Fe3C Carbides Research Articles

Preparation of magnetic composite sorbent based on exfoliated graphite with metallic iron, cobalt and nickel using melamine as a reducing agent

Known preparation methods of the magnetic sorbents based on exfoliated graphite (EG) with metallic phase take several hours and requires the use of a large amount of reducing gas at a high temperature, which is not technologically advanced. The aim of the work is to obtain EG modified with iron, cobalt, and nickel using new simple method. This method includes the thermal treatment in nitrogen atmosphere of the mixture of expandable graphite, Mn+ nitrates (Mn+ = Fe3+, Co2+, Ni2+) and melamine. The thermal decomposition of melamine leads to formation of ammonia, which reduces the products of the Mn+ nitrates decomposition (α-Fe2O3, Co3O4/CoO, NiO) to metallic iron, cobalt and nickel. The use of different amount of melamine as reducing agent leads to the formation of mixtures with different compositions containing iron oxides Fe3O4, FeO, metallic iron α-Fe, Fe-C alloy γ-(Fe,C) and iron carbide Fe3C, which were confirmed by Mossbauer spectroscopy. Metallic Co and Ni on the EG surface is formed in nitrogen atmosphere by reduction with carbon even without melamine. Obtained EG/Fe has the highest saturation magnetization of 54.7 emu/g. EG/Co and EG/Ni have lower saturation magnetization of 40.1 and 12.0 emu/g, which is due to lower saturation magnetization of the individual metals.

Influence of prior austenite grain size on martensite-austenite islands morphology and decomposition characteristics during two-step tempering in Mn-Ni-Mo steel

Martensite-austenite (M/A) islands are typical in the quenched microstructure of Mn-Ni-Mo steel, frequently utilized for fabricating reactor pressure vessels (RPV). These islands can decompose into harmful long rod-shaped Fe3C carbides during tempering above 600°C, quantifying the negative effect of M/A island decomposition on mechanical properties and indicating a potential issue in RPV fabrication. To address this issue, the present study investigates a two-step tempering process on three different prior austenite grain size (PAGS) specimens (5 µm, 47 µm, and 119 µm). The effect of PAGS on M/A island decomposition characteristics during sub-600°C tempering and their impact on precipitated carbide morphology is investigated. A preliminary pre-tempering treatment at 200°C, 350°C, and 500°C for 2 hours was followed by a 3-hour final tempering step at 650°C. The findings revealed unique temperature-dependent decomposition characteristics of M/A islands. For example, at 200°C, partial M/A island decomposition took place, resulting in long rod-shaped carbides during final tempering. Meanwhile, the 350°C pre-tempering specimen revealed aggregated carbide precipitates of varying sizes in former M/A island locations. In contrast, the 500°C pre-tempering specimen showed coarse globular carbides and the absence of carbide precipitate clustering. This result can be attributed to the decrease in surface energy associated with the carbide-matrix interface as size increases. After tempering, all specimens produced three types of precipitates: rod-shaped Fe3C carbides, rhombus-shaped Mn7C3, and globular-shaped Mn23C6 carbides. The formation of long rod-shaped carbides during M/A island decomposition reduces the critical cleavage stress for micro-crack initiation, jeopardizing steel mechanical properties. Aggregated carbides cause the simultaneous formation of numerous micro-voids in close proximity, which eventually merge to form a large crack, resulting in material failure. When compared to other pre-tempered and directly tempered specimens, pre-tempering at 500°C produced the most favorable microstructure in terms of impact properties.

Numerical and experimental study of underwater friction stir welding of 1Cr11Ni2W2MoV heat-resistant stainless steel

Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ's width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.

Modeling of temperature-concentration ranges of phase stability and liquidus surface in the ternary Fe–P–C system and phase composition experimental study

