Alumina-forming Austenitic Steel Research Articles

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.

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.

Oxidation and dissolution behavior of FeCrNiAl based high entropy alloy and alumina-forming austenitic steel exposed to lead-bismuth eutectic (LBE)

Developing corrosion-resistant alloys is essential for the application of lead-cooled fast reactors. Herein, two types of FeCrNiAl alloys (high entropy alloy, HEA and alumina forming austenitic steel, AFA) are fabricated and their long-term corrosion (up to 3000 h) in lead-bismuth eutectic (LBE) at high temperature are investigated. Both alloys illustrate oxidation and dissolution behaviors, a Cr-Al enriched layer forms on AFA and the B2-NiAl phase in HEA demonstrates outstanding oxidation resistance and stability in LBE. This work offers guidelines for alloy design, especially optimizing the phase distribution of new nuclear materials that to be used in LBE.

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.

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.

Preferential distribution behavior of B and its enhancement on creep strength of alumina-forming austenitic heat-resistant steel

The addition of a trace amount of 20 ppm B significantly enhanced the creep performance of the 2.5Al-AFA alloy at 650 °C, demonstrating sustained improvement even after prolonged thermal aging. Boron (B) preferentially distributed in the Laves phase within the matrix, rather than in the NiAl or Cr - rich phases. APT analysis revealed M23C6 may serve as a reservoir of B, gradually allowing B diffusing into the surrounding Laves phase at grain boundaries (GBs). Creep cracks propagated mainly along GBs, with the σ phase serving as initiation sites. The average volume fraction of σ phase exhibited a linear correlation with the reciprocal of creep rupture time. B accelerated the precipitation of Laves with a larger size and higher volume fraction, which played a dual role of improved GB strengthening and more effective suppression of σ phase growth.

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.

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%.

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.

Irradiation accelerated corrosion of alumina-forming austenitic steels in supercritical CO2: The oxide scale formed within an individual grain or affected by grain boundary

The irradiation accelerated corrosion within monolayer grains of a new AFA steel exposed to sCO2 are studied. The presence of irradiation induced dislocation loops accelerate element diffusion and reduces corrosion resistance by ∼44%. Many oxide scale features are firstly found, including a two-phase inner layer consisting of Ni-rich austenite and Cr-rich spinel, and Al2O3/SiO2 oxide precipitates. Irradiation leads to an additional Cr-rich oxide layer at the bottom of inner layer and a narrower Al2O3 band in the internal oxidation layer. Irradiation also induces Cr depletion at grain boundaries, thereby impeding the formation of a protective Cr-rich oxide scale.

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%.

Strengthening and fracture mechanisms of Fe–20Ni–14Cr–2Cu alumina-forming austenitic steel during creeping

Strengthening and fracture mechanisms of Fe–20Ni–14Cr–2Cu alumina-forming austenitic steel during creeping

Mechanical behavior in liquid lead of Al2O3 coated 15-15Ti steel and an Alumina-Forming Austenitic steel designed to mitigate their corrosion

The mechanical behavior in liquid lead of the 15-15Ti steel coated by an alumina layer and an Alumina Forming Austenitic (AFA) steel, with a chemical composition allowing for the formation of alumina at the surface of the steel, has been studied. To investigate the liquid metal embrittlement (LME) sensitivity by liquid lead, tensile tests in air and in liquid lead have been carried out at 400 °C and 500 °C. Then analyses of the cracking and of the fracture surfaces have been performed. No LME sensitivity at the tested conditions was observed for the Al2O3 coated 15-15Ti steel. The AFA steel is not sensitive to LME at 400 °C but suffers from LME at 500 °C. For the AFA steel, the liquid lead promotes at 500 °C an intergranular propagation of surface cracks which suggests a grain boundary wetting-dominated LME mechanism of this steel at 500 °C.

Influence of precipitates evolutions in δ-ferrite and austenite matrix on mechanical properties of alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel with great application potential in components of power plants has been developed due to its excellent high-temperature properties and corrosion resistance. Evolution of precipitates was characterized using multiple methods, and its effect on mechanical properties of Fe–20Ni–14Cr–3Al alumina-forming austenitic (3Al-AFA) steel after isothermal-aging at 973K was also assessed. The results showed that the particle size and volume fraction of precipitates (mainly Laves, B2–NiAl and σ phase) increase with the duration of isothermal-aging. The volume fraction of δ-ferrite increases significantly also with the extension of isothermal-aging time, which is regarded as a harmful factor to mechanical properties of AFA steel. The strength and Vickers hardness firstly increase and then decrease with the extension of isothermal-aging time, which is inversely proportional to the elongation at the initial stage of isothermal-aging. Both strength and elongation decrease with the isothermal-aging time between 500 h and 1000 h due to the δ-ferrite and coarse precipitates therein. Cr element firstly prone to segregate and diffuse in δ-ferrite, which promotes the diffusions of other alloying elements from matrix to δ-ferrite, resulting in the coarsening of precipitates in δ-ferrite region. The highest yield strength (1171 MPa) is achieved when the size of Laves and B2–NiAl precipitates in matrix is about 100–150 nm.

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.

High-temperature creep property deterioration of the alumina-forming austenitic steel: Effect of σ phase

The microstructural evolution of the alumina-forming austenitic (AFA) steel subjected to different cold rolling reductions during creep test at 973 K and 120 MPa was investigated by multiple characterization methods, and the high temperature creep property was also examined. It was found that the creep property of CR10 sample is not satisfactory although cold rolling truly improved it. The creep cavities were observed to nucleate at the interface between B2–NiAl phase and σ phase in the δ-ferrite. The serious coarsening of B2–NiAl phase and σ phase in δ-ferrite was considered as the main factors for the decline of creep property. In addition, the results also suggested that the shape of B2–NiAl phase changes from round to plate with increasing creep time, which resulted from the increase of elastic strain energy. Simultaneously, it was noteworthy that σ phase always grows adjacent to plate-shape B2–NiAl phase, which indicates the coarsening of σ phase is inseparable from that of B2–NiAl phase.

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.

Understanding the stress corrosion cracking growth mechanism of a cold worked alumina-forming austenitic steel in supercritical carbon dioxide

The crack growth behavior of a cold worked alumina-forming austenitic (AFA) steel in supercritical carbon dioxide (sCO2) and Ar is studied. AFA steel exhibits higher crack growth rates in sCO2 than in Ar in the temperature range of 550 °C~650 °C. Creep cavities are widely observed near the crack growth paths and ahead of the crack tips on the specimens tested in both environments. Higher cavity density is observed at the crack tip of the specimen tested in sCO2, which is caused by oxidation and carburization. The high cavity density weakens the grain boundaries and accelerates the fracture of the materials.