Transition Carbides Research Articles

Carbon distribution in lath martensite and quench embrittlement

Effect of water quenching and low-temperature tempering (LTT) at T≤280 °C on fracture mechanisms upon tension and during the Charpy V-notch (CVN) impact test of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was studied. The steel exhibits quench embrittlement (QE). Tensile specimens fractured in elastic region after quenching due to easy crack initiation by intergranular fracture mechanisms. CVN impact toughness was ∼5 J/cm2. Low fracture toughness is attributed to superposition of intergranular fracture and transgranular quasi-cleavage fracture within packets during crack propagation; crack initiation occurs locally at the notch surface. QE is attributed to carbon segregations on high-angle boundaries if a sufficient portion of bulk carbon could not be consumed for the formation of Cottrell clouds on dislocations. LTT provides precipitation of transition carbides that leads to depletion of carbon from martensite and boundary segregations that decreases severity of QE due to suppression of intergranular fracture, increase of fracture brittle stress and contribution of dimple fracture to overall fracture process. The onset of fracture takes place after yielding in tension and fracture toughness increases by a factor of ∼2 after LTT.

Transitional surface Pt carbide formation during carbon nanotube growth

By correlating structural information with catalytic activity, it is possible to determine the active structure of a catalyst. However, this is far from straightforward and the active structure remains debated even in the well-studied reaction of graphitic carbon formation using noble metal catalysts such as platinum (Pt). One major hindrance is that static observations do not provide access to transitional catalyst states. Here we prove the formation of transitional surface Pt carbide several layers deep as well as a Pt-carbon composite phase during the growth process of carbon nanotubes using atomic-resolution gas in situ transmission electron microscopy combined with density functional theory. Knowledge of the active structure of noble metal Pt is of great interest due to its usage in heterogeneous catalysis. Most importantly, it opens up new avenues to suppress catalyst coking. The unwanted build-up of carbon is the major source of catalyst deactivation in important industrial reactions including propane dehydrogenation, with major financial and environmental consequences.

Impact of carbon segregation on transition carbides and cementite precipitation during tempering of low carbon steels: Experiments and modeling

Tempering of low-carbon steels (less than about 0.3 wt.% C) may differ from the traditional precipitation sequence, which includes transition carbide precipitation, retained austenite decomposition and cementite precipitation. The main difference is the partial or total absence of transition carbides. In the present work, the effect of the carbon segregation on the mitigation of the precipitation of the latter is analyzed by combining multi-scale experimental investigations with a new precipitation model accounting for the carbon heterogeneities induced by the segregation. The full precipitation sequence is considered in order to study the impact of the segregation not only on the transition carbides, but also on the cementite which forms afterwards. The experimental work includes APT characterization of carbon segregation, in-situ HEXRD experiments to reveal precipitation kinetics, and TEM observations for carbide size measurements.The here-in developed precipitation mean-field and physics-based model combines two previous ones dedicated to the nucleation and growth of transition carbides and cementite and to the segregation of carbon at dislocations. It is shown that, even after water-quench, carbon atoms are already segregated on dislocations. The mitigation of transition carbide precipitation is caused by the presence of such segregations, which decrease the driving force for the transition carbide nucleation and enhance cementite precipitation. In agreement with previous experiments, the model also demonstrates that inside a martensitic microstructure, the precipitation sequence is different between the first (formed close to Ms temperature) and the last martensite (formed at room temperature) formed upon cooling, because of the difference in dislocation density, which influences the intensity of the segregation phenomenon.

On the role of chemical heterogeneity in carbon diffusion during quenching and partitioning

Carbon partitioning is the core design of quenching and partitioning (Q&P) process, which significantly improves the stability of retained austenite (RA). However, the inevitable precipitation of transition carbides leads to a substantial consumption of carbon atoms. Conversely, in the present study, we have realized the full inhibition of transition carbides in martensitic lath and, in turn, improved the carbon utilization efficiency to stabilize austenite. During fast austenitization from Mn-partitioned pearlite, Mn-heterogeneous high-temperature austenite is produced and more carbon atoms are trapped in the Mn-enriched region. Following Q&P, the alternative film RA and lath martensite in nanoscale is obtained, in which RA is enriched with Mn and C while the lath martensite is depleted with Mn and C. On one hand, the depletion of Mn and C in lath martensite strongly reduces the driving force and nucleation rate for carbide precipitation. On the other hand, the nanoscale microstructure and heterogeneous Mn distribution effectively accelerate carbon diffusion from lath martensite into austenite. Therefore, carbide precipitation is substantially inhibited in lath martensite due to the kinetic mismatch between fast carbon diffusion and sluggish carbide precipitation. This study demonstrates that chemical heterogeneity provides a novel pathway to enhance carbon partitioning efficiency and tensile properties in Q&P steels.

