Carbide Morphology Research Articles

Effect of the Morphology, Size, Distribution and Homogeneity of Carbides and Matrix on Wear Resistance in High Cr-Alloys White Cast Iron

The aim of this work was to study the wear behavior of high-chromium white cast iron of families ASTM A-532 II (B, D) and III A, used in mining equipment, in order to establish relationships between the wear resistance, hardness and microstructure of the alloys, with the ultimate purpose of predicting their resistance to abrasion. Samples from these cast irons were subjected to mechanical wear tests by rotating drum, then their micro/macro hardness was measured and microstructure analyzed by optical and scanning electron microscopy .It was found that when the macroscopic hardness differences were significant there was a strong correlation between the hardness and the loss of mass due to abrasion-impact wear. By contrast, when the alloys had similar hardness, the wear resistance was determined by morphology, size, and the distribution and connectivity of carbides and matrix and therefore was not predictable by an only simple hardness test.

Synthesis of Microporous Mo2C-W2C Binary Carbides by Thermal Decomposition of Molybdenum-Tungsten Blues.

Molybdenum and tungsten carbides are perspective catalytic systems. Their activity in many reactions is comparable to the activity of platinum group metals. The development of the synthesis method for of highly dispersed binary molybdenum and tungsten carbides is an important task. Dispersions of molybdenum-tungsten blue were used as a precursor for synthesis of binary molybdenum and tungsten carbides. The synthesis of carbides was carried out by thermal decomposition of molybdenum-tungsten blue xerogels in an inert atmosphere. The binary carbides were characterized by XRD, TGA, SEM and nitrogen adsorption. The influence of the molar ratio reducing agent/Me [R]/[ΣMe], molar ratio molybdenum/tungsten [Mo]/[W] on phase composition, and morphology and porous structure of binary carbides was investigated. Samples of binary molybdenum and tungsten carbides with a highly developed porous structure and a specific surface area were synthesized.

Synthesis of Transition Metal Carbide Catalysts for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) proposed by Skyllas-Kazacos’ group in the 1980s [1] have recently attracted considerable attention for large-scale energy storage due to their desirable features such as flexible and safe design, high efficiency, and long cycle life. Graphite or carbon materials have been the most common catalysts for VRFBs [2]. However, they often show limited activity and reversibility. This issue may be circumvented with the development of new and efficient catalysts. Transition metal carbides are regarded as attractive candidates because they possess good electronic conductivity, low cost, high abundance, and outstanding thermal and chemical stabilities [3]. However, transition metal carbides fabricated by traditional carbothermal reaction methods exhibit a small specific surface area and inevitable aggregation at high reaction temperatures.A transition metal carbide fabrication process is under development at Hawaii Natural Energy Institute, which favors the formation of nano-sized particles that are expected to possess a large specific surface area. Figure 1a shows X-ray diffraction patterns for graphite as carbon source and the resulting vanadium carbide. The dominant phase is V8C7 and the particle size is approximately 50 nm. As shown in Figure 1b, cathodic currents for both electrodes in 3 M H2SO4 are due to the hydrogen evolution reaction. Although the cathodic current remarkably increases for graphite in 1 M (V2++V3+) + 3 M H2SO4 in Figure 1c, a significant anodic current is not detected indicating that the reversibility of the graphite toward the V2+/V3+ redox reactions is poor. In contrast, anodic and cathodic currents substantially increase on vanadium carbide, signifying an improvement in catalytic activity and reversibility toward the negative electrode reactions in comparison to graphite. The undesired hydrogen evolution reaction appears to have an important influence on the V3+ to V2+ reduction reaction. This influence was removed to more clearly show the vanadium V2+/V3+ redox reactions partial currents. Figure 1d shows the catalytic activity of graphite and vanadium carbide after background current subtraction. Vanadium carbide reveals more pronounced redox peaks than graphite signaling that vanadium carbide has a higher catalytic activity and enhanced reversibility toward V2+/V3+ redox reactions than graphite. The vanadium carbide composition, structure, morphology, and particle size will be tuned to improve activity.Figure 1. (a) X-ray diffraction patterns and (b, c, d) cyclic voltammograms (positive scan) for vanadium carbide and graphite. Acknowledgments Authors are grateful to the Office of Naval Research (award N00014-18-1-2127) and the Hawaiian Electric Company.

