Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr–Mn–N Austenitic Stainless Steels

The effect of intergranular precipitation on the internal oxidation behavior of Cr–Mn–N austenitic steels at 1000 °C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M23C6 carbide morphologies play an important role on the internal oxidation behavior of Cr–Mn–N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M23C6 carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M23C6 carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M23C6 carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr–Mn–N steels.     

Theoretical Analysis of Anomalies in High-Temperature Oxidation of Fe–Cr, Fe–Ni, and Fe–Ni–Cr Alloys

The following anomalies are theoretically analyzed: weakening of the protective ability of dense Cr₂O₃ film during its long-term thermal exposure (because of iron oxidation under the film); lowering of the heat resistance of Fe–Cr and Fe–Ni–Cr alloys during the oxidation (800°C) with an increase in the chromium content over 40 at. %; improving of the protective ability of the films formed at Fe–Ni alloys because of nickel oxidation under the dense FeO film; and the internal oxidation of the Fe 30Ni alloys under the FeO films with the internal formation of FeO oxides and spinel of NiFe₂O₄ type. It is shown that these anomalies can be explained, and the composition of the most heat-resistant alloys calculated, if one takes into account that associates with significantly stronger interatomic bonds than those in ideal solutions can form in solid solutions and cause unlimited solubility of the metallic components in each other.     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

Corrosion of High-Strength Cr–Mn Steels in Chloride Solutions

High-nitrogen Cr-Mn steels 12X18A18 and 05X18A19 are attacked by pitting in NaCl solutions at > 0.1 g-ion/l and T > 313 K because the protective properties of their passive surface films deteriorate (their resistance decreases by a factor of three to six). The passivity range of these steels narrows to 87 and 90 mV at their corrosion depth indices of 1.2 × 10^–3 and 8 × 10^–4 mm/year, respectively. Pitting of 60X38H8B Cr-Mn-Ni steel exposed to NaCl solutions was observed at > 0.3 g-ion/l and T > 303 K. Its corrosion depth index increases to 1.48 mm/year. At < 0.1 g-ion/l, all the steels studied are highly resistant to corrosion.     

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O₂, CO₂, and H₂O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.     

Modeling of Oxidation Kinetics of Y-Doped Fe–Cr–Al Alloys

Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of -Al₂O₃forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be 3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.     

Effect of Microstructure on the Internal Oxidation of Two-Phase Fe–Y Alloys

This work demonstrated the role of microstructure on the internal oxidation rate of two-phase alloys. Fe–Y alloys with Y contents between 1.5 and 15 wt% were employed as a model system. Alloys were prepared by arc-melting and the starting structures were as-solidified mixtures of Fe + Fe₁₇Y₂ intermetallic. An alloy with 1.5 wt% Y was cold-rolled to alter the intermetallic morphology. Oxidation was conducted in an Fe–FeO Rhines pack at 600, 700, and 800 °C up to 72 h. Pre- and post-oxidation microstructures were characterized with electron microscopy. Consistent with other studies, only the Fe₁₇Y₂ phase oxidized. Transmission electron microscopy showed the Fe₁₇Y₂ transformed into nanometer-scale oxides. Oxidation rates were always greater than those predicted by Wagner theory. Parabolic kinetics were obeyed until approximately 10 h. During this time the parabolic rate constants decreased with wt% Y. The effect of alloy microstructure on oxidation kinetics was attributed to connectivity of the Fe₁₇Y₂ phase.     

Oxidation Behavior of ODS Fe–Cr–Al Alloys: Aluminum Depletion and Lifetime

The oxidation kinetics of two ODS Fe–Cr–Al alloys, PM 2000 and MA 956, were studied in oxygen and in air under isothermal conditions from 1000 to 1300°C. They both form an -alumina scale and have good oxidation resistance, without any mass loss. Although the aluminum content in these alloys is higher than the minimum Al content necessary to ensure the growth of a continuous alumina scale, an aluminum depletion occurred in the substrate. This depletion allows the determination of aluminum diffusion coefficients in the ODS alloy. This method is very original and interesting as no Al-stable isotope is available. Moreover, the evolution of the aluminum concentration in the substrate allows one to determine the lifetime of these alloys: indeed, when the aluminum content decreases and becomes lower than a critical value, alumina can no longer form, and less-stable oxides grow very rapidly compared to alumina.     

