Alumina-forming Austenitic Stainless Steel Research Articles

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.

Investigations on the Accelerated Creep Testing of Alumina-Forming Austenitic Stainless Steel

Investigations on the Accelerated Creep Testing of Alumina-Forming Austenitic Stainless Steel

A Systematic Investigation of Precipitates in Matrix and at Grain Boundaries in an Alumina-Forming Austenitic Steel During Creep Testing at 700 °C

A systematic investigation of the microstructure in an alumina-forming austenitic (AFA) stainless steel at different time (0 to 2821 hours) reveals the diverse evolution of precipitates both at grain boundaries (GBs) and in matrix. In the initial stage at 700 °C, the second-phase precipitates in the AFA steel occurred in the sequence of M23C6 carbide at GB (0.5 hour), B2-NiAl phase at GB (1 hour), MC carbide in matrix (5 hours), and L12-Ni3Al in matrix (10 hours). During creep test, the coarsening of L12-precipitates in the matrix followed Oswald ripening law, i.e., the size was proportional to t1/3 (t as the creep time). However, the width of precipitation free zone near GB was proportional to t1/2 and B2 phase at GB increased linearly with time. Meanwhile, the M23C6 carbides, which mainly precipitated at GB, kept at nanoscale within 500 hours and grew ultimately to micrometer size. Consequently, the strength of GBs weakened faster than the grain interior and became similar to the strength of the grain interior after the equicohesive time tE. The creep rupture mode correspondingly changed from transgranular to intergranular after long-term creep rupture. Hence, both strengthening and weakening by precipitation ought to be considered for creep life prediction.

The precipitation control in aged alumina-forming austenitic stainless steels Fe-15Cr-25Ni-3Al-NbWCu by W addition and its effect on the mechanical properties

The effect of Tungsten (W) on the precipitate evolution and mechanical properties of the alumina-forming austenitic stainless steels (AFAs) Fe-15Cr-25Ni-3Al-NbWCu during aging at 700 °C was systematically investigated. After aging at 700 °C, microstructural characterization revealed that both of the secondary NbC and L12-ordered Ni-Cu-Al phases were precipitated in the matrix of the two steels for strengthening alloys, but W-added AFA (AFA-W) steel exhibited a smaller average size of Ni-Cu-Al phase than AFA steel. In addition, most of the precipitates in AFA steels were δ-ferrite phase with prolonging the aging time, and few Fe2Nb-type Laves phases were observed in the 1000 h-aged AFA samples. However, the addition of W promoted the precipitation of Fe2W-type Laves phases and increased its volume fraction during the aging period. The continuity of δ-ferrite at the grain boundary of AFA-W steels was also interrupted by the co-precipitated Fe2W Laves phase and B2-NiAl phase, avoiding the formation of detrimental δ-ferrite network. Compared to AFA steels, AFA-W steels exhibited higher room temperature tensile properties at the same aging time, which may be caused by the strengthening effect of more dispersed Fe2W phases and smaller sizes of Ni-Al-Cu phases in the grain interiors. Meanwhile, the stress concentration on the triple junctions caused by δ-ferrite was decreased by Fe2W precipitates; thus, further resulting in an increased ductility compared to the AFA steels without W addition.

Investigation of microstructure and liquid lead corrosion behavior of a Fe-18Ni-16Cr-4Al base alumina-forming austenitic stainless steel

The microstructure of an alumina-forming austenitic stainless steel with a composition design of Fe-18Ni-16Cr-4Al-2Mo-0.4 Nb (in wt%) is characterized. The steel contains about 75 vol% austenitic phase and 25 vol% ferritic phase. B2-NiAl precipitates with round shape can be found only in the ferritic phase in the as-rolled sample. The corrosion behavior in static liquid lead with different oxygen content of 10−9 and 10−6% by mass at 700 °C for 1000 h is investigated. After exposure for 1000 h in liquid lead with oxygen content of 10−9%, obvious lead penetration combined with nickel dissolution is found. In the case of liquid lead with 10−6% oxygen content, a thin oxide layer can be formed on the surface, thus protecting the steel from liquid lead attack. After the corrosion test, significant precipitations are found in both austenitic and ferritic phases in the matrix of the steel.

