Transformation of Carbides in Prolonged Rail Operation

The evolution of the carbide phase in the surface layers of bulk-quenched rails (after the passage of 500 and 1000 million t of traffic) and differentially quenched rails (after the passage of 691.8 million t) to a depth of 10 mm at the central axis of the rail cross section and at the nearby rounded section is studied by transmission electron-diffraction microscopy. The grains of plate pearlite, ferrite–carbide mixture, and structure-free ferrite are analyzed. The carbide phase in the surface layers of the steel changes in two mutually complementary processes during rail operation: (1) cleavage of cementite particles with subsequent entrainment in ferrite grains or plates (in the pearlite structure); (2) cleavage and dissolution of cementite particles, with transfer of carbon atoms to dislocations (in Cottrell atmospheres and in dislocational cores), which transport them to the ferrite grains (or plates), where cementite nanoparticles are formed again. In the previous location of the plates, fragmented dislocational substructure appears. The boundaries of the fragments are found at the positions previously occupied by cementite α-phase boundaries. The solution of cementite is mainly due to the energy of carbon atoms at dislocation cores and subboundaries in comparison with the cementite lattice. The binding energy of the carbon atom and the dislocations is 0.6 eV and the binding energy of the carbon atom and the subboundary is 0.8 eV, as against 0.4 eV for the carbon atom in cementite. Elastoplastic stress fields are formed; their stress concentrators are intra- and interphase boundaries of ferrite and pearlite grains, cementite plates and ferrite of the pearlite colonies, and globular cementite and ferrite particles. Those are also the basic sources of curvature and torsion in the crystal lattice of the rail steel. On approaching the contact surface, the number of stress concentrators increases, and the internal long-range stress fields are of greater amplitude.     

Formation of internal stress fields in rails during long-term operation

The structure and the internal stress fields in R65 rails withdrawn from operation because of side wear after long-term operation are studied and estimated. A high scalar dislocation density (higher by a factor of 1.5–2), the fragmentation of cementite lamellae, and the precipitation of carbide particles are detected in the layers adjacent to the roll surface. The stresses at the boundaries of the particles with the ferrite matrix can exceed the ultimate strength of the steel.     

Long-term operation surface changes in differentially quenched 100-m rails

By optical microscopy and transmission electron diffraction microscopy, the evolution of the structural and phase states in the surface layers over a depth of 10 mm in the head of differentially quenched rail (category DT350) is studied, as the rail is subjected to passed tonnage of 691.8 million t at the experimental loop of AO VNIIZhT. In the initial state, the following structural components are present in the rail head: plate-pearlite grains (relative content 0.7); mixed ferrite–carbide grains (0.25); and grains of structure-free ferrite. After experiencing a passed tonnage of 691.8 million t, this state only remains beyond a depth of 10 mm. At that depth, a large quantity of bend extinction contours is observed. That indicates elastoplastic distortion of the material’s crystal lattice. The stress concentrators in the steel are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. Structural transformations are observed at the macro level: microcracks appear, running at acute angles from the surface to a depth of 140 μm; and a decarburized layer is formed. At the micro level, elastoplastic stress fields are formed, and the cementite plates in the pearlite colonies break down. The stress concentrators in that case are intraphase and interphase boundaries of the ferrite and pearlite grains, cementite and ferrite plates in pearlite colonies, and globular cementite and ferrite particles. In structure-free ferrite grains, cementite nanoparticles are formed. The results are compared with the evolution of the structural and phase states at the surface of a recess in bulk-quenched rail as the rail is subjected to gross loads of 500 million t: the transformation of the structural and phase states in the surface layers is more pronounced. Plate pearlite is characterized by solution of the cementite plates. That leads to the formation of chains of globular carbide particles at the sites of the cementite plates. This may be associated with transfer of the carbon atoms from the cementite lattice to dislocations.     