This paper reports thermodynamic properties and phase equilibria in the iron-phosphorus-carbon system for developing a technology for smelting ferrophosphorus as well as for utilizing them in the processes of phosphorus-alloying of metals. According to the results of thermodynamic calculations, we plotted a ternary Fe–P–C phase equilibrium diagram (liquidus surface) in the form of graphical dependences, which indicate the concentration fields of phase coexistence. The results of simulation allowed finding out five nonvariant points, three of which are new: E1 eutectic (teutectic=12170С), U1 peritectic (tperitectic=10950С), and Е3 eutectic (teutectic=3380С). The experimental studies of the phase composition of ferrophosphorus showed that the phosphorus microstructure is represented by Fe3P phosphide, Fe3C carbide, and carbophosphide eutectic. We carried out the thermodynamic calculation of the temperature-concentration ranges of phase stability in the Fe–P–C system that are at equilibrium with the smelts of both the carbon solid solutions and phosphorus ones in - and -iron, FeP, Fe2P, Fe3P phosphides, and graphite. In this study, three previously unknown nonvariant equilibria have been determined: a eutectic equilibrium at 1216.580C; a peritectic one with a transformation point at 1095.190C; and a eutectic one at a temperature of 337.510C. In experiments, we obtained an equilibrium concentration of carbon which decreases in the Fe–P–C system when the content of phosphorus increases. The research findings largely reveal special features of preparation of ferrophosphorus by carbon reduction of phosphorites.

Phase composition, structural and magnetic properties of core-shell nanoparticles based on iron carbide Fe3C synthesized by the sol-gel method

A new one-stage anhydrous sol-gel method for the synthesis of core-shell nanoparticles based on Fe3C and carbon is proposed. Powders of iron (III) nitrate Fe(NO3)3·9 H2O and polyacrylonitrile (PAN) were used as initial reagents, dimethylacetamide (DMAc) was used as a solvent. The synthesis was carried out in an inert argon atmosphere at various temperatures (Tsyn) up to 1000 °C. The morphology, structure, elemental and phase compositions, magnetic and electronic properties of the obtained core-shell nanocomposites were studied by various methods, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDXS), Raman and Mössbauer spectroscopy, and magnetometry. It was found that at temperatures Tsyn ≈ 400–500 °C, iron oxide γ-Fe2O3 predominates in the synthesis product. At Tsyn ≈ 600–700 °C, iron carbide Fe3C is formed, whose content increases and reaches a maximum value (about 75%) with increasing Tsyn to 800–1000 °C. At Tsyn ≈ 1000 °C, nanocomposites of the core@shell type – Fe3C@C and Fe2O3 @C, as well as those with a double shell Fe3C@Fe2O3 @C – are formed. In the Fe3C@Fe2O3 @C composites, Fe3C is a core, Fe2O3 and carbon C are shells. As Tsyn increases from 500 °C to 1000 °C, the amount of iron oxide γ-Fe2O3 decreases, and at Tsyn ≈ 1000 °C, γ-Fe2O3 oxide completely transforms into α-Fe2O3. In the sample synthesized at Tsyn ≈ 1000 °C, particles with a crystallite size of 29.6 nm exhibit magnetic properties with the highest saturation magnetization value of 68 emu/g. The temperature dependence of the magnetization M(T) of this sample has a specific character – there are three sharp drops at temperatures close to the N é el/Curie point for bulk Fe3C, Fe3O4, and α-Fe. Such peculiar behavior of magnetization seems to be important, since it can find application in technology.

Effect of accelerated cooling after cross-helical rolling on formation of structure and low-temperature fracture toughness of low-carbon steel

The effect of accelerated cooling after cross-helical rolling of X70 low-carbon steel on the formation of structures and mechanical properties under static tension and impact bending was investigated. The use of interrupted accelerated cooling of steel after cross-helical rolling with exposure at 530 °C (mode I) and continuous accelerated cooling (mode II) leads to the formation of different types and ratios of structures in steel. After rolling according to mode I, the structure is characterized by the presence of ferrite, troostite, granular bainite, and fine Fe3C carbides. After rolling according to mode II, the structure is characterized by the formation of lath bainite and large sections of the martensitic-austenitic (MA) component up to 1 – 2 µm in size. It is shown that a decrease in the fineness of ferrite grains in steel after cross-helical rolling in modes I and II from 12 to 4.6 – 4.3 μm, the formation of a bainitic phase, and hardening of the matrix with carbides led to an increase in the yield strength of steel up to 440 and 490 MPa and tensile strength up to 760 and 880 MPa. Carrying out helical rolling according to mode I makes it possible to significantly increase the low-temperature fracture toughness of steel (KCV–70 °С = 160 J/cm2) compared to the hot-rolled state (KCV–70 °С = 11 J/cm2) and reduce the cold brittleness of steel to the temperatures below –50 °C. The use of continuous accelerated cooling (mode II) does not allow increasing the cold resistance of steel due to the formation of the lath bainite structure and large areas of the MA component.

