Stable Carbide Research Articles

The composition of iron-containing microphases in the structure of multiwalled carbon nanotubes

The phase composition of carbon nanotubes (CNTs) with encapsulated iron atoms was studied by 57Fe Mossbauer spectroscopy. The synthesis of CNTs was shown to stabilize iron atoms as both thermodynamically stable iron carbide and oxide phases and phases not characteristic under synthesis conditions (γ-Fe and γ-Fe2O3). In addition, an analysis of the data obtained led us to suggest the existence of Fe-CNT atomic complexes in interlayer positions. CNTs and iron-containing phases were shown to be stable when subjected to prolonged annealing in air at temperatures up to 670 K.

Microstructural and mechanical behavior of 4.5Cr–2W–0.25V–0.1C steel

In this paper the microstructural and mechanical properties of a hot rolled Cr–W–V steel were studied. The 4.5Cr–2W–0.25V–0.1C steel was austenitized at 1000 °C for 30 min and then tempered at 600 and 700 °C for 1 h. The physical metallurgy of this steel was studied by optical, scanning electron and transmission electron microscopes and X-ray and electron diffraction patterns. The results showed that four types of carbide were precipitated as MC, M 3C, M 7C 3 and M 23C 6 along prior austenite grains and lath boundaries and within martensitic matrix. The average sizes of the M 7C 3 and M 23C 6 carbides in the specimens tempered at 600 °C were about 100 and 180 nm, and in specimens tempered at 700 °C were about 110 and 210 nm, respectively. When the tempering temperature was increased, weight percent of extracted precipitates and the formation rate of the stable carbides such as M 23C 6 and dissolution rate of the metastable carbides such as M 3C and M 7C 3 were also increased. The results of hot tension tests showed the strength and elongation of tempered specimens were decreased when the test temperature was increased. The strength decrease was slow at range of 200–500 °C but sharp at test temperature of 600 °C. The results suggested tempering at 700 °C has better tensile properties at operating temperature (500–600 °C) when compared to tempering at 600 °C.

Semimetallic TiO2 Nanotubes

Orchestrating conductors: A carbo-thermal reduction treatment of a TiO2 nanotube layer in acetylene converts the tube walls into highly conductive and stable oxy carbide compounds. The resulting semimetallic TiO2 nanotube layers (TiOxCy) are electrode materials with properties that differ from other TiO2 nanotubes (see picture). They have a high oxygen overpotential and can be used as a catalyst support, for example, for methanol oxidation.

Effect of chemical potential of carbon on phase transformation and corrosion of JLF-1 steel in a static lithium

The corrosion and the phase transformation of RAFM (Reduced Activation Ferritic/Martensitic) steel, JLF-1(Fe–9Cr–2W–0.1C), in static lithium (Li) were investigated. The specimens were exposed to static Li at 600 °C for 250 h. The carbon potential in Li was controlled by the carbide formation on inner surface of the crucible, which was made of Mo and Nb. These materials were expected to form stable carbide, and worked as carbon trap in Li, which resulted in the decrease in the carbon potential of Li. The effect of low carbon potential on the corrosion and the phase transformation was characterized by comparison with the test performed in the crucible made of Fe–9Cr and SUS316L (18Cr–12Ni), which has no effect as carbon traps. The low carbon potential caused by Nb and Mo caused the dissolution of carbon from the surface of JLF-1 specimen to Li. The depth of the phase transformation observed after the test in Nb crucible was deeper than that tested in Mo crucible. This was because the stability of the Nb carbide was larger than that of Mo carbide causing larger driving force for the dissolution of carbides from the surface.

