High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO<Subscript>2</Subscript> as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO₂ gas mixtures with CO/CO₂ ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 °C to 1000 °C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO ↔ 2Cr₂O₃ + Cr₂₃C₆ was identified to lie between 1173 K and 1223 K (900 °C and 950 °C) at CO/CO₂ ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO₂ ratio of 1272 at all temperatures. The surface carbide, Cr₇C₃, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr₇C₃ precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr₂O₃ + Cr₇C₃ → 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 °C) in impure helium will be limited by oxidation.     

Interface stability during displacement reactions between Cu<Subscript>2</Subscript>O and Co<Subscript>1−<Emphasis Type="Italic">X</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript> alloys at 1223 K

The stability of the reactive interface during the solid-state displacement reaction, Cu₂O+Co_1−X Fe _X =2Cu+(Co_1−X Fe _X )O, is studied as a function of Co-Fe alloy composition at 1223 K. For X≤0.03, the reaction zone has a layered structure, and the cation diffusion in (Co, Fe)O is the rate-limiting step. The interface is unstable in the early stages of the reaction; the instability decreases with time as the oxide thickness increases, and the interface becomes planar at long times. The time required for the attainment of interface planarity increases with the value of X. The reaction kinetics are consistent with the available cation-diffusion data in (Co, Fe)O. For X≥0.045, the product zone is a composite of Cu+(Co, Fe)O, and the rate is limited by the oxygen transport in copper. The transition to interface instability occurs when the oxide can support a cation flux that exceeds the maximum possible oxygen flux in copper. During the reaction, composition gradients develop in (Co, Fe)O because of higher diffusion rates for iron than for cobalt.     

Oxidation Behavior of a Pd<Subscript>43</Subscript>Cu<Subscript>27</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> Bulk Metallic Glass and Foam in Dry Air

The oxidation behavior of both Pd₄₃Cu₂₇Ni₁₀P₂₀ bulk metallic glass (Pd4-BMG) and its amorphous foam containing 45 pct porosity (Pd4-AF) was investigated over the temperature range of 343 K (70 °C) to 623 K (350 °C) in dry air. The results showed that virtually no oxidation occurred in the Pd4-BMG at T < 523 K (250 °C), revealing the alloy’s favorable oxidation resistance in this temperature range. In addition, the oxidation kinetics at T ≥ 523 K (250 °C) followed a parabolic-rate law, and the parabolic-rate constants (k _p values) generally increased with temperature. It was found that the oxidation k _p values of the Pd4-AF are slightly lower than those of the Pd4-BMG, indicating that the porous structure contributes to improving the overall oxidation resistance. The scale formed on the alloys was composed exclusively of CuO at T ≥ 548 K (275 °C), whose thickness gradually increased with increasing temperature. In addition, the amorphous structure remained unchanged at T ≤ 548 K (275 °C), while a triplex-phase structure developed after the oxidation at higher temperatures, consisting of Pd₂Ni₂P, Cu₃P, and Pd₃P.     

Precipitation in Nb-Stabilized Ferritic Stainless Steel Investigated with <Emphasis Type="Italic">in-situ</Emphasis> and <Emphasis Type="Italic">ex-situ</Emphasis> Transmission Electron Microscopy

A Nb-stabilized Fe-15Cr-0.45Nb-0.010C-0.015N ferritic stainless steel is studied with transmission electron microscopy (TEM) to investigate the morphology and kinetics of precipitation. Nb_x(C,N)_y\hbox{Nb}_{x}\hbox{(C,N)}_y and MnS precipitates are present in the steel in the initial condition. Ex-situ TEM analysis is performed on samples heat treated at 973 K, 1073 K, 1173 K, and 1273 K (700 °C, 800 °C, 900 °C, and 1000 °C). Within this temperature range, both Fe₂Nb\hbox{Fe}_2\hbox{Nb} and Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} (with X = C or N) precipitates form. Fe₂\hbox{Fe}_2Nb is observed at 1073 K (800 °C).   Fe₃Nb₃X_x\;\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} precipitates form at the grain boundaries between 973 K and 1273 K (700 °C and 1000 °C). Up to at least 1173 K (900 °C) their fraction increases with time and temperature, but at 1273 K (1000 °C) they lose stability with respect to Nb_x(C,N)_y.\hbox{Nb}_{x}\hbox{(C,N)}_{y}. With in-situ TEM, no phase transition is observed between room temperature and 1243 K (970 °C). At 1243 K (970 °C) the precipitation of Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} is observed in the neighborhood of a dissolving Nb₂\hbox{Nb}_2(C,N) precipitate. For sections of grain boundaries where no Nb_x(C,N)_y\hbox{Nb}_x\hbox{(C,N)}_y precipitates are present, Fe₃Nb₃X_x\hbox{Fe}_3\hbox{Nb}_3\hbox{X}_{x} does not form. It is concluded that the precipitation of Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_x is directly related to the dissolution of Nb₂\hbox{Nb}_2(C,N) through the redistribution of C or N.     

