K. V. Morozov

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.

Surface layer structure degradation of rails in prolonged operation

By methods of optical, scanning and transmission electron microscopy and microhardness measurement the transformation regularities of structure-phase states, defect substructure, fracture surface and mechanical properties of rail surface layer up to 10 mm deep in process of long-term operation (passed tonnage of gross weight 1000 mln. tons) were revealed. According to the character of fracture and level of structure imperfection the three layers were detected: surface, transition and boundary ones. It has been shown that the surface layer ~20 μm in thickness has a multiphase, submicro- and nanocrystalline structure and it contains micropores and microcracks. The increased density of bend extinction contours at 2 mm depth from the tread contact surface was noted, and it was shown that the maximum amplitude of stress fields was formed on the interphase boundary the globular cementite particle–matrix. The evaluation of stress fields was done.

Structure-phase states evolution in rails during a long operation

By methods of scanning and transmission electron microscopy the transformation regularities of structure-phase states, defect substructure, fracture surface of rail surface layer up to 10 mm deep in process of long-term operation (passed tonnage of gross weight 1000 mln tons) were revealed. It has been shown that the surface layer ∼20 μm in thickness has a multiphase, submicro- and nanocrystalline structure and it contains micropores and microcracks. The increased density of bend extinction contours at 2 mm depth from the tread contact surface was noted. The analysis of structure morphological constituents and internal stress fields, created by intra- and interphase boundaries after long operation was carried out. It was shown that the maximum amplitude of stress fields was formed on the interphase boundary the globular cementite particle–matrix. The evaluation of stress fields was done.

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 of rail steel: Degradation of structure and properties of surface layer

By methods of optical, scanning and transmission electron diffraction microscopy and microhardness and tribology parameters measurement the changes regularities of structure-phase states, defect substructure of rails surface after the long term operation (passed tonnage of gross weight 500 and 1000 mln. tons) were established. It is shown that the wear rate increases in 3 and 3.4 times after passed tonnage of gross weight 500 and 1000 mln. tons, accordingly, and the friction coefficient decreases in 1.4 and 1.1 times. The cementite plates are destroying absolutely and cementite particles of around form with the sizes 10–50 nm are forming after passed tonnage 500 mln tons. The appearance of dynamical recrystallization initial stages is marked after the passed tonnage 1000 mln tons. The possible mechanisms of established regularities are discussed. It is noted that two competitive processes can take place during rails long term operation: 1. Process of cutting of cementite particles followed by their carrying out into the volume of ferrite grains or plates (in the structure of pearlite). 2. Process of cutting, the subsequent dissolution of cementite particles, transition of carbon atoms to dislocations (into Cottrell atmospheres), transition of carbon atoms by dislocations into volume of ferrite grains or plates followed by repeat formation of nanosize cementite particles.

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.

Physical nature of rail strengthening in long term operation

Regularities of changes in structure-phase states and the defect substructure of rail surface layers up to 10 mm along the fillet in long-term operation (the gross tonnage 1000 mln tons) were determined by methods of transmission electron diffraction microscopy and by measuring microhardness. The possible reasons of the observed regularities were discussed. It was noticed that two competitive processes may proceed in rail operation: (1) cementite segregation followed by their carrying to the volume of ferrite grains or plates (in the pearlite structure) and (2) cutting, subsequent dissolution of cementite particles, transition of carbon atoms at dislocations (Cottrell atmospheres), and carbon atom transfer by dislocations into the volume of ferrite grains (or plates) followed by the formation of cementite nanoparticles. A qualitative analysis of rail hardening mechanisms at different distance from the tread surface along the fillet after long-term operation was done. It was shown that hardening had a mult...

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.

Formation Structural Phase Gradients in Rail Steel During Long-Term Operation

The paper presents results of the structural and phase analysis of the surface layer composition in the type R65 rail steel in its original state and after long-term operation. It is shown that long-term operation of rail steel is accompanied by its structural and phase modification at a depth of not less than 2 mm. The structural elements are detected that can be stress concentrators.

Formation of gradients of structure, phase composition, and dislocation substructure in differentially hardened rails

A layer by layer analysis of rails, differentially hardened in various modes, has been carried out using transmission electron microscopy on various scale levels. It has been shown that the differential hardening of rails is accompanied by the formation of a morphologically different structure, which is formed according to the diffusion mechanism of γ-α transformation and consisting of plate perlite grains, free ferrite grains, and grains of a ferrite-carbide mixture. The gradient character of modifications of structure, phase composition, and dislocation substructure parameters along the cross section of rail head has been established. It has been revealed that the interfaces between globular cementite particles and the matrix are the most dangerous stress concentrators.