Alumina-forming Austenitic Research Articles

High-temperature oxidation behavior of modified 4Al alumina-forming austenitic steel: Effect of cold rolling

The oxidation behavior and mechanism of as-received and 30 % cold-rolled alumina-forming austenitic (AFA) steel were investigated in dry air at 700 °C. The results show that the mass gain per unit area curves of as-received and 30 % cold-rolled steels subject to near-parabolic law before 100 h oxidation time. Two samples both show higher high-temperature oxidation resistance due to the formation of dense Al2O3 oxide scale. Gradual spallation of outer scale results in the formation of continuous and dense alumina scale. Dislocations can act as short-circuit diffusion channel for the diffusion of Al from alloy matrix to surface, and also provide nucleation sites for B2-NiAl phase, which ensure the continuous formation of Al2O3 scale.

Corrosion behaviour of Al-added high Mn austenitic steels in molten lead bismuth eutectic with saturated and low oxygen concentrations at 450 ℃

Corrosion tests were performed on newly developed alumina-forming austenitic (AFA) steels in stagnant lead bismuth eutectic (LBE) with saturated and low oxygen concentrations at 450 ℃ for 430 h. The steels exhibited enhanced corrosion resistance to the LBE environments with the increasing of Al content. A continuous and protective Al-rich oxide scale formed on the steel specimens that were exposed to LBE with a low oxygen concentration, whereas a non-protective and stratified oxide scale formed in the oxygen saturated LBE.

Oxidation Behavior of Alumina-Forming Austenitic Steels in Superheated Steam at 700 °C

Abstract In this study, three alumina-forming austenitic (AFA) steels (Fe-18Ni-12Cr-AlNbC, Fe-18Ni-12Cr-Al, and Fe-18Ni-16Cr-AlNbC) were exposed to superheated steam (SHS) at 700 °C for 1000 h to study their oxidation behavior. The Fe-18Ni-16Cr-AlNbC alloy showed the best performance as it had the least weight gain due to a denser surface oxide layer and the lack of internal oxidation. The Fe-18Ni-12Cr-AlNbC alloy suffered from internal oxidation and had a greater weight gain than Fe-18Ni-16Cr-AlNbC, while Fe-18Ni-12Cr-Al experienced weight loss due to perhaps external oxide spallation, leaving a less protective Fe-oxide on the external surface.

The Development of Alumina Forming Austenitic Alloy for Core Application in Advanced Nuclear Reactors

The development of materials for core components which can serve in high temperature corrosive environments for a long service time is crucial to realize high efficiency and high-burnup operation of advanced nuclear reactors. Alumina forming austenitic (AFA) alloy is a kind of promising materials with improved corrosion resistance as well as strength at elevated temperature. The progress on the composition design and characterization of AFA alloys are reviewed in this work for evaluation their potential applications in advanced nuclear reactors. AFA alloys without the addition of carbon have been fabricated. Microstructures were observed by SEM and TEM. Mechanical properties were measured at room temperature and high temperature.

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.

Oxidation behavior and microstructure evolution of alumina-forming austenitic & high entropy alloys in steam environment at 1200 °C

Oxidation tests of alumina-forming austenitic (AFA) alloys and high entropy alloys (HEA) have been performed in steam at 1200 °C. AFA alloys with composition formula Fe-3Al-16Cr-(19-21)Ni (wt.%) and HEA with formula Al(7.9-8.9)Cr(21.4-23.2)Ni(34.3-35)Febal (at.%) show low oxidation rate due to the formation of α-Al2O3 scale. By adding yttrium to AFA or niobium to HEA, the samples show no evidence of scale exfoliation. HEA with titanium addition forms a thick oxide layer based on TiO2, Cr2O3 and internal Al oxidation. The growth of oxide scale is discussed based on the oxygen partial pressure of metal oxides and dissociation oxygen pressure of the steam.

Microstructure and mechanical property evolution of an AFA alloy with simple composition design during ageing at 700 °C

The development of core component materials is one of the key issues for the engineering application of advanced energy systems. A Fe–18Ni–12Cr–3Al alumina-forming austenitic (AFA) alloy without the addition of C and Nb was fabricated by vacuum induction melting and post hot deformation treatments. In order to evaluate its stability during long term service, the evolution of microstructure and mechanical properties during aging at the target application temperature of 700 °C was investigated. Laves phase begins to precipitate along grain boundaries after aging for 10 h. After aging for 100 h, the amount of Laves phase along the grain boundaries increase obviously, but still almost no precipitate forms inside the grains. Apart from continuously increased Laves phase along the grain boundaries, a lot of NiAl particles precipitate inside grains after aging for 1000 h. Obvious age hardening occurs after aging for 1000 h, due to the precipitation of NiAl phase, which results in significant increase of tensile strength at room temperature.

