Mo2C Carbides Research Articles

Capacity of hydrogen traps affects H-assisted crack initiation and propagation mechanisms in martensitic steels

Low-carbon martensitic steels are candidate materials for different hydrogen applications. Hydrogen embrittlement (HE) of the steels can be mitigated by designing trap site characteristics. In this study, the different capacities of hydrogen trapping, and reversibility of the trap sites are studied to reveal the extent of HE susceptibility of the steels and H-assisted crack initiation and propagation mechanisms. The trap sites in Ti-contained and Mo-contained martensitic steels were identified and discussed. The trap sites in Ti-contained martensitic steel include basic martensitic microstructure, the interface of the TiC/matrix interface and carbon vacancies inside the TiC carbides. The trap sites in Mo-contained martensitic steel including basic martensitic microstructure and interface of the Mo2C/matrix interface. Ti-contained steel indicates a mitigated HE susceptibility, controlled by carbon vacancies inside TiC acting as strong hydrogen trap sites, and a higher possibility of building up a critical hydrogen content at Mo2C carbide due to lower hydrogen capacity. These results prove that hydrogen trapping capacity at the designated hydrogen trap sites is a determinant factor for H-assisted crack initiation and propagation in martensitic steels. A critical local hydrogen amount should be built up at the trap site to assist the crack to develop. It was also revealed that crack initiation originated from Mo2C carbides accompanied by interfacial decohesion at the interface of martensitic matrix/Mo2C carbides and propagated in quasi-cleavage patterns along {011} planes.

Influence of Mo carbides and two-stage tempering methodology on the susceptibility of medium carbon martensitic steel to hydrogen embrittlement

The objective of the present study was to enhance the hydrogen embrittlement (HE) resistance of the quenched and tempered martensitic steel via the interplay of heat treatment variance and precipitation of nanosized carbides. For this purpose, one-stage tempering and two-stage tempering methodologies were implemented, and steel was alloyed with Mo to instigate the precipitation of Mo carbides. The results revealed that two-stage tempered steel exhibited superior resistance to HE, as a result of reduced dislocation density and higher quantity of Mo2C. To discern the role and trapping behaviour of Mo2C carbides, Thermal Desorption Spectroscopy (TDS) combined with electrochemical hydrogen charging was utilized. Precipitated nanosized Mo2C exhibited the ability to trap hydrogen. On the contrary, an increase of dislocations and higher diffusible hydrogen content in one-stage tempered steel promoted deterioration of mechanical properties which was investigated by Slow Strain Rate Test (SSRT) and fracture surface morphology analysis. In addition, the effective diffusion coefficient for one-stage tempered steel was lower, as dislocations served as additional trap sites.

Carbide Precipitation Behavior of High‐Alloy Steel During Solidification and Cooling Process

The precipitation, growth, and transformation of carbides rich in Cr and Mo during solidification and cooling process of high‐alloy steel for cold working dies are investigated. The Fe–C pseudobinary phase diagram of high‐alloy steel is calculated by Thermo‐Calc software to ascertain the critical phase transformation temperatures. The temperatures of phase transformation during the solidification process are determined by a differential scanning calorimeter. Then the microstructure of the samples at these characteristic temperatures is observed in situ by high‐temperature confocal microscope. The results show that the characteristic temperatures for phase transformation and carbide precipitation are 1380, 1315, 1140, and 980 °C, respectively. At 1315 °C, thick rod‐like or island‐like M7C3 carbides enriched in Cr and Fe are formed. At 1140 °C, secondary M7C3, adhering to the primary eutectic M7C3, grows rapidly in a rod‐like morphology and maintains a similar composition. The secondary M6C carbides of segregated Mo elements precipitate in fan‐shaped clusters near M7C3 through the dissolution of M7C3 and the conversion of M2C. Thermodynamic calculation further indicates that the eutectic Cr7C3 precipitates at 1213 °C, and secondary Cr7C3 remains under conditions favorable for precipitation below the solidus, preceding the precipitation of secondary carbides Cr23C6 and Mo2C.