A new approach for the determination of silicon in airborne particulate matter using electrothermal atomic absorption spectrometry

In this work a new procedure for element specific analysis of silicon in airborne particulate matter is presented. The method is based on a preliminary treatment of the aerosol samples with nitric acid and perchloric acid leading to a mineralization of the organic sampling substrate, dissolution of soluble material and a homogeneous suspension of the remaining non-soluble sample fraction. ETAAS measurement of the derived slurries was performed using a Zr-treated graphite tube which prevents the formation of stable silicon carbide during sample measurement. Losses of volatile silicon species during sample pyrolysis were overcome by using Co(II) as matrix modifier and a pyrolysis temperature of only 300 °C. Furthermore this low pyrolysis temperature prevents charring of organic material which enables accurate ETAAS analysis. The method including the developed pretreatment procedure was evaluated using the Standard reference material ® 2709 (San Joaquin Soil) from NIST (National Institute of Standards and Technology, Gaithersburg, MD, USA). Suitability for measurement of Si in airborne particulate matter with an aerodynamic diameter ≤10 μm (PM10) was demonstrated by the analysis of selected aerosol samples and comparison of derived results with the findings obtained for the same samples after microwave digestion and subsequent ETAAS measurement. Finally the developed procedure was applied for the analysis of silicon in PM10 collected at an urban site in Vienna (Austria). Matrix matched calibration has been used for quantification of derived absorption signals. With the use of 20 μL sample injection volume for ETAAS analysis an instrumental detection limit of 52.2 μg L −1 was obtained, which translates to method detection limits of approximately 0.52 μg m −3 when considering the volumes of air collected per investigated aerosol sample. The reproducibility of analysis given as the relative standard deviation was 4.4% ( n = 12). Derived concentrations for Si in PM10 varied between 0.8 and 7.2 μg m −3 which is in good accordance with the literature findings.

高温高圧純水環境下の600合金用被覆アーク溶接金属の応力腐食割れ感受性に及ぼすC，Nbの影響

The influence of Nb content on the SCC (Stress Corrosion Cracking) susceptibility of Inconel alloy 600 weld metals with C contents of 0.03—0.07mass% has been investigated, since in the recent nuclear power plant there is a tendency to increase the Nb addition to the weld metal to avoid the occurrence of SCC caused by the grain boundary depletion of Cr due to the preferential precipitation of Cr carbides. The SCC susceptibility of the weld metal was evaluated from the maximum depth and number of cracks occurring during the CBB (Creviced Bent Beam) test in high temperature pressurized water using plate specimens cut from shielded metal arc weld metals. For the weld metal specimens in the as-welded state, the Nb and C contents had only slight influences on the SCC susceptibility. When the specimen received a heat treatment consisting of stress relief annealing (SR) for 72 ks at 893 K and subsequent ageing (LTA) for 720 ks at 673 K, however, the significant influences of the increases in C and Nb contents on the SCC susceptibility were observed; the susceptibility of weld metals with higher C contents (∼0.07mass%) decreased with increasing the Nb content up to ∼2.6mass%, but a further increase in the Nb content enhanced the SCC susceptibility remarkably. The decreased SCC susceptibility with an increase in Nb content (less than 2.6mass%) observed in the weld metal of the higher C content can be explained as resulting from the suppression of Cr depletion layer at the grain boundary due to the formation of stable Nb carbides, since TEM-EDS analyses revealed that no Cr depletion at the grain boundary occurred at Nb contents of 2.6mass% or more. When the weld metal of the higher C content was subjected to the SR+LTA treatment, the hardness increased remarkably with the Nb content, suggesting that the higher stress was applied to the specimen during the CBB test, as the Nb content was increased. This increase in the applied stress is a possible factor that contributes to the increase in the SCC susceptibility with the Nb content in the weld metal free from the grain boundary depletion of Cr. For the weld metals with lower C contents (∼0.03mass%), the SCC susceptibility decreased with the Nb content after the SR+LTA treatment, probably owing to the suppression of the Cr carbide precipitation at the grain boundary and resulting Cr depletion.

The influence of hot-working conditions on a structure of high-manganese steel

The high-manganese steels with the austenitic structure belong to a group of modern steels predicted to use in the automotive industry. The chemical composition of the steel containing 25% Mn, solution hardened by Si and Al was developed. Microadditions of Nb and Ti introduced into the steel creating stable nitrides and carbides should act by precipitation hardening and inhibit a grain growth of recrystallized austenite. The aim of the work was to determine the influence of various hot-working conditions on a structure of the investigated steel. The processes controlling work-hardening and removing strengthening after finishing the hot-working were identified. The preliminary upset forging by the use of eccentric press with a degree of deformation in a range of 20 to 60% and at temperatures of 850 and 1000 °C was carried out. On the basis of determined conditions the multi-stage axial compression tests ensuring the fine-grained austenite structure were performed.