Evaluating the Diffusion Coefficient of Sulfur in Low-Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The chemical diffusion coefficient of sulfur in the ternary slag of composition 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was measured at 1680 K, 1700 K, and 1723 K (1403 °C, 1427 °C, and 1450 °C) using the experimental method proposed earlier by the authors. The P_\textS2 P_{{{\text{S}}_{2} }} and P_\textO2 P_{{{\text{O}}_{2} }} pressures were calculated from the Gibbs energy of the equilibrium reaction between CaO in the slag and solid CaS. The density of the slag was obtained from earlier experiments. Initially, the order of magnitude for the diffusion coefficient was taken from the works of Saito and Kawai but later was modified so that the concentration curve for sulfur obtained from the program was in good fit with the experimental results. The diffusion coefficient of sulfur in 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was estimated to be in the range 3.98 to 4.14 × 10⁻⁶ cm²/s for the temperature range 1680 K to 1723 K (1403 °C to 1450 °C), which is in good agreement with the results available in literature     

The effect of carbon in slag on steel reoxidation and carbon analysis by x-ray photoelectron spectroscopy in the CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-MnO-Fe<Subscript><Emphasis Type="Italic">t</Emphasis></Subscript>O system

An equilibrium experiment was carried out at 1873 K to investigate the effect of carbon in CaO-SiO₂-Al₂O₃-MgO-MnO-Fe _t O slag systems on their Fe _t O and MnO activity coefficients, representing the slag’s thermodynamic potential for steel reoxidation. The activity coefficients of Fe _t O and MnO showed not only a sharp increment but also a simultaneous slow decrement with increasing carbon content in slag, suggesting opposite roles of the carbon in slag according to its stable forms. X-ray photoelectron spectroscopy (XPS) was introduced to determine the stable forms of carbon in slag. The XPS results proved that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. The simultaneous consideration of the activity coefficients of Fe _t O and MnO and stable carbon forms showed that carbonate ions increase the activity coefficients of Fe _t O and MnO, but that carbide decreases them. This article suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (Fe _t O + MnO) in slag to some allowable amount.     

Reduction kinetics of Fe<Subscript>2</Subscript>MoO<Subscript>4</Subscript> fine powder by hydrogen in a fluidized bed

Iron molybdate (Fe₂MoO₄) powders with an average particle size of 100 μm were reduced by hydrogen using a fluidized-bed batch reactor in the temperature range of 923 to 1173 K. The extent of the reaction was followed as a function of time by gas chromatography. The fluidizing-gas velocity was set at about 1.5 times the minimum fluidization velocity. The ratio of the height of the static bed to its diameter is about 1. Under the prevailing experimental conditions, it was found that the chemical reaction was the rate-controlling factor. The activation energy for this process was 158±17 kJ/mol. The crystal size of the Fe₂Mo powder produced at lower temperatures was in the nanometer range, indicating the possibility of mass production of alloys and intermetallics in the nanorange, using a fluidized bed.     

Infrared Brazing Fe<Subscript>3</Subscript>Al Using Ag-Based Filler Metals

The microstructural evolution and bonding shear strength of infrared brazed Fe₃Al using Ag and BAg-8 (72Ag-28Cu in wt pct) braze alloys have been studied. The Ag-rich phase alloyed with Al dominates the entire Ag brazed joints, and the shear strength is independent of the brazing time. The BAg-8 brazed joint contains Ag-Cu eutectic for all brazing conditions, and its shear strength increases slightly with increasing brazing time. The highest shear strength of 181 MPa is acquired from the joint infrared brazed at 1073 K (800 °C) for 600 seconds. A thin layer of Fe₃Al is identified at the interface between the brazed zone and the substrate for both braze alloys. An Al depletion zone in the Fe₃Al substrate next to the interfacial Fe₃Al is identified as the α-Fe phase. The dissolution of Al from the Fe₃Al substrate into the molten braze causes the formation of α-Fe in the Fe₃Al substrate.     