Structural and phase states in high-quality rail

The structural and phase states and dislocational substructure in high-quality bulk-quenched rail are assessed quantitatively by transmission electron diffraction microscopy. On the basis of the morphological features, the following structural components of the rail steel are identified: plate pearlite (68%); mixed ferrite–carbide grains (28%); and structure-free ferrite grains (4%). Analysis of the flexural extinction contours shows that the stress concentrators in the steel are the boundaries between cementite plates within the pearlite grains; the boundaries between the pearlite grains and the ferrite grains; and the boundaries between globular particles of secondary phase and the ferrite matrix. The particle–matrix boundaries are the most significant stress concentrators and may be regarded as the primary sites of crack formation.     

强磁场下保温时间对Fe 0.76%C钢先共析 铁素体组织形貌的影响

 借助于光学显微镜研究了磁场（12 T）对Fe 0.76%C合金在807 ℃奥氏体化保温不同时间(10 min、30 min、60 min)后以2 ℃/min的冷速冷却后,先共析铁素体显微组织的形貌变化。结果表明：在相同奥氏体化保温时间下,经强磁场热处理样品的先共析铁素体的面积分数和晶粒数量明显高于无磁场热处理样品。这可归结为强磁场降低了先共析铁素体形核所需的驱动力。随着奥氏体化保温时间的延长先共析铁素体晶粒沿着强磁场方向伸长的趋势明显变弱。这主要是由于奥氏体晶粒随着奥氏体化保温时间的延长逐渐增大,导致铁素体晶核之间的距离增大,从而造成奥氏体中的Fe原子向先共析铁素体晶粒扩散的距离增大所致。     

Microstructure of stainless steel after heat treatment: Data from atomic-force microscopy

The micro- and nanostructure of 40Kh13 stainless steel is studied by optical, scanning electron, and atomic-force microscopy. The images of the steel’s structure and phase composition in three different states (after annealing, quenching, and high-temperature tempering) are compared. The optical images of the ferrite–pearlite structure with considerable content of (Cr, Fe)₂₃C₆ globular carbides obtained after annealing are compared with the results of scanning electron and atomic-force microscopy. It is found that the qualitative conclusions regarding the microstructure of the steel obtained by atomic-force and scanning electron microscopy not only agree with the results of optical microscopy but also provide greater detail. Data from the scanning electron microscope indicate that large carbides are located at the boundaries of ferrite grains. Some quantity of carbides may be found within the small ferrite grains. The size of the inclusions may be determined. The structure formed after quenching consists of coarse acicular martensite. Images from the atomic-force microscope show the acicular structure with greater clarity; three-dimensional images may be constructed. The undissolved carbides are also globular. The size of the martensite plates may be determined. The structure of the steel after high-temperature tempering (tempering sorbite) is formed as a result of the decomposition of martensite to ferrite–carbide mixture, with the deposition of regular rounded carbides. As confirmed by spectral analysis, the individual and row carbides (Cr, Fe)₂₃C₆ that appear contain chromium, which rapidly forms carbides. This structure is stronger than martensite. Data from uniaxial tensile tests are presented for all the states; the hardness HB is determined.     

透射-电子背散射衍射技术表征双相钢中马氏体-奥氏体岛的相结构及晶粒取向

为研究高强度低屈强比双相钢中马氏体-奥氏体岛(M-A岛)的精细结构及晶粒取向差关系,利用自制的透射-电子背散射衍射(T-EBSD)样品台和透射电镜样品,在扫描电镜上完成了纳米级高分辨电子背散射衍射(EBSD)表征。通过优化工作电压、倾转角度、工作距离、探头距离、步长等参数,确定了最佳实验条件:工作电压为30kV,倾转角度为30°~38°,步长为15nm,最终获得了高质量的透射菊池花样,实现了微观组织的精细表征,将常规EBSD的分辨率提升了一个数量级。同时,利用透射电镜的高分辨透射电子显微术(HRTEM)进行了验证。EBSD分析结果表明,LYR690钢中M-A岛内部存在超细铁素体、铁素体与奥氏体混合相、残余奥氏体和马氏体等4种结构。残余奥氏体主要以两种形态存在:一种为数百纳米级别的独立晶粒,其与周边铁素体存在超过15°的大角度取向差;另一种为铁素体晶粒中分布的细微奥氏体结构,其与铁素体晶粒的取向差小于10°。     

