Alumina-forming Austenitic Research Articles

Influence of liquid lead and lead-bismuth eutectic on three alumina forming austenitic (AFA) steels through slow strain rate testing

Liquid metal embrittlement (LME) in three newly developed alumina-forming austenitic (AFA) alloys, two 50 kg batches and one 5-ton heat, was studied in the temperature range 350–600 °C in liquid Pb and 140–600 °C in LBE using slow strain rate testing (SSRT) in a low-oxygen environment. No significant decrease in the engineering strain was observed in either environment. However, the presence of secondary cracks along the length of the specimen and brittle intergranular areas on the fracture surfaces indicates that the AFA alloys do show a minor degree of embrittlement above 570 °C. This appears to be related to grain boundary wetting by Pb/LBE. At temperatures below 570 °C, this wetting effect does not seem to be strong enough to induce LME in the alloys, and their ability to form a sufficiently protective oxide means that they remain unaffected by LME. The results indicate that the AFA alloy group can perform sufficiently well in liquid Pb/LBE environments, and long-term testing should be carried out to determine their viability as candidate materials for use in Pb- and LBE-based cooling systems.

New insight on the formation of uneven oxide scales on AFA steel in supercritical CO2: Roles of recrystallization degree on the high temperature corrosion resistance

Uneven oxide scale is a common phenomenon in high-temperature corrosion, but the formation mechanism has not been fully uncovered. A new alumina-forming austenitic (AFA) stainless steel with superior corrosion resistance in 650°C/15MPa SC-CO2, has been systematically investigated. It is demonstrated that the uneven oxide was formed by the significant impact of the recrystallization behavior of AFA steel on its microstructural evolution and subsequent corrosion behavior. Compared with the thin oxide scale, the thick oxide scale was caused by the relatively higher density of dislocations and grain boundaries of matrix resulting from delayed recrystallization, which facilitated the rapid outward diffusion of ions.

Strength stability at high temperatures for additively manufactured alumina forming austenitic alloy

Several fast-spectrum nuclear reactors designed to generate high power (∼450 MWe) rely on forced convection of media such as supercritical CO2, sodium, or liquid lead to cool the nuclear core, operating at temperatures up to 600 °C. Cost-effective, high-strength Fe-based alumina forming austenitic (AFA) alloys are a promising candidate for the fabrication of critical nuclear components. This study investigated laser powder bed fusion (LPBF) processing of an AFA alloy composition optimized for improved creep resistance. Electron microscopy revealed an elongated grain structure along the build direction with a fine sub-grain cellular structure decorated with (Cr,Fe,Nb)23C6 carbide precipitates at the intercellular boundaries. At temperatures of 20–900 °C, the LPBF alloy's superior tensile properties compared to its arc-melted counterpart and other advanced steels (e.g., SS316) were attributed to the distribution of nano-sized carbide precipitates, whereas the high ductility was attributed to the LPBF alloy's elongated grain structure.

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.

Cast Alumina-Forming Austenitic Stainless Steels for High Temperature Heat Treatment Furnace Rolls

Abstract Alloy developers worldwide have struggled to create creep-resistant alumina-forming, iron-based austenitic stainless steels for use as high-temperature structural alloys, but with limited success in balancing alloy cost, oxidation, and creep resistance. This article describes the research and development of a novel cast alumina-forming austenitic stainless steel. This work won the prestigious Engineering Materials Achievement Award presented at IMAT 2023 in Detroit.

Creep deformation and damage behavior of an alumina-forming austenitic alloy strengthened by intermetallic compounds

Based on Fe–30Ni–15Cr, a new alumina-forming austenitic (AFA) alloy, free of carbon, was designed for potential use in ultra-supercritical generator set. The creep behavior of alloy has been investigated by analyzing the creep constitutive equation, creep damage tolerance factor (λ), microstructure images, and oxide layer structure under different stresses at 750 °C. The creep microstructure and oxide morphology were obtained by electron microscopy. The minimum creep rate (ε˙m) is positively correlated with the stress, and when the stress increases from 120 MPa to 250 MPa, ε˙m increases from 6.26E-3 1/h to 6.34E-4 1/h, almost ten times of the latter. The creep constitutive equation at 750 °C was determined as: ε˙m=9.29×10−11σ3.3. B2–NiAl and L12-Ni3Al were precipitated during creep. The relationship between the misfit of nanoscale L12-Ni3Al phase and γ matrix are perfectly coherent. The L12-Ni3Al phase grows more slowly and remains at the nanoscale size after creep at 750 °C for 869h, which contribute to increase creep strength and reduce the creep rate of the material. However, the coarsened σ-FeCr phase reduced the creep productivity of the alloy. The microstructural degradation caused by precipitate coarsening was found to be responsible for creep failure, as indicated by the creep damage tolerance factor (λ) and creep fracture morphology. The elemental distribution of the oxide layer indicates the absence of a dense alumina protective scale at 750 °C. The extensive presence of oxidation cracks in the matrix suggests that the effect of oxidation cannot be ignored in the analysis of creep life. The fact that σ-FeCr is detrimental to creep damage indicates that the alloy composition needs further optimization.