EBSD analysis and mechanical properties of alumina-forming austenitic steel during hot deformation and annealing

Microstructural evolutions during hot deformation and annealing of alumina-forming austenitic (AFA) steel were investigated using electron backscatter diffraction (EBSD). Tensile properties of deformed and as-received AFA steels were also tested. The yield strength and ultimate tensile stress of deformed AFA steel increase gradually with the increase in rolling reduction, while the fracture elongation decreases rapidly. Specifically, after hot-rolling with the reduction of 40%, the yield strength of the annealed specimen is high to 773.2 MPa, and the fracture elongation of this sample is about 43.6%. The excellent mechanical properties can be attributed to the smaller average grain, larger Schmid factor and coincident site lattice (CSL) boundaries. Such a rapid decline of the elongation of AFA steel occurs due to increasing low angle grain boundaries (LAGB, <15°), the crystal defects and the degree of work hardening with the increase in rolling deformation. An approachable strategy to improve the tensile properties of annealed samples is to effectively hinder the dislocation motion during annealing at 700 °C when few LAGBs could transform into high angle grain boundaries (HAGB, >15°).

Corrosion evaluation of alumina-forming alloys in carbonate molten salt for CSP plants

The use of carbonate molten salts for the new generation of concentrated solar power (CSP) plants have been considered and analysed during the last years to improve the efficiency of energy generation. However, the high temperature and corrosivity of the salts pose a risk on safety and profitability of the technology requiring more resistant materials. In this study, two alumina-forming austenitic (AFA) alloys corresponding to modified OC4 and HR224 grades, were exposed to the eutectic ternary Li2CO3K2CO3Na2CO3 (32.1–34.5–33.4 wt%) salt mixture at 650 °C for 1000 h. The evolution of weight change along the exposure time and analysis of the resulting scales by means of scanning electron microscopy (SEM), X-Ray diffraction (XRD), and glow discharge optical emission spectroscopy (GDOES) revealed a good performance of both steel grades associated with the formation of multi-layered corrosion products. Whereas both alloys undergo external oxidation with the formation of NiO, internal oxidation with the formation of two spinels is the case of HR224. The rate of corrosion is significantly lower than those reported in molten carbonated in non-AFA alloys, allowing to recommend OC4 and HR224 for use in the carbonate-based CSP.

High Temperature Deformation Characteristics of an Alumina‐Forming Stainless Steel

This investigation study deformation characteristics of an innovative stainless steel, alumina‐forming austenitic (AFA) Fe–20Cr–30Ni–0.6Nb–2Al–Mo steel, at elevated temperature, and those isothermal hot compression tests are performed with various temperatures and strain rates. The results show that as the temperature decreases (or as strain rate rised), the stress level increases. On the other hand, in order to better predict the flow stress behavior, a modified constitutive relationship model is established based on the strain‐compensated Arrhenius‐type equation, and the accuracy of this modified model is examined statistically. Furthermore, by comparing with the experimental results, this modified constitutive model is proved to be able to precisely predict the high temperature flow behaviors of the AFA alloy. In addition, processing maps of this alloy are also constructed, and expanded instability regions are found with higher strain rate values (above 0.18 s−1). Moreover, from the microstructure characterization, the features of both adiabatic shear bands and flow localization are formed in those samples of instability regions. Eventually, the optimum high temperature deformation parameters can be determined as 1050–1120 0.01–0.1 s−1 and 1120–1150 °C/10−0.5–10−1.5 s−1 for this AFA alloy.

