VC Carbides Research Articles

Formation and growth of transition metal carbides in ferrite

Carbide nano-precipitates are commonly used to improve mechanical properties of steel. It has been experimentally observed that TiC, NbC, and VC carbide precipitates initially form as ‘plate-like’ particles oriented in the {100} planes of the ferrite lattice. These platelets share similarities with Guinier-Preston zones in Al-Cu alloys.The clustering of group IV and V transition metal atoms (M = Ti, Zr, Hf, V, Nb, Ta) in ferrite is studied using density functional theory. It is deduced that the transition metal carbides all form in a similar way. Furthermore, the transition from an initial M–C cluster to a NaCl-structured platelet to a NaCl-structured precipitate is examined through atomistic simulations using Modified Embedded Atom Method potentials. A route is established along which transition metal carbides form and transform into precipitates that possess the Baker-Nutting orientation relation with the ferrite matrix.

Hydrogen trapping and embrittlement of titanium- and vanadium carbide-containing steels after high-temperature hydrogen charging

This work studies the effect of TiC and VC precipitate sizes on hydrogen trapping and embrittlement. Two experimental ferritic HSLA steels containing either TiC or VC carbides for precipitation strengthening are annealed in nitrogen and hydrogen gas. This results in a hydrogen uptake of up to 0.91 and 0.44 wppm in the TiC and VC steels, respectively. TEM and TDS analysis indicate that semi-coherent TiC particles trap hydrogen in misfit dislocations with an activation energy of 43 kJ/mol. Coherent VC particles are suggested to trap hydrogen in interface carbon vacancies, with an energy between 53 and 72 kJ/mol. Carbon vacancies are the likely trapping site in incoherent precipitates, where SIMS imaging confirms that incoherent TiC precipitates trap preferentially near the interface, whereas incoherent VC precipitates trap throughout their bulk. Neither alloy is embrittled in SSRT tests after hydrogen absorption, which shows that these precipitates can be used as both a hydrogen sink and a strengthening mechanism in steels.Graphical abstract

Influence of Cooling Rate and Alloying by “Cr, V, and Ni” on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

AbstractSiMo ductile irons, typical heat-resistant materials, are subjected to varied wear environments during operation in high-temperature applications. SiMo ductile iron castings of different thicknesses were cast in investment and greensand molds, achieving a wide range of cooling rates. The present work aims to investigate the effect of the cooling rate and alloying elements (Cr, V, and Ni) on the microstructure and the abrasive wear behavior of these grades of SiMo ductile iron at high-temperature 700 °C under different loads. Thermodynamic calculations were used to propose the phase diagrams, critical transformation temperatures, and phase volume fractions in all SiMo alloys by using the Thermo-Calc software then verified by and Differential Scanning Calorimetry (DSC). The microstructure of unalloyed SiMo ductile cast iron consists of graphite nodules and carbides embedded in the precipitates at the grain boundary regions in a ferrite matrix. The alloyed SiMo microstructure contains nodular graphite and the carbides promoted by the alloying elements (Cr and V). The alloyed SiMo alloys exhibit higher wear resistance than unalloyed ones. These wear results support that the microstructure plays a chief role in wear loss. The combination of M6C, VC, and M7C3 carbides embedded in a ferrite-pearlite matrix (alloyed SiMo) seems to be more resistant to wear than the ferritic matrix with lamellar pearlite and eutectic M6C carbides (unalloyed SiMo).

Effect of interstitial carbides on tribological properties of Co21Cr21Fe21Ni21V14.5C1.5 high entropy alloy at elevated temperature

High-entropy alloys (HEAs) with high wear resistance at elevated temperatures are desired for applications like turbine engine blades and internal combustion engines. By reducing the proportion of transition metal elements, the interstitial carbide in the form of doped non-metallic element C can reduce the preparation cost of HEA and improve its mechanical properties. The effect of 1.5 at% carbon doping at different sintering temperatures on microstructure, mechanical properties, and high-temperature tribological behavior was systematically investigated. The compressive strength and strain rate of C-doped HEAs sintered at 1200 °C were 969 ± 21 MPa, 8.43 ± 0.36%, which was improved by solution strengthening and carbide pegging dislocations. The wear mechanisms of C0 alloy at high-temperature are mainly oxidation wear, adhesion wear and delamination wear, while the wear mechanism of C10 alloy is mainly abrasive wear, adhesion wear, fatigue wear and oxidation wear. The combined effect of VC, M23C6 and M7C3 carbides and σ phase plays the role of anti-wear lubrication at RT∼800 °C, and the tribo-chemical reaction generates Cr2O3, NiCr2O4, NiO, and other compounds with high-temperature stability and oxidation resistance to form a thin dense oxide glaze layer, resulting in better anti-wear and friction reduction performance of C-doped alloy than undoped alloy.

