Fe3C Carbides Research Articles

Catalytic graphite mechanism during CVD diamond film on iron and cobalt alloys in CH4-H2 atmospheres

Iron and cobalt alloys were exposed to a CH4-H2 mixture at about 670 °C during the chemical vapor deposition (CVD) of diamond film. Severe carburization is observed on pure Fe and Co-10Al alloy, resulting voluminous graphite preferentially formed before diamond deposition. To clarify the catalytic graphite mechanism, microstructures and compositions around the substrates-graphite interfacial region have been comprehensively analyzed by transmission electron microscopy (TEM). Severe internal carburization on pure Fe leads to a metastable Fe3C carbide layer formed and then decomposed, finally resulting in the precipitation of graphite. Meanwhile, graphitization also occurs on low-Al alloy (Co-10Al), the graphite is precipitated from AlCo3C carbide as substrate carburization occurs. As a comparison, such a graphite interlayer is inhibited when the substrate carburization is effectively hindered on high-Al alloys (Fe-25Al, Co-30Al), attributed to the protection of an alumina-rich layer on the alloy surface.

Effects of Low-Temperature Tempering on Microstructure and Properties of the Laser-Cladded AISI 420 Martensitic Stainless Steel Coating

Post-treatment is crucial to improve the comprehensive performance of laser-cladded martensitic stainless steel coatings. In this work, a low-temperature tempering treatment (210 °C), for the first time, was performed on the laser-cladded AISI 420 martensitic stainless steel coating. The microstructure and properties of the pre- and post-tempering specimens were carefully investigated by XRD, SEM, TEM, a micro-hardness tester, a universal material testing machine and an electrochemical workstation. The results show that the as-cladded AISI 420 stainless steel coating mainly consisted of martensite, austenite, Fe3C and M23C6 carbides. The phase constituent of the coating remained the same, however, the martensite decomposed into finer tempered martensite with the precipitation of numerous nano-sized Fe3C carbides and reverted austenite in the as-tempered specimen. Moreover, a slight reduction was found in the micro-hardness and tensile strength, while a significant increase in elongation was achieved after tempering. The fractography showed a transition from brittle fracture to ductile fracture accordingly. The as-tempered coating exhibited a striking combination of mechanical properties and corrosion resistance. This work can provide a potential strategy to enhance the overall properties of the laser-deposited Fe-based coating for industrial applications.

Correlation between heat treatment process parameters, phase composition, texture, and mechanical properties of 12H18N10T stainless steel processed by selective laser melting

The phase composition, texturing degree, oriented stresses and strains of the crystal lattice, the physical and mechanical properties of austenitic steel 12H18N10T (AISI 321) manufactured by selective laser melting (SLM) before and after heat treatment with various temperatures and protective gas composition have been investigated. SLM stainless steel has better mechanical properties, given that the phase volume content is as follows: γ-Fe ∼ 65%, α-Fe ∼20%, NiTi ∼ 5%; the texturing degree equals to T111∼0.70 and the dominant crystal-lattice orientation is (111). Such a phase composition is formed in the steel after annealing at 620 °C in argon. As the temperature rises during the stabilizing annealing process up to 825 °C, the phase transformation γ→α takes place in SLM steel with a change in the phase concentration γ-Fe and α-Fe in the range from 40 to 50%. In the high-temperature intermetallic phase NiTi, dissolution at 825 °C and a multiple rise of the FeO concentration lead to a change in the crystal lattice parameter of the γ-Fe and α-Fe phases and a decrease in the mechanical properties of SLM steel. This study showed that the improvement of mechanical properties was due to the separation of high-temperature intermetallic phase from austenite and an increase in the concentration of iron carbide Fe3C in SLM steel.