透射-电子背散射衍射技术表征双相钢中马氏体-奥氏体岛的相结构及晶粒取向

为研究高强度低屈强比双相钢中马氏体-奥氏体岛(M-A岛)的精细结构及晶粒取向差关系,利用自制的透射-电子背散射衍射(T-EBSD)样品台和透射电镜样品,在扫描电镜上完成了纳米级高分辨电子背散射衍射(EBSD)表征。通过优化工作电压、倾转角度、工作距离、探头距离、步长等参数,确定了最佳实验条件:工作电压为30kV,倾转角度为30°~38°,步长为15nm,最终获得了高质量的透射菊池花样,实现了微观组织的精细表征,将常规EBSD的分辨率提升了一个数量级。同时,利用透射电镜的高分辨透射电子显微术(HRTEM)进行了验证。EBSD分析结果表明,LYR690钢中M-A岛内部存在超细铁素体、铁素体与奥氏体混合相、残余奥氏体和马氏体等4种结构。残余奥氏体主要以两种形态存在:一种为数百纳米级别的独立晶粒,其与周边铁素体存在超过15°的大角度取向差;另一种为铁素体晶粒中分布的细微奥氏体结构,其与铁素体晶粒的取向差小于10°。     

Influence of the Speed of High-Carbon Steel Billet in the Patenting Unit on Its Final Structure and Mechanical Properties

Prestressed ferroconcrete structures are widely used at present. As a result, compressive stress is created in the concrete and tensile stress in the reinforcing rope. The stressed reinforcing rope is better able to withstand the external loads that it experiences throughout the life of the construction. Consequently, larger loads may be applied or, with unchanged load, the size of the construction may be decreased, with accompanying savings of concrete and steel. Today, it is important to develop a manufacturing technology for nanostructured reinforcing rope that may be used in prestressed concrete-steel constructions. This technology is based on patenting, in which the steel acquires the structure of a fine ferrite–carbide mixture characterized by high strength and improved deformability. In the present work, the influence of increased billet speed in the patenting unit on the final structure and mechanical properties of steel 80, 70, and 50 is investigated, with a view to increasing the productivity in patenting, without loss of strength or plasticity of the steel, in the production of blanks for nanostructured reinforcing rope that may be used in prestressed concrete-steel constructions. To determine the heat-treatment time and temperature, the Gleeble 3500 system is used to plot diagrams of the isothermal decomposition of undercooled austenite. In qualitative and quantitative analysis of the microstructure, the interlamellar spacing of the ferrite–carbide mixture is determined for different billet speeds in the patenting system. The mechanical properties are studied in tensile tests. It is found that, for all billet speeds, the interlamellar spacing of the ferrite–carbide mixture is practically the same and is optimal for subsequent drawing: 0.1–0.2 μm. Thanks to the fine structure of the ferrite–carbide mixture formed in patenting, the strength of the billet is increased. Hence, in subsequent drawing, the billet may withstand greater compression without fracture. In the production of patented billet for nanostructured reinforcing rope, its speed in the patenting unit may be increased to 5 m/min. Consequently, the productivity may be increased without loss of strength and plasticity of the billet.     

