Alumina-forming Austenitic Steel Research Articles

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.

Elucidating microstructural evolution and its effect on the mechanical properties of FSWed alumina-forming austenitic steel for power plant applications

Alumina-forming austenitic (AFA) steel is considered as a promising candidate used for advanced energy systems. Fusion welding of austenitic steels usually requires the introduction of a certain delta (δ) ferrite to avoid thermal cracking, but for the AFA steel, the presence of δ ferrite will greatly accelerate the precipitation of hard and brittle sigma (σ) phase at its target service temperature.In this work, for the AFA steel with a typical composition, we demonstrated a superior capability of friction stir welding (FSW) in avoiding the formation of δ and σ phases, and developing the favorable weld microstructures under various welding conditions. The microstructure evolution of AFA steel during FSW with various rotational speeds (300–700 rpm) was studied in detail. The resulting FSW joints all exhibit higher yield strength (YS) and ultimate tensile strength (UTS) than the base metal, especially at low rotational speed, while retaining reasonable ductility, despite decreasing with reducing rotational speed. The associated relationship between the microstructure and mechanical properties of the resulting FSW joints was well correlated. This work provides an effective method to obtain the fully-austenitic welded joints of AFA steel with excellent strength-ductility synergy.

Compatibility of Alumina-Forming Austenitic Steels in Static and Flowing Pb

Alumina-forming austenitic (AFA) steels were evaluated for the lead fast reactor application in static and flowing high-purity Pb. Two AFA composition ranges with different Ni, Mn, Al, Cr and Nb contents were targeted, and Zr foil was used in an attempt to getter O in some capsules. Based on two rounds of capsule testing at 500–800°C, two AFA compositions were selected for evaluation in flowing Pb with a peak temperature of 650°C in a thermal convection loop made of pre-oxidized FeCrAlMo tubing. After exposure for 1000 h, the mass losses for both AFA alloys were generally low, suggesting good compatibility under these conditions without preoxidation.

Co-strengthening of dislocations and precipitates in alumina-forming austenitic steel with cold rolling followed by aging

The microstructural evolutions of the alumina-forming austenitic (AFA) steel subjected to different cold-rolled reductions and then aging at 700 °C were investigated by multiple characterization methods, and the mechanical properties were also examined. The results show that the increased dislocations (especially, edge dislocations) after cold rolling can effectively enhance the volume fraction and refine the particle size of precipitates during initial aging and subsequently accelerate the coarsening rate of precipitates in the AFA steel, resulting in a high coarsening exponent. The coarsening rate of B2–NiAl phase is higher than that of Laves phase. Excellent mechanical properties with a combination of yield strength (1152 ± 32 MPa) and elongation (44.8 ± 4.1%) of cold rolled AFA steel after aging for 2 h can be obtained due to co-strengthening contribution derived from dislocations and precipitates. With the duration of aging, the dislocation strengthening constantly decreases, the effect of precipitation strengthening is contrarily larger than that of dislocations.

Effect of Cu addition on microstructure and properties of Fe–20Ni–14Cr alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel with high creep strengths and excellent high-temperature corrosion resistance has a promising potential being applied to high-temperature components of ultra-supercritical power plants. The effect of Cu addition on microstructure and tensile properties of Fe–20Ni–14Cr alumina-forming austenitic (AFA) steel after aging at 973 K has been investigated using various morphology microscopic characterizations and tensile test. The obtained results show that Cu addition promotes the formation of nanoscale B2–NiAl phase in AFA steel, which further increases the tensile strength and ductility of the materials to 1464 MPa and ductility of 23.81%, respectively. By occupying Fe and Ni atoms locations in the nanoscale B2–NiAl phase of AFA steel, Cu atoms reduce the chemical driving force of the nanoscale B2–NiAl phase growth. Such phenomenon results in reduction of the corresponding phase formation energy, and thus improves the phase stability. The occupancy behavior of Cu and its effect on the particle size in nanoscale B2–NiAl phase were quantitatively evaluated by first-principle calculation. The present research work proves that the precipitation strengthening plays a dominant role in the yield strength improvement of AFA steel.