Temperature Dependent Behavior of Elastic and Ultrasonic Proprieties of Transition Metal Carbide Mo2C Superconductor

Abstract: Transition metal carbides exhibit peculiar chemical and physical properties, making them integral to industrial applications that demand performance under high temperatures In the case of the hexagonal transition metal carbide Mo2C superconductor, higher-order elastic constants were calculated to be temperature-dependent using an interactional potential model. Second-order elastic constants are used to determine other allied ultrasonic variables. Second-order coefficients are used to analyze the temperature variation of ultrasonic velocities along the z-direction of the superconductor. Furthermore, the temperature difference of Debye average velocity and thermal relaxation time are considered along the same direction. The temperature dependence of ultrasonic properties is explored in relation to thermal, elastic, and mechanical properties. Ultrasonic attenuation , resulting from phonon–phonon (p–p) interactions, is calculated at different temperatures .The study establishes that thermal conductivity is a core provider of the observed ultrasonic attenuation, particularly at higher temperatures. The mechanical and thermal properties of the Mo2C superconductor are superior at lower temperatures. Keywords:Transition metal carbide superconductor, Thermal conductivity, Ultrasonic properties, Elastic properties, Mechanical properties. PACS numbers: 43. 35. Cg; 62.20.Dc; 63.20.Kr.

Degradation of a Cr-Mo steel by carbide precipitation over long-term service

Microstructure of the 40-year service-exposed 1Cr-0.5Mo steel extracted from pressure vessels through the boat sampling technique was characterized using x-ray diffraction, field-emission scanning electron microscopy, energy dispersive spectrometry, electron backscatter diffraction, focused ion beam, and transmission electron microscopy. The microstructure degradation during long-term service was identified. Statistical crystal plasticity models were developed to correlate the microstructure and mechanical properties quantitatively. The steel is found significantly strengthened but slightly embrittled. Nanoscale Mo2C carbides have nucleated within the ferrite matrix over the long-term service, resulting in significant strengthening of the matrix. The coarser grain boundary Cr23C6 carbides, on the other hand, act as the damage accumulation sites and lead to the degradation.

Multiphase precipitation behavior and tensile properties of a Fe-Mn-Al-Mo-C austenitic lightweight steel

In order to improve the strength and strain hardening rate, multiple precipitates were introduced by annealing at 850–950 °C and aging at 550 °C after hot or cold rolling in a Fe-26Mn-8Al-3Mo-1.2C austenitic lightweight steel. The results show that the combination of rolling, annealing at 850 °C and aging produces a microstructure consisting of nano-sized κ-carbides and Mo2C carbides within equiaxed austenite grains and submicron-sized Mo6C carbides along grain boundaries. Further increasing annealing temperature to 950 °C reduces the precipitation of Mo2C and decreases the density of dislocation. The yield strengths increase by ∼300–500 MPa, and ultimate tensile strengths increase by ∼200–400 MPa by comparison with the steel processing by solid solution and two-step aging. Cold rolling before heat treatments leads to an improvement in strength compared to hot rolling, mainly due to grain boundary strengthening and Mo2C precipitation strengthening. Decreasing annealing temperature after rolling from 950 to 850 °C increases strength primarily ascribed to Mo2C precipitation strengthening and dislocation strengthening. The introduction of a large number of fine Mo2C precipitates distributed evenly within the steel matrix through cold rolling and subsequent annealing at 850 °C increases the strain hardening rate effectively.

Effect of co-doped additives on microstructure and mechanical properties of microwave-sintered WC-10Co cemented carbide tool materials

Microwave sintering is an energy-efficient technology of preparing materials. High-performance WC-10Co cemented carbide tool materials were fabricated by microwave sintering. The effects of co-doped additives of metal carbide Cr3C2, VC, Mo2C and rare earth oxide Y2O3 on microstructure and mechanical properties of WC-10Co tool materials were studied. Compared to the existing cemented carbides with single inhibitor or pure metal carbide, the microwave-sintered cemented carbide with co-doped rare earth elements and metal carbides had more excellent mechanical properties. The co-doped additives showed evident effect on grain growth inhibition. The WC-10Co-0.3Cr3C2-0.3VC-0.3Mo2C sintered at 1400 ℃ with the holding time of 10 min showed the best mechanical properties. Its grain size, relative density, Vickers hardness (HV30), fracture toughness and flexural strength were 0.26 ± 0.04 µm, 98.2 ± 0.5%, 19.6 ± 0.2 GPa, 16.9 ± 1.0 MPa·m1/2 and 1888.5 ± 153.1 MPa, respectively.