Effects of B content on microstructure and high-temperature stress rupture properties of a high chromium polycrystalline nickel-based superalloy

Effects of boron (B) addition on microstructure and high-temperature stress rupture properties of a heat-treated high chromium polycrystalline Nickel-based superalloy were investigated by adjusting B content (0, 0.016 wt%, 0.048 wt%, 0.09 wt%). The results indicated that the grain boundary characteristics were altered by B addition, such as the morphologies of the MC carbides, M23C6 carbides and precipitation of γ′ particles along grain boundaries. Meanwhile, it was confirmed that B existed in the form of Cr-rich M5B3 borides in the boron-containing alloys. Moreover, boron addition aggravated the inhomogeneity of microstructures, and reduced the volume fraction of secondary γ′ particles. Additionally, the rupture life was remarkably improved as the B content was increased to 0.016 wt%, whilst the elongation tended to improve as the B content was increased to 0.048 wt%. The improvement of stress rupture properties was mainly contributed to the modification of the grain boundary characteristics which retarded crack nucleation and propagation at grain boundaries. However, with excessive addition of B, the stress rupture properties were reduced significantly, as the solid solution strengthening and precipitation strengthening tended to be weakened by B addition, in addition the incipient melting regions and lager size borides promoted the intragranular crack nucleation and propagation.

Multiscale characterization of the 3D network structure of metal carbides in a Ni superalloy by synchrotron X-ray microtomography and ptychography

Synchrotron X-ray microtomography and ptychography were used to characterize the 3D network structure, morphology and distribution of metal carbides in an as-cast IN713LC Ni superalloy. MC typed carbides were found to distribute mainly on the grain boundary between the matrix γ and γ' phase. The differences in solidification cooling rate had a minor influence on the volume fraction of the MC type carbides, but significantly affected the carbide size, distribution and network morphology. Depending on the local composition of the remaining liquid phase and geometric constraints, the carbides can form either spherical or strip or network morphologies. The research demonstrated clearly the advantage and technical potential of using the two complementary tomography techniques synergistically to characterize non-destructively complex multiple-phase structures in three dimensional space with a spatial resolution of ~30 nm.

Influence of Intermediate Cryogenic Treatment on the Microstructural Transformation and Shift in Wear Mechanism in AISI D2 Steel

AISI D2 steels are an interesting family of tool steels that generally have applications in knife blades, shaper blades, and forging and punching dies due to its optimum toughness, lower wear rate, and relatively high hardness. The present research work aims to design a new heat treatment cycle consisting of single tempering followed by cryogenic treatment and soft tempering to improve the wear properties of these steels. Specimens of AISI D2 tool steel were subjected to three different heat treatment cycles: hardened and double tempered at 450 °C (HTT450), hardened and double tempered at 450 °C followed by cryotreatment and soft tempering at 250 °C (HTT450CT250), and hardened and single tempered at 450 °C followed by cryotreatment and soft tempering at 250 °C (HT450CT250). The microstructures of the samples were characterized for hardness, carbide density, impact energy, wear loss, wear surfaces, and residual stresses. The influence of these measured parameters on the wear behavior was determined using a pin-on-disc wear testing machine. The results confirmed that HT450CT250 is the optimized heat treatment cycle out of the three cycles studied, which showed enhanced wear resistance index, minimum compressive residual stresses, and optimum impact toughness due to increased volume fraction and finer distribution of carbides in the tempered martensite structure. It is further revealed that there is a change in morphology of carbides from globular to needle-shaped, resulting in wear transition, which is well corroborated by scanning electron microscopy studies and wear surface analysis.