The abnormal carbon redistribution in lath martensite during tempering in Mn-patterned steels

Martensite tempering is a critical step to achieve high strength with decent ductility. It is well-known that, during tempering, the carbon is sequentially arranged as segregation, transition carbide and cementite. Abnormally, this study shows the absence of carbide precipitation in Mn-depleted martensite during tempering when Mn distribution is heterogeneous between austenite and martensite. After fast heating and short austenitization from Mn-depleted ferrite and Mn-enriched cementite, the high-temperature austenite with Mn heterogeneous distribution has formed, resulting in the formation of alternating Mn-depleted lath martensite and Mn-enriched film retained austenite (RA) after quenching. In comparison with the carbide precipitation in conventional lath martensite with Mn homogeneous distribution during tempering at 150–500°C, carbide formation is completely inhibited in Mn-depleted lath martensite. Instead, carbon atoms diffuse from Mn-depleted martensite to its neighboring Mn-enriched film RA, leading to an increased carbon content in RA. This is mainly attributed to the strong interaction between Mn and carbon, further assisted by high dislocation density and small width of lath martensite. Additionally, this RA can promote carbon diffusion in the interior due to its gradient Mn distribution.

Tempering behavior of an ultra-high-strength steel with 1.6 wt% Si at low to medium temperatures

Tempering behavior of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was examined. Water quenching produced lath martensite structure with a 10.5% volume fraction of retained austenite (RA) with film-like shape. Low temperature tempering (LTT) leads to precipitation of transition carbides and carbon partitioning between martensite and RA. The following precipitation sequence was found: martensite →non-stoichiometric η-carbide→stoichiometric η-carbide→Fe3C. Highest density of non-stoichiometric Fe2C η−carbides was found after isochronal tempering at 280 °C that provided attractive combination of high yield stress (YS) (1890 MPa) with a ductility of ∼6% and a Charpy V-notch (CVN) impact energy of ∼10 J/cm2. At 400 °C, the replacement of non-stoichiometric η-carbide by the stoichiometric one decreases strength and increases ductility due to 30% decrease in its volume fraction. No tempered martensite embrittlement was found due to the fact that Si effectively suppresses the precipitation of cementite at T ≤ 400 °C. The replacement of transition η-carbide by boundary cementite after isochronal tempering at 500 °C leads to considerably decrease in YS down to 1360 MPa, while elongation to failure increases up to 9.3%. Effect of decomposition of RA on strength, ductility and fracture toughness is insignificant.

Characterization of the precipitation sequence of transition carbides from carbon clusters by transmission electron microscopy

Characterization of the precipitation sequence of transition carbides from carbon clusters by transmission electron microscopy

TRANSFORMATION OF RETAINED AUSTENITE AND PORTEVIN-LE CHATELIER EFFECT IN 44CRMN2SI2MO STEEL UNDER TENSION

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: yield strength σ0.2 = 1140 MPa, ultimate strength σВ = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain Fe2C transition carbides. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which is referred to as the Portevin-Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.

Structure, Phase Composition, and Mechanical Properties of a High Strength Steel with Transition Carbide η-Fe2C

Structure, Phase Composition, and Mechanical Properties of a High Strength Steel with Transition Carbide η-Fe2C

Relationships between Strength, Ductility and Fracture Toughness in a 0.33C Steel after Quenching and Partitioning (Q&P) Treatment

