Effect of Alloying Additions on Phase Equilibria and Creep Resistance of Alumina-Forming Austenitic Stainless Steels
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The high-temperature creep properties of a series of alumina-forming austenitic (AFA) stainless steels based on Fe-20Ni-(12-14)Cr-(2.5-4)Al-(0.2-3.3)Nb-0.1C (weight percent) were studied. Computational thermodynamics were used to aid in the interpretation of data on microstructural stability, phase equilibria, and creep resistance. Phases of MC (M: mainly Nb), M23C6 (M: mainly Cr), B2 [β-(Ni,Fe)Al], and Laves [Fe2(Mo,Nb)] were observed after creep-rupture testing at 750 °C and 170 MPa; this was generally consistent with the thermodynamic calculations. The creep resistance increased with increasing Nb additions up to 1 wt pct in the 2.5 and 3 Al wt pct alloy series, due to the stabilization of nanoscale MC particles relative to M23C6. Additions of Nb greater than 1 wt pct decreased creep resistance in the alloy series due to stabilization of the Laves phase and increased amounts of undissolved, coarse MC, which effectively reduced the precipitation of nanoscale MC particles. The additions of Al also increased the creep resistance moderately due to the increase in the volume fraction of B2 phase precipitates. Calculations suggested that optimum creep resistance would be achieved at approximately 1.5 wt pct Nb in the 4 wt pct Al alloy series.
KeywordsCreep Resistance Lave Phase Solution Heat Treatment Sigma Phase Formation Scanning Electron Microscope Backscatter Electron Image
The authors thank E.P. George, C.T. Liu, and J.H. Schneibel for helpful comments on this manuscript. This work was funded by the United States Department of Energy (USDOE) Fossil Energy Advanced Research Materials program. The Oak Ridge National Laboratory is managed by UT–Battelle, LLC (Oak Ridge, TN), for the USDOE under Contract No. DE-AC05-00OR22725. The authors also acknowledge the SHaRE User Facility at the Oak Ridge National Laboratory, sponsored by the USDOE Office of Basic Energy Sciences, Division of Scientific User Facilities.
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