Mo2C Carbides Research Articles

Co-precipitation kinetics, microstructural evolution and interfacial segregation in multicomponent nano-precipitated steels

We investigated the co-precipitation kinetics, microstructural evolution and interfacial segregation in multicomponent nano-precipitated steels strengthened by Cu-rich precipitates (CRPs), NiAl-type particles and Mo2C carbides. Aging hardness evolution of Fe-2Cu, Fe-2Cu-5Ni-3Mn-1.5Al and Fe-2Cu-5Ni-3Mn-1.5Al-1Mo (wt%) steels with elevating aging temperatures showed typical aging hardening-peak hardening-overaging response. The results of co-precipitates characterized by atom probe tomography and transmission electron microscopy showed that Mo addition obviously slowed down the diffusion of Mn and Cu, delaying the evolution of elemental concentrations, time of peak hardness, and the nucleation of Ni(Al, Mn) precipitates. The sequence of co-precipitates involved CRPs and NiAl precipitates during aging was as follows: CRPs + NiAl particles → Cu/NiAl adjacency structure + NiAl particles → Cu-core/NiAl-shell structure, and Mo addition was incapable of changing the sequence of co-precipitation. Mo2C carbides nucleated heterogeneously behind co-precipitates of CRPs and Ni(Al, Mn) precipitates. Co-precipitation kinetics using differential scanning calorimetry (DSC) analysis and Johnson-Mehl-Arvami (JMA) equation indicated that the activation energy values for the nucleation and growth of CRPs in Fe-Cu steel were 97.3 kJ/mol and 182.6 kJ/mol, respectively, and the two activation energy values for the nucleation and growth of co-precipitates in the Fe-Cu-Ni-Mn-Al steel were 74.1 kJ/mol and 187.7 kJ/mol, respectively.

Processing and Characterization of Refractory Quaternary and Quinary High-Entropy Carbide Composite.

Quaternary high-entropy ceramic (HEC) composite was synthesized from HfC, Mo2C, TaC, and TiC in pulsed current processing. A high-entropy solid solution that contained all principal elements along with a minor amount of a Ta-rich phase was observed in the microstructure. The high entropy phase and Ta-rich phase displayed a face-centered cubic (FCC) crystal structure with similar lattice parameters, suggesting that TaC acted as a solvent carbide during phase evolution. The addition of B4C to the quaternary carbide system induced the formation of two high-entropy solid solutions with different elemental compositions. With the increase in the number of principal elements, on the addition of B4C, the crystal structure of the HEC phase transformed from FCC to a hexagonal structure. The study on the effect of starting particle sizes on the phase composition and properties of the HEC composites showed that reducing the size of solute carbide components HfC, Mo2C, and TiC could effectively promote the interdiffusion process, resulting in a higher fraction of a hexagonal structured HEC phase in the material. On the other hand, tuning the particle size of solvent carbide, TaC, showed a negligible effect on the composition of the final product. However, reducing the TaC size from −325 mesh down to <1 µm resulted in an improvement of the nanohardness of the HEC composite from 21 GPa to 23 GPa. These findings suggested the possibility of forming a high-entropy ceramic phase despite the vast difference in the precursor crystal structures, provided a clearer understanding of the phase transformation process which could be applied for the designing of HEC materials.

Gleeble-Simulated and Semi-Industrial Studies on the Microstructure Evolution of Fe-Co-Cr-Mo-W-V-C Alloy during Hot Deformation.