Variety of yield strength caused by precipitations of L12-Ni3Al and B2-NiAl phases in alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel exhibits excellent high-temperature properties and has attracted significant attention in thermal power generation industry. Microstructure evolutions and yield strengths of 20Ni-AFA steel during isothermal aging are investigated. The results show that five types of phases are found in 20Ni-AFA steel during isothermal ageing: NbC, L12-Ni3Al, M23C6, B2-NiAl and Laves phases. Addition of Cu causes precipitation of L12-Ni3Al phase in 20Ni-AFA steel at the early stage of isothermal aging. L12-Ni3Al phase grows and gradually transforms into B2-NiAl phase which possesses a more stable structure with the extension in isothermal aging time. The transformation leads to yield strengths of 20Ni-AFA steel to decrease anomalously during 24-120 h isothermal aging. Yield strengths of 20Ni-AFA steel increase again as isothermal aging time continues to increase.

Gradient Cr–AlN composite coating prepared on alumina-forming austenitic steel

A gradient composite coating of Cr–AlN with an outer Cr-rich layer and an inner AlN-rich layer was deposited on the alumina-forming austenitic (AFA) steel surface by pack-cementation chromising. The phase composition, element distribution, and microstructure of the coating were characterised and analysed. The microhardness and the friction and wear performance at 600 °C were tested. The results show that the outer Cr-rich layer mainly consists of a polycrystalline Cr–Fe solid solution and a small amount of Cr2C and (Cr, Fe)2N1− x particles. The inner layer contains plenty of AlN particles with different sizes dispersed on the Cr–Fe solid solution. The formation of AlN particles is closely related to the spontaneous chlorination and nitridation reaction. The addition of Y2O3 particles in the encapsulated powder promotes the uniformity of Cr plating and the growth of AlN particles. The gradient composite coating remarkably improves the microhardness and high-temperature wear resistance of AFA steel.

Mechanisms for high creep resistance in alumina forming austenitic (AFA) alloys

Castable alumina forming austenitic (AFA) alloys have demonstrated superior creep life and oxidation resistance at temperatures exceeding 800⁰C. Despite the success in the applicability of these alloys in extreme environments, there is a limited understanding of the deformation modes and the influence of each alloying element guiding the alloy design strategies that could further enhance the creep strength of these AFA alloys, particularly at temperatures at and above 900⁰C. In this study, we reveal the mechanism underpinning the superior creep performance of castable AFA alloys that involves suppressing primary carbide formation through minor compositional modification. This approach results in a three-fold increase in creep strength at 900⁰C and 50 MPa. By employing integrated characterization techniques, we analyzed the microstructures of two AFA alloys, both before and after the creep process. We discovered that the suppression of primary carbides permits the in-situ clustering of now-available interstitial elements such as C, Si, and O during high-temperature creep. This improved solid solution strengthening and reduced stacking fault energy of the alloy. Moreover, it also enabled controlled secondary carbide formation during testing, further improving the creep resistance. These findings underline the important interplay between alloy composition, microstructure, and creep properties, and offer a promising design strategy for developing economical high-temperature Fe-based alloys suitable for advanced applications.

Corrosion behavior of AFA steel in lead-bismuth eutectic alloy with saturated oxygen at 500°C

Alumina-forming Austenitic (AFA) steel is one of the candidate corrosion-resistant structural materials for lead-cooled fast reactors, exhibiting excellent high-temperature corrosion resistance. However, its corrosion mechanisms in lead-bismuth eutectic (LBE) environment at the design operating temperature of lead-cooled fast reactors have not been fully elucidated. In this study, AFA steel was immersed in static LBE with saturated oxygen at 500 °C for 3000 hours. The corrosion behavior of AFA samples was obtained by analyzing the morphology of corrosion cross section and the distribution of elements. The results showed that the AFA steel did not form an oxide film that could effectively resist LBE. Dissolution corrosion of the matrix becomes more severe the longer the exposure time. When the NiAl phase in the matrix is located at the surface, oxide nodules are generated. It has a double-layer oxide structure, which has better resistance to LBE corrosion. Nb undergoes oxidative rupture and has a tendency to detach from the surface of the substrate when it is located on the surface. Cracks occur on the surface of the matrix after 3000 hours of corrosion, and LBE penetrates into the matrix along the cracks.

