Carbide Content Research Articles

Effect of Fe3O4 content on the CO2 selectivity of iron-based catalyst for Fischer-Tropsch synthesis

In this study, a series of catalysts with different Fe3O4 to iron carbide ratios were obtained by carburizing the α-Fe2O3 precursor prepared by co-precipitation method, under various carburization conditions. XRD, Mössbauer spectroscopy, XPS, and Raman spectroscopy were used to characterize the bulk and surface phase compositions of the Fe-based catalysts. The results show that increasing the carburization temperature and prolonging the carburization time lead to higher iron carbide concentration. To explore the active phase of CO2 formation, the catalysts were tested under different reaction conditions by tuning either CO conversion or H2O partial pressure. It turns out that the catalytic performance of the Fe-based catalyst in the FTS and water-gas shift (WGS) reactions is influenced by both the content of iron carbide and the degree of carbon deposition. Under typical Fischer-Tropsch reaction condition, the CO2 selectivity is determined by the CO conversion rather than the Fe3O4 content in the catalyst, meaning that the WGS reaction is here limited by the kinetic factors. On the contrary, adding H2O to the reaction gas results in the trend that higher CO2 selectivity is promoted by higher content of Fe3O4 in the Fe-based catalyst. It seems that Fe3O4 is the main active phase for the WGS reaction in the iron-based catalyst for FTS. These results provide a new insight into the active phase of CO2 generation on the Fe-based catalysts, which could be the theoretical basis for the design of new industrial FTS catalysts with low CO2 selectivity.

The Structure and Properties of Laser-Cladded Inconel 625/TiC Composite Coatings.

This article presents production results concerning metal matrix composite-coatings made using the laser-cladding technology. The enhancement of the wear resistance of the material surface is the one of the main goals accompanying the manufacturing of composite coatings. Nickel-based superalloys are used in several industries because they are characterized by a number of desirable properties including high tensile and fatigue strength as well as resistance to high-temperature corrosion in aggressive environments. One of the most interesting materials from the group of superalloys is Inconel 625, used as a matrix material in tests discussed in this article. However, nickel-based superalloys are also characterized by an insufficient wear resistance of the surface, therefore, in relation to the tests discussed in this article, Inconel 625-based composite coatings were reinforced by adding 10%, 20% and 40% of titanium carbide particles. The addition of hard phases, i.e., TiC, WC or SiC particles can have a positive effect on the erosion resistance of cladded specimens. The aim of the experiment was to determine the impact of the titanium carbide content on the structure of the alloy and its resistance to corrosive wear, enabling the extension of the service life of Inconel 625/TiC composite coatings. The investigation included microhardness tests, corrosion resistance analysis, penetrant tests, macrostructure and microstructure analyses and X-ray diffraction (XRD) tests. The TiC particles increased the hardness of the coatings and, in general, had a negative impact on the corrosion resistance of pure Inconel 625 coatings. However, the increased homogeneity of composite coatings translated into the improvement of corrosion resistance.

Microstructure and oxidation resistance of Al50(Fe–Cr)(50-x)(SiC)x composite coated low-carbon steel prepared by mechanical milling

In this study, the influence of silicon carbide (SiC) content on microstructure and oxidation behavior of the Al50(Fe–Cr)(50-x)(SiC)x composite coating on low-carbon steel was studied for the first time. The correlation between the microstructure and oxidation behavior of the Al50(Fe–Cr)(50-x)(SiC)x composite coating at different SiC content was determined using X-ray diffraction (XRD), optical microscope, and scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The result confirms that the phase and microstructure of the coating is a composite structure consisting of highly densified and porous regions. The addition of SiC (5 at.%) on the composite coating improves the microstructure quality, but increasing SiC more than 5 at.% causes the increment of micro-porosity. The high-temperature cyclic oxidation results at 800 °C for 200 h revealed that all composite coating followed a parabolic rate law. The best oxidation resistance is shown by the addition of 5 at.% SiC on the Al50(Fe–Cr)(50-x)(SiC)x composite coating. It indicated a lower Kp of 0.84 × 10−3 mg2/cm4.s than the other samples. Moreover, the protective oxides of Al2O3 were formed on the composite coating surface. This oxide scale is dense and fully adhered to the coating, thus improving oxidation resistance.