Oxidation Characteristics of Fe–18Cr–18Mn-Stainless Steel Alloys

Air oxidation studies of Fe–18Cr–18Mn stainless steels were conducted at 525, 625, and 725 °C. Alloys were evaluated with respect to changes in oxidation properties as a result of interstitial additions of nitrogen and carbon and of minor solute additions of silicon, molybdenum, and nickel. Interstitial concentrations possibly had a small, positive effect on oxidation resistance. Minor solute additions significantly improved oxidation resistance but could also reduce interstitial solubility resulting in formation of chromium carbides. Loss of solute chromium resulted in a slight reduction in oxidation protection. Oxidation lasting over 500 h produced a manganese rich, duplex oxide structure: an outer sesquioxide and an inner spinel oxide.     

The influence of Cr alloying on microstructures of Fe–Al–Mn–Cr alloys

The effects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12α-Mn and DO₃ phase-field regions.     

The Effect of Water Vapor on Selective Oxidation of Fe–Cr Alloys

Binary Fe–Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 °C in Ar–20%O₂, Ar–7%H₂O and Ar–4%H₂−7%H₂O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar–O₂ gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an “easy” explanation of the frequently observed effect that Fe–Cr alloys with intermediate Cr contents (e.g. 10–20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.     

High-Temperature Oxidation Behaviour of SS 216L (Fe–16Cr–6Ni–6Mn–1.7Mo)

Oxidation behaviour of a 216L austenite stainless steel (Fe–16Cr–6Ni–6Mn–1.7Mo) was evaluated at temperature between 700 and 900?°C by thermogravimetric analysis and compared with that of SS 316L. Transmission electron microscopy in combination with energy dispersive X-ray analysis was used to study surface morphologies and chemical composition of the oxide scales formed. Replacement of Mn with Ni in SS 316L enhances its oxidation rate. SS 216L exhibits an anomalous temperature dependence of the oxidation behaviour. A kinetic inversion was observed at temperature 900?°C. Surface analysis reveals domination of Mn and iron mixed oxides in oxide scale.     

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050?°C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr₂O₄ spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.     

Water Vapour Effects on Fe–Cr Alloy Oxidation

Isothermal oxidation at 700 °C of binary Fe–Cr alloys containing 9, 17 and 25 wt% chromium was measured using continuous thermogravimetric analysis. All alloys developed thin, protective chromia scales in Ar–20O₂ (vol%). Chromia scale growth on the 17 and 25 Cr alloys was faster in Ar–20O₂–5H₂O and Ar–5O₂–20H₂O. In these gases, the Fe–9Cr failed to form a chromia scale and suffered rapid breakaway oxidation, growing iron-rich oxides instead. A low oxygen potential gas, Ar–10H₂–5H₂O caused chromia scaling on Fe–17Cr and Fe–25Cr, but internal oxidation of Fe–9Cr. Application of Wagner’s criterion for sustaining external scale growth is shown to account satisfactorily for these observations.     

Characterization of irradiated single crystals of Fe and Fe–15Cr

Pure Fe and Fe–15Cr single crystals with three different orientations, [1 0 0], [1 1 0] and [1 1 1], were irradiated in the BR2 reactor of SCK-CEN, at a temperature of 300 °C to a dose of 0.2 displacements per atom. Irradiation-induced microstructure changes were studied by transmission electron microscopy. The size distribution and defect densities were measured and the Burgers vectors and the nature of the loops were determined in detail. In the pure Fe specimens, mainly a〈1 0 0〉 edge-type interstitial dislocations loops could be identified. Their average density and size were (4.1 ± 0.4) × 10²¹ m^?3 and (8 ± 2) nm, respectively. In Fe–15Cr, on the other hand, no defects could be observed.     

