A new family of alumina-forming au.slenitic stainless steels is under development at Oak Ridge National Laboratory for structural use in aggressive oxidizing environments at 600-900[degrees]C. Data obtained to date indicate the potential to achieve superior oxidation resistance compared to conventional Cr^sub 2^O^sub 3^- forming iron- and nickel-based heat-resistant alloys, with creep strength comparable to state-of-the-art advanced austenitic stainless steels. A preliminary assessment also indicated that the newly developed alloys are amenable to welding. Details of the alloy design approach and compositionmicrostructure-property relationships are presented. INTRODUCTION

Oxidation resistance is one of the primary considerations that determine the durability of heat-resistant alloys. The key to good oxidation resistance is to establish an external, continuous layer of a slow-growing, thermodynamically stable oxide phase such that subsequent oxidation is limited by diffusion of metal or oxygen species across this layer. For high-temperature applications (i.e., >600[degrees]C), Cr^sub 2^O^sub 3^ and Al^sub 2^O^sub 3^ are the principal oxides used for the protection of metallic alloys.

In many high-temperature environments Al^sub 2^O^sub 3^ scales offer a superior degree of protection to Cr^sub 2^O^sub 3^.8-15 Alumina scales grow at a rate that is 1 to 2 orders of magnitude lower than that of Cr^sub 2^O^sub 3^ (Figure 1a). Alumina is also significantly more thermodynamically stable than is Cr^sub 2^O^sub 3^ (Figure 1b). Alumina scales have proven to be particularly beneficial in the presence of aggressive carbon- or sulfur-species encountered in combustion and chemical process industry applications.8,9 Further, in combustion environments, a key advantage of Al^sub 2^O^sub 3^ over Cr^sub 2^O^sub 3^ is greater stability in the presence of water vapor.16 With both oxygen and water vapor, volatile chromium oxy-hydroxide species can form and significantly reduce oxidation lifetime, in part by constantly thinning the protective Cr^sub 2^O^sub 3^ scale that is established, resulting in linear oxidation kinetics.16,17 Such attack is particularly relevant for thin-walled components such as heat exchangers.18 However, despite the many advantages of protective Al^sub 2^O^sub 3^ scales, virtually all iron-based, heat-resistant structural alloys for use above ~600[degrees]C utilize Cr^sub 2^O^sub 3^-based scales for protection.8'10 This is due to the extensive solid solubility and excellent metallurgical compatibility of chromium in Fe/Fe(Ni), which permits ready formation of a protective Cr^sub 2^O^sub 3^-based scale with ample alloy design flexibility to co-optimize oxidation resistance with other needed properties such as creep resistance, weldability, etc.

Ferritic Fe-Cr-Al-based alloys capable of Al^sub 2^O^sub 3^ formation are widely used in specialty applications such as heating elements and furnace liners. Despite their outstanding oxidation resistance, they are not suitable for structural applications above ~500-600[degrees]C due to poor creep resistance resulting from their open body-centered cubic structure. To obtain creep resistance above ~600[degrees]C in conventional cast or wrought iron-based alloys, an austenitic face-centered cubic structure is needed. Oxide dispersion strengthened (ODS) ferritic Fe-Cr-Al-based alloys10 and nickel- based alloys8-10 capable of alumina scale formation and with excellent high-temperature creep resistance are also available, but their high cost limits their use.

The interest to create Al2O3-forming austenitic (AFA) stainless steels for use as heat-resistant structural alloys dates back at least 30 years (e.g., References 17, 20-23). A major complication for developing a successful AFA stainless steel is that aluminum is a strong ferrite stabilizer. Further, the alloys also require the addition of significant quantities of chromium, and a ferrite stabilizer, to help promote protective Al^sub 2^O^sub 3^ scale formation. (Additions of chromium reduce the critical level of aluminum in an alloy needed to form a protective Al^sub 2^O^sub 3^ scale, often referred to as the third-element effect.10,24,25) Typically explored alloying addition levels of ~4-6 wt.% aluminum and ~10-25 wt.% chromium can destabilize the parent austenitic matrix structure, resulting in duplex ferritic/austenitic microstructures and a loss of creep resistance (e.g.. References 2, 17). The desired austenitic matrix structure can be stabilized in these alloys by large additions of nickel, but the levels needed usually result in a nickel-based alloy rather than an iron-based alloy, and the cost advantages are lost. This paper overviews recent efforts at Oak Ridge National Laboratory (ORNL)1-7 devoted to developing creep-resistant AFA stainless steels with relatively low levels of nickel, comparable to existing advanced austenitic stainless steels and alloys.