Corrosion monitoring and mitigation techniques on advanced thermal energy storage materials for CSP plants

High temperature corrosion is one of the most important issues for materials selection, structure design and service life prediction of engineering parts that are exposed to high temperature environments. The prevention of high temperature corrosive attacks on materials plays a critical role in aspects such as reliability, quality, safety and profitability of any industrial sector associated with high temperature process and in the study case, concentrated solar power (CSP) market, using inorganic molten salts as storage material. The aim of this research is the development of corrosion tests through conventional gravimetric techniques focussed on thermal energy storage (TES) materials as well as electrochemical impedance spectroscopy (EIS) analysis for storage systems monitoring. To mitigate corrosion, different alumina forming austenitic (AFA) alloys were exposed to the most promising molten salts to be proposed for the new generation of CSP plants, nitrate, carbonate and chloride molten salts. Ni base alloys showed a better behaviour in the corrosive environments and electrochemical impedance tests revealed the formation of a protective layer in the first 100 h. The gravimetric results and the scanning electron microscopy (SEM) analyses that were performed, confirmed the excellent behaviour of the steels tested in nitrate salts reducing the corrosion in these promising novel molten salts compared with the solar salt currently used in commercial CSP. On the other hand, carbonate and chloride molten salts showed a higher corrosion rate in the materials proposed, especially chloride molten salts.

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.

Corrosion behavior of alumina-forming and oxide dispersion strengthened austenitic 316 stainless steel in supercritical water

The corrosion behavior of alumina-forming austenitic (AFA) 316 stainless steel (SS) and oxide dispersion strengthened (ODS) 316 SS in aerated supercritical water (SCW) at 650 °C/25 MPa was investigated. The results show that the weight gain curves of AFA and ODS 316 SS follow near-parabolic law. Both AFA and ODS 316 SS show higher corrosion resistance in SCW than normal 316 SS, which is because that Al-Cr rich oxide and Cr-rich oxide layer are formed on AFA and ODS 316 SS, respectively. The corrosion process and mechanism of AFA and ODS 316 SS are discussed.

Improved creep strength of alumina-forming austenitic heat-resistant steels through W addition

We investigated the effect of W addition on the creep and precipitation behavior of alumina-forming austenitic (AFA) heat-resistant steels at 700°C and 160MPa. Our results showed that the creep rupture time of the W containing AFA (WAFA) increased more than that of the AFA due to enhanced precipitation hardening of Laves phase and suppression of sigma phase formation. W increased the volume fraction of Laves phase and enhanced precipitation hardening by higher driving force for the precipitation and affinity of the Laves phase. Therefore most W were consumed for the precipitation of the Laves phase, however Mo content contributed to the precipitation of M23C6 as well as precipitation of Laves phase due to the lower affinity of Laves phase. The M23C6 in AFA contained enough Mo to form the sigma phase, which served as an initiation site for cavitation and crack propagation. The sigma phase grew and coarsened at the expense of M23C6 and Mo-rich Laves phase to match the stoichiometric composition of the sigma phase, and finally creep strength of AFA was greatly reduced.

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.

Evolution of precipitates in a Cu-containing alumina-forming austenitic steel after short-term mechanical tests

Tensile tests at room temperature and 700°C under different strain rates were performed on a Cu-containing alumina-forming austenitic (AFA) steel subject to 0–10% pre-strains after the same thermal-mechanical treatment. The sample with 3% pre-strain showed the longest equilibrium stage of strain hardening and dynamic recovery during testing at 700°C and its creep/rupture life doubled compared to the unstrained condition. Both MC carbide precipitates and L12-ordered precipitates were refined after creep/rupture at 700°C for 50h when dislocations introduced by pre-strain acted as nucleation sites. Precipitation kinetics showed that both L12-ordered phase and MC carbides almost completed during the first 50h of creep/rupture. The evolution of MC and L12-ordered precipitates during tensile tests at sufficiently low strain rate makes it possible to correlate the strain hardening plateau length at dσ/dεp≈0 with the creep/rupture life. Thus, tensile tests at low strain rate can act as a guide to set an appropriate pre-treatment in alloy development.

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.

Development of 1100 °C Capable Alumina-Forming Austenitic Alloys

Recently developed alumina-forming austenitic (AFA) alloys based on ~12–32 weight % (wt%) Ni offer an attractive combination of oxidation resistance and creep resistance at relatively low alloy cost. However, they exhibit a transition to internal oxidation and nitridation of Al above ~750–950 °C depending on composition and exposure environment. In order to identify AFA compositions capable of higher-temperature operation for applications such as ethylene cracking, the oxidation behavior of a series of developmental, as-cast nominal Fe–(25–45)Ni–(10–25)Cr–(4–5)Al–1Si–0.15Hf–0.07Y–0.01B wt% base alloys with and without Nb, Ti, and C additions was evaluated at 1100 °C in air with 10% water vapor. Protective alumina scale formation was observed at levels of 35Ni, 25Cr, and 4Al with additions of Nb and C, indicating promise for 1100°C capable cast AFA alloys.