Crystallization Behavior and Microstructure of Silica-Free 5K<Subscript>2</Subscript>O-45CaO-50P<Subscript>2</Subscript>O<Subscript>5</Subscript> Bioglass

The crystallization behavior and microstructure of silica-free 5K₂O-45CaO-50P₂O₅ (KCP) bioglass have been studied using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning election microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The activation energy for the KCP bioglass crystallization is found to be 337.4 kJ/mol using a nonisothermal method. The crystalline phases of the glass surface determined by XRD are KCa(PO₃)₃, 4CaO·3P₂O₅, and β-Ca(PO₃)₂ when the KCP bioglass is crystallized at 903 K for 4 hours. The crystalline phase of the powder samples determined by XRD is β-Ca(PO₃)₂ when silica-free KCP glasses crystallized at 873 to 1073 K for 8 hours. Crystallization starts at the surface of the KCP bioglass and then proceeds toward the interior of the glass matrix. The morphology of β-Ca(PO₃)₂ is a fibrillar shape 20 to 180 nm in length and 17 to 20 nm in diameter, with an aspect ratio ranging from 1.0 to 10.6.     

A Eutectoid Reaction for the Decomposition of Austenite into Pearlitic Lamellae of Ferrite and M<Subscript>23</Subscript>C<Subscript>6</Subscript> Carbide in a Mn-Al Steel

We discovered a eutectoid reaction in an Fe-13.4Mn-3.0Al-0.63C (wt pct) steel after solution heat treatment at 1373 K (1100 °C) and holding at temperatures below 923 K (650 °C). The steel is single austenite at temperatures from 1373 K to 923 K (1100 °C to 650 °C). A eutectoid reaction involves the replacement of the metastable austenite by a more stable mixture of ferrite and M₂₃C₆ phases at temperatures below 923 K (650 °C). The mixture of ferrite and M₂₃C₆ is in the form of pearlitic lamellae. The morphology of the lamellae of the product phases is similar to that of pearlite in steels. Thus, we found a new pearlite from the eutectoid reaction of the Mn-Al steel featuring γ  → α + M₂₃C₆. A Kurdjumov–Sachs (K-S) orientation relationship exists between the pearlitic ferrite (α) and M₂₃C₆ (C6) grains, i.e., (110)_α // (111)_C6 and [[`1] \overline{1} 11]<sub>α</sub> // [0[`1] \overline{1} 1]_C6. The upper temperature limit for the eutectoid reaction is between 923 K and 898 K (650 °C and 625 °C).     

Density Measurements of Low Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Density measurements of a low-silica CaO-SiO₂-Al₂O₃ system were carried out using the Archimedes principle. A Pt 30 pct Rh bob and wire arrangement was used for this purpose. The results obtained were in good agreement with those obtained from the model developed in the current group as well as with other results reported earlier. The density for the CaO-SiO₂ and the CaO-Al₂O₃ binary slag systems also was estimated from the ternary values. The extrapolation of density values for high-silica systems also showed good agreement with previous works. An estimation for the density value of CaO was made from the current experimental data. The density decrease at high temperatures was interpreted based on the silicate structure. As the mole percent of SiO₂ was below the 33 pct required for the orthosilicate composition, discrete \textSiO₄^{4 -} {\text{SiO}}_{4}^{4 - } tetrahedral units in the silicate melt would exist along with O^2– ions. The change in melt expansivity may be attributed to the ionic expansions in the order of

Solubilities of Chlorine in CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO Slags: Correlation Between Sulfide and Chloride Capacities

To derive a correlation between sulfide and chloride capacities through our own systematic experimental studies by using a gas equilibrium technique involving Ar-H₂-H₂O-HCl gas mixtures, the solubilities of chlorine were determined for CaO-SiO₂-MgO-Al₂O₃ slags at temperatures between 1673 K and 1823 K (1400 °C and 1550 °C). As a formula to correlate sulfide and chloride capacities, the following equation that is the function of temperature only was obtainable;

Structural formation during sintering of composition α-Fe-C<Subscript>60</Subscript> and α-Fe nanotubes