Microstructural change and stress rupture property of Nimonic 105 superalloy for advanced ultra-supercritical power plants

Microstructural change, stress rupture property, deformation and fracture mechanisms of Nimonic 105 superalloy at 750 °C have been studied. Experimental results showed that the stress rupture strength of the alloy at 750 °C for 105 h is about 200 MPa. γ′ precipitates and M23C6 carbides grew gradually with prolonging the rupture time, while no significant change was observed in MC carbide morphology. After stress rupture test at 750 °C and 250 MPa for 23,341 h, a transition from spherical to cuboidal morphology of γ′ precipitates was found, and nearly continuous chains of M23C6 carbides formed on the grain boundary. Orowan looping and strongly coupled dislocation pairs cutting and microtwinning were the dominant deformation mechanisms at 750 °C and 350–450 MPa, while the main deformation mode was Orowan looping at 750 °C and 250 MPa. The failure of the alloy was mainly attributed to the nucleation, growth and interlinkage of voids.

2D-titanium carbide (MXene) based selective electrochemical sensor for simultaneous detection of ascorbic acid, dopamine and uric acid

Two-dimensional (2D) titanium carbide (MXene) nanosheets exhibited excellent conductivity, flexibility, high volumetric capacity, hydrophilic surface, thermal stability, etc. So, it has been exploited in various applications. Herein, we report synthesis of mixed phase 2D MXene as a catalytic material for simultaneous detection of important biomolecules such as ascorbic acid (AA), dopamine (DA) and uric acid (UA). Crystalline structure, surface morphology and elemental composition of mixed phase titanium carbide (Ti-C-Tx) MXene (Tx = –F, −OH, or −O) nanosheets were confirmed by X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), high-resolution scanning electron microscopy (HR-SEM) and Energy-dispersive X-ray spectroscopy (EDS) mapping analysis. Furthermore, Ti-C-Tx modified glassy carbon electrode (GCE) was prepared and its electrochemical properties are studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that Ti-C-Tx modified GCE (Ti-C-Tx/GCE) showed excellent electrocatalytic activity and separated oxidation peaks of important biomolecules such as AA (at 0.01 V), DA (at 0.21 V) and UA (at 0.33 V). Also, Ti-C-Tx/GCE sensor is enabled their simultaneous detection in physiological pH from 100 to 1000 μM for AA, 0.5–50 μM for DA and 0.5–4 μM & 100–1500 μM for UA. The limit of detection’s (LOD) was estimated as 4.6 μM, 0.06 μM and 0.075 μM for AA, DA and UA, respectively. Moreover, real sample analysis indicated that spiked AA, DA and UA can be determined accurately by Ti-C-Tx/GCE with the recovery ratio in the range between 100.5%–103% in human urine samples. The proposed Ti-C-Tx modified electrode exhibited good stability, selectivity and reproducibility as an electrochemical sensor for the detection of AA, DA and UA molecules.

The Effect of Primary Carbide on the Wear Resistance of Fe-Cr-C Coatings

The wear-resistant Fe-Cr-C coating has been widely used in industrial production owing to its high hardness and good comprehensive performance. Its anti-wear ability is mainly owing to the primary chromium carbides solidified during welding. In this study the effect of carbide morphology and distribution on the wear resistance of Fe-Cr-C coatings was investigated through experiment and simulation. In experiments, 3D microstructures of the typical carbides were examined using a high-resolution 3D X-ray computed tomography, and the 3D structures were then used in finite element simulation. Coatings with different morphologies and distributions of primary carbides were investigated. The results show that the fascicular primary chromium carbides break by the shearing stress during wear, leading to transverse fractures, while the mutual support among the homogeneous carbides contributes to the resistance of contact stress of the entire supporting framework, thereby imparting excellent wear resistance. Furthermore, the simulation results from the real 3D microstructure are consistent with the experimental ones. The fascicular structure produces a localized higher strain and larger stress concentration than the homogeneous one, implying higher abrasion loss in the former. These results will be helpful to optimize the structure design and final performance of the anti-wear Fe-Cr-C coatings. It will also be a new application of real-structure FE analysis in the widely used coatings for mining machines.

