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.     

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.     

Redistribution of Carbon Atoms in Differentially Quenched Rail on Prolonged Operation

The change in structure, phase composition, and defect substructure in the head of differentially quenched rail after the passage of gross traffic amounting to 691.8 million t is investigated over the central axis, at different distances from the top surface, by means of transmission electron microscopy. The results confirm that prolonged rail operation is accompanied by two simultaneous processes that modify the structure and phase composition of the plate-pearlite colonies: cutting of the cementite plates; and solution of the cementite plates. The first process involves cutting of the carbide particles and removal of their fragments, accompanied simply by change in their linear dimensions and morphology. The second process involves the extraction of carbon atoms from the crystal lattice of cementite by dislocations. That permits phase transformation of the metal in the rail, which is associated with marked relaxation of the mean binding energy of the carbon atoms at dislocations (0.6 eV) and at iron atoms in the cementite lattice (0.4 eV). The stages in the transformation of the cementite plates are as follows: the plates are wrapped in slipping dislocations, with subsequent splitting into slightly disoriented fragments; the slipping dislocations from the ferrite lattice penetrate into the cementite lattice; and the cementite dissolves with the formation of nanoparticles. The cementite nanoparticles are present in the ferrite matrix as a result of their transfer in the course of dislocational slip. On the basis of equations from materials physics and X-ray structural data, the content of carbon atoms at structural elements of the rail steel is assessed. It is found that prolonged rail operation is accompanied by significant redistribution of the carbon atoms in the surface layer. In the initial state, most of the carbon atoms are concentrated in cementite particles. After prolonged rail operation, the carbon atoms and cementite particles are located at defects in the steel’s crystalline structure (dislocations, grain and subgrain boundaries). In the surface layer of the steel, carbon atoms are also observed in the crystal lattice based on α iron.     

Degradation of rail-steel structure and properties of the surface layer

The change in the structure–phase states and defect substructure of the rail surface after prolonged operation (passed tonnage of 500 and 1000 million t) is studied by optical microscopy, by scanning and transmission electron diffraction microscopy, and by measurement of the microhardness and tribological characteristics. It is found that the wear rate increases by a factor of 3.0 and 3.4 after passed tonnage of 500 and 1000 million t, respectively, while the frictional coefficient is reduced by a factor of 1.4 and 1.1, respectively. After 500 million t, the cementite plates break down completely, and rounded cementite particles (10–50 nm) are formed. After 1000 million t, the initial stage of dynamic recrystallization is noted. Possible explanations of the observations are discussed. Two competing processes may occur in rail operation: (1) fragmentation of the cementite particles, with their subsequent entrainment in the ferrite grains or plates (in the pearlite structure); (2) fragmentation and subsequent solution of the cementite particles, with transfer of the carbon particles to dislocations (Cottrell atmospheres) and transportation of carbon atoms by dislocations within the ferrite grains (or plates), culminating in the formation of cementite nanoparticles.     

Fragmentation of the grain structure of quenched rails

Transmission electron microscopy permits layer-by-layer structural analysis (along the central axis and in the direction of the rounded corner) of bulk-quenched and differentially quenched rails at distances of 0, 2, and 10 mm from the working surface. Regardless of the direction and the distance from the working surface, the structure of all the rails consists of plate-pearlite grains and ferrite grains, containing cementite particles of different shape (ferrite–carbide mixture) and grains of structure-free ferrite (ferrite grains that do not contain carbide phase, grain-boundary ferrite). The morphology and defect substructure of the phases are studied; the locations of the stress concentrators are established. Formulas are derived for the fragmentation parameters of the grains in the ferrite–carbide mixture as a function of the heat-treatment conditions and the distance from the working surface.     

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.     

A Study of Structure Evolution in Pearlitic Steel Wire at Increasing Plastic Deformation

In this work was studied the influence of the plastic deformation's intensity under deep drawing on microstructures of a pearlitic steel containing 0.8% C. Varying the levels of deformation causes diverse dislocation movements as well as modified structural states in the individual phases of the pearlitic eutectoid steel. It is shown that in the course of plastic deformation there is a reduction of interlamellar distance in a pearlite and increase in dislocation density. In some parts partial spheroidisation cementite plates is observed. The bands formed in dislocation structure are found out. The analysis of failure mechanisms of steel with pearlite structure after plastic deformation is carried out. During the deformation of pearlite, the increase in stress at the phase boundary, owing to the elastic strain incompatibility between the ferrite and cementite phases, resembles the stress concentration at grain boundaries. Pores form at the interface between the surface of the ferritic matrix and the spheroidised carbide particles. Such micro pores occur by means of plastic deformation of the ferrite's interstices around the stronger cementite owing to the reduction of the ferritic interstices and their subsequent cracking.     