Long-term oxidation and chromium evaporation behavior of Al2O3-forming austenitic stainless steel for 900 °C balance-of-plant components applications in solid oxide fuel cells

The long-term operation of balance-of-plant (BoP) components in solid oxide fuel cells (SOFCs) relies on the presence of a stable and durable oxide layer. In this study, we investigate the oxidation and chromium (Cr) evaporation behaviors of two developmental alumina-forming austenitic (AFA) alloys compared to chromia-forming alloy 625 at 900 °C in air with 10% water vapor. Transpiration tests, weight gain tests, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and scanning transmission electron microscopy with energy dispersive X-ray analysis are employed to evaluate the oxidation and Cr evaporation behaviors. Our findings reveal that alloy 625 exhibits significantly higher rates of Cr evaporation compared to OC11 (Y and Hf additions) and OC11LZ (Y and Zr additions), with evaporation amounts ∼56 and ∼28 times greater, respectively. The observed differences between OC11 and OC11LZ can be attributed to variations in the formed oxide scales during long-term operation. Furthermore, we examine the influence of Hf and Zr reactive elements on the long-term oxidation and chromium evaporation behaviors, providing insights into the role of these elements in enhancing the performance and stability of the alloys.

Preliminary Characterization of Alumina-Forming Austenitic–Type Advanced Alloys as Structural Materials for LFRs

The lead-cooled fast reactor (LFR) is one of the most promising Generation-IV nuclear designs currently under development in Europe, China, and the United States. LFRs can ensure enhanced performance and minimal waste production thanks to a closed fuel cycle, but they also have some issues that need to be addressed. One of the most critical is the long-term degradation process initiated in structural materials exposed to liquid Pb. The present state of the art has shown that commercial austenitic steels, such as American Iron and Steel Institute 316L and 15-15Ti can be adopted as structural materials in Pb environments up to 480°C, beyond which they start to experience the dissolution of constituting alloying elements (Ni, Cr, and Fe) if not protected by a coating or by surface modification. In more recent years, a lot of research effort has been done in order to develop new coating technologies and new base materials for operation with liquid Pb at higher temperatures. Among the newest alloys, alumina-forming austenitic (AFA) steels have gained interest in the research community because of their promising corrosion resistance results even at temperatures of 600°C. In this framework, an experimental campaign has been run at the Research Center ENEA of Brasimone that aims to characterize the behavior of two different AFA steels (with low and high Ni content in their composition) in static Pb at 650°C and 750°C with a moderate low oxygen concentration (10−6 wt %). After exposure, the AFA steels were characterized from the point of view of the morphology and composition, and the results are presented and discussed here.

Fe-14Ni-14Cr-2.5Al steel showing excellent corrosion-resistance in flowing LBE at 550 °C and high temperature strength

We investigated the corrosion behavior, microstructural evolutions, and mechanical properties in an alumina forming austenitic (AFA) Fe-14Ni-14Cr-2.5Al steel after exposure in the flowing lead-bismuth eutectic (LBE) at 550 °C for 4008 h and aging in air for 5000 h. The results showed that the AFA steel exhibited not only excellent corrosion-resistance in the flowing LBE but also a strong stability of austenite and high yield strength (589 MPa) at 550 °C. The excellent corrosion-resistance was ascribed to the rapid formation of a thin Al2O3 scale. For the first time, the precipitation of γ'-Ni3Al phase was observed in the AFA steel after long-term aging at 550 °C.

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.

New insights into the initial oxidation behavior of an ultra-corrosion resistant alumina-forming austenitic steel in supercritical carbon dioxide: The effects of precipitated intragranular intermetallic compounds

Effects of precipitated intragranular intermetallic compounds on the initial oxidation behavior of a new alumina-forming austenitic (AFA) steel and its base steel in supercritical carbon dioxide are investigated. Laves phase is the only precipitate in the base steel, which leads to the internal oxidation and destroys the integrity of oxide. B2-NiAl, Sigma and Laves phases are precipitated in AFA steel. B2-NiAl particle acts as Al-reservoir and Laves particle retards the formation of thick Cr-rich oxide. Sigma phase accelerates the growth of Cr/Si-rich oxides, which contributes to the formation of a thicker Cr/Si-rich oxide layer.

Challenges in computationally designing high temperature Fe-based austenitic alloys: Addressing the role of Ni additions

Alumina-forming austenitic (AFA) alloys are relatively inexpensive high performance materials which combine the creep resistance of low-cost austenitic alloys and the oxidation resistance of expensive alumina forming alloys. However, a fundamental understanding of the role of key alloying elements such as Ni, Cr, Al, Nb, Ti, V, B and C in the experimentally observed oxidation behavior of these alloys is still lacking. The present work is a first in a series of studies aiming to quantitatively describe the role of Ni in promoting or disrupting protective Al2O3 scale formation on AFA alloys. Ternary Fe-Al-xNi model alloys with three different Ni contents were isothermally exposed in an atmosphere with a low partial pressure of oxygen between 800–1000 °C for 24 h to evaluate the role of Ni in the observed internal oxidation behavior. Increasing Ni contents had no impact on the internal oxidation behavior of the alloys. The experimental and theoretical analyses in the present work suggested a negligible effect of the internal oxide precipitates on the inward diffusion of oxygen, typically expected in these systems, while simultaneously highlighting the barriers in the development of reliable models for computation-assisted design of these alloys.