Refinement of primary carbides in hypereutectic high-chromium cast irons: a review

As a category of crucial wear-resistant alloys, high-chromium cast irons (HCCIs) are widely used in mining, minerals and cementation industries. The large volume fraction of coarse primary M7C3 carbides (PC) imparts excellent wear resistance. However, coarse carbides also induce brittleness, resulting in high cracking susceptibility, and early failure of components, particularly under impact. To minimize the brittleness and increase the service life of HCCI parts, different techniques have been developed through modifying the carbide morphology and refining its size. This paper comprehensively reviews the currently available methods that have either been used in industry production or in laboratory development to modify the primary M7C3 carbides in various HCCIs. The possible mechanisms that govern the refinement of primary carbides are also discussed in-depth. Based on previously published work, the mechanical performance of HCCIs is correlated with the microstructure of the matrix, and with the size, shape, volume fraction and distribution of primary carbides. This may provide solid fundamental to develop more effective techniques and/or new alloys to further improve the properties of this type of materials, increasing their engineering service life and to tailor their wider applications. In addition, the present work also seeks theoretical feasibility to apply the recently well-established theories/models of grain refinement for cast metals to refinement of the primary carbides in HCCIs.

Investigation of M7C3, M23C6 and M3C carbides synthesized on austenitic stainless steel and carbon fibers using laser metal deposition

This study investigates the mechanism driving the formation of core-shell carbides during laser metal deposition (LMD). The interaction between CFs and 316L metal powder under laser irradiation results in the formation of new composite materials (316L/CFs) because of two-phase (solid-liquid) melting. 316L/CFs comprise a metal matrix with M7C3 and M23C6 carbides. The metal matrix also contains raw 316L powder particles. The composites thus obtained (316L/CFs) were characterized using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and microhardness studies. The characterization revealed an in-situ transformation of M7C3 carbides to M23C6, which either occurs completely or ceases during transition stages resulting in the formation of core-shell carbides. The morphological analysis illustrates that primary carbides are of core-shell types with a single-crystal lattice. Moreover, the heat treatment can be used for the coalescence, coagulation, and partial dissolution of eutectic M23C6 carbides. It was observed that the morphology of eutectic and prime carbides are altered during the heat treatment at 1050 °C (4 h) and smooth cooling to 350 °C (11 h). Thus, the transformation of Cr7C3 (skeletal) → Cr23C6 (skeletal) → Cr23C6 (globular) occurs. Furthermore, the characterization of heat-treated samples confirms the core-shell nature of carbides. On the other hand, the core microhardness decreased by 1.5 times compared to that of the shell.

Martensite tempering kinetics: Effects of dislocation density and heating rates

In this study, the combined effect of dislocation density and rapid thermal cycling on the tempering kinetics of fully martensitic steel has been investigated by straining the sheets to different deformation levels. It was found that the strain-induced dislocations created more nucleation sites for carbide precipitation, and decreased carbide growth time. Strain-enhanced precipitation caused the carbide morphology to change from elongated to small quasi-spherical precipitates. Plastic deformation also accelerated the decomposition of carbides and film-like retained austenite in the as-received martensite laths. After tempering the deformed martensite had higher microhardness than tempered unstrained martensite, resulting from finer and semi-coherent precipitates, a high retained dislocation density, and the presence of untempered and partially tempered martensite blocks.

