A. B. Yur’ev

Evolution of the structure and phase states of rails in prolonged operation

The transformation of the structural and phase states and defect substructure of the surface layer (depth up to 10 mm) in rails during prolonged operation (with a total load amounting to 1000 million t) is analyzed on the basis of metal physics. The microhardness is plotted, and decrease in strength of the rail’s contact surface after prolonged operation is noted. In rail operation, a multilayer structure is formed. The surface layer (about 20 μm) has a multiphase submicrocrystalline and nanocrystalline structure; it contains micropores and microcracks. The structure at a distance of 2 mm from the contact surface is morphologically similar to the steel structure before operation: it consists primarily of pearlite grains (mainly plates), mixed ferrite-carbide grains, and structure-free ferrite grains. The density of the flexural extinction contours increases at a distance of 2 mm from the contact surface. The amplitude of the stress field is greatest at the phase boundary between a globular particle and the matrix.

Using Nitrovan alloy in the production of low-temperature rail steel

A production technology for low-temperature rail steel using Nitrovan alloy instead of nitrided ferrovanadium has been developed.

Comparative Analysis of the Structure and Phase States and Defect Substructure of Bulk and Differentially Quenched Rails

Optical and transmission electron microscopy (TEM) methods are used for layer by layer comparative analysis of low-temperature reliability rails with increased wear resistance and contact-fatigue strength of the highest quality after bulk quenching and differential hardening by different regimes. Quantitative relationships are established for changes in structural parameters, phase composition, and dislocation substructure over the central axis and fillet at different distances from the running surface. The degree of structure and phase composition inhomogeneity and defective substructure is revealed. It is shown that with respect to structural component content and interlamellar distance the structure after bulk hardening compared with differential hardening is more uniform in a layer 2 mm thick and less uniform at a distance of 10 mm from the running surface. With respect to stress concentration density, the rail structure after bulk hardening (compared with differential hardening) is less uniform in a layer 2 mm thick and more uniform in a layer at a distance of 10 mm from the running surface.

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.

Structural and phase states of bulk-quenched rail and differentially quenched rail

For high-quality rail bulk-quenched in oil and DT350 rail differentially quenched in different conditions, the low-temperature reliability, wear resistance, and contact-fatigue strength are subjected to layer-by-layer analysis along the central axis and at the shoulder, by up-to-date physical methods. On the basis of the results the structure, phase state, defect substructure, and internal stress fields are compared.

Structure, phase composition, and defect substructure of differentially quenched rail

Differential quenching of rail by compressed air is a promising hardening method. Transmission electron microscopy is used for layer-by-layer analysis of differentially quenched rail. Quantitative parameters of the structure, phase composition, and dislocational substructure are determined and compared for different quenching conditions. Differential quenching of rail by compressed air in different conditions is accompanied by diffusional γ → α transformation. Three morphologically distinct components are formed: grains of plate pearlite, structure-free ferrite, and ferrite-carbide mixture. Gradient behavior is noted in the resulting structure: the state of the surface layer in the rail steel depends not only on the quenching conditions but also on the direction of observation and the depth of the layer being analyzed. Dislocational substructure is obtained; not only dislocational chaos, but also reticular, cellular, and fragmented dislocational substructure.

Rail Strengthening Nature in the Course of Long-Term Operation

Using the methods of transmission electron diffraction microscopy and measurement of microhardness, the regularities of the change in the structural-phase states and defective substructure of the surface layers of rails up to 10 mm by fillet after long-term operation (the passed tonnage of 1000 million tons gross) are established. The possible causes of the observed regularities are discussed. A quantitative analysis of the mechanisms of strengthening of rails at different distance from the rolling surface by fillet after long-term operation is carried out. It is shown that strengthening is multifactorial in nature and is due to substructural strengthening caused by the formation of nanoscale fragments; dispersion strengthening by carbide phase particles; strengthening caused by the formation of Cottrell and Suzuki atmospheres on dislocations; internal stress fields, formed inside; and interphase boundaries.

Rail strengthening in prolonged operation

In rail operation (with traffic corresponding to passed tonnage of gross loads of 500 and 1000 million t), the surface layer of the steel is significantly strengthened. Electron-microscope data permit quantitative analysis of the contribution of different mechanisms to rail strengthening in prolonged operation, at different distances from the contact surface. The strengthening is multifactorial: it involves substructural strengthening associated with nanofragment formation; dispersional strengthening by carbide particles; the formation of atmospheres at dislocations; and polar stress due to interphase and intraphase boundaries. The significant increase in the surface strength of rail steel after prolonged operation (passed tonnage of gross loads of 1000 million t) is due to the presence of long-range internal stress fields and to the fragmentation of material with the formation of nanostructure.

Geometric conditions in bar rolling

Analysis of the scope for more efficient bar rolling is considered; a method of rapid analysis is outlined. By this means, the geometric parameters of the rolling pathways may be represented by alternative groups of repeatedly transformed dimensionless factors, and the results of repeated transformation may be statistically analyzed, to obtain an assessment of competitiveness. The multivariant representation of the geometric conditions yields significantly more information, including unexpected technological reserves of the process. Such analysis provides a set of recommendations regarding the local correction of the deformation region in one or more passes of the steel through the mill. As an example, this approach is employed in assessing the potential for increasing the product yield in rolling structural-steel bar for cold extrusion and upsetting on small-bar and wire mills at OAO EVRAZ ZSMK. The proposed method may be used to improve the production of a wide range of steel bar.

Influence of melt oxidation on the quality of rail steel

The production of vacuum-treated rail steel with discharge of metal containing at least 0.10% C from the electrofurnace and the addition of carbon-bearing material in the ladle at discharge is described. This technology significantly reduces the oxygen content in the steel and the presence of rows of brittle-fracturing oxides and hence permits the production of higher-quality steel.