Understanding the recrystallization and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses

M50 steel is a commonly used material for aeroengine main shaft bearings. In this study, a gradient nanostructured M50 steel was fabricated by an efficient surface nanocrystallization method, i.e., ultrasonic shot peening (USP) technology. The recrystallization behavior and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses were investigated. Compared with the USPed sample, the ferrite structure size was 32.5% smaller, and the structure size uniformity was 51.5% greater in the 0–50 μm region after USP treatment and electrical pulsing treatment (EPT) with 38 A/mm2–1 s. Athermally induced ferrite recrystallization by EPT accompanied with obvious precipitation of VC and Mo2C carbides, and decomposition of austenite occurred during ferrite structure coarsening. EPT has different responses to different depths of gradient M50 steel, and there is an optimal current density to achieve structure refinement in different depths. However, the coarse-grain tempered M50 steel did not undergo obvious ferrite structure refinement under the same EPT parameters. The results revealed that the EPT promoted static recrystallization of incompletely recrystallized structures after USP processing by activating the release of stored deformation energy. Static recrystallization under the athermal effect of EPT was achieved by promoting the migration of the subgrain boundary to absorb the surrounding dislocations and increase the misorientation between itself and the surrounding ferrite structure. The mechanism of the athermal effect of EPT was based on electron-defect interactions, such as dislocations and lattice distortions. High-density nonequilibrium grain boundaries, acting as “active sites”, accelerating the process of recrystallization under the effect of electric pulses. This study proposed a method for optimizing EPT parameters through the change in instantaneous resistance to evaluate the degree of recrystallization of the gradient structure.

Structure and Oxidation Behavior of Multicomponent (Hf,Zr,Ti,Nb,Mo)C Carbide Ceramics.

Multicomponent ceramics based on transition metals carbides are widely known for their excellent physicomechanical properties and thermal stability. The variation of the elemental composition of multicomponent ceramics provides the required properties. The present study examined the structure and oxidation behavior of (Hf,Zr,Ti,Nb,Mo)C ceramics. Single-phase ceramic solid solution (Hf,Zr,Ti,Nb,Mo)C with FCC structure was obtained by sintering under pressure. It is shown that during the mechanical processing of an equimolar powder mixture of TiC-ZrC-NbC-HfC-Mo2C carbides, the formation of double and triple solid solutions occurs. The hardness of (Hf,Zr,Ti,Nb,Mo)C ceramic was found at 15 ± 0.8 GPa, compressive ultimate strength-at 1.6 ± 0.1 GPa and fracture toughness-at 4.4 ± 0.1 MPa∙m1/2. The oxidation behavior of the produced ceramics in an oxygen-containing atmosphere was studied in the range of 25 to 1200 °C by means of high-temperature in situ diffraction. It was demonstrated that (Hf,Zr,Ti,Nb,Mo)C ceramics oxidation is a two-stage process accompanied by the change of oxide layer phase composition. As a possible mechanism of oxidation, diffusion of oxygen into the ceramic bulk results in the formation of a complex oxide layer made of c-(Zr,Hf,Ti,Nb)O2, m-(Zr,Hf)O2, Nb2Zr6O17 and (Ti,Nb)O2 was proposed.

First-principles calculation of structural stability and mechanical properties of Cr doped Mo2C carbide

First-principles calculation of structural stability and mechanical properties of Cr doped Mo2C carbide

Study of the Influence of V, Mo and Co Additives on the Carbide Formation and Microhardness during Thermal Diffusion Chrome Planting of X35CrNi2-3 Steel

Saturation diffusion with chromium has not been adequately studied among all the surface thermochemical treatment (STCT) processes of steels. Especially, the complex saturation behavior when several elements are added directly for chemical treatment needs to be systematically studied. This work aims at determining the effect of V, Mo, and Co on the parameters of chromium thermal saturation diffusion (thickness, phase composition, microstructure, and microhardness) of the surface layer in X35CrNi2-3 steel. The process was carried out at a temperature of 1000 °C for 24 h. The results showed that complex structural chromium plating together with the addition of strong carbide-forming elements (V and Mo) has a significant effect on the phase composition of the fabricated layer, where the formation of VC and Mo2C carbides significantly increases the microhardness of the samples to 2000 HV and 2500 HV, respectively. On the other hand, the addition of Co with a less carbide-forming affinity has little effect on the phase composition of the coating, and nitride compounds predominated in the microstructure similar to the single-element chromium plating. The results indicate the possibility of improving and accelerating the traditional thermal chromium plating processes and opening up new horizons for obtaining gradient coatings with superior tribological properties.