Microstructure evolution and creep behavior of nitrogen-bearing austenitic Fe–Cr–Ni heat-resistant alloys with various carbon contents

In the petrochemical industry, centrifugal cast Fe–Cr–Ni heat-resistant alloy tubes are critical components that are exposed to severe conditions for long-time service. The life of the alloy tubes is primarily limited by creep damages, which can evolve into catastrophic failures. In order to improve the creep properties in a low-cost approach, the influence of N + C content on the microstructure evolution and creep behavior of the 25Cr35NiNb alloys has been studied in this work. Creep tests were carried out at 1173 K (900 °C) and 1323 K (1050 °C) with different stress levels. Microstructure investigation on precipitates revealed that the C/N ratio significantly affects the content and morphology of Cr-rich carbides and the phase transition of Nb-rich carbonitrides after thermal exposure. By adopting the threshold stress analysis, the true creep activation energy and stress exponent of all the alloys were found to be ∼270 kJ mol−1 and ∼5, respectively, which indicated that dislocation climb is the rate-controlling mechanism of creep. Furthermore, the tailored novel N-bearing alloy possessed superior comprehensive properties of creep resistance and damage tolerance compared to the 25Cr35NiNb grade alloys. Our current findings not only realize improved creep properties by the combined modification of N addition and C/N ratio, but also provide new insights into understanding the microalloying mechanisms of high-temperature materials in general.

Analysis of thermal stresses synergy in surface layer of carburised creep-resistant casts during rapid cooling processes

ABSTRACT During the carburising processes, treated elements are transported on a specially designed technological equipment of the furnaces. This equipment works in particularly hard conditions. This is because in each work cycle, apart from the load carried by the charge, it is subjected to harmful effect of many other factors such as the highly carburising technical atmosphere of the furnace and high and rapidly changing temperature. These factors influence fatigue changes in the equipment’s parts, the observable effect of which may be cracks on the surface, propagating deeper into the material in subsequent cycles. A pivotal factor influencing the kinetics of fatigue processes in the carburising furnaces equipment are thermal stresses appearing in each operating cycle, with each temperature change. The thermal stresses in this equipment are caused by the temperature gradient across the section of the heated or cooled element and different thermal expansion of the structural components of the carburised alloy. In the paper, on the developed FEM model of the subsurface layer of a carburised rib of a pallet for transporting the charge in the furnace, analyses of stress distributions during rapid cooling were carried out. These analyses were performed with taking into account the separate impact of the above-mentioned sources of thermal stresses and their simultaneous action. In the analyses which consider the influence of the temperature gradient, the main load factor was the temperature distribution determined on the basis of the heat flow analyses carried out for the analysed section of pallets rib. The conducted analyses made it possible to assess the impact of local stresses caused by different thermal expansion of structural components of the carburised alloy on the change in the distribution of global stresses caused by the temperature gradient formed during rapid cooling. Based on the obtained results, it was assessed how the increased concentration of carbide precipitates, characteristic for the grain boundary of the carburised alloy, favours the formation of cracks observed in real castings, and whether the analysed stress sources have a significant impact on the kinetics of fatigue processes.

ВЛИЯНИЕ КАЛЬЦИЙ-СТРОНЦИЕВОГО КАРБОНАТА И УСЛОВИЙ ОХЛАЖДЕНИЯ ПРИ ЗАТВЕРДЕВАНИИ НА СТРУКТУРУ И СВОЙСТВА КОМПЛЕКСНО-ЛЕГИРОВАННЫХ БЕЛЫХ ЧУГУНОВ СИСТЕМЫ Fe–C–Cr–Mn–Ni–Ti–Al–Nb