Corrosion behavior of ferritic stainless steel alloyed with different amounts of niobium in hydrochloric acid solution

The corrosion behavior of stainless steel alloys containing corrosion-resistant elements was investigated. Ferritic stainless steel (FSSs) electrodes were synthesized by applying a scan rate of 1 mV s−1. Stainless steels were used unalloyed and alloyed with about 0.5, 1, and 3 wt% elemental Nb. The samples were obtained from casting and forging. The samples were classified into three groups. In the first group, samples were unhomogenized and remained in production condition. In the second and third groups, samples were exposed to homogenization at 1,100 °C for 30 min or 180 min, respectively, and then quenched. The corrosion performance of the FSSs was investigated in 0.3 M HCl acid solution using electrochemical impedance spectroscopy (EIS). Corrosion resistance was calculated using the Stearn–Geary equation. SEM investigations of samples immersed in 0.3 M HCl acid solution for 60 and 360 min were performed. SEM micrographs showed generalized pitting. Consequently, it was determined that niobium has a beneficial effect on the corrosion resistance of FSS since niobium reacts with carbon to form stable carbides.

Carbides in stainless steels: Results from ab initio investigations

The useful properties of steels are due to a complicated microstructure containing iron and chromium carbides. Only some basic physical properties of these carbides are known with high precision, although the carbides may have a vital impact on the performance and longevity of the steel. To improve on this situation, we have performed extensive density-functional theory calculations of several carbides. The quantitative results are in perfect agreement with the relative empirical stability of the carbides. Also, in contradiction with experimental data, we find that Cr23C6 responsible for the hardness of stainless steels is not the most stable chromium-dominated carbide.

A thermodynamic model for carbon trapping in lattice defects

A thermodynamic model in the framework of the CALPHAD method is proposed to describe the distribution of interstitial solute (carbon) atoms in bcc-iron during tempering of martensite. In the model, the crystal defects are introduced as a separate sublattice, which is a highly favored lattice position for carbon compared to undisturbed interstitial sites or precipitate sites in cementite. With the model, which is convenient for multicomponent, multiphase systems, the fraction of carbon atoms trapped by the defects during tempering can be calculated. It is demonstrated that, when martensite is heavily dislocated, the precipitation of cementite on tempering is greatly suppressed because the high dislocation density provides a sufficient number of stressed sites which can trap most of the carbon atoms. The calculations for an alloy steel show that, unlike cementite, the defect density and carbon trapping phenomenon influence the precipitation of stable carbides only marginally.

Accessing Metal−Carbide Chemistry. A Computational Analysis of Thermodynamic Considerations

The electronic structures of terminal metal carbide complexes are calculated using DFT. This study outlines the factors that give rise to stable carbide complexes, which can be used to help in the synthesis of new carbide complexes and to tune their stability as desired. The calculations reveal the presence of a strong Ru≡C triple (σ + 2π) bond. The C atom is nearly unhybridized, such that the C-component of the Ru−C σ-bond has 90% 2p character. This leaves a very stable carbon-based lone pair that is almost entirely 2s in character, which accounts for the lack of Lewis base character exhibited experimentally. Calculations predict a Ru−C bond dissociation energy of 147.4 kcal mol−1 in a typical Ru carbide complex. This large bond strength is not unique to the Ru≡C bond, as revealed by an extension of the study to identify schemes by which to chemically tune the metal−carbide bond strength. Methods examined to achieve this tuning include changing the identity of the central metal and altering the metal lig...

