Search

In this study, the high-temperature evolution of eutectic carbides in a medium-alloy steel was investigated as well as their precipitation mechanism. Results revealed that MC and M6C eutectic carbides in the steel appeared during the final solidification due to the segregation of Mo and V and the melt undercooling. These eutectic carbides consist of blocky shapes with varying orientations, and rod-like shapes with the same orientation. Further investigation was confirmed that the carbides (MC and M6C) exhibited an intergrowth behavior. The cooling rate had a direct influence on the secondary dendrite arm spacing (SDAS), which decreased from 180 to 130 μm as the rate ranged from 0.6 to 21.9 °C/min. After holding for 0, 25, 50, and 80 h, respectively, the volume percentage of eutectic carbides exhibited a significant decreasing trend from 2 to 0.2%. Additionaly, the carbides exhibited irregular morphology after holding for 0–50 h, respectively, and which was transformed into the rod-like shape with a small size distributed along the grain boundaries after holding for 80 h. This phenomenon was attributed to the mutual diffusion between Mo, V, and other alloying elements in the carbides and Fe element in the steel matrix.

This study investigates the effect of a tungsten carbide-nickel coating on the wear resistance of plain carbon steel. The primary phases identified in the coating were Fe3W3C, Fe4W2C, and a eutectic structure containing Fe3W3C and (Fe–Ni)-α. Wear behavior was evaluated through a friction coefficient test and Scanning Electron Microscopy (SEM) analysis. The coated samples exhibited a significantly lower friction coefficient compared to the uncoated sample, indicating superior wear resistance. This improvement is attributed to the presence of a network of (Fe, W)3C carbides and the strong interfacial bonding between the coating and substrate. Coated surfaces displayed parallel scratches and small wear particles, indicative of a scratch-type wear mechanism. Microhardness measurements revealed a distinct profile across the coating thickness. The high hardness near the surface is attributed to the presence of simple carbides. Hardness increased further in the center of the coating due to optimized welding parameters. Hardness decreased towards the interface with the substrate due to a decrease in alloying elements and a shift towards complex carbides.

Microstructures of as-cast 2.17%C–16.23%Cr high chromium white cast iron(HCWCI) alloyed with 1.03%V–0.1%B–1.45%Si are studied. The constituents of the as-cast microstructure include M 7 C 3 chromium rich eutectic carbides, globular boro-carbides and austenite dendrite matrix. The equilibrium phase diagram calculation for the alloy composition is performed with ThermoCalC software using TCFE9 database. The alloy sample is heat-treated in different heat treatment conditions and the microstructures are studied. The precipitation of secondary boro-carbides has increased in both normalized and quenched and tempered sample. Although the matrix microstructure in the normalized sample remains austenite, the matrix microstructure of quenched and tempered sample has mostly become martensitic with needle-like microstructure. The hardness of the quenched and tempered sample has increased to 52HRC from 45 HRC in the as-cast condition. The normalized sample despite of showing a very small improvement in its hardness compared to as-cast condition, it has shown the highest resistance to sliding wear. [ABSTRACT FROM AUTHOR]

The presence of the eutectic phase (delta ferrite + M23C6) in martensitic stainless steels brings significant deterioration of the in-service mechanical properties of the critical components such as turbine shaft made of these alloys. In the present study, thermodynamic and kinetics of eutectic phase formation during solidification and the reheating stages before forging of a large size X38CrMo16 martensitic stainless steel ingot are investigated. Material characterization and microstructural evolution were characterized in three different zones of a large size ingot. It was observed that the forging temperature and the solidification rate were the two most effective parameters influencing the volume fraction of the eutectic phase and its morphology. Optical and electron microscopy observations along with Energy Dispersion Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) measurements were used in the investigation. The results showed that the eutectic phase precipitated primarily at the grain boundaries. Furthermore, a clear evolution on the morphology and volume fraction of the eutectic phase including thin film-like and skeleton-like carbides was found from the outer surface to the center of the ingot along the radial direction. EDS analysis revealed the substantial presence of chromium, molybdenum, and carbon within the M23C6 along the grain boundaries. Phase transformation and the precipitation phase sequences were analyzed as a function of temperature and composition of eutectic transformation using the Thermo-Calc software and the predictions were validated with experimental findings.

High Cr white irons with various fractions of primary dendrite have been prepared through the modification of their chemical composition. Increasing C and Cr contents decreased the primary dendrite fraction. Eutectic solidification occurred with the phase fraction ratio of austenite: M7C3 = 2.76:1. The measured primary dendrite fractions were similar to the calculated results. ThermoCalc calculation successfully predicted fractions of M7C3, austenite, and M23C6. Conventional heat treatment at high temperature caused a destabilization of austenite, releasing it’s solute elements to form M23C6 carbide. Precipitation of M23C6 during destabilization preferentially occurred within primary (austenite) dendrite, however, the precipitation scarcely occurred within austenite in eutectic phase. Thus, M23C6 precipitation by destabilization was relatively easy in alloys with a high fraction of primary dendrite.

Abstract Melting of primary M 7 C 3 and MC eutectic carbides in a cobalt-base superalloy has been investigated. It is found that the melting phenomenon of primary M 7 C 3 and MC eutectic carbides occurs at approximately 1280 and 1320 °C, respectively, in particular the melting of MC eutectic carbide is firstly found. The melting process of primary M 7 C 3 eutectic carbide is that M 7 C 3 first transforms into M 23 C 6 and then M 23 C 6 melts rather than directly melts at or above 1280 °C, and the re-solidified structure is still M 23 C 6 eutectic carbide, but its lamellar spacing becomes finer with respect to primary M 7 C 3 eutectic carbide. Additionally, the higher the heat treatment temperature, the less the difference in the lamellar spacing of re-solidified M 23 C 6. On the other hand, after primary MC eutectic carbide melts, the finer Chinese script MC eutectic carbide associated with compositional change is produced upon re-solidification in comparison to the original as-cast structure. Graphical abstract Image 1 Highlights • Melting of primary carbides, especially MC, is observed in cobalt-base superalloys. • M 7 C 3 firstly transforms into M 23 C 6 and then melts at high temperatures. • The lamellar spacing of re-solidified phase is finer than that of primary phase. [ABSTRACT FROM AUTHOR]

The excellent abrasion resistance of high chromium cast irons (HCCIs) stems from the dispersion of the hard iron-chromium eutectic carbides. The surrounding matrix on the other hand, provides sufficient mechanical support, improving the resistance to cracking deformation and spalling. Prior knowledge of the microstructural characteristics is imperative to appropriately design subsequent heat treatments, and in this regard, employing computational tools is the current trend. In this work, computational and experimental results were correlated with the aim of validating the usage of MatCalc simulations to predict the eutectic carbide phase fraction and the elemental distribution in two HCCI alloys, in the as-cast condition. Microstructural observations were carried out using optical microscopy and SEM. The chemical composition and fraction of each phase was measured by electron probe microanalysis and image analysis, respectively. In all cases, the values predicted by the pseudo-equilibrium diagrams, computed with MatCalc, were in accordance with the experimentally determined values. Consequently, the results suggest that time and resource intensive experimental procedures can be replaced by simulation techniques to determine the phase fraction and especially, the individual phase compositions in the as-cast state.

In-situ observation of eutectic solidification of Cr12MoV steel was conducted using high temperature confocal laser-scanning microscopy. The semi-solid isothermal treatment temperature of the steel was determined by thermodynamic calculation using Thermo-cal software. At the same time, the influences of isothermal treatment temperature and time on eutectic carbides in the steel were also studied. The results show that when the liquid metal cools at the rate of 47 °C·min-1, the eutectic reaction occurs rapidly at 1,214.7 °C in one second with the reticular liquid around austenite dendrites, transforming into a network of eutectic structure. After being held at 1,300 °C for 30 min, the carbide network is broken due to the impingement of refined primary austenite dendrites and secondary dendrites arms, and the thickness of eutectic structure is reduced.

Recently, many researchers have studied on the variety of alloys and tempt to develop wear resistant materials. Multi-component white cast iron has been reported to exhibit good erosive wear and abrasive wear resistance. Niobium (Nb) is also one of the carbide-forming elements in the same manner as vanadium (V), but not much is known about the effect on the wear resistance by Nb addition into the cast irons. Therefore, multi-component white cast irons with about 2 mass% carbon (C) and about 5 mass% chromium (Cr), molybdenum (Mo), tungsten (W) and V or Nb with 0, 5 mass% cobalt (Co) were prepared as the experimental materials in this study. As results, the amount of wear loss of the multi-component white cast irons containing Nb is less compared to that of the multi-component white cast irons containing V, showing good abrasive wear resistance. Especially, material 5Nb-5Co showed the best abrasive wear resistance. Area ratio of eutectic carbides and secondary carbides in any of the specimens were at the same level. However, eutectic MC carbide of multi-component cast iron containing Nb was larger as compared to that of multi-component cast iron containing V, showing higher resistance to the contact with the abrasive grains. Therefore, it was suggested that by adding Nb as an alternative to V, it's possible to develop a new wear resistant material. [ABSTRACT FROM AUTHOR]

The present work investigated the solidification microstructure of AISI M2 high speed steel manufactured by different casting technologies, namely iron mould casting and continuous casting. The results revealed that the as-cast structure of the steel was composed of the iron matrix and the M2C eutectic carbide networks, which were greatly refined in the ingot made by continuous casting process, compared with that by the iron mould casting process. M2C eutectic carbides presented variation in their morphologies and growth characteristics in the ingots by both casting methods. In the ingot by iron mould casting, they have a plate-like morphology and grow anisotropically. However, in the ingot made by continuous casting, the carbides evolved into the fiber-like shape that exhibited little characteristics of anisotropic growth. It was noticed that the fiber-like M2C was much easier to decompose and spheroidize after heated, as a result, the carbides refined remarkably, compared with the case of plate-like carbides in the iron mould casting ingot.

The characteristics of abrasive wear damage to the carbides and eutectic phases in a Fe–Cr–W–Mo–V–C alloy were systematically investigated using point scratch testing on heat-treated specimens with preserved eutectic structures from a characteristic eutectic carbide formation temperature of 1240 °C. A deep-etching method is applied to study the detailed morphologies of the eutectic carbides by optical microscopy (OM) and field emission scanning electron microscopy (FESEM). The types of the eutectic carbides were identified by X-ray diffraction (XRD) as V-rich eutectic MC and Mo-rich eutectic M 2 C. Single-pass scratch tests were carried out on polished and deep-etched surfaces of the specimens to investigate the micro-scale wear behavior of the eutectic carbides with and without protection by the matrix phase. It was found that, when a surrounding matrix is present, the micro-cracks initiate from the eutectic region during the scratch tests and slip-lines appear in scratched edges of the matrix phase. Without matrix protection as revealed in the deep-etched specimens, the main failure form is carbide dendrites break-off forming wear debris. The detailed differences in response and deformation mode of carbide microstructures with/without matrix support to single-point abrasion behavior are schematically depicted for main eutectic MC and M 2 C carbides under different loading conditions. Potential improvement on wear performance through the composition design is also discussed based on the different fracture and wear behavior of V-rich eutectic MC and Mo-rich eutectic M 2 C identified. [ABSTRACT FROM AUTHOR]

In this study, the effects of vanadium on the morphology and wear behavior of a eutectic Cr-Fe-C hardfacing alloy were discussed. The alloys tested contained different amounts of vanadium, ranging from 0 to 2.39 wt%. A fibrous VC was found when the alloy contained 0.93 wt% vanadium. The addition of vanadium was found to decrease the fraction of eutectic MC and increase the width of the interspaces between the eutectic cells. The DTA results revealed that VC formed just before the eutectic α+MC during solidification. The surface hardness was shown to increase with increasing vanadium content, which also caused the hardness deviation and wear loss to decrease; however, the addition of vanadium was not shown to affect the hardness of eutectic α+MC. The VC could be scratched off during the wear test due to the increase in the width of the interspaces between the eutectic cells; therefore, the alloys that contained 0.93 and 2.39 wt% vanadium exhibited similar wear loss results. [ABSTRACT FROM AUTHOR]

The as-cast microstructure of HK heat resistant steel produced by the investment casting process has been investigated. The pouring temperature and cooling method effects on the austenite grain size and shape, segregation of alloying elements and formation of eutectic carbides have been evaluated herein. Microscopic studies have shown that the microstructure of the steel consists of dendritic austenite with carbides in the grain boundaries and dendritic arm spaces. Decreasing the pouring temperature converts large columnar grains to small equiaxed grains and prevents the large eutectic carbides in the grain and dendritic boundaries. Also, increasing the cooling rate can decrease the formation of grain boundary carbides. [ABSTRACT FROM AUTHOR]

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute (AISI) M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M2C-type carbide decomposed continuously. M6C-type carbide nucleated at the interface of M2C/ γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M6C/M6C and inner of M6C. With the increasing decomposition of the metastable M2C-type carbide, the rod-shaped construction of eutectic carbide began necking, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less significantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M2C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature. [ABSTRACT FROM AUTHOR]

Abstract: The morphology and distribution of the eutectic carbides has an important influence on the properties of high-speed steel (HSS) rolls. Manufacturing methods reported previously have not achieved effective refinement and dispersal of the eutectic carbides. In order to investigate the effect of the solidification rate (SR) on the eutectic carbides and find a way to refine them, the morphology and distribution of the carbides has been studied systematically under various solidification rates, which were controlled by using different moulds with several initial temperatures. The morphology and distribution of the eutectic carbides was assessed by optical microscopy (OM) and scanning electron microscopy (SEM), as well as by X-ray diffraction (XRD) and X-ray energy dispersive spectrum (EDS). It was found that, with the increase of SR, the eutectic carbides became finer and more uniformly dispersed. A high SR can be recommended as a good way to improve the wear resistance and thermal fatigue properties of high-speed steel rolls. [Copyright &y& Elsevier]