Mechanism of Crack Propagation during Hot-Rolling Process of a Medium-Carbon 40Cr Alloy Steel

In this work, the mechanism of crack propagation during hot-rolling process of a typical medium-carbon 40Cr alloy steel is investigated by cracks characterization, thermodynamic calculation, and confocal laser scanning microscopy (CLSM). The depth of cracks is about 2.5 mm and its length along with rolling direction can even reach 2000–3000 mm. Thermodynamic calculations show that the oxide phases including MnSiO₃ and MnCr₂O₄ can generate when the oxygen content is 0.3–1.0 wt%, suggesting that low oxygen is beneficial to the selective oxidation of Si, Mn, and Cr elements from the medium-carbon low alloy. Furthermore, in situ experiment by CLSM indicates that the submicron Cr–Mn–Si–O particles can refine austenite grains. In addition, the contents of chain proeutectoid ferrite in the steel containing Cr–Mn–Si oxides increase by 6.3% and 12.0% at the lower cooling rates of 5 and 10 °C min⁻¹, respectively, comparing with that of no-oxide particles steel. The submicron Cr–Mn–Si–O particles can refine austenite grains, which induces the precipitation of chain proeutectoid ferrite. Thus, the serious surface cracks propagate along the chain proeutectoid ferrite with the submicron Cr–Mn–Si–O particles during the hot-rolling process.     

Effect of Magnetic Field Intensity on Microstructure and Orientation of Proeutectoid Ferrite in Fe‐0.76%C Alloy

Effect of high magnetic field annealing on microstructure and orientation of proeutectoid ferrite in Fe–0.76%C alloy was investigated. It was found that the high magnetic field increases the number of proeutectoid ferrite grains considerably, and the elongation tendency of proeutectoid ferrite grains along the magnetic field direction is increased by the applied magnetic field, too. These effects increased with the enhancement of magnetic filed intensity. Crystallographic orientation analysis by EBSD showed that no preferential orientations exist in the proeutectoid ferrite when applied a magnetic field with different intensity. Possible reasons of the above phenomena were discussed in the paper.     

Austenite Formation in Plain Low-Carbon Steels

In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite structures in a plain low-carbon steel was investigated using dilation data and microstructural analysis. Different stages of microstructural evolution during heating of the HR and CR samples were investigated. These stages include austenite formation from pearlite colonies, ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrystallization of deformed ferrite and spheroidization of pearlite lamellae before transformation were evident at low heating rates. An increase in heating rate resulted in a delay in spheroidization of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Furthermore, a morphological transition is observed during austenitization in both HR and CR samples with increasing heating rate. In HR samples, a change from blocky austenite grains to a fine network of these grains along ferrite grain boundaries occurs. In the CR samples, austenite formation changes from a random spatial distribution to a banded morphology.     

Study of the ferrite grain coarsening behind the transformation front by electron backscattered diffraction techniques

The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening.     

A STUDY OF THE FACTORS CONTROLLING GRAIN SIZE IN SINTERED HARD-METAL

Abstract

Previous experimental work concerning the grain growth observed during the sintering of tungsten carbide–cobalt alloys is reviewed. Particle-sizing methods suitable for the examination of hard-metal powders are described, and techniques for the evaluation of the carbide grain size in the sintered compacts are discussed.

By using a Model A Coulter Counter to examine the size distribution of the carbide grains (obtained from the milled hard-metal powders by dissolution of the cobalt with hydrochloric acid), and by counting techniques on electron photomicrographs of carbon replicas of the sintered compacts, it has been established that the increase in grain size during sintering is quantitatively related to the carbon content of the material after pre-heating. The results presented indicate that the cobalt content exerts little influence on the average grain size of the sintered structures A cobalt content >10% by weight is shown to exert a strong damping effect on the rate of comminution during milling.

The linear relationships between the specific surface area of the carbide grains in milled powders (obtained using a Perkin–Elmer Sorptometer) and the specific surface of the carbide phase in sintered compacts are given. The influence of sintering temperature and time on average grain size and contiguity in a commercial alloy is shown. Some preliminary work indicates that the morphologies of the initial carbide powders may be important factors with respect to the grain size of sintered hard-metal.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

The effect of carbides on fracture toughness of steels of ferritic matrix

The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries.     