Understanding carbide evolution and surface chemistry during deep cryogenic treatment in high-alloyed ferrous alloy

The study investigates the effect of deep cryogenic treatment (DCT) on a high-alloyed ferrous alloy (HAFA) and its effectiveness on carbide evolution and chemical shifts of alloying elements. With ex-situ and in-situ observations ranging from the microscopic to the nanoscopic level, we uncover the atomistic mechanism by which DCT affects carbide precipitation, resulting in a 50% increase in carbide volume fraction. Synchrotron-based scanning photoelectron microscopy provides insight into the agglomeration of carbon during exposure to DCT. We find that Mo plays a crucial role in DCT through its modification of chemical bonding states, which is postulated to originate from the loosely-formed primordial Mo2C carbides formed during exposure to cryogenic temperatures. These in turn provide energetically favorable nucleation zones that accelerate the formation of M7C3 carbides, which serve as intermediate states for the formation of M23C6 carbides, which most strongly impact the mechanical properties. These results are supported by atom probe tomography, showing the preferential formation of Mo-rich M7C3 carbides in DCT samples, resulting from greater solute mobility. This work clarifies the fundamental mechanisms on how DCT affects HAFA, solving a long-elusive problem.

Optimization of thermal stability and mechanical properties of Mo2C carbides via multi-element doping: experimental and theoretical calculations

Optimization of thermal stability and mechanical properties of Mo2C carbides via multi-element doping: experimental and theoretical calculations

Redistribution of substitutional alloying elements between α-Fe matrix and cementite after long-term tempering in a low alloying steel

ABSTRACT The partitioning behaviour of non-carbide forming elements (Si, Cu and Ni) and carbide forming elements (Mn and Mo) between α-Fe matrix and cementite (θ) after long-term tempering a quenched reactor pressure vessel (RPV) model steel has been studied by atom probe tomography (APT). It has demonstrated that a non-carbide forming element enriched layer near the interface of α/θ in the α-Fe matrix side is a common feature for tempered steels containing non-carbide forming element, which is widely accepted as the mechanism by which substitutional elements retard the coarsening of cementite. In addition, a carbide forming element enriched layer near the interface of α/θ in the cementite side, Si, Cu and Ni enriched clusters formed in the cementite and Mo2C carbide formed beside cementite were observed, and all this is taken as valuable mechanisms by which addition of substitutional alloying elements can increase tempering resistance of alloying steels.

Selective Deposition of Mo2C-Containing Coatings on {100} Facets of Synthetic Diamond Crystals.

An efficient way to improve the properties of metal–diamond composites (mechanical strength, wear resistance, thermal conductivity) is the preliminary modification of the diamond surface to improve its wettability by the metal matrix. In the present work, Mo2C-containing coatings were deposited on the diamond crystals under different conditions: hot pressing (atmosphere of argon), spark plasma sintering (forevacuum), and annealing in air. The influence of the sintering parameters on the morphology and phase composition of the coatings deposited on diamond was studied. Mo2C-containing coatings were selectively deposited on the facets of synthetic diamond microcrystals by annealing of the latter with a molybdenum powder. Experiments were carried out to deposit coatings under different conditions: during hot pressing (argon atmosphere), spark plasma sintering (forevacuum), and annealing in air. The process parameters were the temperature, holding time, and concentration of molybdenum in the initial mixture. Experiments with a pre-oxidized molybdenum powder were also conducted. The coated diamond crystals were investigated by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The deposition was enabled by the gas phase transport of molybdenum dioxide, MoO2, contained in the starting powder. The following sequence of the coating formation stages was proposed. First, MoO2 sublimes and is adsorbed mainly on the {100} facets of diamond. Then, it is reduced to metallic molybdenum by carbon of the diamond, which further reacts with carbon to form the Mo2C carbide phase. These processes occurred during treatment of the mixtures in the hot press and the spark plasma sintering facility. When the mixture was annealed in air, no selective deposition was observed. During annealing, MoO3 particles adhered to the diamond surface.

Characterization and cause analysis of the “bright spots” on the surface of a GH2674 large-size gas turbine disk

After surface turning process, many surface defects (visually shown as “bright spots”) were found on both sides of the large-size turbine disk. Macro- and microscopic inspections illustrated the distribution of the spots, hardness variation and morphologies inside the spot regions. MC carbides, Laves phases, Mo2C carbides, η phases, G phases were found in the spot regions by SEM, TEM and EDS. The depth of the aggregated precipitates was shown by 3D X-ray tomography. It is indicated that the optic difference of the “bright spots” is due to the change of surface roughness after the cutting tool passed through the segregation regions with higher hardness. The bright spots are essentially freckle defects, which originated from the Ti segregated zones in the ingot. The effect of these defects on the tensile and fatigue properties were further analyzed. The remedial methods are then proposed to alleviate the detrimental effect of the bright spots.

