On the Loss of Protective Scale Formation in Creep-Resistant, Alumina-Forming Austenitic Stainless Steels at 900°C in Air

Abstract

A family of creep-resistant, Al2O3-forming austenitic (AFA) stainless steels was recently developed. The alloys exhibit excellent oxidation resistance up to ∼800°C, but are susceptible to internal attack of Al at higher temperatures. In the present work, higher levels of Ni, Cr, Al, and Nb additions were found to correlate with improved oxidation behavior at 900°C in air. The alloys generally appeared to be initially capable of external Al2O3 scale formation, with a subsequent transition to internal attack of Al (internal oxidation and internal nitridation) that is dependent on alloy composition. Compositional profiles at the alloy/scale interface suggest that the transition to internal oxidation is preceded by subsurface depletion of Al in the lower-Al compositions. In higher Al-containing compositions, NiAl second-phase precipitates act as an Al reservoir, and Al depletion may not be a key factor. Alloy design directions to increase the upper-temperature limit of protective Al2O3 scale formation in these alloys are discussed.

Effect of Alloying Additions on Phase Equilibria and Creep Resistance of Alumina-Forming Austenitic Stainless Steels

“…Considering the second-phase precipitate strengthening described earlier, the creep properties of the 4Al-1.5Nb alloy would be expected to be the best among the series of AFA alloys. The combination of high Al and high Nb additions also raise the upper limit temperature of alumina scale formability to approximately 900°C, [9][10][11][12] so that improved oxidation resistance would also be expected due to the higher Nb additions.…”

“…Increased oxidation temperatures, above~750°C to 800°C, also resulted in a transition to the internal oxidation of Al. Subsequent studies showed that higher levels of Al, Nb, and Ni additions increased the upper-limit temperature of Al 2 O 3 scale formation tõ 900°C, [9][10][11][12] with the strongest effect observed when the Nb level was increased to the 1 to 3 wt pct level.…”

“…[7][8][9][10][11][12] These alloys exhibited a unique combination of creep resistance via MC-type (M: mainly Nb) carbide strengthening and improved high-temperature oxidation resistance through the formation of a protective Al 2 O 3 scale rather than through conventional alloys protected by Cr 2 O 3 scales. Initial efforts to develop creep-resistant AFA alloys were made by modification of the hightemperature ultrafine-precipitate-strengthening family of advanced austenitic steel alloys.…”

Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels

“…[3,4] A family of alumina-forming austenitic (AFA) stainless steel alloys is currently under development by the authors. [5][6][7][8][9][10][11][12][13][14][15] These alloys are targeted for potential use as superheater/reheater tubes in fossil-fired steam plants, among other applications for chemical and petrochemical processing and energy production. The AFA alloys possess the ability to form an external, protective Al 2 O 3 scale at 873 K to 1173 K (600°C to 900°C), which affords superior oxidation and corrosion protection to the Cr 2 O 3 scales that grow on conventional stainless steel alloys in many industrially relevant environments.…”

Combined experiment and first-principles study of the formation of the Al2O3 layer in alumina-forming austenitic stainless steel

“…Thus, the equilibrium concentrations of Al, Cr, and Ni are important to obtain the single austenitic matrix and to form the continuous protective Al 2 O 3 scale. 14,15,[18][19][20][21] Moreover, Si has been found to be a signi-cant alloying atom to enhance the anti-oxidation and the mechanical properties of the alloys because it promotes the formation of the Cr 2 O 3 scale. The AFA stainless steel (Fe-25Ni-18Cr-3Al) shows that the addition of the right quantity of Si can improve the oxidation resistance at 800 C in water-steam environment.…”