SOME FEATURES OF THE PROCESS OF INTERACTION OF IRON WITH METHANE AT A TEMPERATURE OF 900°C AND A PRESSURE OF 100 MPA

Using the original high-gas pressure unit (IHPV), experiments were conducted for the first time on the interaction of iron with methane at a temperature of 900°C and a pressure of 100 MPa. Complex methods (microprobe, Raman spectroscopy, chromatography, mass balance calculations) are used for a thorough analysis of fluid compositions and metallic phases formed in experiments. For the first time, experimental and theoretical quantitative data on the composition of the fluid and the composition of the fluid components dissolved in the metal were obtained. Unlike the previously studied Fe3C–H2 system, in experiments, when Fe reacts with methane, there is an active interaction of carbon formed due to the pyrolysis of methane with iron up to the synthesis of Fe3C carbide. The experiments have shown that increasing pressure inhibits significantly hydrogen yield during methane conversion on metallic iron. Carbon saturation of iron with the formation of Fe3C is not complete within the entire volume of the metal during 24 h runs at 900°С. Employing molybdenum containers facilitates CH4 decomposition.

Some Features of the Process of Interaction of Iron with Methane at a Temperature of 900°C and a Pressure of 100 MPa

Using the original high-gas pressure unit (IHPV), experiments were conducted for the first time on the interaction of iron with methane at a temperature of 900°C and a pressure of 100 MPa. Complex methods (microprobe, Raman spectroscopy, chromatography, mass balance calculations) are used for a thorough analysis of fluid compositions and metallic phases formed in experiments. For the first time, experimental and theoretical quantitative data on the composition of the fluid and the composition of the fluid components dissolved in the metal were obtained. Unlike the previously studied Fe3C–H2 system, in experiments, when Fe reacts with methane, there is an active interaction of carbon formed due to the pyrolysis of methane with iron up to the synthesis of Fe3C carbide. The experiments have shown that increasing pressure inhibits significantly hydrogen yield during methane conversion on metallic iron. Carbon saturation of iron with the formation of Fe3C is not complete within the entire volume of the metal during 24 h runs at 900°С. Employing molybdenum containers facilitates CH4 decomposition.

Evolution of structural-phase state and properties of hypereutectoid steel rails at long-term operation

The methods of modern physical materials science were used to analyze the evolution of microhardness, tribological properties, dislocation substructure and phase composition of the rails with increased wear resistance and contact endurance of DT 400 IR category after missed tonnage of 187 million gross tons on the experimental ring of Russian Railways. It is shown that extremely long-term operation of the rails is accompanied by a decrease (3.1 times) in wear parameter of the rolling surface and an increase (1.4 times) in microhardness, scalar dislocation density (1.5 times) and Fe3C carbide content (1.24 times). Operation of the rails led to a decrease in the crystal lattice parameter, which correlates with an increase in the content of iron carbide. We made the assumptions about physical causes of the change in parameters.

Modeling the Composition of the Earth’s Liquid Outer Core

The paper suggests expressions, coherent with experimental data, for the calculation of the sound velocity and the temperature of a condensed medium under pressures in the Earth’s liquid outer core through parameters of the shock Hugoniot of the medium. Additives to the Earth’s liquid outer iron-nickel core were fitted so that the calculated pressure, density and sound wave velocity corresponded to geophysical values and the temperature did not exceed the admissible limits. It was established that the iron carbide Fe3C containing (4.0 ± 0.5) wt.% carbon with additives of FeS2 and SiC with a total content of Si and S up to 1.5 wt.% meets the required criteria as the main light additive in the upper layers of the Earth’s core. Less probable is the additive of carbon in the diamond phase up to 0.5 wt.%. In the Earth’s outer core, at the boundary with the solid inner core, among the possible light additives are the iron carbide Fe3C with (3.5 ± 0.4) wt.% carbon or diamond phase carbon of (4.0 ± 0.5) wt.%. Additives at the bottom of the Earth’s liquid outer core, containing significant amounts of Si, S, are hardly probable. The content of H in the Earth’s liquid outer core can be significant at the minimum possible core temperatures. The content of O is insignificant.