Fe-Co-Cr-Mo-W-V-C alloy is one of the most important materials for manufacturing drills, dies, and other cutting tools owing to its excellent hardness. However, it is prone to cracking due to its poor hot ductility during continuous hot working processes. In this investigation, the microstructure characteristics and carbide transformations of the alloy in as-cast and wrought states are studied, respectively. Microstructural observation and first-principles calculation were conducted on the research of types and mechanical properties of carbides. The results reveal that carbides in as-cast Fe-Co-Cr-Mo-W-V-C alloy are mainly Mo2C, VC, and Cr-rich carbides (Cr7C3 and Cr23C6). The carbides in wrought Fe-Co-Cr-Mo-W-V-C alloy consist of Fe2Mo4C, VC, Cr7C3, and a small amount of retained Mo2C. For these carbides, Cr7C3 presents the maximum bulk modulus and B/G values of 316.6 GPa and 2.48, indicating Cr7C3 has the strongest ability to resist the external force and crack initiation. VC presents the maximum shear modulus and Yong’s modulus values of 187.3 GPa and 465.3 GPa, which means VC can be considered as a potential hard material. Hot isothermal compression tests were performed using a Gleeble-3500 device to simulate the flow behavior of the alloy during hot deformation. As-cast specimens were uniaxially compressed to a 70% height reduction over the temperature range of 1323–1423 K and strain rates of 0.05–1 s−1. A constitutive equation was established to characterize the relationship of peak true stress, strain rate, and deformation temperature of the alloy. The calculated results were in a good agreement with the experimental data. In order to study the texture evolution, the microstructures of the deformed specimens were observed, and an optimal deformation temperature was selected. Using the laboratorial optimal temperature (1373 K) in forging of an industrial billet resulted in uniform grains, with the largest size of 17 µm, surrounded by homogenous spherical carbides.

Intercalation of Platinum into a Graphene Film Formed on Molybdenum Carbide Mo2C

It has been shown that platinum atoms evaporation on graphene covering molybdenum carbide Mo2C at T = 1000 K causes platinum intercalation into the graphene. In such a case, a continuous multilayer platinum film grows under the graphene, which makes it possible to distinguish graphene-related components in the complex Auger spectrum of carbon.

Intercalation of C60 Fullerene Molecules under Single-Layer Graphene on Molybdenum Carbide

It is shown that heating of a fullerite film several monomolecular layers thick deposited onto single-layer graphene formed on a substrate of molybdenum carbide Mo2C at T = 700–800 K leads to the intercalation of C60 molecules under the graphene layer. The direct deposition of C60 molecules at T = 650 K also leads to the intercalation of C60 molecules under graphene; the maximal amount of fullerene accumulated under graphene is one single layer.

Effect of strain rate and temperature on the serration behavior of SA508-III RPV steel in the dynamic strain aging process

Dynamic strain aging (DSA) effect on SA508-III reactor pressure vessel (RPV) steel was investigated. The SA508-III RPV steel was subjected to tension tests at different strain rates (1.1 × 10−5 s−1 and 6.6 × 10−5 s−1) and different temperatures (500 and 550 °C) to evaluate the influence of strain rate and temperature on the serrated flow behavior, which is the repetitive and discontinuous yielding phenomenon on the stress–strain curves. The higher temperature leads to the higher density of precipitates, M23C6 carbides and needle-like Mo2C carbides. It was found that the samples under tension test of 6.6 × 10−5 s−1 and 500 °C possess superior mechanical properties and mainly show A-type serrations on the tension test curves. Then, the local regress method was used to filter the DSA curves, thus to show the real trend of the curves. It has been found that the less time of interaction between dislocations and precipitates under higher strain rates leads to a higher strength of the sample. The more tiny-stress drops on the 550 °C serration curve can be attributed to the hardening phase, M23C6 carbides and needle-like Mo2C carbides. The higher percentage of the small stress drops on the serration curves represents the higher mechanical strength.

Cobalt-modified molybdenum carbide as a selective catalyst for hydrodeoxygenation of furfural

