rate, and its length reached 920 nm with aspect ratio of 7.4 after aging up to 1000 h. Although the GB precipitate coverage was 74%, it did not impair the tensile ductility.The precipitating process of 18-12-Al based AFA steel was relatively slower than that of 18-12-AlNbC based AFA steel during aging at 700℃. The majority of precipitates were observed after aging for 1000 h, resulting in obvious increase in tensile strength at RT. In this specimen, most of the precipitates were B2-NiAl, which exerted significant effect of precipitation strengthening. However, it had some negative effects on the tensile ductility.The creep properties of these two AFA steels were tested at 700℃ under different stresses. The creep stress exponents of these two specimens were around 7, indicating that their creep behavior belonged to dislocation creep. Also the creep damage tolerance factor （λ） of them was larger than 5, therefore their creep damage was due to the microstructual degradation. For 18-12-AlNbC based AFA steel, the creep rate decreased during the primary stage, and then it was stable during the steady state stage. Therefore the minimum creep rate and the steady state creep rate of 18-12-AlNbC based AFA steel were almost the same. However, there was a gradual increase in the creep rate after it reached the value of minimum creep rate for 18-12-A1 based AFA steel. Compared with 18-12-Al based AFA steel,18-12-AlNbC based AFA steel exhibited better creep properties at 700℃, such as lower minimum creep rate, longer creep rupture life and smaller creep deformation. NbC precipitates would result in better creep resistance. Moreover intermetallics of Laves phase and B2-NiAl did not deteriorate the creep ductility.