Effects of Nb/Ti/V/Ta on phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic stainless steels

The present work investigated the effects of strong carbide-forming elements M (M = Nb, Ti, V, Ta) on the phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic (AFA) stainless steels (Fe-20Ni-(18-23)Cr-2.5Al-2.3Mo-0.08C-Nb/Ti/V/Ta wt%). The designed alloy rods were solid-solutioned at 1523 K for 1.5 h, and then aged at 1073 K for 24 h or oxidized at 1073 K for 500 h, respectively. Besides primary MC carbides, B2-NiAl, Fe2M, and σ-FeCr phases would be precipitated from the γ-austenitic matrix in these aged AFA alloys. Furthermore, the precipitation of σ-FeCr could be accelerated by the B2-NiAl formation, which is related to the γ stability. The formation of MC and Fe2M can be controlled via adjusting the ratio of M/C. The oxidation resistance of these AFA alloys can be improved effectively by V and Ta addition due to the formation of compact Al2O3 protective scales. This work will provide a new approach to obtain a good combination of creep strength and oxidation resistance through co-alloying with Nb/Ta/V and controlling the M/C ratio within 1.0-2.0 (in molar ratio) simultaneously.