Under severe operating conditions, such as aggressive abrasives, high temperatures and harmful working environments, machine and equipment parts are favoured that are made mainly of white cast irons. The structure and the amount of carbide phase in these cast irons influence their mechanical properties and their ability to resist wear at elevated temperatures. White cast irons owe their particular wear resistance to the high content of primary MC carbides and eutectic carbides such as M7C3 in the matrix, which is mainly composed of austenite or austenite and ferrite. There is a wide variety of technological methods that are designed to improve the special characteristics of cast irons, centred mainly on improving the structure of the materials. The most effective methods for improving the mechanical and special properties of foundry alloys are refining and complex modification. The heat resistance of cast iron is improved as a result of removal of harmful impurities from the melt and formation of refractory inclusions such as SrO and available MgO, Al2O3, SiO2 in carbonate. These inclusions serve as crystallisation centres, which leads to melt supercooling and structure refinement. The modifying effect of carbonate is due to the formation of SrO oxide, these inclusions act as modifiers of the second kind. When cast iron is heated, an oxide film of strontium chromate (SrCrO4) is formed on the surface of castings, which has a higher density and better adhesion to the metal base than spinel oxide (FeO·Cr2O3). This results in better resistance to scale formation.

Spatial analysis of CO2 hydrogenation to higher hydrocarbons over alkali-metal promoted iron(ii)oxalate-derived catalysts

The kind of alkali metal in ferrous oxalate dihydrate affect the concentration and spatial distribution of iron carbides formed in situ in CO2 hydrogenation. These structural changes impact the rate of CH4 formation from CO2 but not from CO.

Assessment of the effect of small additions of some rare earth elements on the structure and mechanical properties of castings from hypereutectic chromium white irons

<abstract> <p>Article considers the influence of additions of rare earth elements such as Sm, La, Ce, Nd, and Y on the structure and properties of hypereutectic high-chromium white cast iron of grade G-X300CrMo27-2. To obtain an increased content of carbides in the studied cast iron samples, the carbon content was 3.75–3.9 and 4.1–4.2 wt%. The amount of rare earth elements additives added to the melt is 0.2% by weight. Data were obtained on the effect of overheating and cooling rate in the crystallization interval on the effect of rare earth additives, the structure and properties of white cast iron castings are given. According to the results of the microprobe analysis, it was shown that, under the chosen crystallization conditions, Sm, La, and Ce can form solid solutions with primary and eutectic carbides (FeCr)<sub>7</sub>C<sub>3</sub>. La and Ce form solid solutions with austenite. Nd and Y do not dissolve in iron chromium phases. All listed rare earth elements form phosphides and oxyphosphides. Experimental data are presented on the effect of rare earth elements on the size of primary (FeCr)<sub>7</sub>C<sub>3</sub> carbides and a hypothesis is proposed on the effect of rare earth elements on the crystallization process of hypereutectic chromium white cast irons. Experimental data are presented on the effect of REE additives on the microhardness of phases, hardness, strength, and resistance to abrasive wear of cast iron castings. It was found that the introduction of these additives into hypereutectic chromium white cast iron does not contribute to the modification of the structure and leads to an increase in the size of primary crystals, as well as a decrease in their mechanical properties. However, the addition of Y increases the abrasive wear resistance, but reduces the strength of castings made from such white cast iron.</p> </abstract>

Effect of Composition and Method of Mechanical Preparation of a Composite TiC-Ni Mixture on of Titanium Carbide Content in Coatings Formed by Cold Spraying

Effect of Composition and Method of Mechanical Preparation of a Composite TiC-Ni Mixture on of Titanium Carbide Content in Coatings Formed by Cold Spraying

High Temperature Fatigue Properties of Boron-doped Superalloy B1914

The B1914 superalloy is engaged in the power and aerospace industry as a material for critical hot section of gas turbines. Typical findings in these critical components with a high number of operations are damages by low cycle fatigue where high stresses comparable to the material yield strength are developed due to mechanical loading or thermal gradients causing large plastic deformation. The present contribution refers to the high-temperature low cycle fatigue behaviour of polycrystalline nickel-based superalloy B1914. Cylindrical specimens were fatigued under strain control with constant total strain amplitude in symmetrical cycling at 800°C and 900°C in air. The microstructure is based on a γ matrix (face-centered cubic lattice) with coherent L12 γ´ precipitates and γ/γ´ eutectic. The higher content of grain boundary stabilizing boron allows for lower carbon contents and thus reduces the carbide content that can facilitate fatigue crack initiation. Cyclic hardening/softening curves and fatigue life diagrams were determined. An increase in testing temperature accelerated fatigue crack initiation at grain boundaries associated with carbides. The SEM and TEM analysis of fatigued material further the discussion of fatigue behaviour of the B1914 superalloy.