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.

Probing the origin of transition metal carbide VC for oxygen reduction reaction: A DFT study

The development of oxygen reduction reaction (ORR) electrocatalysts with high activity, high stability and low-cost is still a grand challenge. Transition metal carbides (TMCs)-based catalysts have been reported as one species of oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) trifunctional catalyst with high efficient and favorable stability. Particularly, TMCs exhibit excellent ORR activity in electrochemical catalysis, even be profitable comparable with Pt in some situations, but the clarification of the intrinsic catalytic mechanism of VC remains a challenge. Herein, the potential performance of VC system on the ORR by using the first-principles calculation method based on density functional theory and quantum mechanics. The results showed that both end-on and side-on adsorptions on the surface of VC system all possess have satisfactory ORR catalytic activity. In this study, our work proposed a reasonable design scheme for the development of multifunctional electrocatalysts with the conversion and storage of renewable energy, and provided a new possibility for the application of TMCs.

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.

Interpretation of microstructure evolution and mechanical properties under aging treatments of a Fe–24Mn-0.6C–2Al-0.6V austenitic steel for cryogenic application

Aging treatment is an effective way of strengthening high-Mn strip steels based on automotive industry. However, the role of this method in enhancing yield strength and cryogenic toughness (−196 °C) of high-Mn steels has still unclear so far. Besides, solute segregation has always been a vital factor affecting the mechanical properties of high-Mn steels, whereas it is still lacking in adequate attention. In the present work, the microstructural evolution, including solute macrosegregation, grain boundary segregation and precipitation, and the resultant mechanical properties under different aging treatment were elaborated. It was found that the banded segregation of C, Mn and V elements formed during aging treatment strongly affects the cryogenic toughness. Tridimensional EPMA analyses demonstrate that there exists dense banded segregation of C, V and Mn elements in longitudinal section and cross section for both aged steels (aging at 500 °C and 800 °C). However, more obvious banded segregation of C and V elements developed in the steel aged at 800 °C (800A), and more micron-scale VC carbides tend to precipitate at segregation band, which leads to the easy initiation and propagation of microcracks along the segregation band. Moreover, numerous nano-scale VC precipitates significantly increase the yield strength of 800A steel by 100 MPa, but there exist severer grain boundary segregation and precipitation after aging treatment, which enhances the twining stress by increasing local SFE of grain boundary regions. The twinning formation hence became harder, further deteriorating the cryogenic toughness of the 800A steel.

Microstructure, Phase Transformations, and Mechanical Properties of Shape Memory Fe-Mn-V-C Austenitic Steels

The structure and mechanical properties of new dispersion-hardened Mn-(Cr)-Si-V-C steels with a shape memory effect (SME), which undergo strengthening due to the precipitation of VC carbides in a steel matrix, were analyzed. The stabilizing and the destabilizing carbide aging at different temperatures allows one to control and thus adjust the desired strength characteristics (σ0,2 = 250–880 MPa) and amount of reversible deformation (up to 2.7%). The recovery of the shape of the samples was carried out as a result of both the γ-ε transition in the course of heating after preliminary deformation in the initial austenitic state (i.e., due to the transformation of the γ to the ε phase) and the shear re-twinning of martensite in the martensitic ε phase (i.e., due to the occurrence of transformation of the ε to the twinned εtw phase).

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.

Carburization and decarburization behavior of Grade 91 ferritic-martensitic steel in liquid sodium environments

This paper presents a study of carbon transfer and its effect on microstructure and tensile properties of Grade 91 (G91) ferritic-martensitic steel exposed to sodium at 550–650 °C. Sodium exposure tests were conducted in Argonne's forced convection sodium loops up to exposure times of ∼40,000 h. Thermal aging study of G91 steel was conducted in parallel to isolate the thermal aging effect from the sodium effect. It was found that sodium exposures at 650 °C dissolved M23C6 carbides, eliminated the martensite subgrain structure resulting in excessive grain growth and reduced the tensile strength by >50%, while sodium exposures at 550 and 600 °C had an insignificant effect on its microstructure and tensile properties. These effects were attributed to the carburization/decarburization process of G91 steel in sodium environments.Carbon concentrations in sodium were determined by a foil equilibration method. The estimated carbon concentration was in the range of 0.8–1.2 ppm in the SMT-1 loop and 0.3–0.7 ppm in the SMT-2 loop. Thermodynamic analysis of the carburization – decarburization process was conducted for G91 steels exposed in sodium environments. The carbon activity-concentration relationship for G91 was evaluated by considering four phases in G91, i.e. bcc ferrite, M23C6, NbC and VC carbides. It was found that the carburization-decarburization process in G91 steel was dictated by M23C6 carbides at high carbon activities, while NbC and VC carbides dominated the process at low carbon activities. The calculated carburization-decarburization boundary showed that G91 would undergo decarburization at 650 °C and carburization at 550 °C in the sodium loop environments, which was consistent with our experimental observations. This experimental and theoretical analysis provided a basis for predicting the effect of carbon transfer on the integrity of reactor components in sodium environments and for the design of new alloys used in sodium-cooled fast reactors.