Design of alumina-forming austenitic stainless steel using genetic algorithms

Alumina-forming austenitic stainless steels (AFA-SSs) are candidates for high temperature applications to replace conventional stainless steels that form unstable chromium-oxide. In this work, a new AFA-SS composition was designed, through thermodynamic considerations, oxidation model calculations, and Genetic Algorithm-based optimization, which is expected to form alumina scale with only few number of alloying additions. The model incorporates thermodynamic stability of alumina with the kinetics of diffusion of aluminum and oxygen, and then considers the role of chromium as the third element, to predict alumina scale formability. In the present study, this model was utilized to design a new AFA-SS with minimum number of alloying additions. In addition, the oxidation behavior of the designed alloy was investigated at 800 °C to demonstrate its alumina forming capability and compared with a well-known alumina-forming steel. The cross-sections of the scales were studied using scanning electron microscopy and energy dispersive spectroscopy. The results confirmed the formation of the alumina layer in the designed alloy and validated the utility of the model and Genetic Algorithm-based optimization.

Effect of tungsten on the oxidation of alumina-forming austenitic stainless steel

As more W replaced Mo in alumina-forming austenitic stainless steels, weight gain by oxidation decreased after 336 h at 1053 K. Electron microscopy revealed slower growth of scale in the presence of more numerous second phases by W addition. The retardation of oxidation was attributed to the necessary partitioning of W in front of the metal-oxide interface. The W-rich second phases interacted with growing oxides and finally transformed to fine particles of metallic W alloy within the scale.

Effect of Aluminizing on the High-Temperature Oxidation Behavior of an Alumina-Forming Austenitic Stainless Steel

Due to their excellent oxidation resistance and creep resistance at elevated temperature, alumina-forming austenitic (AFA) stainless steels have emerged as potential alloys for high-temperature applications. In this study, a reactive air aluminizing method was used to aluminize the surface of AFA 25 stainless steel. The high-temperature oxidation behavior of both as-received and aluminized AFA 25 was investigated at 850°C in dry air. The mass gain per unit area was higher for aluminized AFA 25 than for as-received material. X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive x-ray spectroscopy, performed on the oxide scales following high-temperature oxidation tests, confirmed formation of alumina scale on both as-received and aluminized AFA. However, the aluminized alloy exhibited a thicker alumina scale on the surface, as well as more extensive penetration of the scale into the subsurface region of the alloy.

Ultrahigh stability and strong precipitation strengthening of nanosized NbC in alumina-forming austenitic stainless steels subjecting to long-term high-temperature exposure

For precipitation-strengthened heat-resistant steels, property degradation resulted from phase coarsening is always one of the most critical challenges, and therefore, knowledge regarding the long-term influence of temperature on mechanical properties and microstructure is prerequisite for their applications. In this work, long-term thermal aging up to 10,000 h of typical alumina-forming austenitic (AFA) stainless steels was conducted, and evolution of microstructure and mechanical properties of these promising heat-resistant alloys were studied systematically. It was found that mechanical performance of the steel strengthened by secondary nanosized NbC showed not only much enhanced creep resistance, superior to that of most traditional heat-resistant steels, but also stable high-temperature strength even after long-term aging. The spheroidal secondary NbC particles showed extreme coarsening resistance with a low coarsening kinetic constant (almost six orders of magnitude smaller than that of the Laves Fe2Nb phase). The low interfacial energy of nanosize NbC, which was resulted from the cube-on-cube orientation relationship and semi-coherent interfacial structure with the matrix, gave rise to its sluggish coarsening kinetics. Our current findings not only confirm that the AFA alloys strengthened by dense precipitated, nanosized semi-coherent NbC is a promising structural material for long-term high-temperature applications, but also sheds new insights into understanding precipitation hardening mechanism for high-temperature materials in general.