Effect of the Parameters of Semi-Solid Processing on the Elimination of Sharp-Edged Primary Chromium Carbides from Tool Steel

Although conventional tool steels have been heat treated on a routine basis for decades, the search continues for new ways to eliminate their troublesome sharp-edged primary chromium carbides, which impair toughness. One of the available techniques is semi-solid processing, which involves partial melting of the workpiece. The structure after semi-solid processing consisted of a austenite and carbide-austenite network. The network can be broken up and its fragments distributed uniformly by subsequent forming with appropriate parameters. In this experimental study, X210Cr12 tool steel was heated to a semi-solid state, and after cooling to a solid state, worked in a hydraulic press. Suitable soaking temperatures were sought within an interval between 1200 °C and 1280 °C. The workpieces were quenched from the forming temperature in water or oil. In order to improve formability and reduce hardness, tempering was tested as well. Additional experimental regimes included conventional quenching and tempering. Once the appropriate parameters were chosen, the elimination of primary chromium carbides was successful. The resultant microstructures were fine and consisted of M-A constituent with a size of approximately 1 μm, and very fine Fe3C and Cr7C3 carbides. The hardness was in excess of 800 HV10. They were examined using optical and scanning electron microscopes. The carbides were characterized on transparent foils in a transmission electron microscope. Mechanical characteristics were determined in micro-tensile tests.

Microstructure and wear properties of Hardox 450 steel surface modified by Fe-C-Cr-Nb-W powder wire surfacing and electron beam treatment

Structural phase states and tribological properties of the coating surfaced onto Hardox 450 martensite low-carbon steel with powder wire Fe-C-Cr-Nb-W and modified by subsequent electron-beam processing are studied by methods of modern physical material science. It is shown that irradiation of ∼ 5 mm thick surfaced layer with high intensity pulsed electron beams results in the formation of ∼ 20 μm thick surface layer with the master phases of α-Fe and NbC, Fe3C and M6C(Fe3W3C) carbides. It is established that wear resistance of the surfaced layer after electron-beam processing increases more than 70-fold relative to wear resistance of Hardox 450 steel and friction coefficient decreases significantly (∼3-fold).

Experimental constraints on the sound velocities of cementite Fe3C to core pressures

Sound velocities of cementite Fe3C have been measured up to 1.5 Mbar and at 300 K in a diamond anvil cell using the nuclear resonant inelastic X-ray scattering (NRIXS) technique. From the partial phonon density of states (pDOS) and equation of state (EOS) of Fe3C, we derived its elastic parameters including shear modulus, compressional (VP) and shear-wave (VS) velocities to core pressures. A pressure-induced spin-pairing transition in the powdered Fe3C sample was found to occur gradually between 10 and 50 GPa by the X-ray Emission Spectroscopy (XES) measurements. Following the completion of the spin-pairing transition, the VP and VS of low-spin Fe3C increased with pressure at a markedly lower rate than its high-spin counterpart. Our results suggest that the incorporation of carbon in solid iron to form iron carbide phases, Fe3C and Fe7C3, could effectively lower the VS but respectively raise the Poisson's ratio by 0.05 and 0.07 to approach the seismically observed values for the Earth's inner core. The comparison with the preliminary reference Earth model (PREM) implies that an inner core composition containing iron and its carbon-rich alloys can satisfactorily explain the observed seismic properties of the inner core.

Gradient Structure of the Layer Applied to Hardox 450 Steel by Fe–C–Cr–Nb–W Powder Wire after Electron-Beam Treatment

Surfacing with composite coatings strengthened by carbide, boride, and other particles is currently of great interest in materials physics. The performance of the applied layer is primarily determined by the phase composition of the coating. To permit the selection of coatings capable of withstanding extremal operating conditions, including high loads and abrasive wear, their properties and structure must be investigated in detail. In the present work, state-of-the-art techniques in materials physics are used to study the structure, phase composition, and tribological properties of coatings applied to Hardox 450 low-carbon martensitic steel by Fe–C–Cr–Nb–W powder wire and then subjected to electron-beam treatment. The electron-beam parameters are as follows: in the first stage, energy density per pulse ES = 30 J/cm2; pulse length τ = 200 μs; and number of pulses N = 20; in the second stage, ES = 30 J/cm2; τ = 50 μs; and N = 1. These conditions are selected on the basis of calculations of the temperature field formed in the surface layer of the material by a single pulse. It is found that electron-beam treatment of an applied layer of thickness about 5 mm leads to modification of a thin surface layer (about 20 μm), consisting largely of α iron and the carbide NbC; small quantities of the carbides Fe3C and Me6C (Fe3W3C) are also present. This modified surface layer differs from the unmodified coating mainly in terms of the morphology and dimensions of the secondary-phase inclusions. In the modified surface layer, the inclusions are smaller and take the form of thin layers along the grain boundaries. In the unmodified coating, the inclusions are mainly rounded particles, chaotically distributed within the grain. After electron-beam treatment, the wear resistance of the applied layer increases by a factor greater than 70 with respect to Hardox 450 steel, while the frictional coefficient is significantly less (about a third as much).