The hydrodeoxygenation (HDO) reaction is crucial to the upgrading of biomass-derived furfural to produce a promising fuel additive, 2-methylfuran. In order to enhance the stability of the molybdenum carbide (Mo2C) catalyst, this work utilizes cobalt (Co) modification to tune the oxygen and furfural binding energies on Mo2C. Density functional theory (DFT) calculations of the adsorption configuration of furfural on Mo2C(0001) and cobalt-modified molybdenum carbide (Co/Mo2C(0001)) reveal that the CO bond of furfural is elongated, facilitating the selective CO scission to produce 2-methylfuran. The reduced oxygen and furfural binding energies on Co/Mo2C(0001) allow the facile removal of surface oxygen and furfural to improve the stability of the catalyst. Temperature-programmed desorption (TPD) experiments on model surfaces confirm the enhanced stability and overall HDO performance of Co/Mo2C/Mo(110). Based on the results from high-resolution electron energy loss spectroscopy (HREELS), 2-methylfuran-like intermediates are observed on both Mo2C/Mo(110) and Co/Mo2C/Mo(110). Parallel reactor evaluations over the corresponding powder catalysts further demonstrate the enhanced stability of Co/Mo2C over Mo2C at ambient pressure. This work illustrates the important roles of oxygen and furfural binding energies in the furfural HDO reaction on both model surfaces and powder catalysts, which in turn provides insights into designing selective and stable carbide-based catalysts for HDO reactions.

A systematical analysis with respect to multiple hydrogen traps influencing sulfide stress cracking behavior of API-5CT-C110 casing steel

The irreversible hydrogen traps including Cr-rich M7C3, Mo2C and MC carbides are characterized by scanning transmission electron microscope (STEM) after the quenching-and-tempering process performed on the API-5CT-C110 casing steel. Combined with our previously published work [Mater. Sci. Eng. A 688 (2017) 378–387], a systematic analysis with respect to the hydrogen interactions with reversible traps (grain boundaries and dislocations) and precipitates influencing sulfide stress cracking (SSC) behavior is presented. Grain boundaries provide channels for hydrogen diffusion. Dislocations trap hydrogen and act as hydrogen “transporters” during moving. Precipitates are introducing traps to retain hydrogen and restrict dislocation movement, then improve resistance to SSC. However, the mobile dislocations can transport the trapped hydrogen from precipitates into potential crack sites in the presence of external stress, leading to the initiation of SSC cracks. Therefore, SSC susceptibility significantly depends on the density of dislocations. The steel with lower dislocation density exhibits superior SSC behavior mainly attributed to its finite hydrogen “transporters”.

EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.

Optimization by Using Taguchi Method of the Production of Magnesium-Matrix Carbide Reinforced Composites by Powder Metallurgy Method

The aim of this study was to determine the optimum production parameters in the production of magnesium matrix carbide-reinforced composites by using the powder metallurgy method. The parameter levels maximizing density (%), hardness (HB10), and bending strength (MPa) values were found by using the Taguchi method. The type of reinforcement, the amount of reinforcement, the sintering time, the sintering temperature, additive type, and additive rate were selected as the production parameters. Since the production of Mg and its alloys by using casting methods is problematic, the hot pressing method, a powder metallurgy method, was preferred in this study. Ceramic-based carbide particles were used as reinforcing materials in Mg matrix composite materials. B4C, SiC, Mo2C, and TiC carbides were preferred as the carbide. Microstructure and phase composition of the produced materials was examined with scanning electron microscope (SEM), X-ray diffractogram (XRD) and X-ray energy dispersive spectrometry (EDS). The hardness of the materials was measured by using a Universal Hardness device. The relative densities of the materials were determined according to Archimedes’ principle. The bending strength properties of the materials were determined by using the three-point bending test. The optimum conditions were a sintering temperature of 500 °C, sintering duration of 5 min, additive type of B4C and additive rate of 2.5%, and the results obtained at these conditions were found to be as follows; relative density of 98.74 (%), hardness of 87.16 HB10 and bending strength of 193.65 MPa. SEM images taken from the fracture surfaces showed that the carbides added to the matrix had a relatively homogeneous distribution. XRD analyses revealed that the matrix was oxidized very little, and no phase formation occurred between the matrix and the carbides. Carbide addition caused a distinct hardness increase by showing the effect of distribution strengthening in the matrix.

