Transition Carbides Research Articles

Mechanical property-microstructure relationships of an X70 pipeline steel at elevated temperatures

Mechanical property-microstructure relationships of an X70 pipeline steel were evaluated at temperatures up to 400 °C using tensile testing, scanning and transmission electron microscopy, and synchrotron wide-angle X-ray scattering characterization techniques. The X70 steel had a bainitic microstructure consisting of quasi-polygonal ferrite, retained austenite, martensite-austenite islands, and carbide/nitride microconstituents. Results are compared to an X52 steel with a ferrite-pearlite microstructure. Both steels exhibited dynamic strain aging (DSA) as evidenced by serrated yielding, reductions in strain rate sensitivity, increased strengths, and reduced dutilities in the approximate temperature ranges of 100–250 °C. For temperatures above the DSA range, the X70 steel exhibited unique properties of an increase in strength simultaneously with an increase in uniform strain, both features interpreted due to microstructural changes during testing due to dynamic tempering, indicated by decomposition of the retained austenite and associated cementite and transition carbide nucleation and coarsening, i.e. microstructural changes normally associated with static tempering at temperatures above 400 °C. However, if the X70 steel was tempered at the test temperature prior to testing, dynamic microstructural changes were absent and the steel exhibited behavior similar to that observed for the X52 steel, i.e. a decrease in strength and increase in ductility at temperatures above the DSA range. New alloying strategies are suggested to improve microstructure stability during isothermal holds at elevated temperature, while maintaining the strength benefits from strain assisted bainitic tempering during plastic deformation.

Recent development of high-entropy transitional carbides: a review

Recent development of high-entropy transitional carbides: a review

Modeling competitive precipitations among iron carbides during low-temperature tempering of martensitic carbon steel

Upon tempering of martensitic carbon steels, transition carbides (TCs), as precipitated precursors that compete for available C atoms among multiple iron carbides, have tremendous influences on the microstructural evolution and the final properties of steels. Although two similar structures of TCs, i.e. the hexagonal ε-Fe2C and the orthorhombic η-Fe2C, were successively identified, they still cannot be clearly distinguished experimentally, so far. It is timely to grasp the corresponding atomic-scale nature to predict the real structure of TCs. Based on the maximal entropy production principle and the atomic-scale computations, a Fokker-Planck type equation (FPE) following a modified multi-scale modeling framework, is solved for competitive precipitations among the potential iron carbides (including ε-Fe2C, η-Fe2C and θ-Fe3C) during the low-temperature tempering of carbon steel. The obtained microstructural evolution path dominated by synergy of thermodynamics and kinetics indicates a precipitation sequence for TCs, i.e. ε → η, which is further verified by the minimum energy path in terms of the solid-state nudged elastic band (SSNEB) method. On this basis, the potential hexagonal-orthorhombic transformation (HOT) of TCs is discussed, by which the probable structure of TCs can be accurately identified in experiment. The current modeling framework is expected to be a worthy paradigm for reference to predict the structural evolution of TCs in engineering materials.

Tailoring strength/ductility combination in 2.5 wt% Si-alloyed middle carbon steel produced by the two-step Q-P treatment with a prolonged partitioning stage

The present work aimed at studying the effect of the two-step “Quenching-and-Partitioning” (Q-P) processing with a prolonged partitioning stage on the mechanical properties of middle carbon steel with the composition of 0.56 wt% С, 2.50 wt% Si, 1.70 wt% Mn, 0.50 wt% Cr, 0.21 wt % Mo, 0.12 wt% V, 0.05 wt% Nb. A total of 28 regimes with the quenching temperature of 160 °C and 200 °C, the partitioning temperatures of 200–300 °C and the partitioning time of 10–300 min were applied. Under Q-P treatment the UTS value ranged between 1535 and 2374 MPa, the total elongation ranged between 8 and 23% and the U-notched impact toughness ranged between 48 and 117 J/cm2, and TEL and KCU values were twice those of the “Quenching-Tempering” (Q-T) treatment. The Q-P-treated specimens possessed a multi-phase structure consisting of 38–61 vol% of tempered martensite (with transitional carbide precipitates), lower carbide-free bainite and 11–28 vol% of retained austenite enriched with 0.80–1.23 wt% carbon. The tensile strength gradually increased with the decrease in the partitioning temperature and partitioning time. In contrast, the ductility and the impact toughness tended to increase with the increasing partitioning temperature and partitioning time, and this phenomenon could be attributed to martensite tempering, bainite transformation and the increase in volume fraction of retained austenite. Q-P regimes were tailored for different strength/ductility combinations. The Q-P treatment with the regime of quenching at 160 °C, and partitioning at 270 °C for 50–120 min ensured the most advanced mechanical properties with the combination of 1743–1830 MPa UTS, 20–21% TEL, 36.6 GPa⋅% PSE, 75–79 J/cm2 KCU. The combination of the ultra-high tensile strength of 2374 MPa with 9% TEL and 61 J/cm2 KCU was obtained by quenching at 160 °C and partitioning at 200 °C for 90 min. The structural characteristics of the Q-P-treated steel were analyzed by SEM, TEM, XRD, nanoindentation, and dilatometric measurements.

