Alumina-forming Austenitic Steel Research Articles

Annealing effects on precipitation and high-temperature properties of a Cu-containing alumina-forming austenitic steel

A novel Cu-containing alumina-forming austenitic (AFA) stainless steel was annealed at 1080°C and 1230°C following the same treatments. Annealing at different temperatures renders different microstructure and properties of the AFA steels. In comparison to alloy A1080, the grain size of alloy A1230 after annealing is larger and the high temperature properties of A1230 is better, i.e. higher strain hardening ability and longer creep/rupture life at 700°C. Coherent L12-ordered precipitates of 10–20nm appear in both alloys and the amount of nanoscale MC precipitates is obviously higher in alloy A1230. Evidence of L12-ordered precipitates cut by dislocation and dislocation pinned by MC precipitates is clearly shown. The precipitation behavior of MC carbides agrees with the calculated Time-Temperature-Transformation (TTT) curves. The results show that nanoscale MC precipitates formed during creep testing contribute to the longer creep/rupture life of alloy A1230.

Microstructural evolution and mechanical properties of an Fe–18Ni–16Cr–4Al base alloy during aging at 950°C

The development of Gen-IV nuclear systems and ultra-supercritical power plants proposes greater demands on structural materials used for key components. An Fe–18Ni–16Cr–4Al (316-base) alumina-forming austenitic steel was developed in our laboratory. Its microstructural evolution and mechanical properties during aging at 950°C were investigated subsequently. Micro-structural changes were characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Needle-shaped NiAl particles begin to precipitate in austenite after ageing for 10 h, whereas round NiAl particles in ferrite are coarsened during aging. Precipitates of NiAl with different shapes in different matrices result from differences in lattice misfits. The tensile plasticity increases by 32.4% after aging because of the improvement in the percentage of coincidence site lattice grain boundaries, whereas the tensile strength remains relatively high at approximately 790 MPa.

The effect of thermo-mechanical treatment on the high temperature tensile behavior of an alumina-forming austenitic steel

Two different thermo-mechanical treatments (TMT) were performed on an alumina-forming austenitic (AFA) stainless steel, DAFA 29, which was recently developed at Oak Ridge National Laboratory for application in advanced ultra supercritical power plants. The resulting fine-grained TMT materials showed yield stresses more than double the yield stress of the as-received material at room temperature and more than 50% higher at 600ºC, but lower yield stresses (but greater elongations) at 700ºC and above. Strain rate jump tests performed at both 600ºC and 700ºC showed that the TMT materials have greater strain rate sensitivity and lower activation volumes at both temperatures. The strain rate has a power law relationship with the flow stress with a stress exponent of ~5. The as-received AFA alloy showed serrated stress–strain curves at 600ºC suggesting the occurrence of dynamic strain aging.

Effect of tungsten addition on high-temperature properties and microstructure of alumina-forming austenitic heat-resistant steels

High-temperature tensile and creep properties of W-added Alumina-Forming austenitic (AFAW) heat-resistant steel were investigated as compared with AFA steel without W. High-temperature tensile properties of two steels were similar to each other, but creep lifetime of AFAW steel was increased. Microstructural examination using SEM and TEM revealed that creep rate rapidly decreased when Laves phase initially precipitated. This indicated that the precipitation of Laves phase played an important role in hardening of AFA steel. It is also found that AFAW steel exhibited finer and denser Laves phase compared with AFA steel, which is due to partial substitution of W for Mo. The finer and denser distribution of Laves phase contributed to improved creep properties of AFAW steel by enhancement in precipitation hardening.

Precipitation sequence and its effect on age hardening of alumina-forming austenitic stainless steel

The precipitation sequence during ageing of Fe–14Cr–20Ni–0.9Nb–2.5Al based alumina-forming austenitic (AFA) steel was explored through a transmission electron microscopy analysis and a small angle neutron scattering experiment. The samples were aged at 700°C for up to 504h. Particles of NbC, M23C6 and Ni3Al-type L12 were observed in the early stage of ageing. Metastable L12 particles were formed both in grain interior and along grain boundary. M23C6 carbides precipitated along grain boundary accompanied with precipitation of L12 particles. After ageing for longer than 48h, particles of B2-NiAl and Laves-Fe2Nb were newly formed. We suggest the possibility of phase transition from L12 to B2 with increase in ageing time. Finally, this study examined the change of mechanical properties during ageing through a Gleeble hot tension test and a Vickers hardness test, and then the relationship between precipitation behavior and mechanical properties was carefully investigated and discussed in terms of precipitation behavior.

Formation of L12-ordered precipitation in an alumina-forming austenitic stainless steel via Cu addition and its contribution to creep/rupture resistance

Addition of 2.8wt.% Cu to an alumina-forming austenitic (AFA) stainless steel promotes formation of a L12-ordered phase with the dominating elements Ni, Cu and Al, instead of B2-NiAl and Cu-rich phases as predicted by the phase equilibrium calculation. At 700°C/150MPa, the creep/rupture life of the AFA steel increases from 1537h to 2047h with the occurrence of the L12-ordered phase. The nano-size L12-ordered Ni–Cu–Al precipitates are uniformly distributed in the austenitic matrix with an average diameter of 35±7nm even after 2047h creep/rupture testing.