The formation of the structure of semiconductor compositions α-Fe-C₆₀ and α-Fe nanotubes is studied at 650 and 880°C. The concentration of fullerenes and nanotubes varied from 1 to 2 wt %. The samples were pressed at 600 MPa. It is shown that the fullerene of compositions α-Fe-fullerene partially transforms into graphite while another part interacts with Fe. As a result of this interaction, fullerene-containing Fe _x C₆₀ phases are formed. At 880°C, fullerenes decompose more intensely than at 650°C. It is established that, during the sintering of composition Fe nanotubes at t = 650 and 880°C, the nanotubes partially decompose with the formation of fullerene-containing phases and this process is more active at 650°C than at 880°C. An increase in the amount of nanotubes introduced into the compositions leads to the activation of the synthesis of fullerenes.     

Synthesis and Characterization of Microwave Absorbing SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanocomposite

Zinc ferrite and strontium hexaferrite; SrFe₁₂O₁₉/ZnFe₂O₄ (SrFe_11.6Zn_0.4O₁₉) nanoparticles having super paramagnetic nature were synthesized by simultaneous co-precipitation of iron, zinc and strontium chloride salts using 5 M sodium hydroxide solution. The resulting precursors were heat treated (HT) at 850, 950 and 1150°C for 4 h in nitrogen atmosphere. The hysteresis loops showed an increase in saturation magnetization from 1.040 to 58.938 emu/g with increasing HT temperatures. The ‘as-synthesized’ particles have size in the range of 20–25 nm with spherical and needle shapes. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate shape with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties were estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches −26.51 dB (more than 99% power attenuation) at 10.636 GHz which suits its application in RADAR absorbing materials.     

Oxidation of Fe-V Melts Under CO<Subscript>2</Subscript>-O<Subscript>2</Subscript> Gas Mixtures

The oxidation mechanism of liquid Fe-V alloys with V content from 5 to 20 mass pct under different oxygen partial pressures using CO₂-O₂ mixtures with CO₂ varying from 80 pct to 100 pct was investigated by thermogravimetric analysis between 1823 K and 1923 K (1550 °C and 1650 °C). The products after oxidation were identified by scanning electron microscopy energy-dispersive spectrograph and X-ray diffraction. The results indicate that the oxidation process can be divided into the following steps: an apparent incubation period, followed by a chemical reaction step with a transition step before the reaction, and diffusion as the last stage. At the initial stage, a period of slow mass increase was observed that could be attributed to possible oxygen dissolution in the liquid iron-vanadium coupled with the vaporization of V₂O. The length of this period increased with increasing temperature as well as vanadium content in the melt and decreased with increasing oxygen partial pressure of the oxidant gas. This analysis was followed by a region of chemical oxidation. The oxidation rate increased with the increase of the O₂ ratio in the CO₂-O₂ gas mixtures. During the final stage, the oxidation seemed to proceed with the diffusion of oxygen through the product layer to the reaction front. The Arrhenius activation energies for chemical reaction and diffusion were calculated, and kinetic equations for various steps were setup to describe the experimental results. The transition from one reaction mechanism to the next was described mathematically as mixed-control equations. Thus, uniform kinetic equations have been setup that could simulate the experimental results with good precision.     

Study of the Reduction of Industrial Grade MoO<Subscript>3</Subscript> Powders with CO or CO-CO<Subscript>2</Subscript> Gases to Prepare MoO<Subscript>2</Subscript>

Industrial grade MoO₂ powders have a plenty of advantages relative to MoO₃ in the direct alloying steelmaking processes. In this work, the reduction of industrial grade MoO₃ powder with CO gas or the mixed gases of CO and CO₂ has been investigated in detail in order to prepare industrial grade MoO₂ powder. It is found that reaction temperature has a significant effect on the product composition. Using pure CO as the reducing gas, for temperatures below 868 K (595 °C), the main product is MoO₂ with some whisker carbon; for temperatures above 868 K (595 °C) the main reaction products are MoC and amorphous carbon; as the reaction temperature further increased, the final reaction product is Mo₂C. In addition, Mo₄O₁₁ is always formed as an intermediate product during the reaction processes both at lower and higher temperatures, which is similar to that observed on reduction of MoO₃ by H₂. It is found that adding CO₂ to the reducing gases eliminated carbon formation but still allows the formation of MoO₂ during the reaction process. This method may be applied to produce industrial grade MoO₂.