Tempering effects of athermal martensite during quenching and reheating of a SAE 52100 bearing steel

Athermal martensite formation is normally described as temperature-dependent. By dilatometer experiments, it will be shown that time-dependent effects occur during martensite formation of a bearing steel SAE 52100 (DIN EN 100Cr6). A series of different cooling rates or isothermal holding stages below martensite start temperature were applied. The nature of the time-dependent effects during cooling is enlightened by analyzing the tempering effects during a subsequent reheating. It was found that in particular the low temperature tempering effects were activated during cooling. Mainly the formation of transition carbides and the loss of tetragonality depended on the cooling condition. Retained austenite transformation was only affected by austenite stabilization and cementite formation was unaffected by the cooling condition.

Evolution of microstructure and mechanical properties during tempering of M50 steel with Bainite/Martensite duplex structure

The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.

Strain-Ageing of Low-Alloyed Multiphase High-Strength Steels

The strain-aging of low alloyed, multiphase high-strength steels with strain-induced austenite to martensite transformation was studied. The influence of prestrain, aging time, and temperature dependence of the static strain aging was carried out. Ageing temperatures between 60 and 220 ∘ C and aging times from 20 to 10,000 min were investigated. The choice of steel composition allowed studying the influence of alloying elements, such as Si and Al, on the static strain aging behavior. Samples after aging were studied using light-optical microscopy, X-ray diffraction, and in-depth transmission electron microscopy (TEM). The Harper model was used to describe the precipitation mechanisms occurring during aging. The study of thin foils after aging using TEM showed the precipitation of low temperature transition carbides in the microstructure, which was observed between 60 and 5000 min. By using X-ray diffraction, it was revealed that aging at 170 ∘ C for a long time caused a slight decrease of the retained austenite volume fraction, but the C content remained constant.

Improvement in the wear resistance of a hypereutectoid rail via heat treatment

The rail, as the most important component in railways, needs to be routinely checked to maintain proper service conditions to prevent catastrophic accidents by any premature failure of the rail. The severe plastic deformation of rails due to cyclic wheel/rail contact forces forms a superficial deformed layer, referred to as the white etching layer, because of cementite decomposition and the formation of the carbon-supersaturated nanostructured ferrite phase. This supersaturated superficial layer is highly prone to the initiation and propagation of rolling contact fatigue cracks. However, it has been observed that transition carbides and cementite can precipitate when the first and third stages of tempering starts at approximately 150 and 400 °C, respectively, due to carbon ejection from the body-centered tetragonal structure. A controlled laboratory heat treatment was conducted on a used hypereutectoid rail at 200 and 500 °C. Then, the wear resistances of the heat-treated samples were compared with the as-received rail sample using pin-on-disc testing. A significant increase in the wear resistance of the sample treated at 200 °C confirmed the improvement in the wear resistance of used pearlitic rails, which is a great interest for railway industries.

Microstructural and Mechanical Investigation of WC-TiC-Co Cemented Carbides Obtained by Conventional Powder Metallurgy

WC-Co cemented carbide is one of the widely hard materials used for cutting in machining industry, due to its microstructural and mechanical stability even at high temperature. However, diffusion wear is the most serious problem that WC-Co suffers from. One of the most applied approaches to improve the WC–Co cemented carbide performances is the addition of transition metal carbides such as: TiC, TaC and NbC which prevents diffusion wear thanks to the gamma phase (Ti,Ta,Nb,W)C which is formed during sintering. Therefore, and in order to understand the thermal metallurgical reactions occurred between WC-Co cemented carbide and (Ti, Ta, Nb)C transition carbides and theirs effects on the microstructural and mechanical properties. The WC–TiC– TaC– NbC–Co cemented carbide was elaborated by conventional powder metallurgy then thermal, microstructural and mechanical investigations were performed on the elaborated carbide. A temperature of sintering was determined to be more than 1347 oC by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Scanning electronic microscopy (SEM) coupled with energy dispersive spectrometer (EDS) observations showed that the microstructure consists in a mixture of angular WC grains and (W,Ti)C rounded grains embedded in the Co-rich binder. X-ray diffraction analysis confirmed the presence of these three phases with free carbon. The results of EDS analysis highlight the solution-reprecipitation phenomena caused by liquid phase sintering and clearly revealed the presence of small amount of free carbon. The mechanical characterizations showed that the WC–TiC– TaC– NbC–Co cemented carbide exhibits excellent hardness-fracture toughness combination.