Physicochemical laws of the interaction of nickel aluminides with alloying elements: II. Interaction of nickel aluminides with alloying elements and/or interstitial phases

The Ni-Al-X (X is an interstitial element or phase) phase diagrams are analyzed to reveal systems that can be used as the basis for designing promising alloys and natural composites based on nickel aluminides reinforced by interstitial phases (natural composites I). The most thermally stable materials are shown to be heterophase alloys and composite materials (CMs) located in the eutectic-type (including degenerate eutectic) pseudobinary sections of ternary or multicomponent phase diagrams. They exhibit insignificant (or zero) dissolution of interstitial phases at operating temperatures and the absence of an intense interaction between CM components (natural composites II). Natural composites I based on the NiAl-or Ni3Al-interstitial phase alloys produced upon cooling from a melt can be reinforced by the refractory thermally stable rigid interstitial phases, namely, borides and carbides, that are present in pseudobinary sections in equilibrium with these nickel aluminides, since the elements forming these phases dissolve completely in matrix melts and the mutual solubility of these phases in the solid state is low. Such borides are TiB2 and HfB2 in equilibrium with β-NiAl, and such carbides are, e.g., TiC and HfC in equilibrium with β-NiAl and La2C3, NbC, and TaC in equilibrium with γ′-Ni3Al. Natural composites II should be produced using solid-phase methods (NiAl with AlN, Y2O3, Al2O3) or a combination of methods, where a refractory interstitial phase of the Al2O3 or Y2O3 type is solid and the intermetallic NiAl or Ni3Al matrix is liquid. NiAl-TiB2 (HfB2), NiAl-Al2O3 (Y2O3), and Ni3Al-La2C3 (NbC, TaC) composites are considered as examples.

Tin–Transition Metal–Carbon Systems for Lithium-Ion Battery Negative Electrodes

Magnetron cosputter deposited ternary libraries of (, V and Co) ( and ) have been studied structurally and electrochemically using combinatorial and high-throughput methods. Each of the sputtered binary systems shows an amorphous composition range where the specific capacity for lithium decreases with M content. Adding carbon to the amorphous binaries, to make ternaries, causes the precipitation of crystalline Sn in the cases when or V, but not when . We believe this is because stable carbides of Ti and V exist but stable Co carbides do not. The sputtered system was found to have a large amorphous range and the initial amorphous atomic arrangement in certain compositions are stable over at least 27 charge-discharge cycles of . Crystalline Sn was found to precipitate in composition ranges having competitive specific capacity in the and libraries causing rearrangement of the atoms during cycling leading to poor capacity retention.

Wetting and interface interactions in the B 4C/Al–Me (Me [dbnd]Cu, Sn) systems

The wettability of B 4C in contact with non-carbide and non-boride forming liquid metals (such as Cu or Sn) has been the subject of several studies. These metals do not wet boron carbide unless a reactive element is added to the melt. The present study is concerned with the addition of Al which completes the series of reactive elements added to the non-wetting metals. While Si represents the elements that form stable carbides and Ti represents the elements that form stable borides, Al belongs to the group of elements that form ternary borocarbides. The wetting experiments in the B 4C/(Me–Al, Me Cu, Sn) systems have shown that a ternary product, namely Al 8B 4C 7 was formed at the interface and that wetting is governed by the thermodynamic properties of the binary liquid system.

Mechanism and Kinetics of Plasma Nitriding of the Nb-Alloyed PM Tool Steel

The aim of this work was to describe the mechanism and kinetics of plasma nitriding of a Nb-containing PM (powder metallurgy) tool steel. Material containing 2.5 wt.% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W was prepared by nitrogen melt atomization and hot isostatic pressing. Heat-treated steel (quenching from 1100 °C, triple tempering at 550 °C for 1h) was plasma nitrided at temperatures ranging from 470 °C to 530 °C / 30 - 180 min. Light microscopy, TEM, SEM and WDS were used to study the nitrided steel. It has been shown, that nitriding at 470°C leads to the formation of thin layers composed only of a diffusion zone containing nitrogen-rich martensite and fine nitride precipitates, no layer of nitrides is formed on the surface. Nitriding is probably controlled by the nitrogen diffusion in martensite to the material or by the processes in the nitriding atmosphere at this temperature. Nitriding at the temperature of 500°C and more leads to the formation of a continuous layer of nitrides and carbonitrides on the surface that limits further nitrogen diffusion. Niobium, as a prospective element in tool steels, was not found to play a role in the formation of the nitrided layer directly. Niobium replaces vanadium in very thermodynamically stable primary MC carbides. This results in higher vanadium content in others less stable carbides and in the matrix. Due to this effect, higher portion of vanadium can precipitate as VC carbides and VN nitrides during heat treatment and nitriding, respectively.