Concurrent (Fe,Mo)2C carbides and Cu-rich clusters precipitation in ferritic steel containing copper during aging

ABSTRACT The concurrent (Fe,Mo)2C carbides and Cu-rich clusters precipitation in the ferritic steel containing copper after aging at 400°C for 4000 h have been investigated using transmission electron microscopy (TEM) and atom probe tomography (APT). The results show that spinodal decomposition and an atomic ordering reaction occur in the ferritic steel containing copper. After aging, the retained austenite decomposed by the spinodal mechanism into C-rich and C-lean regions. The C-rich phase transforms into the (Fe,Mo)2C carbides via the ordering reaction, while Cu-rich clusters formed in the C-lean regions. The experimental results can be interpreted on the basis of phase separation via spinodal decomposition followed by chemical ordering. The small size of Cu-rich clusters can be attributed to the lower diffusion coefficient of Cu in the retained austenite.

Redistribution of elements in the main and secondary phases and its effect on the microstructure of the Mo–Fe–B cermet alloyed with Cr

C-containing cermets based on Mo2FeB2–Fe with various Cr concentration were synthesized. The influence of Cr on the phase composition and microstructure of sintered materials was studied. It was shown that an increase in the Cr content in cermet leads to a growth of its amount in ternary boride Mo2FeB2 and carbide Mo2C. It was found that Cr atoms replace a fraction of Fe atoms in Mo2FeB2. At high Cr content, the formation of a binder based on Cr solid solution and M6C carbide was observed. The last one precipitated from the liquid at the final stage of synthesis. It was shown that high Cr concentration suppresses the abnormal growth of boride particles.

Grain refinement mechanisms of alloying molybdenum with carbon manufactured by laser powder bed fusion

Pure molybdenum produced by additive laser powder bed fusion (LPBF) processes has a coarse, epitaxially-grown columnar grain structure with cracks located at high-angle grain boundaries, which are embrittled through oxygen segregation. The tensile strength of these pure molybdenum specimens is only 50 MPa, or 11% of the strength of powder metallurgically sintered, deformed, and recrystallized molybdenum. Alloying molybdenum with carbon can reduce cracks, increase density to 99.5%, and thus increase tensile strength to 650 MPa. The addition of carbon prevents embrittled grain boundaries and high residual porosity by outgassing oxygen, trapping any residual oxygen in the oxygen-soluble Mo2C carbide, and increasing the grain boundary surface on which the oxygen is distributed. In this study, we investigated the grain size, tensile strength, and hardness of five different molybdenum-carbon alloys produced via LPBF at constant process parameters. Our alloys ranged from pure molybdenum to 2.2 wt% carbon, which corresponds to a near-eutectic composition. In the absence of extrinsic nucleant particles in the melt, the relationship between grain size and solute concentration—and thus the growth restriction factor—follows an inverse exponential Arrhenius-type equation. We attributed grain refinement at higher carbon contents to increasing thermal and constitutional supercooling. The supercooling allows grains to nucleate and grow on the underlying solidified layer, where the fastest growth direction 〈100〉 is better aligned to the thermal gradient than that of the epitaxially growing grains.

SUBSTANTIATING THE PROCESS OF PRODUCTION OF FERROMOLYBDENUM FROM TEGHUT MOLYBDENITE CONCENTRATE UNDER PRODUCTION CONDITIONS

Studies of the production process of ferromolybdenum under industrial conditions feom Teghut molybdenite concentrate by using different reducing materials are carried out. Molybdenum can be reduced by carbon at low energy costs. However, the reduction of molybdenum oxides with carbon is accompanied by the formation of undesirable carbides Mo2C and MoCl-x. Due to the limitation of the carbon content in ferromolybdenum (≤0.05…0.5% C), it is obtained by the metallothermal method. In production, ferromolybde¬num is smelted by the out-of-furnace silicoaluminothermal method with an upper fuse from fired molybdenum concentrate (50.% Mo) and the use of ferro-silicon-aluminum (FSA) containing 60…65% Si and 10…12% Al, aluminum grains, iron ore, steel shavings and lime. The process takes place only due to the heat released by the reduction reactions of molybdenum, iron and other elements oxides with silicon and aluminum. From the standpoint of thermodynamics, the reduction reaction of molybdenum oxides by the carbothermal method has a positive value of enthalpy and entropy (due to the formation of CO), the total value of the change in Gibbs energy (∆G_T^0) remains negative and becomes smaller in absolute terms. That is why carbohydrate recovery requires external energy expenditure. On the contrary, when reducing by the aluminothermic and silicothe¬r¬m¬ic methods, the reactions are too negative, since these reactions are highly exothermic, and here the entropy factor does not affect the negative value of the change in Gibbs energy (∆G_T^0) (all reactants and the resulting products are in solid form), therefore the reactions are recovered due to the exothermicity of their own reactions and do not require external energy costs, which is the potential advantage of this method.