A Study of two Dimensional Metal Carbide MXene Ti3C2 Synthesis, characterization conductivity and radiation properties

Since the discovery of exceptional properties of graphene, a lot of researchers focused on the discovery of another nobel two-dimensional (2D) materials. Recently, an elegant exfoliation approaches was proposed as a method to synthesis a new family of transitional 2D metal carbide or nitrades of MXene from a layered MAX phase. A layered MAX phase of Ti3AlC2 was synthesized through pressureless sintering (PLS) the initial powder of 3TiH2/1.1Al/2C without preliminary dehydrogenation under argon atmosphere at 1350 oC. An elegant exfoliation approach was used to eliminates Al from its precursor to form a layered-structure of Ti3C2. In this study, thermal conductivity of MAX phase and MXene were studied using absolute axial heat flow method to measure the abilities sample to conduct heat and the data was collected using Picolog 1216 Data Logger. Electrical conductivity of these two materials was also compared by using two-point probe, due to its simplicity. Radiation properties of 2D MXene Ti3C2 was studied by using an established radon monitor, placed in closed, fabricated container. Morphological and structural properties of this 2D material were also studied using an established FESEM and XRD apparatus. SEM images shows two types of morphology which is a layer of Ti3C2 and the agglomerates Al2O3 with graphite. XRD pattern reveals three phases in this material which is a rhombohedral Al2O3, rhombohedral graphite and rhombohedral Ti3C2 phases, respectively. Thermal and electrical conductivity of MXene were proven higher than MAX phase. Radon concentration for this material for five consecutive days explains the radiation level of this material which is under the suggestion value from US Environmental Protection Agency (EPA). From this finding, it is can conveniently say that the MXene material can be promising material for electronic application.

Phase transformation mechanisms during Quenching and Partitioning of a ductile cast iron

The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.

Critical Assessment 34: Are χ (Hägg), η and ϵ carbides transition-phases relative to cementite in steels?

ABSTRACTHägg carbide (χ) is, during the tempering of carbon-rich martensite, referred to as a transition carbide which eventually gives way to cementite. However, there are Fe–C binary phase diagrams estimated using thermodynamic data, that define a low-temperature phase field where a mixture of Hägg carbide and ferrite is more stable than that of cementite and ferrite. In this scenario, it may be cementite which is the transition carbide. Evidence is presented here that the predominance of Hägg carbide over cementite in the circumstances described is unlikely to be correct based on historical and new observations. Literature data are also interpreted to show that η and ϵ-carbides are best regarded as transition-phases relative to mixtures of cementite and ferrite.

Modeling Competitive Precipitations Among Iron Carbides During Low-Temperature Tempering of Martensitic Carbon Steel

Upon tempering of martensitic carbon steels, transition carbides (TCs), as precipitated precursors that compete for available C atoms among multiple iron carbides, are usually regarded as either the hexagonal e-Fe2C or the orthorhombic η-Fe2C, both of which have tremendous influences on the microstructural evolution and the final properties of steels, but still cannot be clearly distinguished experimentally, so far. It is timely to grasp the corresponding atomic-scale nature to predict the real structure of the TCs. Based on maximal entropy production principle (MEPP) and atomic-scale computations, a Fokker-Planck type equation (FPE) following a modified multi-scale modeling framework, is solved for competitive precipitations among the potential iron carbides, including e-Fe2C, η-Fe2C and θ-Fe3C, during the low-temperature tempering of carbon steel. The obtained microstructural evolution path dominated by synergy of thermodynamics and kinetics indicates a precipitation sequence for TCs, i.e. e → η, which is further verified by the minimum energy path (MEP) in terms of the solid-state nudged elastic band (SSNEB) method. On this basis, the potential hexagonal-orthorhombic transformation (HOT) of TCs is discussed, by which the probable structure of TCs can be accurately identified in experiment. The current modeling framework is expected to be a worthy paradigm for reference to predict the structural evolution of TCs in engineering materials.

Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Martensite Embrittlement Regime

While tempering reactions under furnace conditions have been extensively studied, short-time tempering at higher temperatures, as might apply to induction tempering, has not been thoroughly explored. The mechanical behavior and phase development of short-time (1, 10, and 100 seconds) and conventionally (3600 seconds) tempered 4340 steel are compared at an equivalent degree of tempering defined by the Hollomon–Jaffe tempering parameter. The tempering parameter accurately predicts hardness values across short-time and conventional tempering conditions, but is less able to describe phase evolution associated with short-time tempering. Room temperature Charpy impact toughness and ductile-to-brittle transition temperature systematically improve with shorter tempering times at an equivalent tempering parameter. In particular, rapid tempering significantly increases toughness within the tempered martensite embrittlement regime. Relative to conventional tempering, shorter tempering times exhibit higher retained austenite content for a given tempering parameter, although no systematic or significant difference in cementite or transition carbide content is observed between time conditions. The retained austenite decomposition behavior indicates that the relationship between the classical tempering stages is altered at short tempering times, where increased overlap and/or “re-ordering” of stage II and III tempering are observed.