Evolutions of Micro- and Macrostructure by Cerium Treatment in As-Cast AISI M42 High-Speed Steel

The as-cast M42 high-speed steels containing different contents of cerium (Ce) were manufactured to investigate the effects of Ce on the solidification structures at the micro- and macroscale. The results indicate that the addition of Ce could modify MgO·Al2O3 and MnS into the Ce-containing inclusions. The addition of Ce refined the dendrite structure and eutectic carbides, which can be ascribed to the heterogeneous nucleation of primary austenite on Ce2O2S or Ce2O3 and the increase in both the compositional supercooling at the dendrite forefront and the restriction on the dendrites coarsening. Both the secondary dendrite arm spacing (SADS) and total eutectic carbides content decrease gradually with increasing Ce content. M2C and M6C carbides are the predominant precipitates. The increase of Ce content made the morphology of M2C carbides change from long lamellar or straight-rod morphology into shorter curved-rod or honeycomb morphology because of the overgrowth of eutectic austenite, but it had no significant effect on the morphology of M6C carbides. Ce2O2S and Ce2O3 can serve as the very effective heterogeneous nucleation sites for M6C carbides because of the low lattice disregistry between them, hence Ce addition improved markedly the macroscopic distribution of M6C carbides in the cast ingot and promoted the formation of M6C carbides at the expense of M2C carbides.

Mechanism of the scratching of monocrystalline silicon carbide with a diamond grit at different wear stages

An investigation was conducted to explore the mechanisms of the scratching of monocrystalline silicon carbide with a single diamond grit. The scratching was repeated on a silicon carbide workpiece to generate different wear shapes of the diamond grit. The forces were recorded during each scratching and the wear of the diamond grit together with the silicon carbide morphologies was monitored at a fixed interval. Based on the different diamond wear shapes determined through scratching experiments, a smoothed particle hydrodynamics method was used to simulate the scratching process. In addition to the items monitored in the experiments, the simulation was also used to analyze the change of subsurface damages on silicon carbide and to predict the mechanisms of diamond damage. It is shown that double-edged abrasive grits might lead to a better silicon carbide surface quality in scratching. The simulation results indicate that the maximum equivalent stress distribution might be used to predict the damage of the diamond grits during scratching. The findings of this article will be of benefit to the optimal selection of machining parameters and the optimal design of diamond tools for abrasive machining of monocrystalline silicon carbide.

On surface carbides in low-temperature carburized austenitic stainless steels

Surface carbides are occasionally found at the surface of austenitic stainless steels subjected to low-temperature carburizing surface treatments which are intended to induce an interstitially supersaturated surface layer. The crystallographic nature and chemical composition of the carbides can have a significant influence on the surface properties, especially on the corrosion resistance of the steel. However, their phase identification and chemical characterization via conventional methods (e.g. X-ray diffraction) is difficult due to their complex structure and metastable nature. In this paper, a combined metallographic and electron microscopic approach is applied to characterize the morphology and chemical composition of surface carbides found on AISI 304L and SS2343 after the same Kolsterising® treatment. Exclusively M5C2, also known as Hägg or χ-carbide was found at the surface of 304L, preferentially oriented along (111) deformation bands. In SS2343, however, χ-carbide coexisted with M7C3 (or ω-carbides) and M23C6. The carbides had preferential orientation relationships with the matrix, suggesting a carbide evolution during carburizing. All the carbides show elemental partitioning, with Cr enrichment and Ni depletion. A microstructure development sequence regarding precipitation and growth of surface carbides is proposed in this paper.

Microstructure evolution and mechanical properties of simulated HAZ in a Ni-17Mo-7Cr superalloy: effects of the welding thermal cycles

Welding thermal cycles with different peak temperatures were reproduced by a Gleeble testing machine to simulate the heat-affected zone of a Ni-17Mo-7Cr superalloy-welded joint. The effects of the thermal cycle times and peak temperatures on the microstructure and tensile properties of the thermal simulation specimens were systematically studied. The results show that no eutectic carbides are observed below 1300 °C, even when the number of thermal cycles is increased to 5. However, the eutectic reaction occurs above 1300 °C and causes a significant change in the carbide morphology because it transforms from a granular to lamellar structure. Moreover, the carbide composition transforms from (Ni, Cr)3Mo3C for the primary carbides to Ni3(Mo, Cr)3C for the lamellar carbides after the eutectic reaction. Tensile tests show that the thermal cycle has a minor effect on the tensile properties, except when the specimen underwent 5 thermal cycles with a peak temperature of 1340 °C. This is mainly due to the severe liquation reaction caused by elevated temperatures from the welding heat source, which induces cracking along the grain boundaries and results in a degradation of the tensile properties.