Surface composition and morphology of a carbon matrix/Mo2C composite material

The surface composition and morphology of a carbon matrix/Mo2C composite have been studied by scanning electron microscopy and X-ray photoelectron spectroscopy. The results demonstrate that the carbon matrix has the form of entangled filaments of carbon nanotubes. The surface layer of the composite contains 57 carbon atoms per molybdenum atom. Molybdenum is present here in the form of the Mo2C carbide (39 at %) and the Mo2O5 (50 at %) and MoO2 (11 at %) oxides, with E b(Mo 3d 5/2) = 228.2, 229.6, and 231.9 eV, respectively. The presence of the Mo4+ and Mo5+ oxides in the surface layer is due to active reaction of the Mo2C in the composite with atmospheric oxygen and moisture during the sample preparation process and can be accounted for by the small particle size of the material. Based on analysis of the structure of the C 2s and C 2p valence electron spectra, we assume that the carbon nanotubes of the composite are graphitelike carbon structures. The composite studied here does not become charged when exposed to an X-ray beam, which suggests that it is a weak dielectric.

Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15vol% VC or/and Mo2C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1h at 1420°C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo2C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12vol% Ni cermet had a modest Vickers hardness (HV30) of 1077±22kg/mm2 and an indentation toughness of 9.1±0.5MPa·m1/2. With the addition of 10–15vol% VC, the hardness increased to 1359±15kg/mm2, whereas the toughness increased to 11.3±0.1MPa·m1/2. Addition of 5 and 10vol% Mo2C into a NbC-12vol% Ni mixtures generated the same values in HV30 and KIC when compared to VC additions. A maximum flexural strength of 1899±77MPa was obtained in the cermet with 20vol% Ni binder and 4vol% VC+4vol% Mo2C addition, exhibiting a high fracture toughness of 15.0±0.5MPa·m1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400°C from 0.1 to 10m/s.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of Nitrogen-Alloyed High-Mn Austenitic Hot Work Die Steel

In view of the requirements for mechanical properties and service life above 650 °C, a high-Mn austenitic hot work die steel, instead of traditional martensitic hot work die steel such as H13, was developed in the present study. The effect of heat treatment on the microstructure and mechanical properties of the newly developed work die steel was studied. The results show that the microstructure of the high-Mn as-cast electroslag remelting (ESR) ingot is composed of γ-Fe, V(C,N), and Mo2C. V(C,N) is an irregular multilateral strip or slice shape with severe angles. Most eutectic Mo2C carbides are lamellar fish-skeleton-like, except for a few that are rod-shaped. With increasing solid solution time and temperature, the increased hardness caused by solid solution strengthening exceeds the effect of decreased hardness caused by grain size growth, but this trend is reversed later. As a result, the hardness of specimens after various solid solution heat treatments increases first and then decreases. The optimal combination of hardness and austenitic grain size can be obtained by soaking for 2 h at 1170 °C. The maximum Rockwell hardness (HRC) is 47.24 HRC, and the corresponding austenite average grain size is 58.4 μm. When the solid solution time is 3 h at 1230 °C, bimodality presented in the histogram of the austenite grain size as a result of further progress in secondary recrystallization. Compared with the single-stage aging, the maximum impact energy of the specimen after two-stage aging heat treatment was reached at 16.2 J and increased by 29.6%, while the hardness decreased by 1–2 HRC. After two-stage aging heat treatment, the hardness of steel reached the requirements of superior grade H13, and the maximum impact energy was 19.6% higher than that of superior grade H13, as specified in NADCA#207-2003.

Characterization of interfaces between TiCN and iron-base binders

This work includes a study of the interface between matrix and reinforcement in Fe-Ti(C,N) systems. A study of wettability and solubility between metal and ceramic phases is first performed. It combines wetting experiments at high temperature with kinetics simulation. In doing so, dense Ti(C,N) is produced by spark plasma sintering and used as substrate. On the other hand, Fe, Fe-WC and Fe-Mo2C alloys are used as metal matrix to evaluate the influence of WC and Mo2C carbides on the formation, composition and properties of the interface. These experiments are complemented by a micromechanical study of the interface. Hardness and elastic modulus are obtained from analysis of nanoindentation arrays in the matrix and into the reinforcement for each system. The sliding contact response at the interface (and neighboring regions) is evaluated by cross-section nanoscratches conducted at different load levels.

