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

Abstract

A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the~923 K to 1173 K (650°C to 900°C) temperature range due to the formation of a protective Al 2 O 3 scale rather than the Cr 2 O 3 scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe 2 (Mo,Nb)-Laves, Ni 3 Al-L1 2 , etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative to compositional and microstructural factors.

CRADA No. NFE-10-02715 Assessment of AFA Stainless Steels for Tube Products in Chemical Processing and Energy Production Applications

“…Creep rupture properties for select OC-4 -OC-10 alloys are shown in Table 3. Alloy OC-5 exhibited superior creep rupture life at 750°C and 100MPa, consistent with composition-creep property studies in the AFA alloys system, which have shown that optimum creep resistance occurs with Nb levels in the 1 wt.% range, although higher levels of Nb (typically in the 2-3 wt.% range are needed for optimum oxidation resistance [1,2]. At 700°C/170MPa and 650°C/250MPa, alloy OC-5 also exhibited good creep resistance.…”

“…This behavior was superior to that observed for the baseline OC-4 alloy. The OC-5 alloy also exhibited good oxidation resistance under this aggressive test condition, despite it's lower Nb level which was optimized for creep resistance [1,2]. Figure 4 shows oxidation data for select OC AFA alloys in a simulated exhaust environment of air with 10% water vapor at 650 and 800°C.…”

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

“…Aluminum, Cr, and Si were selected based on their general importance to achieving oxidation resistance [3]. Increased carbon was selected based on the potential to influence second phase MC and Fe 2 (Mo,Nb) Laves phase precipitation [9]. Boron was selected based on suggestions of its beneficial effect on oxidation in water vapor for chromia formers [13][14][15][16], although it should be noted that there is anecdotal concern for hot workability with high levels of B.…”

“…However, none of these previous AFA stainless steels sufficiently balanced mechanical properties and oxidation resistance to enable commercial adoption. Recently, a new family of AFA stainless steels with a promising combination of mechanical properties and oxidation resistance has been developed at Oak Ridge National Laboratory (ORNL) [4, 8,9]. These ORNL AFA alloys are based on Fe- (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)Ni-(12-15)Cr-(2.5-4)Al-(0.6-3)Nb-0.1C weight percent (wt%), have balanced levels of Al, Cr, and Ni to maintain a single phase austenitic matrix microstructure, and utilize MC-base precipitates (M primarily Nb) for creep strength.…”

Development of Creep-Resistant and Oxidation-Resistant Austenitic Stainless Steels for High Temperature Applications

“…Wrought aluminum forming austenitic (AFA) alloys were developed at ORNL in 2006, based on the matrix composition of the HT-UPS steels described earlier [19][20][21] (Table II). The AFA alloys won an R&D 100 Award in 2009.…”