Alumina-forming Austenitic Stainless Steel Research Articles

Characterization of the oxide film formed on alumina-forming austenitic stainless steel in deaerated, DH, DO, and OT supercritical water

The effects of water chemistry (deaerated (DO < 10 ppb), dissolved hydrogen (DH 1.6 ppm), dissolved oxygen (DO 2 ppm), and oxygen treatment (OT 300 ppb O2 and 100 ppb NH3) on the corrosion behavior of alumina-forming austenitic stainless steel (AFAs) were systematically investigated. A dual-layer oxide scale composed of an inner Fe/Cr/Al oxide layer and an outer Fe-rich oxide layer was observed on the AFAs. Water chemistry was found to have a negligible effect on the inner Fe/Cr/Al oxide layer; however, the morphology of the outer Fe-rich oxide layer was found to depend significantly on the chemistry. No facetted and dense hematite, which effectively protected the alloy and led to lower weight gain (less oxidation) over prolonged exposure, was formed in the outer oxide layer in oxidizing environments (DO and OT). Conversely, columnar magnetite with a porous structure was formed in outer oxide layer in reducing environments (deaerated and DH) in the absence of oxygen, with a higher weight gain observed. The addition of NH3 increased the pH of the supercritical water (SCW), thereby promoting alumina dissolution and rendering the OT conditions undesirable for use in a supercritical water-cooled reactor (SCWR). This study provides valuable insight into the behavior of AFAs in SCW and highlights the importance of carefully controlling SCW chemistry to optimize material performance.

Effect of recrystallization on aging embrittlement and high temperature creep properties of AFA steel

As a kind of typical precipitate strengthened steel, alumina-forming austenitic (AFA) stainless steels usually suffer obvious aging embrittlement issue. Recrystallization treatment was performed by cold rolling followed with heat treatment at 1080 ℃ for a Fe-25Ni-15Cr-3.5Al based AFA steel to evaluate the effect of recrystallization on aging embrittlement. The materials before and after recrystallization were aged at 700 ℃ for 120 h to evaluate the effect of recrystallization on precipitation behavior. After recrystallization, the average grain size decreased and the density of grain boundaries increased obviously. The size of precipitates along grain boundaries was smaller and more uniform after recrystallization, and the precipitation of Cr rich phase was inhibited. Room temperature tensile test, Charpy impact, and creep behavior at high temperature of samples with different treatment states were measured and compared. The aging embrittlement of AFA steel was alleviated obviously after recrystallization. The creep resistant was also improved for sample treated by recrystallization and aging. The relevant mechanisms were discussed.

Anomalous oxidation rate-temperature dependence of alumina-forming austenitic stainless steels exposed to 500–600 °C supercritical water

The oxidation behavior of austenitic stainless steels with 0 and 2.5 wt% Al exposed to 500–600 °C supercritical water was studied. The oxidation rate-temperature dependence of base steel (0 wt% Al) followed the conventional Arrhenius law, while the alumina-forming austenitic stainless steel (AFA steel, 2.5 wt% Al) exhibited an anomalous oxidation rate-temperature dependence, which is attributed to the special phase transition and oxidation behavior of B2-NiAl precipitates in the AFA steel.

Effect of Mn content on the high-temperature oxidation behaviors of Mn-substituted-for-Ni alumina-forming austenitic stainless steel

We studied high Mn-substituted-for-Ni AFA steel (Fe-14Cr-(6–12) Mn-10Ni-2.5Al-3Cu-Nb-C AFA steel) oxidation behaviors at 800 °C in air. (6–12) Mn AFA steel formed a three-layered scale: the outer layer is (Mn, Fe)-rich oxide, such as (Mn0·983Fe0.017)2O3, the intermediate is Cr-rich oxide, and the inner layer is Al2O3. (6–8) Mn-AFA steels formed a protective Al2O3 layer with stronger oxidation resistance than (10–12) Mn-AFA steels. The oxidation rate constants of 6Mn- and 8Mn-AFA steels are 1.7 × 10−3 and 2.4 × 10−3 mg2/(cm4·h), respectively, approximately 10 times higher oxidation resistance than that of 10Mn- and 12Mn-AFA steels. (10–12) Mn-AFA steels cannot form a continuous Al2O3 layer. The discontinuous Al2O3 layer allows the elements to diffuse outward, eventually forming Mn-Fe-Nb-rich oxide nodules on the surface of the steel. Mn concentrations with the potential to form protective alumina at 800 °C were identified.

Corrosion Behavior of Alumina-Forming Austenitic Steel in Supercritical Carbon Dioxide Conditions: Effects of Nb Content and Temperature.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr2O3 oxide film and an inner Al2O3 oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ'-Ni3Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe2(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.