Wear Resistance of H13 and a New Hot-Work Die Steel at High temperature

The friction and wear behaviors of a new hot-work die steel, SDCM-SS, were studied at high temperature under dry air conditions. The wear mechanism and microstructural characteristics of the SDCM-SS steel were also investigated. The results showed that the SDCM-SS steel had greater wear resistance compared with H13 steel; this was owed to its high oxidizability and temper stability. These features facilitate the generation, growth, and maintenance of a tribo-oxide layer at high temperature under relatively stable conditions. The high oxidizability and thermal stability of the SDCM-SS steel originate from its particular alloy design. No chromium is added to the steel; this ensures that the material has high oxidizability, and facilitates the generation of tribo-oxides during the sliding process. Molybdenum, tungsten, and vanadium additions promote the high temper resistance and stability of the steel. Many fine Mo2C and VC carbides precipitate during the tempering of SDCM-SS steel. During sliding, these carbides can delay the recovery process and postpone martensitic softening. The high temper stability postpones the transition from mild to severe wear and ensures that conditions of mild oxidative wear are maintained. Mild oxidative wear is the dominant wear mechanism for SDCM-SS steel between 400 and 700 °C.

Effect of tempering temperature on the microstructure and mechanical properties of a reactor pressure vessel steel

The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investigated after tempering at different temperatures ranging from 580 to 700 °C for 5 h. With increasing tempering temperature, the impact toughness, which is qualified by Charpy V-notch total absorbed energy, initially increases from 142 to 252 J, and then decreases to 47 J, with a maximum value at 650 °C, while the ultimate tensile strength varies in exactly the opposite direction. Comparing the microstructure and fracture surfaces of different specimens, the variations in toughness and strength with the tempering temperature were generally attributed to the softening of the bainitic ferrite, the agminated Fe3C carbides that resulted from decomposition of martensite/austenite (M/A) constituents, the precipitation of Mo2C carbides, and the newly formed M/A constituents at the grain boundaries. Finally, the correlation between the impact toughness and the volume fraction of the M/A constituents was established, and the fracture mechanisms for the different tempering conditions are explained.

High-Temperature Corrosion of UNS N10003 in Molten Li2BeF4(FLiBe) Salt

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In t...

Effect of Heat Treatment on the Microstructure and Hardness of 17Cr-0.17N-0.43C-1.7 Mo Martensitic Stainless Steel

The microstructure and hardness of a nitrogen-containing martensitic stainless steel were investigated as a function of heat treatment using optical microscopy, electron microscopy, amount of retained austenite, and hardness measurement. The steel was subjected to three heat treatments: hardening, cryo treatment, and tempering. The hardness of the steel in different heat-treated conditions ranged within 446-620 HV. The constituents of microstructure in hardened condition were lath martensite, retained austenite, M23C6, M7C3, MC carbides, and M(C,N) carbonitrides. Upon tempering at 500 °C, two new phases have precipitated: fine spherical Mo2C carbides and needle-shaped Cr2N particles.

Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe3C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo2C (aspect ratio ranging from 10 to 25) and plate-shaped M4C3 carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe3C, and interlath-retained austenite decomposed into ferrite and elongated carbide.

Thermal stability and decomposition products of a MoCl1.9 ± 0.1(C30 ± 1H30 ± 1) composite

We have studied the thermal stability of a MoCl1.9 ± 0.1C30 ± 1H30 ± 1 composite in vacuum (10 Pa) at temperatures from 70 to 1150°C. Using chemical and physicochemical analysis data (gas liquid chromatography, MALDI TOF mass spectrometry, and X-ray diffraction), we have identified the nature of the decomposition products of the composite. The solid residue after the decomposition has been shown to consist predominantly of hexagonal molybdenum carbide Mo2C, with finely dispersed carbon as an impurity phase. Volatile components have been identified as a mixture of alkyl-substituted anthracenes (phenanthrenes) and polycyclic hydrocarbons with saturated and aromatic rings. These results were used to gain insight into the structure of the hydrocarbon matrix of the composite.