Effects of Mn content on austenite stability and mechanical properties of low Ni alumina-forming austenitic heat-resistant steel: a first-principles study

Low Ni alumina-forming austenitic (AFA) heat-resistant steel is an advanced high-temperature stainless steel with reduced cost, good machinability, high-temperature creep strength, and high-temperature corrosion resistance. Using the First-principles approach, this study examined the effect of Mn content on austenite stability and mechanical properties at the atomic level. Adding Mn to low Ni-AFA steel increases the unit cell volume with an accompanying increase in the absolute value of formation energy; the austenite formed more easily. The austenitic matrix binding energy decreases and remains negative, indicating austenite stability. As the Mn content increases from 3.2 to 12.8 wt%, the system's bulk modulus (B) rises significantly, and the shear modulus (G) falls. In addition, the system's strength and hardness decrease, and the Poisson ratio of the austenite matrix increases with improved elasticity; the system has excellent plasticity with an increase in the B/G. For the Fe22–Cr5–Ni3–Al2 system, with the increase of Mn content, the electron density distribution between the atoms is relatively uniform, and the electrons around the Mn atoms are slightly sparse, which will slightly reduce the structural stability of the matrix. The experiment demonstrated the matrix maintains the austenitic structure when adding 3.2–12.8 wt% Mn elements to low Ni-AFA steel. At an Mn content of 8 wt%, the overall mechanical properties of the high-Mn AFA steel are optimal, with a tensile strength of 581.64 MPa, a hardness of 186.17 HV, and an elongation of 39%.

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.

Advanced characterization of two novel Fe-rich high entropy alloys developed for laser powder bed fusion in the Al-Co-Cr-Fe-Ni-Zr system

Laser powder bed fusion (LPBF) was used to manufacture two high entropy alloys (HEAs) within the Al-Co-Cr-Fe-Ni-Zr system. The selected compositional ranges were similar to alumina forming austenitic (AFA) steels but omitting interstitials and replacing Nb with Zr as the Laves forming element. The Fe-rich austenitic HEAs were prepared with varying Al and Zr content to evaluate the influence on the presence, size, and distribution of the intermetallic (IM) precipitates. The LPBF process combined with a single (950 °C 6 h) heat treatment formed large, elongated grains with a fine dispersion of multiple different nano-sized IM phases. Synchrotron high energy x-ray diffraction (HEXRD) revealed the cubic M23Zr6 as the main Zr-rich IM phase, stabilized by the multi-element mixture (M=Co, Fe, Ni) and the high cooling rates of the LPBF process. Further HEXRD in-situ compression was performed from room temperature to 900 °C to evaluate the phase stability, thermal expansion, and the strength contribution of the austenitic matrix and the M23Zr6 and NiAl B2 IM phases. The evolution of lattice strain and full width at half maximum of the reflexes was tracked using a statistical model, enabling quantitative analysis along the deformation.

Investigation of isothermal continuous hot deformation behavior of the new as-cast alumina-forming austenitic alloy

Multi-pass isothermal continuous hot deformation experiments were designed for homogenization and recrystallization in the as-cast alumina-forming austenite (AFA) heat-resistant alloy. Flow curves and microstructure characterization were used to study the grain evolution with different deformation parameters in each pass. In particular, attention was paid to the dendrites and segregated phase. Under different hot deformation conditions, the alloys show significant work-hardening characteristics, which result from the strengthening effect of the primary segregated MC-type carbide. The increase in deformation temperature contributes significantly to dynamic recrystallization (DRX), and the main mechanism of DRX is discontinuous DRX (DDRX). The continuous DRX (CDRX) is less occurring. Static recrystallization (SRX) occurs during the isothermal holding time between deformation passes. SRX becomes more and more apparent with temperature and the holding time. The increase in the deformation passes leads to a gradual disappearance of the alloy dendrites and an increase in the recrystallization volume fraction. Meanwhile, the segregated phase gradually decomposes into particles and partially dissolves. However, the segregated phase particles eventually show band-like distribution. SRX grains around the segregated phase will exhibit similar orientations due to particle-stimulated nucleation (PSN) mechanisms. Until the grain size increases to the point that it can escape from particle restraint, it will transform to random orientation. The above study confirms that multi-pass isothermal continuous hot deformation can improve the as-cast microstructure and guide practical industrial production.