Pre-Oxidation to Improve Liquid Metal Compatibility

Liquid metals represent an aggressive environment where selective degradation can attack structural alloys. Pre-oxidizing structural alloys, particularly those able to form alumina scales, have shown promise for Sn, Pb and Pb–Li concepts for fusion and fission applications. For Pb at 700 °C, a dramatic reduction in mass loss was observed for an alumina-forming austenitic (AFA) steel after pre-oxidation. Likewise, a ferritic FeCrAlMo alloy showed better compatibility in static Sn at 400–500 °C and flowing PbLi when pre-oxidized at 1000 °C to form α-Al2O3. Recent experiments with flowing PbLi at 675–700 °C has shown significant degradation suggesting that alumina-forming alloys are limited to ~ 650 °C in this environment. While the concept is promising, most experiments have been relatively short-term and long-term degradation mechanisms need to be evaluated.

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

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

Creep behaviors of Fe-18Ni-12Cr based alumina-forming austenitic steels with ultralow carbon

Two kinds of Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steels with ultralow carbon content were prepared by vacuum induction melting. Creep rupture tests were carried out with the applied stress range between 80 and 180 MPa at the target application temperature of 700 °C. Microstructure evolution was characterized using glancing X-ray diffraction, scanning electron microscopy equipped with a detector of electron backscatter diffraction and transmission electron microscope. Compared with 18-12-Al, 18-12-AlNb exhibited superior creep properties including ten times longer creep rupture life ( $${t}_{\mathrm{rup}}$$ ) and one magnitude smaller minimum creep rate ( $$\dot{\varepsilon }_{{\min }}$$ ). For example, the $${\dot{\varepsilon }}_{\mathrm{min}}$$ at the applied stress of 140 MPa was 4.4 × 10–5 and 2.1 × 10–4 h–1 for 18-12-AlNb and 18-12-Al, respectively. The strengthening effect of B2-NiAl would weaken at 700 °C due to its ductile-to-brittle transition. The addition of Nb not only promoted the formation of Laves-Fe2(Mo,Nb), but also resulted in formation of nanoscale NbC. The GB Laves-Fe2(Mo,Nb) did not impair the creep ductility by strengthening GBs, and all the creep ruptured AFA steels exhibited typical ductile fracture mode. The GB Laves-Fe2(Mo,Nb) and NbC precipitates contributed to the superior creep resistance of 18-12-AlNb.

Analysis of the initial oxidation layer of W-substituted alumina-forming heat-resistant steel

ABSTRACT High-temperature oxidation behaviour of an alumina-forming austenitic (AFA, Febalance–20Ni–14Cr–2.5Al–0.15Si–2Mn–2.5Mo–0.9Nb, in wt%) steel and W-substituted AFA steel was examined. The isothermal oxidation tests were performed on the two steels at 780°C for 2 weeks in air, which indicated that the substitution of W for Mo was beneficial to improve the oxidation resistance of the AFA steel. After oxidation at 780°C for 30 min, more compact and protective initial oxide layer against cation diffusion was formed on the W-substituted AFA steel than that of the original AFA steel, which was considered to be responsible for the reduced weight gain after the long-term oxidation.

Alumina-Forming Austenitic (AFA) steels and aluminium-based coating on 15-15 Ti steel to limit mechanical damage in presence of liquid lead-bismuth eutectic and liquid lead

To limit corrosion of the steels in contact with liquid metal (Pb or Pb-Bi), both different solutions based on the presence of alumina at the surface of the steels were selected: 2 Alumina-Forming Austenitic (AFA) steels and the 15-15 Ti steel coated by alumina layer. These technical options to mitigate corrosion by lead or Pb-Bi were investigated in terms of mechanical performances in liquid lead or liquid Pb-Bi. So the liquid metal embrittlement (LME) sensitivity of the different materials selected for their good corrosion resistance was evaluated by tensile tests or small punch tests carried out in presence of liquid metal, and by post-mortem analysis of the cracking and of the fracture surfaces. No LME sensitivity for the tested conditions has been observed for the Al2O3 coated 15-15Ti steel and for the two AFA steels (16Ni-14Cr-2.5Al-2.5Mn-1Nb), one without and one with 2% wt.% W and 0.02% wt.% Y in their as received state. But a 650 °C thermal aging promotes modifications of the microstructure specially precipitations and then LME sensitivity of the AFA steels, according the nature of the precipitation.