Effects of Nb/Ti/V/Ta on phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic stainless steels

The present work investigated the effects of strong carbide-forming elements M (M = Nb, Ti, V, Ta) on the phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic (AFA) stainless steels (Fe-20Ni-(18–23)Cr-2.5Al-2.3Mo-0.08C-Nb/Ti/V/Ta wt%). The designed alloy rods were solid-solutioned at 1523 K for 1.5 h, and then aged at 1073 K for 24 h or oxidized at 1073 K for 500 h, respectively. Besides primary MC carbides, B2-NiAl, Fe2M, and σ-FeCr phases would be precipitated from the γ-austenitic matrix in these aged AFA alloys. Furthermore, the precipitation of σ-FeCr could be accelerated by the B2-NiAl formation, which is related to the γ stability. The formation of MC and Fe2M can be controlled via adjusting the ratio of M/C. The oxidation resistance of these AFA alloys can be improved effectively by V and Ta addition due to the formation of compact Al2O3 protective scales. This work will provide a new approach to obtain a good combination of creep strength and oxidation resistance through co-alloying with Nb/Ta/V and controlling the M/C ratio within 1.0–2.0 (in molar ratio) simultaneously.

Preliminary creep testing of the alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2Nb-5Al

The alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2 Nb-5Al was given a solutionizing anneal at 1250 °C followed by anneals at 800 °C for 0, 2.4, 24, 240 h to produce B2 (ordered b.c.c.) and Laves phase precipitates of different sizes with different extents of grain boundary coverage. Both tensile tests and constant-stress (43 MPa) creep tests were performed on the heat-treated materials and on the as-cast alloy at 760 °C. The precipitates grew during the creep testing. In addition, L12 (ordered f.c.c.) precipitates nucleated and grew during the creep testing to similar particle sizes after 500 h independent of the prior heat treatment at 800 °C. The specimens given the shortest heat treatment of 2.4 h, which had the smallest initial particle sizes, showed both the highest yield strength and the smallest creep strain after 500 h. The extent of grain boundary coverage by precipitates did not appear to affect the creep rates. No grain boundary cracking or precipitate cracking was found in the heat-treated specimens after creep testing, but the as-cast material failed around 600 h.

Alumina-Forming 오스테나이트계 스테인리스강의 냉간가공을 통한 미세 NbC 석출 촉진과 크리프 특성 향상

Alumina-Forming 오스테나이트계 스테인리스강의 냉간가공을 통한 미세 NbC 석출 촉진과 크리프 특성 향상

High temperature deformation of Laves phase precipitates in alumina-forming austenitic stainless steels

Laves phases with the C14 type structure are present in recently-developed, alumina-forming austenitic (AFA) stainless steels. Here, for the first time, we show the Laves phase undergoes plastic deformation when such AFA steels are deformed at 700°C at strain rate of 5×10−4s−1. The gliding dislocations in the Laves phase leave behind stacking faults that we have analyzed using transmission electron microscopy. The stacking faults are on the basal plane of the Laves phase and are bounded by 1/3 〈101¯0〉 partial dislocations. These slip of dislocations are effective plastic deformation and improve the ductility of AFA materials.

Combined experiment and first-principles study of the formation of the Al2O3 layer in alumina-forming austenitic stainless steel

Combined experiment and DFT research on the formation of the Al2O3 layer in AFA stainless steel is presented. The oxide layer has the multilayer structure with outer Cr2O3 and inner Al2O3 due to the diffusion of Al from the matrix to the Cr2O3 slab.

Fireside Corrosion of Alumina-Forming Austenitic (AFA) Stainless Steels

The performance of four alumina-forming austenitic alloys was tested in simulative fireside corrosion atmospheres. The alloys were exposed at 700 °C to gas atmospheres of oxygen with varying amounts of SO2 with two deposits, one which contained Na2SO4/K2SO4/Fe2O3 in a 1.5:1.5:1.0 molar ratio (SCM) and the other Na2SO4/K2SO4 in a 1:1 molar ratio (M1). The specimens were placed into alumina crucibles and submerged halfway in the powder deposits. Corrosion first initiated in the region where the deposit was thinnest. In the SCM, most alloys exhibited two-stage kinetics: an initiation stage during which degradation was slow followed by a propagation stage during which degradation was rapid. Exposures in the SCM deposit showed a significant effect of alloy composition and microstructure. In the as-processed state, OC4 suffered severe scale spallation and was not protective, while OC8, which contains a higher Cr content, remained protective. The exposures with the various microstructures indicate the significance of the factor on fireside corrosion. The OC8 alloy was extremely resistant in the as-processed state but was rapidly degraded when the microstructure had been equilibrated prior to exposure. Conversely, the OCS and OCT alloys, which were borderline in the as-processed state were quite resistant after their microstructures had been equilibrated. The results indicate that corrosion resistance in these environments comes from alumina, but the alloys need more Cr and in a very accessible microstructure location to get the third-element effect to promote the alumina. The weight changes and the amount of degradation were more severe with M1 than with the SCM. There was more corrosion when the alkali iron trisulfates must be formed from a thermally grown iron oxide scale, than when the iron oxide was already in the deposit. In the latter case, the rate of degradation by “synergistic fluxing” was reduced.