Fabrication and development of neutron shielding materials based on natural rubber and boron carbide

Neutron shielding materials were fabricated based on natural rubber (NR) and boron carbide (B4C) as their main components. Natural rubber, which is abundant in hydrogen, can lower the energy of a neutron. Boron carbide could be utilized as a neutron shielding material due to its high boron content. The shielding materials were fabricated with boron carbide concentrations of 0, 20, 40, 60 parts per hundred rubber (phr), and 50 wt%. Each sample has been fabricated with thicknesses of 5, 10, 15, 20, and 24 mm. The fabricated materials were examined for mechanical and neutron attenuation tests. The Am/Be at the Radiation Measurement Laboratory at the Nuclear Engineering Department, Faculty of Engineering, Chulalongkorn University, was used as a neutron source. The results show that boron carbide has the effect of reducing the toughness while increasing the hardness of NR composites. The NR sample of 50 wt% in boron carbide allows only 14.1% of neutron transmission according to fast neutron attenuation tests, which is similar to that of a sample of the commercial product obtained from a Japanese company. Our findings will later be compared to those of a Monte Carlo N-Particle (MCNP) transport code.

Thermal properties of cemented carbides used for metal cutting

High temperatures, generated during machining, may have a detrimental effect on the cutting tool. Therefore, tool material's ability to effectively lead away the heat is an important property that needs to be understood and quantified. To accomplish that, thermal conductivity, diffusivity and specific heat of 20 cemented carbide grades were studied using light flash analysis at temperatures between 22 and 800 °C.The results demonstrate how thermal conductivity of cemented carbides increases with coarser WC grain size and lower Co content. Presence of cubic carbides in the material is shown to greatly reduce its thermal conductivity. Specific heat of investigated materials increases with higher Co and cubic carbides content, but it is not affected by WC grain size. Influences of temperature on thermal properties are illustrated and discussed.Most importantly, using regression analysis, an attempt is made to predict thermal conductivity based on reliable, readily available or easily measurable material characteristics. It is shown that thermal conductivity of cemented carbide grades could be estimated with good accuracy from the material's chemical composition and coercivity value.

Boro/carbothermal reduction co-synthesis of dual-phase high-entropy boride-carbide ceramics

Dense, dual-phase (Cr,Hf,Nb,Ta,Ti,Zr)B2-(Cr,Hf,Nb,Ta,Ti,Zr)C ceramics were synthesized by boro/carbothermal reduction of oxides and densified by spark plasma sintering. The high-entropy carbide content was about 14.5 wt%. Grain growth was suppressed by the pinning effect of the two-phase ceramic, which resulted in average grain sizes of 2.7 ± 1.3 µm for the high-entropy boride phase and 1.6 ± 0.7 µm for the high-entropy carbide phase. Vickers hardness values increased from 25.2 ± 1.1 GPa for an indentation load of 9.81 N to 38.9 ± 2.5 GPa for an indentation load of 0.49 N due to the indentation size effect. Boro/carbothermal reduction is a facile process for the synthesis and densification of dual-phase high entropy boride-carbide ceramics with both different combinations of transition metals and different proportions of boride and carbide phases.