Enhanced Corrosion Resistance of an Alumina-forming Austenitic Steel Against Molten Al

The development of corrosion resistant steel for molten Al has always attracted the interests from die casting industry for several decades. In this paper, we reported the enhanced corrosion resistance in 750 °C molten Al of an alumina-forming austenitic steel (AFA) via deliberately producing oxide layer after different high-temperature oxidation processes. The results showed that the corrosion resistance against molten Al of the AFA was improved significantly due to the presence of a stable, compact and continuous oxides barrier, in comparison with H13 tool steel as well as unoxidized AFA without Al2O3 film. Apparently, the non-wetting α-Al2O3 film, which was formed after high-temperature oxidation, effectively protected the substrate from inner-diffusion and penetration of the molten Al. These findings provide one of cost-effective ways to improve the corrosion resistance of steels against molten Al.

Early segregation and precipitation at triple junction in an alumina-forming austenitic steel

Grain boundary triple junctions (TJs) are 1-dimensional microstructural elements on their own with specific thermodynamic and kinetic properties, which are different from the connecting boundaries. Our study compares the segregation and precipitation evolution near the TJs and at grain boundaries (GBs) in an alumina-forming austenitic steel with 2.29 wt% Mo. After annealing at 700 ℃ for 10 min, Mo segregated at the TJs and the concentration increased to ~12 wt%, while the concentration at GBs was ~4 wt%. After 1 h, Mo accumulated to ~8 wt% at GBs. Laves phase (Fe2Mo) formed near the TJs after 10 h. The results suggest that the TJs are the fast diffusion paths of Mo and promote the precipitation of Laves phase, which could render the fracture cracks at TJs during creep rupture.

The effect of isothermal aging on creep behavior of modified 2.5Al alumina-forming austenitic steel

The microstructural evolution and creep behavior of aged 2.5Al alumina-forming austenitic (AFA) steel were investigated by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average particle size, volume fraction of precipitates and dislocation density were statistically measured to evaluate the contribution of precipitation strengthening and dislocation strengthening to the creep strength. The results suggested that the phase boundaries between Laves and B2–NiAl were incoherent interface, and both B2–NiAl and Laves were main strengthened phases in aged 2.5Al-AFA steel. The creep results indicated that the sample aged for 1000 h exhibited creep capability with the smallest creep strain and steady-state creep rate. The isothermal aging treatment can be considered as an effective way to improve the creep strength of 2.5Al-AFA steel.

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.

Microstructure and Mechanical Properties of 4Al Alumina-Forming Austenitic Steel after Cold-Rolling Deformation and Annealing.

Microstructural evolutions of the 4Al alumina-forming austenitic steel after cold rolling with different reductions from 5% to 30% and then annealing were investigated using electron backscattering diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tensile properties and hardness were also measured. The results show that the average grain size gradually decreases with an increase in the cold-rolling reduction. The low angle grain boundaries (LAGBs) are dominant in the cold-rolled samples, but high angle grain boundaries (HAGBs) form in the annealed samples, indicating that the grains are refined under the action of dislocations. During cold rolling, high-density dislocations are initially introduced in the samples, which contributes to a large number of dislocations remaining after annealing. With the sustaining increase in cold-rolled deformation, the samples exhibit more excellent tensile strength and hardness due to the decrease in grain size and increase in dislocation density, especially for the samples subjected to 30% cold-rolling reduction. The contribution of dislocations on yield strength is more than 60%.

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.