Effect of cold rolling and annealing on the microstructure and mechanical properties of Mn-substituted-for-Ni alumina-forming austenitic stainless steel

The microstructural evolution of low-Ni alumina-forming austenitic (8Mn-AFA) steel during cold rolling with different reductions (from 45% to 80%) and annealing was investigated using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques. Compared with conventional AFA steel (low Mn content), 8Mn-AFA steel exhibits a higher strength in the transverse direction (TD) than in the rolling direction (RD) during plastic deformation. This anisotropy increases with the cold rolling deformation. When the cold rolling deformation increases from 45% to 80%, the ultimate tensile strength (Rm) in the RD and TD increases from 1091 to 1306 MPa and from 1162 to 1442 MPa, respectively. The stacking fault energy (SFE) of 8Mn-AFA steel is only 46.79 mJ/m2 due to the substitution of 8Mn for Ni. The low SFE induces the formation of many deformation twins arranged along or close to the TD (the angle between them is less than 30°) in the austenite grain, yielding a higher strength in the TD than in the RD. Compared with cold-rolled samples, Rp and Rm decrease in annealed samples, and the anisotropy in the TD and RD almost disappear as the number of deformation twins decreases. The high strength of the cold-rolled 8Mn-AFA steel can be attributed to the dislocation enhancement, grain boundary enhancement, and texture-induced anisotropy strengthening. The dislocation enhancement contributes the most to the strength, exceeding 70%, followed by grain boundary contribution, about 6.9 – 10.8%, while the contribution of texture-induced anisotropy is 5 – 9.2%.

Corrosion of alumina-forming austenitic alloys under the supercritical carbon dioxide-based venus atmospheric surface conditions

Several grades of alumina-forming austenitic (AFA) stainless steel (SS) alloys were probed after exposure to simulated sCO2-based 467 °C Venus surface atmospheric conditions using the NASA Glenn Extreme Environments Rig (GEER) for 30 days. AFA samples were characterized by gravimetric analysis, X-ray diffraction (XRD), and electron microscopy. All AFA samples formed multilayered scales in which the outer layer is mainly magnetite (Fe3O4) and nickel sulfides (Ni3S4, Ni4S3), and the inner layer is mainly chromia containing aluminum, (Cr,Al)2O3. The most resistant of the AFA alloys exhibited specific weight gain corrosion resistance similar to SS304, which formed an inner (Fe3−xCrxO4) spinel.

Alumina-forming austenitic stainless steel for high durability and chromium-evaporation minimized balance of plant components in solid oxide fuel cells

The chromium evaporation and oxidation behaviors of alumina-forming austenitic stainless steels are systematically investigated at 800 °C in air +10% H2O relative to 310S for 5000 h. Cr evaporation rates of 310S are about 35 times higher than AFA alloys after 5000 h. Relatively rapid oxidation is observed on 310S after only one 500 h cycle, followed by a modest degree of mass loss and spallation, while the AFA alloys show high oxidation resistance throughout the entire test. Continuous inner alumina layer formed on AFA alloys stays compact and stable after 5000 h which greatly reduces the Cr evaporation.

Synergistic effects of microalloying and pre-straining on enhanced nanoprecipitation and creep property of alumina-forming austenitic stainless steels

Non-shearable metal carbide (MC) nanoparticles are one of the most effective strengthening media for improving creep resistance of alloys at elevated temperatures. However, it is difficult to achieve sufficient homogeneous dispersion due to their large crystallographic difference with the matrix. Herein, we report a feasible method to form massive NbC nanoprecipitation in an aluminum-forming stainless steel via the combination of microalloying and pre-straining. The employment of pre-straining generated dislocations uniformly distributed inside the alloy matrix, which provides high-density heterogeneous nucleation sites and enables fast pipe diffusion channels for involved solutes, thus substantially enhancing the nucleation rate of the secondary NbC. Moreover, due to its stronger affinity with C and larger diffusivity as compared with Nb, Ti atoms were found to occupy the Nb sublattice of the NbC at the onset of precipitation. The formation of (Ti,Nb)C nuclei with decreased lattice misfit (from 20.8% to 19.1%) effectively reduced the critical nucleation energy barrier and thus enhanced the nucleation rate accordingly. With the synergistic effect of 0.1 wt% Ti microalloying and 10% pre-straining, massive precipitation of refined secondary (Ti,Nb)C nanosized particles (4.4 ± 0.7 nm) with a much increased volume fraction (almost 6.8%) was practically realized, remarkably reducing the steady-state creep rate and prolonging the creep-to-rupture lifetime to 2044 h at 1023 K and 120 MPa. The current findings provide a feasible strategy to disperse strong yet massive nanoparticles within metallic metrics, which may shed new insights into the development of advanced high-temperature alloys with much enhanced creep properties at no expense of oxidation resistance.