The enhancement of hydrogen attack in steel by prior deformation

An investigation has been made of the cause of enhancement of hydrogen attack (HA) by prior cold working. Bars of 1020 Si-killed steel deformed in three point bending were found to contain microcracks at ferrite/ferrite grain boundaries containing cementite inclusions and at ferrite/pearlite boundaries. Very long cementite plates crack, but shorter cementite inclusions (? ?2 μm) suffer decohesion from the matrix at or near the inclusion ends. The observed anisotropy of microcracking with respect to grain boundary orientation and sign of the deformation stress can be understood in terms of the stress state at the inclusion/matrix interface during deformation. The microcracks are not eliminated by an annealing treatment which is known to cancel the effect of deformation on HA. It is concluded that residual stresses arising from the difference in deformability between cementite particles and matrix are responsible for the influence of deformation on HA. Formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60201     

PD3钢轨钢接触疲劳变形组织的TEM观察

在透射电镜下观察接触疲劳失效后不同状态的ＰＤ３钢轨钢的变形组织。结果表明，接触疲劳变形组织的主要特点是：珠光体片层间距减小，珠光体团被剪切分割成亚团，先析铁素体内形成位错胞结构；铁素体／渗碳体界面上存在高的位错密度；珠光体中渗碳体片发生变形与断裂。同时对珠光体片层间距与变形组织的关系进行了分析讨论。     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Nb对高碳钢珠光体球化的影响

摘要：设计了2种不同Nb含量的高碳珠光体钢（0.025Nb和Free -Nb），采用光学显微镜、扫描显微镜、透射电镜和硬度测试仪对两种试验钢珠光体球化前后的显微组织进行了观察和球化后的硬度进行了测量。结果表明：Nb元素可以细化高碳珠光体钢的片层间距，相同条件下具有更多的铁素体 渗碳体界面，在球化退火的第一阶段提供大量的位错和亚晶界使片状珠光体快速熔断，同时也给第二阶段碳的扩散提供高速扩散通道；细小的片层间距缩短了碳和合金元素的扩散距离，使球化转变速度加快，促进了高碳珠光体的球化。Nb元素的添加获得了细小片层间距以及更多的合金碳化物使试验钢的初始硬度偏高，球化退火前4h硬度值下降幅度较大，球化退火4h后对试验钢硬度的影响不大。     

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.     

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.     

The effect of alloying elements and microstructure on the strength and fracture resistance of pearlitic steel

The processes of ductile and brittle fracture in fully pearlitic steel and their relation to both the scale of the microstructure and the presence of substitutional alloy elements have been investigated at room temperature using smooth tensile and over a range of temperatures using V-notched Charpy impact specimens. The results show that the early stages of cracking, revealed in both types of specimen, are largely the result of shear cracking of the pearlite lamellae. These cracks grow and can reach a size when they impinge upon the prior austenite boundary; afterward the character of fracture can be either microvoid coalescence or cleavage, depending on test conditions and metallurgical variables. Further, the carbide plates of the pearlite lamellae can act as barriers to the movement of dislocations as is the case normally with grain boundaries. For pearlite an optimum spacing of approximately 0.2 μm resulting from a balance between carbide plate thickness and interlamellar spacing was found to enhance toughness, although such changes are much smaller than corresponding changes due to varying alloy elements. Specific alloy elements used herein strengthened the lamellar ferrite in pearlite, inhibiting the movement of dislocations while also usually decreasing the lamellar cementite plate thickness for the same spacing. This dual behavior results in enhanced resistance to the initiation and propagation of microcracks leading to an improvement in strength, ductility, and toughness. The most effective alloy elements for the composition ranges studied in fully pearlitic steels are Si and Ni for strength improvement, and Ni and Mn for toughness.     