Kinetics of Oxidation of Fe–6Si

Surface oxidation of Fe–6Si during annealing in low-pressure air (～10⁴ Pa) in the temperature range 500–550 °C was investigated using resistivity measurements, Mössbauer spectroscopy, X-ray diffraction and scanning-electron microscopy (SEM). The time dependence of the resistivity exhibits an increase in two steps, which indicates changes in the structure and/or phase composition of the alloy. Structure and phase investigations show that the first step can be explained as formation of hematite (α-Fe₂O₃) and the second step is due to transformation of the hematite to magnetite (Fe₃O₄). The kinetics of the transformations were derived from the resistivity data. The activation energies (estimated from Arrhenius plots) of 194 kJ/mol and 165 kJ/mol were obtained for the formation of hematite and transformation of hematite to magnetite, respectively.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Microstructural evolution and strain hardening of Fe–24Mn and Fe–30Mn alloys during tensile deformation

High Mn steels demonstrate an exceptional combination of high strength and ductility owing to their sustained high work hardening rate during deformation. In the present work, the microstructural evolution and work hardening of Fe–30Mn and Fe–24Mn alloys during uniaxial tensile testing at 293 K and 77 K were investigated. The Fe–30Mn alloy did not undergo significant strain-induced phase transformations or twinning during deformation at 293 K, whereas these transformations were observed during deformation at 77 K. A modified Kocks–Mecking model was successfully applied to describe the strain hardening behavior of Fe–30Mn at both temperatures, and quantitatively identified the influence of stacking fault energy and strain-induced phase transformations on dynamic recovery. The Fe–24Mn alloy underwent extensive ε martensite transformation during deformation at both test temperatures. An analytical micromechanical model was successfully used to describe the work hardening of Fe–24Mn and permitted the calculation of the ε martensite stress–strain curve and tensile properties.     

Structure and magnetic properties of a multi-principal element Ni–Fe–Cr–Co–Zn–Mn alloy

A nanocrystalline alloy with a nominal composition of Ni₂₀Fe₂₀Cr₂₀Co₂₀Zn₁₅Mn₅ was produced by mechanical alloying and processed using annealing treatments between 450 and 600 °C for lengths from 0.5 to 4 h. Analysis was conducted using x-ray diffraction, transmission electron microscopy, magnetometry, and first-principles calculations. Despite designing the alloy using empirical high-entropy alloy guidelines, it was found to precipitate numerous phases after annealing. These precipitates included a magnetic phase, α-FeCo, which, after the optimal heat treatment conditions of 1 h at 500 °C, resulted in an alloy with reasonably good hard magnetic properties. The effect of annealing temperature and time on the microstructure and magnetic properties are discussed, as well as the likely mechanisms that cause the microstructure development.     

High-temperature Oxidation Behavior of a High Manganese Austenitic Steel Fe–25Mn–3Cr–3Al–0.3C–0.01N

In this paper, a Fe–Mn–Al–C austenitic steel with certain addition of Cr and N alloy was used as experimental material. By using the SETSYS Evolution synchronous differential thermal analysis apparatus, the scanning electron microscope(SEM), the electron microprobe(EPMA) and the X-ray diffraction(XRD), the high-temperature oxidation behavior microstructure and the phase compositions of this steel in air at 600–1,000 °C for 8 h have been studied. The results show that in the whole oxidation temperature range, there are three distinct stages in the mass gain curves at temperature higher than 800 °C and the oxidation process can be divided into two stages at temperature lower than 800 °C.At the earlier stage the gain rate of the weight oxidized in temperature range of 850 °C to 1,000 °C are extremely lower.The oxidation products having different surface microstructures and phase compositions were produced in oxidation reaction at different temperatures. The phase compositions of oxide scale formed at 1,000 °C are composed of Fe and Mn oxide without Cr. However, protective film of Cr oxide with complicated structure can be formed when the oxidation temperature is lower than 800 °C.