Deconvolution of deterioration of anode-supported cells by chromium poisoning from alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

Al2O3-forming austenitic (AFA) stainless steels are potential replacements for the existing balance of plant (BoP) components in solid oxide fuel cells (SOFCs). In this study, chromium (Cr) poisoning of anode supported cells (ASCs) coupled with various alloys was analyzed by the distribution of relaxation times (DRT). The performance deterioration of ASCs was mainly attributed to the increased polarization resistances of chemisorption of oxygen on the cathode and the oxygen diffusion and reaction in the cathode. The superior performance of ASC coupled with AFA alloys was due to the formed continuous alumina layer which can vastly decrease the evaporated gaseous Cr species, thus alleviating the Cr poisoning on the cathode region.

Research Progress of Alumina-Forming Austenitic Stainless Steels: A Review

The development of Alumina-Forming Austenitic (AFA) stainless steel is reviewed in this paper. As a new type of heat-resistant steel, AFA steel forms an alumina protective scale instead of chromia in a corrosive environment. This work summarizes the types of developed AFA steels and introduces the methods of composition design. Various precipitates appear in the microstructure that directly determine the performance at high temperatures. It was found that alloy elements and the heat treatment process have an important influence on precipitates. In addition, the corrosion resistance of AFA steel in different corrosive environments is systematically analyzed, and the beneficial or harmful effects of different elements on the formation of alumina protective scale are discussed. In this paper, the short-term mechanical properties, creep properties and influencing factors of AFA steel are also analyzed. This work aims to summarize the research status on this subject, analyze the current research results, and explore future research directions.

In-situ TEM investigation of dislocation loop evolution in Al-forming austenitic stainless steels during Fe+ irradiation: Effects of irradiation dose and pre-existing dislocations

Alumina-forming austenitic (AFA) stainless steel with excellent properties has been regarded as a promising candidate of fuel claddings in Supercritical carbon dioxide (S-CO2) gas cooled nuclear reactor. In current work, in-situ irradiation with 400 keV Fe+ in transmission electron microscope were performed on AFA stainless steel to investigate the influences of irradiation dose and pre-existing dislocation lines on loop evolution at 773 K. The irradiation damage caused by irradiation-induced loops was also analyzed. In-situ TEM observation displayed the evolution and characteristics of dislocation loops, including initiation, migration, aggregation, growth, annihilation, and combination. Both Frank loops with b = 1/3<111> and perfect loops with b = 1/2<110> were formed, and at 0.54 dpa, 1/2<110> loops accounted for the majority and was up to 64.6%. Pre-existing dislocation lines obviously affected not only the evolution and distribution of dislocation loops, but also the degree of irradiation hardening. At the same irradiation dose, not only the size and number density of dislocation loops in the region with dislocation lines were smaller than those in the region without dislocation lines, but also the increment of yield strength and irradiation damage was lower before the formation of dislocation network. Increasing pre-existing dislocation density should be beneficial to improve the irradiation resistance of materials, but it needed to be comprehensively considered with the mechanical properties of the material. Al component played an important role in irradiation behavior of AFA steels and the corresponding mechanism was discussed subsequently.

Tensile Deformation Behavior at High Temperature of a New Alumina-Forming Austenitic Stainless Steel

Tensile Deformation Behavior at High Temperature of a New Alumina-Forming Austenitic Stainless Steel

Reason for negative effect of Nb addition on oxidation resistance of alumina-forming austenitic stainless steel at 1323 K

To know reason for negative effect of Nb addition on oxidation resistance in alumina-forming austenitic stainless steels at high temperature, effects of Nb addition on microstructural evolution and oxidation resistance were investigated in Fe-25Ni-10Cr-4.5Al based steels after oxidation at 1323 K. The results showed that the degraded oxidation resistance due to the Nb addition resulted from suppressed outward diffusion of Al atoms by formation of Fe2Nb phase. Pre-aging at 1223 K could improve the oxidation resistance of the Nb-containing Fe-25Ni-10Cr-4.5Al steel due to the pre-precipitated B2-NiAl phase.