SOIL STABILIZATION USING SILICON CARBIDE (SIC) NANOPARTICLES: CONFIRMATION USING XRD, SEM, AND FTIR

The current research focuses on nanoparticles’ ground-improvement potential using clayey soil mixed with varying amounts of the nanoparticles “Silicon Carbide”. With an increase in the amount of nanomaterial, a tendency of improvement has been recorded in liquid and plastic limits, as well as the plasticity index. The maximum reduction in liquid limit (15.8%), plastic limit (13.6%), and plastic index (18.7%) was recorded at 0.25 gm of Silicon Carbide as compared to control (0 gm of SiC). There was a 26.7% and 33.3% increase in the cohesion of soil at 0.25 gm and 0.3 gm of Silicon Carbide, respectively. Furthermore, when the Silicon Carbide content increased from 0.25 gm, the rate of increment of friction angle also increased. It was 87.5% and 137.5% at 0.25 gm and 0.3 gm of Silicon Carbide, respectively. Furthermore, 0.3 gm of Silicon Carbide, is found to be optimal within the scope of the experiment as at this amount of Silicon Carbide both cohesion and angle of friction attained maximum. XRD, SEM, and FTIR were used to confirm the findings. It concludes that by using even a small amount of nanomaterial, an appreciable change in the properties of clayey soil can be obtained in the field.

Structure and Properties of Composite Coatings Cold Sprayed from Powder Mixtures of Aluminum and Boron Carbide

In the present work, an experimental study of the effect of aluminum and boron carbide powder mixture composition on the microstructure and basic properties of the cold sprayed coatings was carried out for the first time. A series of coatings deposited on stainless steel substrates was obtained. The microstructure of the deposited coatings was analyzed by scanning electron microscopy and X-ray diffraction analysis. The microhardness and bonding strength of the coatings were measured. It was shown that an increase from 0 to 72 vol.% in the boron carbide content in the powder mixture leads to an increase in its content from 0 to 15-17 vol.% in the coating, its microhardness – from 46.3 to 72.4 HV0.3, and bonding strength – from 17.4 to 61.4 MPa. The obtained results open up broad prospects for the application of the cold spray method to deposition of functional coatings, which are highly demanded in the nuclear industry.

Properties of NbC/а‐C:H films on titanium bipolar plates for proton exchange membrane fuel cells

AbstractSurface modification of metallic bipolar plates is a crucial subject for the performance elevation of proton exchange membrane fuel cells (PEMFCs). In this work, a series of NbC/а‐C:H films with different Nb/C ratios are prepared by arc ion plating. Film microstructure, composition, mechanical properties, hydrophobility, interfacial contact resistance (ICR), and corrosion resistance in the simulated cathode environment of PEMFCs are systematically studied. The results show that within the experiment conditions, higher NbC content helps to promote the film hardness and adhesion strength as well as the interfacial conductivity. While higher а‐C:H content attributes to a more compact microstructure thus improving the anti‐corrosion performance. The best corrosion resistance and conductivity come with the lowest corrosion current density of 0.09 µA/cm2 and ICR of 0.77 mΩ cm2, respectively. Based on the result of this research, to further improve the comprehensive performance of NbC/а‐C:H film, strategies for increasing the metal carbide content and preventing surface metal oxidation while keeping a dense and fine microstructure need to be considered.

The effects of nanoparticles as a biodiesel ingredient on the performance of a VCR diesel engine

The combustion, efficiency, and emission properties of a single-cylinder variable compression ratio diesel engine were examined in this research (B100A30C30). To create a homogeneous solution, the ultrasonicator was used in conjunction with a 30 ppm concentration of silicon carbide and carbon nanotubes, two common nanoparticles found in fuel combinations. The increased mixing and chemical reactivity offered by nanoparticles' greater surface area to volume ratio during combustion improved the combustion, performance, and emission characteristics of diesel engines. The engine with nanoparticles (B20A30C30) had a thermal efficiency of braking that is 20% higher than the engine with regular particles (B100). Following that, nitrogen oxide emissions reduced by 38%, carbon monoxide emissions by 68%, hydrocarbon emissions by 52%, and smoke emissions by 48%.