The Increase in Wear Resistance of Low Carbon Steel by Flux-Cored Wire Surfacing Followed Be Electron Beam Processing

Structure-phase state and tribological properties of the coating deposited on Hardox 450 martensitic low-carbon steel by flux-cored wire Fe-C-Cr-Nb-W and modified by subsequent electron-beam processing were studied. It is shown that the electron beam processing of ~ 5 mm thick deposited layer leads to the formation of ~ 20 µm thick modified surface layer with the main phases of α-Fe and NbC, Fe3C and M6C(Fe3W3C) carbides. Wear resistance of the weld layer is 70 times higher than the one of the original steel.

EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.

Microstructures and Mechanical Properties of 12Cr1MoVG Tube Welded Joints With/Without Post-weld Heat Treatment

Small-caliber, thick-wall 12Cr1MoVG seamless steel tube welded joints were fabricated in this study by gas tungsten arc welding and shielded metal arc welding techniques, then the microstructures, mechanical properties, and residual stress distributions of the joints with or without post-weld heat treatment (PWHT) were compared. The welded joints are mainly composed of bcc ferrite (F), Fe3C, and M7C3 carbides. PWHT did not cause an apparent microstructure evolution in the joints, but promoted granular pearlite decomposition and growth of F grains and carbides, therefore decreasing the yield, tensile strength, and hardness while increasing the impact toughness and elongation of the welded joints. PWHT also released the circumferential residual stress and altered the stress state in the joint from tensile to compressive. Although the mechanical properties and bending performance of the small-caliber, thick-wall 12Cr1MoVG seamless welded joints without PWHT are acceptable, our results show that the joints with PWHT are more reliable.

First‐principles prediction of Si‐doped Fe carbide as one of the possible constituents of Earth's inner core

AbstractWe perform a computational study based on first‐principles calculations to investigate the relative stability and elastic properties of the doped and undoped Fe carbide compounds at 200–364 GPa. We find that upon doping a few weight percent of Si impurities at the carbon sites in Fe7C3 carbide phases, the values of Poisson's ratio and density increase while VP, and VS decrease compared to their undoped counterparts. This leads to marked improvement in the agreement of seismic parameters such as P wave and S wave velocity, Poisson's ratio, and density with the Preliminary Reference Earth Model (PREM) data. The agreement with PREM data is found to be better for the orthorhombic phase of iron carbide (o‐Fe7C3) compared to hexagonal phase (h‐Fe7C3). Our theoretical analysis indicates that Fe carbide containing Si impurities can be a possible constituent of the Earth's inner core. Since the density of undoped Fe7C3 is low compared to that of inner core, as discussed in a recent theoretical study, our proposal of Si‐doped Fe7C3 can provide an alternative solution as an important component of the Earth's inner core.

Structural phase states and properties of the layer surfaced on low-carbon steel with Fe‒C‒Cr‒Nb‒W powder-core wire followed by electron-beam processing

Structural phase states and tribological properties of the coating surfaced onto Hardox 450 martensite low-carbon steel with powder wire Fe‒C‒Cr‒Nb‒W and modified by subsequent electron-beam processing are studied by methods of modern physical material science. It is shown that irradiation of ~5 thick surfaced layer with high intensity pulsed electron beams results in the formation of ~20 μm thick surface layer with the master phases of α-Fe and NbC, Fe3C and M6C(Fe3W3C) carbides. The main difference of the surface layer modified with electron-beam processing from the unmodified volume of the surfacing is the morphology and dimensions of the second phase inclusions. In the modified layer of the surfacing the inclusions have smaller dimensions and are located in the form of interlayers along the grain boundaries. In unmodified surfacing the particles of the faceted shape located chaotically in the grain volume are the basic morphological type of the inclusions. It is noted that the small value of crystal lattice Nb parameter observed in the experiment may be caused by the high level of vacant interstitial sites having the smaller size in comparison with the occupied interstitial sites. It is established that wear resistance of the surfaced layer after electron-beam processing increases more than 70-fold relative to wear resistance of Hardox 450 steel and friction coefficient decreases significantly (~3-fold).