ORNL AFA DEVELOPMENT APPROACH

Alumina-forming austenitic alloy development efforts at ORNL used the high-temperature ultrafine-precipitation-strengthened (HTUPS) family of austenitic stainless steel alloys26 as a starting point for alloy modification.2 These alloys exhibit among the highest creep strengths ever reported for austenitic stainless steel alloys. However, they were originally developed for advanced liquid-metal nuclear reactor environments where gas-metal oxidation was not a major consideration. Consequently their oxidation resistance is relatively poor. A typical HTUPS composition is Fe-14Cr-16Ni-2.5Mo- 2Mn-0.5Ti0.3V-0.15Nb wt.% base with additions of B, C, and P to form stable nanoscale precipitates such as MC (M = Nb, Ti, V) carbides for strengthening.26 Levels of -2.5 (alloy HTUPS-I) and 3.8 wt.% aluminum (alloy HTUPS-2) were added initially to a baseline HTUPS alloy composition, with the nickel level increased to 20 wt.% in order to stabilize austenite, in an attempt to promote protective Al^sub 2^O^sub 3^ scale formation (Table I).2

Evaluation of creep-rupture life at 750[degrees]C and 100 MPa in air indicated that the HTUPS-1 alloy with 2.5 wt.% aluminum exhibited excellent creep resistance (Figure 2a). For comparative purposes, conventional austenitic stainless steels such as type 347 stainless steel alloys (-Fe-18Cr-I INi wt.% base) ruptured in less than -100-300 h under these conditions. State-of-the-art austenitic stainless steel alloys such as alloy 709 (~Fe-25Ni-20Cr wt.% base) can exhibit creep rupture lives at 750[degrees]C and 100 MPa from - 2.000-6,000 h.

Increasing the level of aluminum to 3.8 wt.% (HTUPS-2) resulted in the formation of delta-ferrite in the microstructure, which converted to sigma phase on exposure at 750[degrees]C and degraded creep resistance.2 However, a short-term oxidation screening at 800[degrees]C in air indicated that neither the 2.5 wt.% nor 3.8 wt.% level of aluminum addition was sufficient to enable Al^sub 2^O^sub 3^ scale formation (Figure 3). Instead, the aluminum was internally oxidized in HTUPS-1 and HTUPS-2, and the external scale consisted of faster-grow ing, less-protective mixed iron- and chromium-rich oxide phases.2 Such behavior is the reason previous alloy development efforts generally utilized higher levels of aluminum (4-5 wt.%) and chromium (>15 wt.%) with the drawback of destabilizing austenite and a loss of creep resistance without large nickel additions.

Protective Al^sub 2^O^sub 3^ scale formation was, however, achieved in an HTUPS type alloy with only -2.5 wt.% aluminum (AFA 2- 1) (Table I and Figure 3c). (All Al^sub 2^O^sub 3^-forming alloys are referred to as AFA, with 2, 3, or 4 designating 2.5, 3, or 4 wt.% aluminum series). This was accomplished by eliminating the titanium and vanadium additions to the alloy and increasing the niobium level to 0.9 wt.% niobium2,4 (Table I). Despite the absence of titanium and vanadium, the AFA 2-1 alloy still exhibited excellent creep resistance at 75O0C and 100 MPa in air with a rupture lifetime of -2,000 h (Figure 2a). Creep-rupture life data for AFA 2-1 and some comparable commercial Cr,O3-forming alloys are presented in the Larson-Miller plot shown in Figure 2b.

Transmission-electron microscopy (TEM) analysis indicated that the creep resistance in AFA 2-1 resulted from nano MC precipitates, similar to those observed in alloy 709 (Figure 4). It should be noted that as with the original HTUPS alloys,26 AFA 2-1 utilized 10% cold work2 to enhance MC precipitation and creep resistance. Subsequent studies of related developmental AFA alloys have indicated that good creep resistance can also be obtained in solution-treated material without prior cold work.