The effect of quenching and partitioning (Q&P) processing on strength, ductility and fracture toughness is considered in a 0.33% C-1.8% Si-1.44 Mn-0.58% Cr steel. The steel was fully austenitized at 900 °C and quenched to 210 °C for 30 s. Partitioning at 350 °C for 600 s produces a martensitic matrix with transition carbides, bainitic ferrite and film-like retained austenite (RA) that is stable against transformation to strain-induced martensite under tension. This processing provided the highest strength and fracture toughness but the lowest ductility and product of strength and elongation (PSE), σB·δ (MPa·%). Partitioning at 500 °C produced RA with a relatively low carbon content and low volume fraction of carbides. The steel after this Q&P processing exhibits the highest ductility and PSE but low YS and Charpy V-notch (CVN) impact toughness. High ductility and PSE correlate with the ability of RA to transform into strain-induced martensite, while high strength and impact toughness are associated with the high-volume fraction of transition carbides in the carbon-depleted martensitic matrix and a lack of transformation of RA to strain-induced martensite. The highest CVN impact energy was attained in the steel exhibiting transgranular quasi-cleavage fracture with the lowest effective grain size for brittle fracture. No correlation between strength, ductility and fracture toughness is observed in Q&P steels if these materials have distinct structural constituents.

Ab initio study of structural, mechanical and electronic properties of 3d transitional metal carbide in cubic rocksalt (rs), zincblende (zb), and cesium chloride (cc) structures by using LDA and GGA Approximation.

This study rigorously investigates three 3d transition metal carbide (TMC) structures via LDA and GGA approximations. It examines cohesive energy (Ecoh), Vickers hardness (Hv), mechanical stability, and electronic properties. Notably, most 3d TMCs exhibit higher cohesive energy than nitrides, and rs-TiC demonstrates a Vickers hardness of 25.66 GPa, outperforming its nitride counterpart. The study employs theoretical calculations to expedite research, revealing mechanical stability in CrC and MnC (GGA) and CrC (LDA in cc structure), while all 3d TMCs in rs and seven in zb structures show stability. Charge transfer and bonding analysis reveal enhanced covalency along the series, influenced by the interplay between p orbitals of carbon and d orbitals of the metal. Most 3d TMCs exhibit metallic properties, excluding zb-TiC and zb-FeC in all phases. An inverse correlation between elastic constant C44 and electronic states near the Fermi level (EF) emerges, guiding applications and design. This study efficiently uncovers 3d TMC properties, offering insights for applications and design. We employed the Vienna ab initio Simulation software (VASP) to perform computations based on density functional theory (DFT). Our approach incorporated both the projector augmented wave (PAW) and PW91 general gradient approximation (GGA) methods within the local density approximation (LDA).

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

The effect of tempering after water quenching on the strength and fracture toughness of two steels with chemical compositions of 0.34%C-1.77%Si-1.35Mn-0.56%Cr-0.2%Mo-0.04%Nb-0.03Ti-0.002B and 0.44%C-1.81%Si-1.33%Mn-0.82%Cr-0.28%Mo was examined. The last steel exhibits quenching embrittlement in an as-quenched condition. At a tempering temperature of 280 °C, the precipitation of transition η–Fe2C carbides in martensitic matrix leads to increasing fracture toughness and eliminates quench embrittlement in the steel with 0.44 wt.%C. Tempered martensite embrittlement at 400 °C appears as decreased values of the Charpy V-notch impact energy, ductility and the product of strength and elongation, σB×δ (MPa×%) and is attributed to increased effective grain size for fracture, mainly. The precipitation of boundary cementite takes place at tempering at 500 °C and provides increased ductility and fracture toughness despite a decohesion along carbide/ferrite interfaces. The low severity of TME in Si-rich low-alloy medium carbon steels is attributed to the suppression of boundary cementite precipitation at tempering temperatures ≤400 °C.

Tempering Behavior of a Si-Rich Low-Alloy Medium-Carbon Steel

Owing to the addition of Si, 0.33C-1.8Si-1.44Mn-0.58Cr steel exhibits a unique tempering behavior. The tempering takes place in two distinct sequential stages that are significantly different from those in steels containing 0.2–0.5 wt.% of Si. Stage I is associated with the precipitation of transition carbides in a paraequilibrium manner, can take place in temperatures ranging from ~200 to ~474 °C, and concurrently increases strength, ductility, and toughness. Stage II is associated with the decomposition of retained austenite to bainitic ferrite and transition carbides. As a result, no significant effect of overlapping of Stage I with Stage II takes place. Stage III does not occur at temperatures below ~474 °C, since the precipitation of cementite in a orthoequilibrium manner is suppressed by the addition of 1.8 wt.% of Si. It was shown that a major portion of carbon atoms redistributes to Cottrell atmospheres under quenching. During low-temperature tempering at 200–400 °C, the precipitation of transition carbides consumes a large portion of carbon atoms, thereby increasing the number of ductile fractures and improving the impact toughness without strength degradation. The formation of chains of cementite particles on boundaries takes place in Stage IV at a tempering temperature of 500 °C. This process results in the full depletion of excess carbon from a ferritic matrix that provides increased ductility and toughness but decreased strength.

Efficient PbSe Quantum Dot Infrared Photovoltaic Applying MXene Modified ZnO Electron Transport Layer

AbstractInfrared (IR) solar cells are potential optoelectronic devices for boosting the power conversion efficiency (PCE) of conventional photovoltaics (such as pervoskite and silicon solar cells) by broadening the utilization range of the sunlight spectrum to short‐wavelength infrared region. PbSe colloidal quantum dots (QDs) are one of the optimal candidates for IR solar cells because of their tunable bandgap in the IR region and flexible solution processibility. At present, the best PbSe QD IR photovoltaics generally adopt ZnO as an electron transport layer (ETL). However, the intrinsic drawbacks and surface defects of ZnO can potentially deteriorate the PCE of devices. Herein, Ti3C2Tx, a representative 2D transition carbide, is combined with sol‐gel ZnO to develop a new hybrid ETL for fabricating high‐performance IR solar cells. This combination effectively suppresses the defects within ZnO by forming new bondings and simultaneously enhances the crystalline of ZnO film. Meanwhile, the introduction of Ti3C2Tx into ZnO film accelerates the transport and collection of photo‐generated carriers by constructing a new electron transport pathway. Consequently, compared to the bare devices, the infrared PCE of PbSe QD solar cells increases by 19.5% to 1.04%. These results demonstrate that this hybrid ETL can offer a bright approach for developing high‐performance optoelectronic devices.

Tailoring Ni3ZnC0.7/graphite heterostructure in N-rich laminated porous carbon nanosheets for highly efficient microwave absorption

Bimetallic transition carbide nanoparticles (Ni3ZnC0.7) encapsulated with graphitic shells were successfully embedded into N-rich laminated porous carbon nanosheets (NC–ZnNi1.5) by one-step pyrolysis of bimetallic organic frameworks. It was found that C atoms penetrated octahedral interstitial spaces of the Ni lattice to form Ni3ZnC0.7. The charge states and distribution of metal atoms were influenced by the interstitial C atoms, which promoted polarization relaxation and facilitated dielectric loss. Simultaneously, the volatilization of Zn influenced recrystallization and rearrangement of the crystalline domains, facilitated graphitization and established a 3D conductive network to optimize the conductive loss. Besides, multi-level heterogeneous interface and nitrogen doping also further optimized the impedance matching. Benefiting from these advantages, the minimum reflection loss of NC-ZnNi1.5 was −69.1 dB at 12.7 GHz, and the effective absorption bandwidth was 6.5 GHz. The mechanism of bimetallic carbide's dielectric loss was explained in detail, providing a new pathway for the development of multi-component carbon-based microwave absorbers in the future.

EVOLUTION OF MICROSTRUCTURE AND EMBRITTLEMENT DURING THE TEMPERING PROCESS IN SiCrCu MEDIUM-CARBON STEELS

The effect of Cu and Si contents on the evolution of embrittlement during a heat treatment consisting of quenching into an oil bath and tempering in the range 200–500 °C was investigated in medium-carbon steels with 0.48 w/% and 0.57 w/% of C. The results showed that the higher silicon content shifted the tempered martensite embrittlement to higher tempering temperatures. The steels alloyed with Cu had lower notch-toughness values, which worsened with increasing tempering temperature compared to the Cu-free samples. In the investigated tempering temperature range, the following microstructural changes occurred: formation of transition carbides, decomposition of retained austenite, and precipitation of cementite and Cu-particles.

Competitive role of film-like austenite and transition carbides on hydrogen embrittlement resistance and impact toughness in bainite-containing quenched and partitioned steel

Competitive role of film-like austenite and transition carbides on hydrogen embrittlement resistance and impact toughness in bainite-containing quenched and partitioned steel

Effect of Double-Step and Strain-Assisted Tempering on Properties of Medium-Carbon Steel.

The present work aimed to study the properties of medium-carbon steel during tempering treatment and to present the strength increase of medium-carbon spring steels by strain-assisted tempering (SAT). The effect of double-step tempering and double-step tempering with rotary swaging, also known as SAT, on the mechanical properties and microstructure was investigated. The main goal was to achieve a further enhancement of the strength of medium-carbon steels using SAT treatment. The microstructure consists of tempered martensite with transition carbides in both cases. The yield strength of the DT sample is 1656 MPa, while that of the SAT sample is about 400 MPa higher. On the contrary, plastic properties such as the elongation and reduction in area have lower values after SAT processing, about 3% and 7%, respectively, compared to the DT treatment. Grain boundary strengthening from low-angle grain boundaries can be attributed to the increase in strength. Based on X-ray diffraction analysis, a lower dislocation strengthening contribution was determined for the SAT sample compared to the double-step tempered sample.

A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying.

The essences of the quenching and partitioning (Q&P) process are to stabilize the finely divided retained austenite (RA) via carbon (C) partitioning from supersaturated martensite during partitioning. Competitive reactions, i.e., transition carbide precipitation, C segregation, and decomposition of austenite, might take place concurrently during partitioning. In order to maintain the high volume fraction of RA, it is crucial to suppress the carbide precipitation sufficiently. Since silicon (Si) in the cementite θ (Fe3C) is insoluble, alloying Si in adequate concentrations prolongs its precipitation during the partitioning step. Consequently, C partitioning facilitates the desired chemical stabilization of RA. To elucidate the mechanisms of formation of transition η (Fe2C) carbides as well as cementite, θ (Fe3C), besides the transformation of transition carbides to more stable θ during the quenching and partitioning (Q&P) process, samples of 0.4 wt% C steels tailored with different Si contents were extensively characterized for microstructural evolution at different partitioning temperatures (TP) using high resolution transmission electron microscopy (HR-TEM) and three-dimensional atom probe tomography (3D-APT). While 1.5 wt% Si in the steel allowed only the formation of η carbides even at a high TP of 300 °C, reduction in Si content to 0.75 wt% only partially stabilized η carbides, allowing limited η → θ transformation. With 0.25 wt% Si, only θ was present in the microstructure, suggesting a η → θ transition during the early partitioning stage, followed by coarsening due to enhanced growth kinetics at 300 °C. Although η carbides precipitated in martensite under paraequilibrium conditions at 200 °C, θ carbides precipitated under negligible partitioning local equilibrium conditions at 300 °C. Competition with the formation of orthorhombic η and θ precipitation further examined via ab initio (density functional theory, DFT) computation and a similar probability of formation/thermodynamic stability were obtained. With an increase in Si concentration, the cohesive energy decreased when Si atoms occupied C positions, indicating decreasing stability. Overall, the thermodynamic prediction was in accord with the HR-TEM and 3D-APT results.

The structure and the properties of WC-Co samples produced by SLM technology and carbon-doped prior to HIP processing

Purpose The purpose of this study is to verify how the carbon doping of the WC-Co cemented carbide (CC) affected their structure before their processing by hot isostatic pressing (HIP) technology. Design/methodology/approach The samples for this experiment were fabricated by selective laser melting technology (SLM) using a YAG fiber laser with a power of P = 40 W and a scanning speed of 83 mm/s. The subsequent carbon doping process was performed in a chamber furnace at 900 0 C for 1, 4 and 12 h. The HIP was performed at 1,390°C and pressures of 40 MPa, 80 MPa and 120 MPa. The changes induced in the structures were evaluated using X-ray diffraction and various microscopic methods. Findings X-ray diffraction analysis showed that the structure of the samples after SLM consisted of WC, W2C, Co4W2C and Co phases. As a result of the increase in the carbon content in the structure of the samples, the transition carbide W2C and structural phase Co4W2C decayed. Their decay was manifested by the coarsening of the minor alpha phase (WC), which occurred both during the carburizing process and during the subsequent processing using HIP. In the samples in which the structure was carburized prior to HIP, only the structural phases WC and Co were observed in most cases. Originality/value The results confirm that it is possible to increase the homogeneity of the CC structure and thus its applicability in practice by additional carburization of the sample structure with subsequent processing by HIP technology.