Effects of solution treatment on the microstructural evolution and tensile properties of GH 3230 superalloy manufactured by laser melting deposition

GH 3230 is a solution strengthening nickel-based superalloy, which is widely used to fabricate hot components involving service temperatures nearly or above 1100 °C. In this study, GH 3230 samples were fabricated by laser melting deposition (LMD) and the effects of solution treatment temperature ranging from 1200 °C to 1320 °C on the microstructure and tensile properties were investigated. Meanwhile, a comparison of the microstructure and properties of the different additive manufactured specimens and forgings were also made. The microstructure and phase evolution of the samples were examined with optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the morphology of grain boundary carbides altered from chain-like to semi-chain short rod by the solution treatment, and the phase composition changed from a large amount of Cr 23 C 6 phases and a small amount of Ni 2 W 4 C phases to a massive number of Ni 2 W 4 C phases. It was found that almost no change occurred with regard to the carbide content and the carbide size varied from 1.09 μm to 0.85 μm with the solution treatment temperature ranging from 1200 °C to 1260 °C. However, when the solution treatment temperature was further increased to 1320 °C, the content of carbides decreased dramatically from 4.35 vol% to 0.5 vol%, and size of carbides decreased from 0.85 μm to 0.80 μm. In general, the solution treatment improved not only the ultimate tensile strength (UTS) but also the elongation (EL) of LMDed samples. The ultimate tensile strength could be increased from 1000 MPa (as-LMDed) to 1062.7 MPa (LMD + HT 1260 °C), and the elongation was enhanced from 16.7% to 24.3%. • Haynes 230 samples were manufactured by laser melting deposition and the effects of heat treatment on the microstructure and properties were studied. • After heat treatment the carbides morphology altered from chain-like to semi-chain short rod and two kinds of carbides changed to one type. • Heat treatment improved both strength and elongation of specimens. The recommended heat treatment temperature was 1260 °C and its strength (1062.7 MPa) was obviously higher than that of forgings + HT 1230 °C (904.0 MPa).

Effect of Na promoter and reducing atmosphere on phase evolution of Fe-based catalyst and its CO2 hydrogenation performance

Although alkali metal promoters have a considerable effect on the product distribution of CO2 hydrogenation over Fe-based catalysts, the mechanism of the active phase transformation is uncertain. The effects of Na modification and pretreatment atmosphere on phase evolution of Fe2O3 sample and the synergistic effect between iron oxides and carbides were investigated using in-situ X-ray diffraction (in-situ XRD). The physiochemical properties of catalyst were characterized by H2-TPR and CO/H2-TPSR-MS. When the reducing environment is H2, Na promoter inhibited the reduction of Fe2O3. However, when the reducing atmosphere is syngas (CO/H2 = 1:2), a suitable amount of Na promoter decreased the reduction and activation temperatures, and increased the iron carbide concentration. The selectivity of light olefins increased from 0.3% to 20.2%, and CO2 conversion increased from 7.3% to 25.8% when syngas was used as the reducing gas compared to using H2 as the reducing gas. Compared with pure Fe2O3, the CH4 selectivity of Na modified Fe2O3 decreased from 43.2% to 14.9%, while the selectivity of C5+ increased from 7.8% to 37.0%, owing to the fact that the Fe5C2 content increased from 8.5% to 38.4%. The ratio of Fe3O4 to Fe5C2 in the catalyst could be effectively controlled by changing the reducing atmosphere and the amount of Na promoter, thereby improving the CO2 hydrogenation activity and target product selectivity.

The effect of niobium element on the tensile behavior in GH3535 alloy at room temperature and 750 °C

The microstructure and tensile behavior of GH3535 and 2 wt% niobium (Nb) modified GH3535 alloys at room temperature (RT) and 750 °C were systematically investigated. It is found that Nb element is distributed in both matrix and M6C carbide, while the type and content of carbide in GH3535 alloy are barely affected by Nb element. The tensile properties at RT and 750 °C are improved by adding Nb element, which can be ascribed to the synergistic effect of the solid solution strengthening, and nano twins strengthening. Additionally, they will be improved by grain refinement at RT. The strengths of both alloys at 750 °C are lower than that of RT, which is related to the softening caused by dislocations climbing and softening of grain boundaries at high temperature.