NOVO AÇO FERRAMENTA DE ALTA RESISTÊNCIA A QUENTE

Hot work tool steels are mainly employed in hot metal forming processes, especially as dies for forging, extrusion or die casting. In such applications, tool steel hot resistance is essential. During forging operations, hot work dies are exposed to high temperatures, in which tempering reactions can occur. Therefore, increased hot resistance can only be attained if such reactions are retarded. The new material described in the present paper, named VHSUPER ® , was developed focusing the increase in tempering resistance, through a different design of chemical composition. Pilot scale and industrial heats were produced and field tests were performed. The tempering resistance test showed the superior behavior of the new steel in comparison to AISI H11 and AISI H13 grades. This mainly results from its higher Mo content, which forms stable carbides during secondary hardening. In spite of the higher tempering resistance, impact tests showed that toughness was not reduced. The field testes, in hot forging dies, resulted in substantial increase in die life for the new

The Role of Metal in Self‐Organization of Carbon into Graphite‐ or Diamond‐Like Nanotubes

The reasons of the self‐organization of carbon‐metal systems into graphite‐ or diamond‐like nanotube were firstly discussed. Non‐transition metals, which do not form carbide, have highly disposed free levels and the metal phase formation does not allow energy relaxation over metal phase, which evolves as a result of recombining carbon‐carbon bonds. Therefore the energy is conserved as a metastable state such as diamond‐like nanotubes or films. Transition metals, which cannot form stable carbides, produce fullerene and graphite‐like nanotube structures because these metals produce relaxing the recombination energy of carbon atoms on forming the stable graphitic structures.

Creep behavior of a new cast austenitic alloy

A new cast austenitic alloy, CF8C-Plus, has been developed by Oak Ridge National Laboratory (ORNL) and Caterpillar for a wide range of high temperature applications including diesel exhaust components and turbine casings. The creep strength of the CF8C-Plus steel is much greater than that of the standard cast CF8C stainless steel and is comparable to the highest strength wrought commercial austenitic stainless steels and alloys, such as NF709. The creep properties of CF8C-Plus are discussed in terms of the alloy design methodology and the evaluation of some long-term creep tested specimens (over 20,000 h). Microcharacterization shows that the excellent creep strength is due mainly to the precipitation of very fine nano-scale and stable MC carbides, without the formation of deleterious intermetallic phases.

Formation of hexagonal η-MoC1−x phase at a temperature lower than 1660°C by solar radiation heating under presence of excess free carbon

There are a number of Mo–C binary phase diagrams available and they are not necessarily in agreement with each other. Anyway, in none of them, presence of mono-carbide MoC is reported at temperatures between 1660 °C and 1150 °C. In this intermediate temperature range, sub-carbide Mo2C is identified as the only stable carbide phase in the binary Mo–C system even under presence of excess free carbon. However, in recent carbide synthesis work started from mixed powders of Mo and excess carbon under solar radiation heating, formation of hexagonal hypo-stoichiometric mono-carbide η-MoC1−x coexisting with sub-carbide Mo2C was detected. The target reaction temperature in the experiment was set to be 1600 °C and it took merely fractions of a second to heat from the ambient temperature to this target temperature in the used solar furnace. The possibility of formation of MoC is discussed with reference to the comparable experimental evidences obtained for W–C system also under solar heating condition with ultra-fast heating rate.

Fabrication of porous SiC-based ceramic microchannels via pyrolysis of templated preceramic polymers

This article reports conversion chemistry of preceramic polymer to ceramic phase during the fabrication of high-temperature stable silicon carbide and silicon carbonitride monolithic porous microchannels. The micromolding in capillariesmethod is used to fabricate porous channels by the initial infiltration of a solution of 1.5-μm diameter silica spheres or 1-μm diameter polystyrene spheres into polydimethylsiloxane channels followed by filling the void space among the spheres by using viscous commercial polymeric precursors. Subsequently, the polymer-sphere composite channel was cured and pyrolysed at 1200 °C under inert atmosphere, and final wet etching step of silica spheres with 10% hydrofluoric acid solution developed the pore structures by removing the silica spheres, whereas polystyrene sphere decomposes at the early stage of pyrolysis.