Impregnation of Tungsten-Carbide Substrates with Low-Carbon Steel

The interaction of tungsten carbide and low-carbon steel in contact and contact-free is investigated in systems with contact and contact-free heating. Substrates of pressed tungsten-carbide powder sintered in a vacuum furnace are impregnated with low-carbon steel of specific chemical composition. A high-speed video recording of the process permits measurement of the contact wetting angle at any time during the experiment. The experiment is conducted at the Center for High-Temperature Studies, Foundry Research Institute, Krakow, Poland. The microstructure of the substrates obtained is investigated. The chemical composition of the reaction products is studied by means of a Jeol JSM-6460 LV scanning electron microscope. All the samples are successfully impregnated. The same structure is observed throughout the substrate cross section. It consists of three phases: tungsten-carbide grains; and iron–tungsten–carbon compounds with different iron content (86.72 and 22.86–23.68%). At the edges adjacent to the upper face, large quantities of the iron–tungsten–carbon compound with 22.86–23.68% Fe may be observed. This is explained in that such regions are impregnated last, and the iron dissolves the carbide more than in other regions. At points of direct substrate–iron interaction, grains of tungsten carbide are clearly identified; they are bound together by iron-based melt (with different iron content in different phases). Partial coverage of the sample surface by an iron film is seen on both samples at the horizontal face of the substrate adjacent to the impregnation region. The edges of the substrate’s horizontal face are completely covered with iron film, under which lie tungsten-carbide grains. Despite the use of different methods to study the interaction of tungsten carbide with low-carbon steel (contact and contact-free heating), no great difference in sample structure is observed.     

Kinetics of Sub-Micron Grain Size Refinement in 9310 Steel

Recent efforts have focused on the development of novel manufacturing processes capable of producing microstructures dominated by sub-micron grains. For structural applications, grain refinement has been shown to enhance mechanical properties such as strength, fatigue resistance, and fracture toughness. Through control of the thermo-mechanical processing parameters, dynamic recrystallization mechanisms were used to produce microstructures consisting of sub-micron grains in 9310 steel. Starting with initial bainitic grain sizes of 40 to 50 μm, various levels of grain refinement were observed following hot deformation of 9310 steel samples at temperatures and strain rates ranging from 755 K to 922 K (482 °C and 649 °C) and 1 to 0.001/s, respectively. The resulting deformation microstructures were characterized using scanning electron microscopy and electron backscatter diffraction techniques to quantify the extent of carbide coarsening and grain refinement occurring during deformation. Microstructural models based on the Zener–Holloman parameter were developed and modified to include the effect of the ferrite/carbide interactions within the system. These models were shown to effectively correlate microstructural attributes to the thermal mechanical processing parameters.     

Dissolution of zinc ferrite samples in acids

The dissolution of rotating discs of synthetic zinc ferrite — the principal constituent of the ‘Moore Cake’ residue in zinc extraction plants — was studied in mineral acids, particularly in 1–5 N H₂SO₄ at 70–99°C. This dissolution was found to be directly proportional to the surface area, and the order of the zinc ferrite-sulphuric acid reaction with respect to proton activity, [H⁺], to be 0.6. The apparent energy of activation was established as 15 kcal/mole, and the chemical reaction on the solid surface as the rate-controlling step.What appeared to be ‘non-stoichiometric’ or preferential dissolution of zinc (over iron) from zinc ferrite was observed during the initial stages of reaction. This was attributed to the existence of trace amounts (undetectable by X-ray methods) of unreacted zinc oxide grains in the zinc ferrite matrix. This is, to our knowledge, the first time that electron microprobe analysis has been used to identify and analyse these grains. Prolonged sintering at 1200°C for 48 hours eliminated the ZnO phase.Dissolution of zinc ferrite in acid is stoichiometric. A typical dissolution rate is ～ 10^?8 mol cm^?2 sec^?1, which corresponds to almost complete extraction of zinc from ‘Moore Cake’ particles in 2–5 N H₂SO₄ solution at 95°C in 1–2 hours.     

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100?kJ?cm^-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.