Underlying structure of bulky oxide nodule on alumina-forming austenitic stainless steel

The bulky oxide nodule and its underlying structure were investigated in an alumina-forming austenitic steel exposed to dry air at 1053 K for 336 h. Some bulky oxide nodules were found to be attached on globular phases in the matrix. By combined application of electron backscattering diffraction and energy dispersive X-ray spectroscopy, the bulky nodule was identified as Cr-rich M3O4 underlying which was Nb-rich MO2. Thermodynamic calculation suggested that the MO2 was transformed from the primary NbC near surface.

Corrosion behavior of an alumina forming austenitic steel exposed to supercritical carbon dioxide

Microstructure characterization of corrosion behavior of an alumina forming austenitic (AFA) steel exposed to supercritical carbon dioxide was conducted at 450–650°C and 20MPa. At low temperature and short exposure times, the oxidation kinetics were parabolic and the oxide scales were mainly composed of protective and continuous Al2O3 and (Cr, Mn)-rich oxide layers. As the temperature and exposure time increased, the AFA steel gradually suffered breakaway oxidation and its oxide scales showed a multilayer structure mainly composed of Fe3O4, (Cr, Fe)3O4, NiFe/FeCr2O4/Cr2O3/Al2O3, FeCr2O4/Al2O3, and NiFe/Cr2O3/Al2O3, in sequence. The corrosion mechanism based on the microstructure evolution is discussed in detail.

Hot corrosion behaviour and its mechanism of a new alumina-forming austenitic stainless steel in molten sodium sulphate

Abstract Hot corrosion behaviour and related mechanism of an alumina-forming austenitic (AFA) stainless steel at 1173 K in molten sodium sulphate were investigated. Compared with the Ni-based super-alloy K438, K417 and the commercial steel 316L, the current AFA steel exhibited a high hot-corrosion resistance with less internal sulphidation. It was found that a dense, continuous Al2O3 scale created at the early corrosion stage enabled formation of a compact Cr2O3 scale on it in the subsequent corrosion. Such dual layers effectively suppressed the sulphur penetration and protected the matrix from the hot corrosion.

Effects of silicon additions on the oxide scale formation of an alumina-forming austenitic alloy

This study investigated the effect of silicon additions on the oxidation behaviour of an alumina-forming austenitic (AFA) stainless steel at high temperatures in air with 10% water vapour. The AFA steels exhibited good oxidation resistance with less than 0.3wt.% Si due to the formation of a dense, continuous and protective Al2O3 scale. However, silicon additions of more than 0.3wt.% stimulated B2-NiAl precipitation and reduced the Al concentration in the austenite matrix, leading to its degradation with oxidation resistance at 800°C in air with 10% water vapour.

Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800 °C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe–20Ni–14Cr–2.5Al and Fe–18Cr–25Ni–3Al base. At 800 °C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe–14Cr–20Ni–2.5Al. At 900 °C in air with 10 % water vapor, however, additions of Mn in these AFA steels based on Fe–18Cr–25Ni–3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn1.5O4 and Cr2O3 oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.

Corrosion of alumina-forming austenitic steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti in supercritical water

The oxidation behavior of alumina-forming austenitic (AFA) steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti has been examined after exposure to supercritical water (SCW) at 500 °C and 25 MPa pressure and with a dissolved oxygen content of 25 wppb, for exposure periods of 1300 h, 1700 h and 3000 h. A double layer oxide structure developed on all samples, consisting specifically of a Fe-rich outer hematite layer and an inner layer of Al–Cr–Fe-rich oxide containing a small fraction of FCC Ni-rich metal. Additionally, a thin Ni-rich layer in the bulk alloy close to the oxide/metal interface was also observed. Based on weight gain measurements, the oxidation resistance of the AFA steel was superior to other austenitic alloys, 800H, D9, and 316 stainless steel, tested under similar conditions. The formation of the protective Al–Cr–Fe-rich oxide layer may provide good long-term protection against corrosion in SCW environment.

Congress objectives

The deformation behavior of alumina-forming austenitic heat-resistant steels at high temperatures was studied to elucidate their hot workability using two types of alloys, i.e. AFA and MAFA. Uniaxial compression tests were performed between the temperature range of 800–1100 °C with strain rates of 0.005–5.0 s− 1. From the constitutive relationship between flow stress, temperature and strain rate, the activation energies for hot deformation were estimated. Both of the investigated alloys exhibited higher activation energies (AFA: 618.8 and MAFA: 711.1 kJ/mol) than other conventional alloys under similar conditions. Processing maps based on dynamic material model and microstructures after the tests indicated that the hot working of both alloys became more efficient and stable with increasing temperature and decreasing strain rate, which was attributed to more active operation of restoration processes such as dynamic recovery and recrystallization. In both alloys, secondary NbCs were precipitated during the test. Due to their pinning effect on dislocations, grain and sub-grain boundaries, the progress of restoration processes was hindered, which contributed to their high activation energy. In spite of the common types of restoration processes, MAFA with increased Nb and C content exhibited degraded hot workability due to more extensive precipitation of NbC.