Analysis of wear of a nitrided AISI H13 hot work tool steel in an aluminium hot extrusion process

The present work present experimental results of hot wear testing of AISI H13 tool steel with the aim of replicating the wear mechanisms occurring during hot extrusion process. The hot wear tests were carried out at a temperature of 550 °C and a sliding speed of 0.5 m/s for sliding distances of 1800, 3600 and 5400 m. A pin-on-disk test fixture was designed and fabricated to investigate the sliding between nitrided AISI H13 steel pins and 6063 aluminium alloy disks in a "flat surface-flat surface" conformal contact mode. The different characterisation techniques used (SEM/EDS, XRD) helped to better explain the complex wear mechanisms leading to surface degradation in the tribological couple. Graphitisation phenomenon caused by the dissolution of metastable iron carbide Fe3C was observed to occur after a sliding distance of 3600 m. In addition, coarsening, coalescence and tearing of carbides in the ultimate lower valleys of the degraded surface were observed to occur after 5400 m sliding distance. Nano-indentation technique was used to evaluate the cross-sectional hardness evolution of the nitrided layer, and results showed that the maximum value of hardness of 12.01 ± 0.63 GPa was obtained at the nearest point of the surface. Volume and surface parameters were used to characterise the surfaces before and after wear damage as function of sliding distance. The progressive variation and evolution of the peak material volume parameter (Vmp), core material volume (Vmc), core void volume (Vvc) and dale void volume (Vvv) parameters reached higher values after wear testing at 3600 m sliding distance indicating a synergy between different and complex wear mechanisms.

Distribution of Iron Nanoparticles in Arrays of Vertically Aligned Carbon Nanotubes Grown by Chemical Vapor Deposition.

Arrays of aligned carbon nanotubes (CNTs) are anisotropic nanomaterials possessing a high length-to-diameter aspect ratio, channels passing through the array, and mechanical strength along with flexibility. The arrays are produced in one step using aerosol-assisted catalytic chemical vapor deposition (CCVD), where a mixture of carbon and metal sources is fed into the hot zone of the reactor. Metal nanoparticles catalyze the growth of CNTs and, during synthesis, are partially captured into the internal cavity of CNTs. In this work, we considered various stages of multi-walled CNT (MWCNT) growth on silicon substrates from a ferrocene–toluene mixture and estimated the amount of iron in the array. The study showed that although the mixture of precursors supplies evenly to the reactor, the iron content in the upper part of the array is lower and increases toward the substrate. The size of carbon-encapsulated iron-based nanoparticles is 20–30 nm, and, according to X-ray diffraction data, most of them are iron carbide Fe3C. The reasons for the gradient distribution of iron nanoparticles in MWCNT arrays were considered, and the possibilities of controlling their distribution were evaluated.

Phase Transformations of 5Cr-0.5Mo-0.1C Steel after Heat Treatment and Isothermal Exposure

This study consists of the experimental and numerical analysis of the phase transformations of 5Cr-0.5Mo.0.1C steel after heat treatment. The microstructure of the as-received steel comprised ferrite and bainite, which is in agreement with the microconstituents predicted by the Calphad-calculated TTT diagram. Calphad-based precipitation calculations show that the cooling stage of normalizing treatment did not cause carbide formation. In contrast, tempering at 700 °C for 15 min promotes the intergranular precipitation of Fe3C, M7C3 and M23C6 carbides, which is consistent with experimental results. Aging at 600 °C for short periods caused the precipitation of both M7C3 and M23C6 carbides; however, M23C6 is the dominant phase after prolonged aging. This is in agreement with experimental results. A rapid decrease in the steel hardness was observed after short aging, which is attributable to bainite transformation. Further reduction in hardness is associated with the diffusion-controlled coarsening of M23C6 carbide.