The influence of metals on the phase transformations of fullerenes at high pressure and high temperatures

Phase transformations in crystalline fullerenes C60 and C70 with additions of two groups of metals (7 at. % each): carbide-forming (V and nichrome Ni20Cr80) and non-carbide-forming (Ag and Ni) at a high pressure of 8 GPa and high temperatures of 500–1100 °C have been investigated by X-ray diffraction and Raman spectroscopy. The measurements were carried out ex situ after a thermobaric treatment. Two groups of phase transformations were found: crystalline fullerenes into amorphous graphite and the formation of carbides VC, V2C and Cr3C2. The addition of V and nichrome stabilizes fullerene molecules and raises the temperature of transformation of crystalline fullerene into amorphous graphite by hundreds of degrees, while the addition of Ag and Ni does not have such an effect. Thus, the described effect is explained by chemical interaction. Carbides in systems with V and nichrome are formed mainly in the interaction of metal with amorphous graphite, and not with crystalline fullerene. The result is explained on the basis of a comparison of the interaction of metal atoms with fullerene molecules and with individual carbon atoms.

Influence of Tempering Temperature on the Microstructure and Wear Properties of Powder Metallurgy High‐Speed Steels

The influence of tempering temperature on the microstructure of powder metallurgy high‐speed steels (PM HSSs) PM20 is investigated by X‐ray diffraction, electron backscattering diffraction, and transmission electron microscopy. With the increasing of tempering temperature, the ratio of low angle grain boundaries decreases, while the volume fraction and size of M6C and VC carbides increase. The wear resistance of PM20 with different tempering temperatures is studied by reciprocating ball‐on‐flat methods. The wear properties of PM20 decrease gradually with the increasing of tempering temperature. The variation of wear properties is explained by the characteristic of the martensite matrix, carbides, and the misorientation angle of boundaries. All samples present a complex wear mechanism of abrasive wear and oxidation wear.

Extremely high temperature carbide precipitation induced intragranular acicular ferrite transformation of M2 steel during semi-solid cooling

Intragranular acicular ferrite (IAF) transformation of M2 steel occurred at extremely high temperature (1180 ∼ 960 °C) during semi-solid cooling. The uncommon IAF transformation was observed in situ via confocal laser scanning microscope. Small VC carbide particles precipitated explosively during semi-solid cooling and they acted as the preferential site for the nucleation of IAF due to the low lattice misfit with ferrite. The length of ferrite lath showed a bimodal behavior that long primary ferrite laths and short secondary ferrite laths both existed. The acicular laths intersected with each other and terminated at the solid boundary or interface. The prior solid austenite grain was filled with acicular ferrite. The IAF might help refine the effective grain size and improve the strength of semi-solid M2 steel.

High carbon alloyed design of a hot-rolled high-Mn austenitic steel with excellent mechanical properties for cryogenic application

Many efforts have been made to enhance the yield strength of high-Mn austenitic steel, but the methods, including grain refinement through severe plastic deformation and hierarchical microstructure design, have certain limitations in industrial application, especially the thick plate, and the cryogenic toughness is also decreased. In the present work, two high-Mn austenitic steels with 0.6/0.8 wt% carbon additions were fabricated to strengthen the high-Mn austenitic steel, and the effects of carbon on microstructural evolution and the resultant strength, cryogenic toughness (−196 °C) were revealed. The results show that the yield strength of 0.8C steel can be dramatically improved by 121 MPa compared to 0.6C steel, but at the same time, the two steels exhibit excellent and similar cryogenic impact energies, both above 120 J. The strengthening modes of 0.6C steel contain four kinds of grain refinement, solid-solution, dislocation and precipitation, so does the 0.8C steel. The substantially increased yield strength of 0.8C steel is derived from solid-solution and precipitation strengthening. Under impact loading at −196 °C, the main deformation modes of 0.6C steel are dislocation glide and cross twinning, resulting in ductile dimpled fracture and high cryogenic toughness. As for 0.8C steel, the difference with 0.6C steel is that there exist fewer deformed grains in as-hot rolled microstructures due to the facilitation effect of high carbon addition on recrystallization and recovery, which promotes dislocation glide and twinning formation. Besides, the fine nanoscale VC carbides are also conductive to maintaining the high toughness while improving strength. Hence, the high cryogenic toughness can be retained in the 0.8C steel. The yield strength and cryogenic toughness of 0.8C steel reach 624 MPa and 123 J, respectively, greatly superior to the reported high-Mn austenitic steels.