V和Ti在高碳钢中的应用

研究了V、Ti在预应力钢绞线及钢丝用高碳钢线材中的应用。高碳钢盘条中加入微量的V、Ti,在降低了珠光体相变温度的同时使珠光体相变与贝氏体相变温度区间发生分离;V的加入可以在细化珠光体片层间距的同时,抑制晶界连续渗碳体的形成。V、Ti在高碳钢中主要以复合碳氮化物的形式在晶界铁素体及珠光体片层间弥散析出,同时有部分V以合金碳化物的形式存在于渗碳体片层中。高温区析出的Ti(C,N)对奥氏体晶粒的长大具有显著的抑制作用,V主要在低温区以碳氮化物的形式起到析出强化的作用,另有部分V原子与Cr类似,与渗碳体结合形成合金碳化物,起到了强化渗碳体的作用。     

Study on the Interaction between Rare Earth and Carbon in High Carbon Steel

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A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels

The precipitation of copper has been detected and studied in three of the main decomposition products of austenite: allotriomorphic grain-boundary cementite, pearlitic cementite, and Widmanstätten cementite plates. The investigation has been carried out on two high-alloy hypereutectoid steels containing copper contents of 1.0 and 2.5 wt pct. The main advantage of these high-alloy steels is that the parent austenite phase remains stable upon cooling to room temperature, thus preserving the parent phase and the parent/product interfaces in the microstructure for subsequent examination. Transmission electron microscopy (TEM) revealed that the copper precipitation occurs in proeutectoid allotriomorphic grain-boundary cementite in association with the transformation interface. The copper particles were dispersed in the form of rows (or sheets) within the allotriomorphs of cementite. Evidence for copper precipitate particles nucleated at structural features imaged at the growth interface was also obtained. Copper precipitation was found to occur in both the ferrite and cementite lamellae of pearlite, and again, examination of partially decomposed structures revealed copper particles nucleated at the austenite/pearlite transformation interface. In addition, copper particles were also observed at the ferrite/cementite interface of pearlite. Copper precipitation observed in Widmanstätten cementite plates revealed a precipitate-free midrib region in the plates and a higher concentration of copper particles toward the broad faces of the plate. Copper particles were also found located at coarse linear interface defects at the broad faces of the plate.     

Atom probe and transmission electron microscopy investigations of heavily drawn pearlitic steel wire

Transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM) observations of pearlitic steel wire show that drawing to a true strain of 4.22 causes fragmentation of cementite lamellae into nanoscale grains. The drawing strain amorphizes some portions of the cementite lamellae in regions where the interlamellar spacing is very small, but most of the cementite lamellae are polycrystalline with nanoscale grains. The carbon concentration in the ferrite is inhomogeneous and varies from 0.2 to 3 at. pct; the carbon concentration in nanocrystalline cementite is less than 18 at. pct, significantly lower than that in stoichiometric Fe₃C. Silicon is segregated to ferrite/cementite boundaries, but, in regions with a small interlamellar spacing, the silicon concentration is uniform across the lamellae. Annealing at 200 °C for 1 hour does not cause noticeable changes in the microstructure. Annealing at 400 °C or above for 1 hour causes spherodization of the cementite lamellae, and the carbon concentrations in ferrite and in cementite return to the predeformation values.     

The microstructural evolution,crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbon

The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-μm long and 0.1-and 0.5 -μm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for 10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates. The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 μm in thickness and can be as long as 0.5 μm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds, and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect to one another and typically 0.5 μm or less in both length and thickness and cementite particles 0.4 μm or less in size. (Occasionally, longer martensite plates were observed; but they never exceeded 1 μm in length.) The microstructures produced here offer the potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing similar microstructures which will also exhibit enhanced high-temperature stability.     

Atom probe and transmission electron microscopy investigations of heavily drawn pearlitic steel wire

Transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM) observations of pearlitic steel wire show that drawing to a true strain of 4.22 causes fragmentation of cementite lamellae into nanoscale grains. The drawing strain amorphizes some portions of the cementile lamellae in regions where the interlamellar spacing is very small, but most of the cementite lamellae are polycrystalline with nanoscale grains. The carbon concentration in the ferrite is inhomogeneous and varies from 0.2 to 3 at. pct; the carbon concentration in nanocrystalline cementite is less than 18 at. pct, significantly lower than that in stoichiometric Fe₃C. Silicon is segregated to ferrite/cementite boundaries, but, in regions with a small interlamellar spacing, the silicon concentration is uniform across the lamellae. Annealing at 200 °C for 1 hour does not cause noticeable changes in the microstructure. Annealing at 400 °C or above for 1 hour causes spherodization of the cementite lamellae, and the carbon concentrations in ferrite and in cementite return to the predeformation values.