The corrosion and stress corrosion cracking behavior of a novel alumina-forming austenitic stainless steel in supercritical water

The general corrosion and stress corrosion behavior of Fe-27Ni-15Cr-5Al-2Mo-0.4Nb alumina-forming austenitic (AFA) steel were investigated in supercritical water under different conditions. A double layer oxide structure was formed: a Fe-rich outer layer (Fe2O3 and Fe3O4) and an Al-Cr-rich inner layer. And the inner layer has a low growth rate with exposing time, which is good for improvement of corrosion resistance. Additionally, some internal nodular Al-Cr-rich oxides were also observed, which resulted in a local absence of inner layer. Stress corrosion specimens exhibited a combination of high strength, good ductility and low susceptibility. The stress strength and elongation was reduced by increasing temperature and amount of dissolved oxygen. In addition, the corresponding susceptibility was increased with decreased temperatures and increased oxygen contents.

Development of Cast Alumina-Forming Austenitic Stainless Steels

Cast Fe-Ni-Cr chromia-forming austenitic stainless steels with Ni levels up to 45 wt.% are used at high temperatures in a wide range of industrial applications that demand microstructural stability, corrosion resistance, and creep strength. Although alumina scales offer better corrosion protection at these temperatures, designing cast austenitic alloys that form a stable alumina scale and achieve creep strength comparable to existing cast chromia-forming alloys is challenging. This work outlines the development of cast Fe-Ni-Cr-Al austenitic stainless steels containing about 25 wt.% Ni with good creep strength and the ability to form a protective alumina scale for use at temperatures up to 800–850°C in H2O-, S-, and C-containing environments. Creep properties of the best alloy were comparable to that of HK-type cast chromia-forming alloys along with improved oxidation resistance typical of alumina-forming alloys. Challenges in the design of cast alloys and a potential path to increasing the temperature capability are discussed.

Evolution of microstructure and tensile properties of Fe–18Ni–12Cr based AFA steel during aging at 700 °C

The effects of aging at 700°C on the microstructure and tensile properties of an alumina-forming austenitic (AFA) stainless steel were investigated. With increasing aging time, B2-NiAl and Laves phase precipitated first on grain boundaries (GB) and then in the grain interior. The GB precipitate coverage reached 74% after aging for 1000h. The GB precipitates not only suppressed grain coarsening during aging, but also influenced the tensile fracture mode at 700°C by partitioning stress concentration on triple junctions. Moreover plate-like and spherical NbC particles precipitated during aging. Spherical NbC with size of around 5nm were stable, while plate-like NbC grew to 89nm after aging for 1000h. These precipitates played an important role on the tensile strength. Age hardening contributed to the increasing tensile strength at RT with aging time, while the softening mechanism of dynamic recovery dominated the tensile tests at 700°C.

Micro-alloying Effects of Yttrium on Recrystallization Behavior of an Alumina-forming Austenitic Stainless Steel

Micro-alloying effects of yttrium on the recrystallization behavior of an alumina-forming austenitic (AFA) stainless steel were investigated. It was found that the grain growth kinetics of the steels doped with different amounts of yttrium (i. e., 0, 0.05 and 0.10 mass% Y) could be described by an Arrhenius type empirical equation. Added Y could interact with carbon and influence the morphology of carbides both inside grains and on the grain boundaries, thus altering the grain boundary mobility and grain growth. The steel doped with 0.05 mass% yttrium showed the highest activation energy of grain growth and the most retarded recrystallization behavior, which mainly resulted from the high density of fine carbides both inside grains and on the grain boundaries. However, excess addition of 0.10 mass% Y induced coarsening and then lowered density of carbides, which alleviated the yttrium effects. The results also manifest that micro-alloying of rare-earth elements such as yttrium is an effective way for controlling grain growth behavior during recrystallization of AFA steels, which may have great implications on engineering applications.