EUTECTIC CRYSTALLIZATION OF Fe-C ALLOYS WITH HYPERCEMENTITE CARBIDE

Рurpose. To investigate the peculiarities of the structure formation in white cast irons with the participation of Fe3C and Fe7C3 carbides.
 Methods of research. By the comparing detailed both microscopic and thermal (DSC) kinetics of the melting and crystallization process to determine the regularities of the eutectic transformation of the white cast iron under significant undercooling of the liquid phase.
 Microscopic analysis of samples using a light microscope Jenaphot 2000 (Carl Zeiss, Germany). Thermal analyses using differential scanning calorimeter (DSC) STA 449C “Jupiter” (Netzsch Geratebau GmbH, Germany).
 Results. It is shown that eutectic crystallization of white cast irons with great undercooling of the melt is a complex process that consists of crystallization and simultaneous dissolution of crystalline phases: γ austenite and two metastable carbides Fe3C and Fe7C3. In general, crystallization proceeds according to scheme L → L+ Fe7C3 +γ → L++ Fe7C3 + γ + Fe3C→ Fe7C3 +γ+ Fe3C. As a result, two eutectics are formed, one Fe7C3 +γ (plate-like) with austenitic, second ledeburite γ+ Fe3C with cementite matrix.
 Scientific novelty. The microscopic kinetics of nucleating and growth of plate-like Fe7C3 +γ eutectics was investigated and relative stability of Fe7C3 carbide under normal conditions was found. A scheme of metastable phase equilibria with the participation of Fe7C3 carbide in system Fe-Fe3C-Fe7C3 is proposed.
 Practical value. A simple liquid phase processing technology is proposed in order to generate significant undercooling of the melt that causes formation a natural composite material with both a matrix of transformed austenite and carbide reinforcing phases.

In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel

During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters.

Corrosion resistance of niobium-added slag-free self-shielded flux-cored welding overlay in neutral solution

The niobium-added iron-based slag-free self-shielded flux-cored wire was developed and the effect of niobium addition on the corrosion resistance in neutral pH solution was investigated. The carbides, including NbC carbides, primary (Cr, Fe)7C3, and eutectic (Cr, Fe)3C carbides, remain intact after neutral corrosion, but the grain interface is corroded preferably for all the niobium-added welding overlays. The eutectic microstructure of 7.2 wt-% niobium-added welding overlay, which has more grain interface, leads to severe corrosion. When niobium addition reaches 14.4 wt-%, the welding overlay possesses the best corrosion resistance since more NbC carbides are formed, prophase austenite dendrite is precipitated, decreasing the grain interface, and chromium content is increased in the matrix of the hypoeutectic microstructure.

Influence of heat treatment on the structure and properties of pseudo-alloy steel – copper alloy obtained by infiltration

The paper presents the results of studies of the effect of heat treatment regimes on changes in the structure and properties of steel-copper alloy pseudo-alloys obtained by infiltration. It is shown that, depending on the composition and initial density of the steel skeleton, the strength of the material increases by 1.3–1.8 times, the hardening effect is realized when the carbon content in the steel skeleton is 0.3–1.5 % and is achieved due to changes in the structure and phase composition of the steel base and copper phase. It has been established that during heating for quenching and during tempering, redistribution of carbon occurs in the iron phase, which is more pronounced in the frame of the pseudo-alloy made of medium-carbon steel. The formation of a “crust” structure in the grains of the skeleton is noted, while in the skeleton made of medium-carbon steel this occurs at a tempering temperature of 200 °C, in low-carbon steel – at a temperature of 500–650 °C. In a high-carbon steel skeleton, carbon stratification in the grain body is less pronounced. An increase in the strength of pseudo-alloys at tempering temperatures of 500–650 °C is associated with the formation of the α′-phase, the precipitation of the Fe3C carbide phase and the metastable Fe2C phase in the iron phase, as well as the precipitation of dispersed phases Fe4Cu3, Fe4Cu3, η-Cu6Sn5 and δ-Cu3Sn8 in the copper phase. Due to the precipitation of phases, the microhardness of the infiltrate in the form of copper in pseudo-alloys after tempering at 550 °C increased from 820–880 to 950–980 MPa, in the form of tin bronze – from 1450 to 1750 MPa. The use of heat treatment leads to an increase not only in the strength, but also in the tribotechnical properties of the pseudo-alloy: the friction coefficient of the pseudo-alloy with a frame of 80 % density made of FeC0.8 steel decreases to 0.008–0.009, the seizure pressure doubles and the wear resistance increases by more than 2.5 times.