Comparative study of the mechanical and tribological properties of a Hadfield and a Fermanal steel

In this study, Fe-12.50Mn-1.10C-1.70Cr-0.40Mo-0.40Si-0.50(max)P-0.50(max)S (Hadfield alloy) and Fe-28.4Mn-0.86C-1.63Al-0.42Cu-1.80Mo-1.59Si-0.60W (Fermanal alloy) (Wt. %) in the aged condition were compared in terms of its tribological and microstructural properties. The x-ray diffraction (XRD) patterns were refined with the lines of the austenitic γ-phase, Chromium Iron Carbide (Cr2Fe14C), Iron Carbide (Fe2C), and Iron Oxide (Fe0.974O (II)) for the Hadfield alloy, and the lines of the austenitic γ-phase, martensite (M), Mn1.1Al0.9 phase and iron carbide (Fe7C3) for the Fermanal alloy. Mossbauer spectra were fit with two sites for the Hadfield alloy, which displayed as a broad singlet because of the austenitic disordered phase, and had a magnetic hyperfine field distribution, which corresponds to the Cr2Fe14C ferromagnetic carbides found by XRD. There were two paramagnetic sites, a singlet, which corresponds to the austenite disordered phase, and a doublet, which can be attributed to the Fe7C3 carbide. The obtained Rockwell C hardness for aged Hadfield and Fermanal alloys were 43.786 and 50.018 HRc, respectively.

Structural, elastic and thermodynamic properties of iron carbide Fe7C3 phases: An ab initio study

Using ab initio spin-polarized density functional theory calculations, the structural, elastic and thermodynamic properties of the orthorhombic and hexagonal phases of the iron carbide Fe7C3 were investigated. The calculated ground-state lattice parameters are in good agreement with the available corresponding theoretical and experimental data. The single-crystal and polycrystalline aggregate elastic constants, sound velocities, Debye temperature, brittle/ductile character and elastic anisotropy have been estimated. The calculated bulk modulus values of both considered phases are very close and are approximately equal to 262GPa, which classifies the title compounds among the hard materials. The temperature and pressure dependencies of the unit-cell volume, bulk modulus, volume thermal expansion coefficient, isochoric and isobaric heat capacity, and Debye temperature were investigated using the quasi-harmonic Debye model.

Nanocrystalline Fe–C composites obtained by mechanical alloying of iron and carbon nanotubes

Mechanical alloying of the elemental powder mixture of iron and multiwalled carbon nanotubes (10, 20 and 30vol.%) was performed in a high energy planetary ball mill. Nanocomposite materials obtained were examined by X-ray diffraction method as well as by transmission and scanning electron microscopy. It is shown that several phases were forming at the different stages of the milling process, namely, α-(Fe,C) and γ-(Fe,C) supersaturated solid solutions along with the Fe3-xC and the Fe7C3 carbide phases. The hardness and magnetic properties (Curie temperature and coercive force) of the nanocomposite materials obtained were studied. The process of the mechanical alloying of materials studied has been analyzed.

XRD and EBSD Measurements of Directional Solidification Fe-C Eutectic Alloy

Abstract In a vacuum Bridgman-type furnace, under an argon atmosphere, directionally solidified sample of Fe - C alloy was produced. The pulling rate was v = 83 μm/s (300 mm/h) and constant temperature gradient G = 33,5 K/mm. The microstructure of the sample was examined on the longitudinal section using an Optical Microscope and Scanning Electron Microscope. The X-ray diffraction and electron backscatter diffraction technique (EBSD) have been used for the crystallographic analysis of carbide particles in carbide eutectic. The X-ray diffraction was made parallel and perpendicular to the axis of the goniometer. The EBSD shows the existence of iron carbide Fe3C with orthorhombic and hexagonal structure. Rapid solidification may cause a deformation of the lattice plane which is indicated by different values of the lattice parameters. Such deformation could also be the result of directional solidification. Not all of the peaks in X–ray diffractograms were identified. They may come from other iron carbides. These unrecognized peaks may also be a result of the residual impurity of alloy.