Effect of V Addition on Microstructure and Mechanical Properties in C–Mn–Si Steels after Quenching and Partitioning Processes

Three C-Si-Mn Q&P steels with different V addition after one-step and two-step quenching and partitioning (Q&P) processes were investigated by means of optical microstructure observation, X-ray diffraction (XRD) measurement, transmission electron microscopy (TEM) characterization and particle size distribution (PSD) analysis. The effect of V addition on strength and ductility of the steels was elucidated by comparative analysis on the microstructure and mechanical properties as functions of partitioning time and temperature. For one-step Q&P treatment, the mechanical properties were mainly controlled by the tempering behavior of martensite during partitioning. V addition was helpful to mitigate the deterioration of mechanical properties by precipitation strengthening and grain refinement strengthening. For two-step Q&P treatment, the satisfying plasticity was attributed to the transformation-induced plasticity (TRIP) effect of retained austenite maintaining the high work hardening rate at high strain regime. The higher volume fraction of retained austenite with high stability resulted from the refined microstructure and the promoted carbon partitioning for the steel with 0.16 wt% V addition. However, the carbon consumption due to the formation of VC carbides led to the strength reduction of tempered martensite.

Effect of NbC and VC carbides on microstructure and strength of high-strength low-alloyed steels for oil country tubular goods

High-strength low-alloyed (HSLA) steels for oil country tubular goods (OCTGs) are usually microalloyed with niobium (Nb) and vanadium (V). In this work, the microstructure and strength of HSLA steels with different Nb and V contents were studied with the purpose of investigating the role of NbC and VC carbides. The results showed that NbC carbides were responsible for restraining the growth of prior austenite grains, and when the austenitizing temperature was below 910 °C, it has been found that the prior austenite grain size was similar in steels with 0.02 and 0.006 wt% Nb addition. VC carbides could impede dislocation annihilation and refine Cr-rich carbides and martensitic blocks during tempering. Moreover, VC carbides mainly contributed to the tempering softening resistance so that the V-bearing steels exhibited a higher strength than the V-free steel after tempering at 690 °C–730 °C. To illustrate the high tempering softening resistance of V-bearing steels, the changes in various strengthening contributions induced by V addition were quantitatively evaluated. The calculation results indicated that the main reason for the high tempering softening resistance of V-bearing steels was precipitation strengthening by nanosized VC carbides, in contrast to dislocation strengthening, substructure strengthening, and precipitation strengthening due to the refinement of Cr-rich carbides.

Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M 6 C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M 6 C and martensite caused by the inconsistency deformation, M 6 C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness. • The type of carbides and interface relationship was obtained. • Evolution of microstructure during heat treatment was studied. • The role of carbides in impact toughness was investigated. • The variation of impact toughness value has been explained by the characteristics of martensite.

Improvement in microstructure and wear-resistance of high chromium cast iron/medium carbon steel bimetal with high vanadium

In the present study, a high chromium cast iron (HCCI) with high content of vanadium (15 wt.%) as the wear resistance layer was fabricated on the medium carbon steel (MCS) substrate by laser cladding process. The effect of high vanadium addition on microstructure, phase, hardness and wear properties of bimetal were investigated. The results show that the microstructure of HCCI layer was significantly improved and refined with addition of high vanadium. No defect such as microcracks and unbonded regions was observed on the interface of the bimetal. The x-ray diffraction revealed that the HCCI (15 wt.% V) layer consists of α-Fe matrix, (Cr, Fe)7C3 and VC carbides. The SEM images shown that round-like VC particles uniformly distributed on the α-Fe matrix and (Cr, Fe)7C3 are of rod-like with distributed on grain boundary, such micromorphology makes it have higher hardness (883HV) and excellent wear resistance than HCCI layer without vanadium addition.

The carbides, tensile properties, and work-hardening behavior of annealed H13 die steels with varied yttrium contents

Abstract In this study, the micro-alloying of rare-earth yttrium (Y) was employed to improve the fracture behavior and work-hardening ability of H13 die steel. The improved work-hardening ability was attributed to modification of Cr23C6 and VC carbides by the addition of Y. The strip-typed Cr23C6 carbides originally distributed along the grain boundary were interrupted and tended to spheroidize with increasing Y content. Compared with 0Y–H13 steel, the quantity of spherical VC carbides significantly increased in 0.013Y–H13 steel. A higher-density dislocations and intensive interaction between precipitates and dislocation led to a higher work-hardening exponent. Additionally, fractographic studies revealed that the fracture mode transition from cleavage failure to ductile rupture with increasing Y content. Moreover, excess Y content (0.044%) resulted in the appearance of inclusion clusters in dimples, which were detrimental to fracture and work-hardening behavior.