Defining the features of structural and phase transformations in the recycling of anthropogenic metallurgical waste containing refractory elements

This paper reports a study into the features of the phase composition and microstructure of a master alloy obtained by using the reduction melting of oxide man-made waste. That was necessary to define those technological indicators that provide for an increase in the degree of extraction of alloying elements during the recycling of anthropogenic raw materials and the subsequent use of the alloying material. It has been determined that the phase composition of the alloy at a Si:C ratio in the charge of 0.11 mainly consisted of a solid solution of elements in α-Fe, as well as carbides Fe3C and Fe3W3C. At the Si:C ratios in the charge of 0.28 and 0.52, along with a solid solution of the elements in α-Fe, Fe8Si2C, Fe5Si3, and FeSiC, FeSi2 manifested themselves, respectively. The microstructure of the alloy demonstrated a clear manifestation of several phases with different content of alloying elements. Changing a Si:C ratio in the charge from 0.11 to 0.28 and 0.52 led to an increase in the residual silicon content (wt %) in the studied areas, from 0.00–0.25 to 0.12–1.79 and 0.20–2.11, respectively. At the same time, the carbon content (wt %) in the examined areas varied from 0.25–2.12 to 0.24–2.52 and 0.45–2.68, respectively. The content of alloying elements in the investigated areas varied within (wt %): W – 0.00–43.06, Mo – 0.00–32.72, V – 0.19–20.72, Cr – 0.69–33.94, Co – 0.00–3.96. Analysis of the study’s results reveals that the most acceptable ratio of Si:C in the charge is 0.52. In this case, there is a certain content of residual silicon along with carbon in the form of carbosilicide and silicide compounds. Such indicators of the alloy provide sufficient reducing capacity of the alloy when used. The properties of the alloy make it possible, when smelting steels, to replace part of those standard ferroalloys that do not have strict carbon restrictions.

Characterization of vein-like structures formed in nitrided layers during plasma nitriding of 8Cr4Mo4V steel

In this study, nitrided 8Cr4Mo4V was characterized, using optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), atom probe tomography (APT), electron probe microanalysis, and Thermo-Calc software, to study the vein-like structures formed during plasma nitriding. The structures existed in a certain zone in the nitrided layers, namely the posterior end of the N-content plateau and the anterior end of the N-content rapid-decline zones. The structures disappeared in the topmost surface layer with increasing nitriding time. For the first time, APT was used to observe these structures. Together, the APT and TEM images revealed that the main components of the structures were Fe3C and alloy nitrides. The structures and matrix were related through the (220) Fe3C//(110)α-Fe, [110] Fe3C//[001]α-Fe orientations. The appearance and disappearance of the structures were closely related to the C and N contents. In the alloy carbides, N replaced C to form alloy carbonitrides and released C and Fe to form Fe3C which appeared as the vein-like structures. In the topmost surface of the nitrided layers, the C content decreased because of decarburization. The Fe3C and alloy carbides were transformed into ferrite and alloy nitrides, and the structures disappeared.

Combustion Characteristics and Mechanisms of Two Gun Barrel Steels by Promoted Ignition Combustion

ABSTRACT The combustion characteristics of 25Cr3Mo2NiWVNb (MPS700V) and 30SiMn2MoV gun barrel steels are investigated in oxygen-enriched atmosphere. For MPS700V, threshold pressure of combustion increases from 0.26 to 0.46 MPa with diameter increases from 2.4 to 4.0 mm, where are higher than that of 30SiMn2MoV in the same size. Three different zones including oxide zone, melting zone, and matrix zone are observed in both 30SiMn2MoV and MPS700V steels. Elements Mo, C in 30SiMn2MoV and Mo, Ni, C in MPS700V are enriched in the melting zone. When pressure is lower than threshold pressure, samples do not entirely burn but numerous ablation pits form on the surface. The average depth and area of ablation pits on MPS700V are less than that on 30SiMn2MoV under the same pressure. Dendritic microstructure is observed around ablation pits in both MPS700V and 30SiMn2MoV. For MPS700V, dendritic microstructure is composed by Mo2C and γ-Fe phases, where Mo, C are enriched in the dendrite and Ni is enriched in interdendrite. For 30SiMn2MoV, dendritic microstructure is composed by Mo2C and Fe3C carbides and ferrite, where Mo, C are enriched in the dendrite. Based on above, the difference of combustion mechanism for 30SiMn2MoV and MPS700V is discussed.