Abstract: High-speed steel M2 strip has been experimentally produced by using a twin roll strip castor. The microstructure of the strip and the eutectic carbide in the cast-rolled strip were examined in detail. The spheroidizing procedure of the eutectic carbide in the twin roll cast M2 high-speed steel strip was investigated with the method of annealing, quenching and tempering. The results show that the eutectic carbide was gradually spheroidized with the increase of heating temperature and holding time through coarsening and accumulation of eutectic carbide, dissolving and breaking up of net work eutectic carbide, and precipitating and coarsening of secondary carbide. The kinetics analysis of coarsening and accumulating of eutectic carbide, and coarsening of secondary carbide in the high-speed steel strip were made based on the experimental results, and the growing transient velocity equations of carbide were deduced. The mechanism of cast-rolling enhancing the spheroidizing of eutectic carbide in M2 steel was discussed. [Copyright &y& Elsevier]

Abstract: The effects of rare earth (RE) on the microstructures and properties of the Fe–5% V–5% W–5% Mo–5% Cr–3% Nb–2% Co–2% C (Fe–V–W–Mo) alloy are investigated. Modification of the Fe–V–W–Mo alloy is effectively achieved through the addition of suitable amount of RE, which refines the prior-austenite grains and eutectic structure. The as-cast microstructure shows that the flake carbides in the eutectic structure become short and fine. After hardening treatment, most of the eutectic carbides are spheroidized and uniformly distributed. The hardness and hot hardness of the modified Fe–V–W–Mo alloy are slightly increased, while the impact toughness is increased by 37.81% and reaches 10.17J/cm2. The rolls made of modified Fe–V–W–Mo alloy show excellent service properties in high speed wire mills, including the prevention of cracks and spalling. [Copyright &y& Elsevier]

High Cr white irons with various fractions of primary dendrite have been prepared through the modification of their chemical composition. Increasing C and Cr contents decreased the primary dendrite fraction. Eutectic solidification occurred with the phase fraction ratio of austenite: M7C3 = 2.76:1. The measured primary dendrite fractions were similar to the calculated results. ThermoCalc calculation successfully predicted fractions of M7C3, austenite, and M23C6. Conventional heat treatment at high temperature caused a destabilization of austenite, releasing it's solute elements to form M23C6 carbide. Precipitation of M23C6 during destabilization preferentially occurred within primary (austenite) dendrite, however, the precipitation scarcely occurred within austenite in eutectic phase. Thus, M23C6 precipitation by destabilization was relatively easy in alloys with a high fraction of primary dendrite. [ABSTRACT FROM AUTHOR]

The evolution of eutectic carbide and granular pearlite, including secondary carbide and matrix ferrite, in 1.12C–8.1Cr–2.3Mo–1.08Si–0.32V alloy steel under spheroidization annealing is investigated. This procedure can be divided into two stages: austenitization in the range of 780–900 °C for 0.5–5 h and second annealing in the range of 740–780 °C for 3–7 h. The area fraction, size, number per unit area, and sphericity of carbides are analyzed under different annealing processes. The result indicates that the mean diameter of secondary carbides increases and the number of carbides per area decreases with the increase in temperature in two ranges of 780–900 and 740–780 °C. At >860 °C, the secondary carbide sufficiently spheroidizes, the eutectic carbide dissolves at the tip with a larger radius of curvature, and ferrite grains of the matrix are basically equiaxed. It can be found that the austenitizing stage plays a crucial role in nucleation and coarsening of secondary carbides, dissolution and segmentation of eutectic carbides, and recrystallization of matrix ferrite grains. The microhardness of the sample presents a decreasing tendency from fast to slow with the increase of the average diameter of secondary carbides. [ABSTRACT FROM AUTHOR]

The excellent abrasion resistance of high chromium cast irons (HCCIs) stems from the dispersion of the hard iron-chromium eutectic carbides. The surrounding matrix on the other hand, provides sufficient mechanical support, improving the resistance to cracking deformation and spalling. Prior knowledge of the microstructural characteristics is imperative to appropriately design subsequent heat treatments, and in this regard, employing computational tools is the current trend. In this work, computational and experimental results were correlated with the aim of validating the usage of MatCalc simulations to predict the eutectic carbide phase fraction and the elemental distribution in two HCCI alloys, in the as-cast condition. Microstructural observations were carried out using optical microscopy and SEM. The chemical composition and fraction of each phase was measured by electron probe microanalysis and image analysis, respectively. In all cases, the values predicted by the pseudo-equilibrium diagrams, computed with MatCalc, were in accordance with the experimentally determined values. Consequently, the results suggest that time and resource intensive experimental procedures can be replaced by simulation techniques to determine the phase fraction and especially, the individual phase compositions in the as-cast state. [ABSTRACT FROM AUTHOR]