Role of script MC carbides on the tensile behavior of laser-welded fusion zone in DZ125L/IN718 joints at 650 °C

Script MC carbides are easily formed in the welding of Ni-base superalloys. In this study, the effect of script MC carbides on the tensile behavior of laser-welded DZ125L/IN718 joints in the fusion zone was investigated. The phase size, phase distribution and element distribution of script MC carbides, and the orientation relationship between script MC carbides and γ matrix were carefully analyzed using SEM, EDS, TEM and HRTEM. The breaking behavior of script MC carbides was investigated through interrupted tensile testing at 650 °C. The results demonstrate that the script MC carbides are distributed in the interdendrites with an average size of 3.5 $$\pm$$ 1.6 μm. They are (Ta, Nb, W, Ti and Mo)C and incoherent with the γ matrix. The fine script MC carbides are beneficial for improving both the strength and ductility of the fusion zone by improving coordinated deformation with γ matrix. However, the script MC carbides are found broken very early. About 9% script MC carbides have been broken after welding. The number of broken carbides increases with the increasing plastic strain. The majority of the cracks are found at the necks of the script MC carbides. The breaking mechanism of script MC carbides reveals that the high breaking tendency of the neck is attributed to the relatively long arm length and, more importantly, to the minimum section width size. Modifying the carbide morphology into a sphere is suggested for lowering the breaking tendency of MC carbides.

Effect of Secondary Aerosol Cooling on the Characteristics of Carbides in M42 High‐Speed Steel Produced by the Electroslag Remelting Withdrawal Process

The characteristics of carbides, such as type, size, and distribution, are important factors that affect the quality of high‐speed steel. To improve the characteristics of carbides produced by the electroslag remelting (ESR) process, an innovative process of electroslag remelting withdrawal with secondary aerosol cooling (ESRW–SAC) is proposed. The structure of the molten bath is obtained using tungsten powder detection, and the characteristics of carbides are statistically quantified by scanning electron microscopy (SEM), X‐ray diffraction (XRD), and the Image‐Pro Plus quantitative analysis software. The 3D carbide morphologies and carbides types are determined by an electrolysis device and SEM–energy‐dispersive spectroscopy (EDS). The results show that, compared with the traditional process, without SAC, the structure of the molten metal bath becomes shallower and flatter with the ESRW–SAC process. Under the influence of SAC, the network spacing and lamellar thickness of carbides considerably decrease, the phase area of carbides increases, and the distribution of carbides becomes more uniform. After the ESRW–SAC process, M2C and M6C carbides are still dominant; however, the morphology of M2C carbides changes from a lamellar to a curved short‐rod form.

Evolution of Transient Nature Nanoscale Softening During Martensite Tempering

In this study, the progression of martensite tempering as a function of tempering parameter has been investigated using instrumented nanoindentation. Three distinct stages of tempering related to the carbon segregation, carbide nucleation, and Ostwald ripening were identified. During the second tempering stage, the nanohardness achieves a plateau which is attributed to the change of carbide morphology, dissolution of intra-lath carbides, and growth of inter-lath and block boundary carbides.

The Morphology of Carbide and Nitride Coatings on Steels after Laser Irradiation

The results of experimental studies of the structure and properties of steels and alloys subjected to laser chemical-heat treatment of carbide and nitride coatings created by electro-spark alloying and ion-plasma sputtering are achieved. It is shown that laser processing of coatings allows to change purposefully the structural state and set properties of the surface layers of the product, and, therefore, modify the basic characteristics of process operation and control the most important output parameters – wear products, and the quality of the surface layers of the irradiated parts. It is established that a rational choice of chemical composition and method of applying coatings on the surface of metal products of various functionalities helps to improve the quality of the surface layers, to increase the hardness of the irradiated working areas by 30-50%, wear resistance-2-3 times, as compared to volumetric-hardened steels.