Molybdenum carbide nanoparticles supported on nitrogen-doped carbon as efficient electrocatalysts for hydrogen evolution reaction

Transition carbides have been studied as one of the most promising non-precious-metal catalysts for hydrogen evolution reaction (HER) due to analogous electronic structure to precious metals. Herein, molybdenum carbide nanoparticles supported on nitrogen-doped carbon (Mo2C/NC) have been successfully synthesized through a facile and environmentally-friendly hydrothermal method. The nitrogen content is adjusted by varying the molar ratio of glucose (G) and glucosamine hydrochloride (GH). With an optimal G/GH ratio of 0.6, Mo2C/NC-0.6 nanoparticles exhibit a low onset reduction potential of 95mV with a Tafel slope of 79.2mVdec−1, an overpotential of 168mV at cathodic current density of 10mAcm−2 and good stability in acidic media. The high electrocatalytic activity of Mo2C/NC-0.6 can be attributed to high specific surface area, synergistic effect between Mo2C and N species, and the conductive N-doped carbonaceous matrix. This study may provide a promising strategy for synthesizing efficient water splitting catalysts with great electrocatalytic activities.

Tailoring the strength and ductility of T91 steel by partial tempering treatment

T91 ferritic/martensitic steel (modified 9Cr-1Mo steel) with high strength and high ductility is considered one of the promising structural materials for various industrial applications. Recent studies show that equal channel angular pressing can improve the yield strength of T91 steel to 1.6 GPa but degrade the uniform elongation to ∼1%. Here, we report that by using a modified quenching and tempering treatment, T91 steel can achieve a high yield strength of 1.4 GPa and a uniform elongation of ∼5.5%. Extensive microstructure analyses show the formation of transition carbides and ultra-fine martensite within bainite grains. This study provides an alternative approach to obtain a combined enhancement of strength and ductility for T91 steel.

Cobalt doped β-molybdenum carbide nanoparticles encapsulated within nitrogen-doped carbon for oxygen evolution.

Herein, we report a novel cobalt doped transition metal carbide-based OER electrocatalyst, cobalt doped β-molybdenum carbide (β-Mo2C) encapsulated by a nitrogen doped carbon framework, Co0.1-β-Mo2C@NC, which shows an unexpected onset overpotential of 200 mV and an overpotential of 262.2 mV at a current density of 10 mA cm-2. This strategy of cobalt doping reduces the amount of cobalt, and increases the density of active sites as well; furthermore, the introduction of molybdenum carbide enhances the stability of the catalyst.

Interface modification of tempered martensite utilizing nano-scale transition carbide via isothermal sub-zero treatment

The present study investigated the influence of isothermal holding time on characteristics of nano-scale transition carbide and interfaces of tempered high-carbon martensitic steel processed by sub-zero treatment (SZT). We interpreted a stress-induced activity of carbon atoms through SZT prior to tempering, which was crucial for obtaining desirable precipitation of transition carbide. The modification upon interfaces under the influence of transition carbide characteristics was recognized for the first time and discussed based on mechanism of precipitate-interface interaction in relation to the nanohardness value.

Formation of eta carbide in ferrous martensite by room temperature aging

For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, (1¯10)α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years.

Cold Sintered Ceramic Nanocomposites of 2D MXene and Zinc Oxide.

Nanocomposites containing 2D materials have attracted much attention due to their potential for enhancing electrical, magnetic, optical, mechanical, and thermal properties. However, it has been a challenge to integrate 2D materials into ceramic matrices due to interdiffusion and chemical reactions at high temperatures. A recently reported sintering technique, the cold sintering process (CSP), which densifies ceramics with the assistance of transient aqueous solutions, provides a means to circumvent the aforementioned problems. The efficacious co-sintering of Ti3 C2 Tx (MXene), a 2D transition carbide, with ZnO, an oxide matrix, is reported. Using CSP, the ZnO-Ti3 C2 Tx nanocomposites can be sintered to 92-98% of the theoretical density at 300 °C, while avoiding oxidation or interdiffusion and showing homogeneous distribution of the 2D materials along the ZnO grain boundaries. The electrical conductivity is improved by 1-2 orders of magnitude due to the addition of up to 5 wt% MXene. The hardness and elastic modulus show an increase of 40-50% with 0.5 wt% MXene, and over 150% with 5 wt% of MXene. The successful densification of ZnO-MXene nanocomposite demonstrates that the cold sintering of ceramics with 2D materials is a promising processing route for designing new nanocomposites with a diverse range of applications.