Effects of Cobalt Addition on the Precipitation Evolution and Tensile/creep Behavior of Fe-15Cr-25Ni-2.5Al-NbWCu Alumina-forming Austenitic Steel at 700°C

The effects of Co on the precipitation evolution and tensile/creep properties of the alumina-forming austenitic (AFA) stainless steel at 700°C were systematically investigated. Two investigated steels based on the Fe-15Cr-25Ni-2.5Al (in mass%) AFA steel were prepared; one is added 2.5 mass% W sorely, and the other is added 2.5 mass% W and 5 mass% Co together. After aging treatment, secondary NbC was precipitated in both steels and dispersed within the matrix. However, after adding Co to AFA steels, nanoscale L12 Ni–Al–Cu phase was not observed, and the amount of Laves and the δ-ferrite phase decreased with the same aging time, indicating that addition of Co improved microstructure stability. The tensile strength of AFA specimens with added Co did not significantly decrease, especially with a long-term aging treatment, compared to samples without added Co. However, introducing Co greatly improved the toughness of AFA steels. Also, the results of the creep fracture tests show that adding Co improved the creep life of AFA steels. The reason for this enhancement is that the formation of detrimental secondary phases at the grain boundary was greatly suppressed, after adding Co to AFA steels.

The formation mechanism, growth, and effect on the mechanical properties of precipitate free zones in the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al during creep

The mechanisms of formation and growth of L12 precipitate-free zones (PFZs) in the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al during creep were studied using scanning electron microscopy and energy dispersive x-ray spectroscopy. The PFZs formed following the dissolution of L12 precipitates due to depletion of nickel and aluminum. Little PFZ growth occurred until the grain boundaries were substantially covered by Laves phase and B2 precipitates large enough to deplete the surrounding grain of nickel and aluminum. This resulted in the dissolution of the L12 precipitates. Additionally, micro-cracks observed in the PFZ at long creep times suggest that the PFZ is ultimately the weakest point in the microstructure, and that fracture initiates there.

Analysis of the elevated temperature deformation mechanisms and grain boundary strengthening of the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al

The deformation mechanisms of an alumina-forming austenitic stainless-steel Fe–20Cr–30Ni–2Nb–5Al (at. %) were investigated at 750 °C using both creep tests and strain rate jump tests. It was found that nano-sized L12-structured precipitates were the dominant precipitate in the matrix and provided the majority of the strengthening. This behavior could be modeled well using a threshold stress term in a power law creep equation. Additionally, there appears to be a grain boundary strengthening mechanism in which the grain boundaries covered by precipitates act as barriers to dislocation movement. This could be again modeled using a power law creep equation. In addition to blocking dislocations, the grain boundary precipitates also prevent grain boundary sliding during creep. While the grain boundaries showed several strengthening mechanisms, all tests ultimately failed along the grain boundaries.

Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr2O3-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H2O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr2O3-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H2O to provide further guidance for SOFC BOP component alloy selection.

High-Temperature Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels Optimized by Refractory Metal Alloying

Alumina-forming austenitic stainless steels are known for their superior high-temperature oxidation resistance. Following our previous work that solved the matching of major alloying elements in their specific 16-atom cluster formula, we here focus on the 800 °C air-oxidation resistance of 0.08 wt. % C alloy series satisfying cluster formula [(Al0.89Si0.05NbxTa0.06−x)-(Fe11.7−yNiyMn0.3)]Cr3.0−z(Mo,W)z, x = 0.03 or 0.06, y = 3.0 or 3.2, z = 0.07 or 0.2, to explore the effect of minor alloying elements Mo, Nb, Ta and W. This cluster formula is established particularly based on alloys which were originally developed by Oak Ridge National Laboratory. All samples are graded as complete oxidation resistance level according to Chinese standard HB 5258-2000, as their oxidation rate and oxidation-peeling mass are generally below 0.1 g/m2 × h and 1.0 g/m2, respectively. In alloys without Ta and W, a Cr2O3-type oxide layer is formed on the surface and Al2O3 particles of sizes up to 4 μm are distributed beneath it. In contrast, in Ta/W-containing alloys, a continuous protective Al2O3 layer is formed beneath the outer Cr2O3 layer, which prevents internal oxidation and provides the lowest weight gain. Instead of internal Al2O3 particles, AlN is formed in Ta/W-containing alloys. The W-containing alloy possesses the thinnest internal nitride zone, indicating the good inhibition effect of W on nitrogen diffusion.

Critical Dynamic Recrystallization Model and Nucleation Mechanisms of an Alumina-Forming Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of an alumina-forming austenitic (AFA) stainless steel was investigated on a Gleeble-3500 simulator. Moreover, the critical dynamic recrystallization model was established, and the critical stress was determined. Microstructural characterization was analyzed using electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The microstructure evolution showed that the degree of dynamic recrystallization (DRX) and the content of high-angle grain boundaries increased significantly with the increasing temperature and the decreasing strain rate. For recrystallization mechanisms, the bulging mechanism was found dominating, while the sub-grain rotation mechanism played an assisting role. Furthermore, discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) coexisted during the hot deformation.