Interaction of Fe and Fe3C with hydrogen and nitrogen at 6–20 GPa: a study by in situ X-ray diffraction

A method of in situ X-ray diffraction at Spring-8 (Japan) was used to analyze simultaneously the hydrogen incorporation into Fe and Fe3C, as well as to measure the relative stability of carbides, nitrides, sulfides, and hydrides of iron at pressures of 6–20 GPa and temperatures up to 1600 K. The following stability sequence of individual iron compounds was established in the studied pressure and temperature interval: FeS > FeN > FeC > FeH > Fe. A change in the unit-cell volume as compared to the known equations of state was used to estimate the hydrogen contents in carbide Fe3C and hydride FeHx. Data on hydride correspond to stoichiometry with x ≈ 1. Unlike iron sulfides and silicides, the solubility of hydrogen in Fe3C seemed to be negligibly low—within measurement error. Extrapolating obtained data to pressures of the Earth’s core indicates that carbon and hydrogen are mutually incpompatible in the iron–nickel core, while nitrogen easily substitutes carbon and may be an important component of the inner core in the light of the recent models assuming the predominance of iron carbide in its composition.

ABOUT THE ISSUE OF CARBIDES FE3C AND FE7C3 FORMATION IN HIGH-CHROMIUM CAST IRONS

Purpose. This article analyzes the formation conditions, transformation and systematization of carbide phases formed in the system Fe – C – Cr.Methodology. Conversion of the elements’ content from mass % into atomic % and vice versa was carried out using standard methods. In order to identify the structural components and etching of carbides the Marble etchant was applied. Cast iron 300Х28Н2 in cast state without heat treatment and after isothermal holding at 1050 °С during 4.5 hours with further normalization was studied. Findings. Isothermal state diagrams of the Fe–C–Cr system don’t take into account the existence of Fe7C3 carbide alloyed with chromium. But there is evidence of the existence of chromic carbides containing 24…37.6 % of chromium, which exceeds its maximum solubility in cementite, but is not enough to form Cr7C3.Analysis of chromium and carbon content in carbide phases which are formed in high-chromium cast irons allowed to substantiate the formation of Fe7С3 carbide, stabilized with chromium. Assessment of the carbide phase by chemical composition in mass percent doesn’t allow determining the main carbide-forming element with sufficient accuracy. It is shown that with the increase of chromium concentration in carbides, mass content of carbon increases. Areas of existence of carbides of different types depending on carbon and chromium content in them were determined. Maximum content of chromium in the carbide (Fe, Cr)7C3 is 44.0 %. Allocation of alloyed cementite occurs on the present carbides Ме7C3 or grain boundaries, and with the increase of cooling rate – in the grain volume. This process is thermodynamically inevitable due to the decrease of carbon solubility in ferrite or austenite at temperatures when chromium diffusion is impeded, and only carbon diffusion is possible. At high chromium concentrations carbide Fe7C3 is formed, which transforms into Cr7C3 carbide as chromium diffusion takes place. Originality. The model of laminated structure of carbides, formed out of the liquid phase in the system Fe–C–Cr was built. The identification of the carbide phase based on the elements’ concentration in atomic percent was suggested. The model of the change of carbon content in carbides of different types depending on chromium concentration was built. Practical value. The suggested system of carbide classification and their structural model allows optimizing the cast irons’ composition and heat treatment modes for different exploitation conditions.

Effect of tempering temperature on the microstructure and mechanical properties of a reactor pressure vessel steel

The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investigated after tempering at different temperatures ranging from 580 to 700 °C for 5 h. With increasing tempering temperature, the impact toughness, which is qualified by Charpy V-notch total absorbed energy, initially increases from 142 to 252 J, and then decreases to 47 J, with a maximum value at 650 °C, while the ultimate tensile strength varies in exactly the opposite direction. Comparing the microstructure and fracture surfaces of different specimens, the variations in toughness and strength with the tempering temperature were generally attributed to the softening of the bainitic ferrite, the agminated Fe3C carbides that resulted from decomposition of martensite/austenite (M/A) constituents, the precipitation of Mo2C carbides, and the newly formed M/A constituents at the grain boundaries. Finally, the correlation between the impact toughness and the volume fraction of the M/A constituents was established, and the fracture mechanisms for the different tempering conditions are explained.

Effect of sulfur concentration on diamond crystallization in the Fe–C–S system at 5.3–5.5 GPa and 1300–1370°C

The paper presents results of experiments aimed at diamond synthesis in the Fe–C–S system at 5.3–5.5 GPa and temperatures of 1300–1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (Fe3C and Fe7C3 carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = Fe7C3) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed.