O. A. Peregudov

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.

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.

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 AND EVOLUTION OF STRUCTURE-PHASE STATES IN RAILS AFTER DRAWN RESOURCE

By optic and transmission electron microscopy, the regularities of the transformation of structural-phase states, the defective substructure of the rail surface layer to the depth of 10 mm under long-term operation (passed gross tonnage 500 and 1000 mln ton) are determined. In the initial state the structure is presented by perlite grains with predominantly lamellar morphology, grains of a ferrite-carbide mixture and structurally free ferrite grains. It is shown that operation of rail steel is accompanied by a complete failure of lamellar pearlite grains in the 15 μm thick surface layer and the formation of a ferrite-carbide mixture with nano-size particles. The strain-induced transformation of steel leads to the increase in the scalar and excessive density of dislocations, the curvature-torsion value of the crystal lattice and the amplitude of internal stress fields.

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.

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.

Physical nature of surface structure degradation in long term operated rails

Here we present research data on the structural-phase state and surface properties of rails after long-term operation with a transported tonnage of gross weight 500 and 1000 mln tons. Using optical, scanning, and transmission electron diffraction microscopy, and measurements of microhardness and tribological parameters, it is shown that the wear rate of the material after transport of 500 and 1000 mln tons increases 3 and 3.4 times, respectively, and the friction coefficient decreases 1.4 and 1.1 times. After transport of 500 mln tons, complete failure of cementite plates occurs resulting in round cementite particles of size 10–50 nm. After transport of 1000 mln tons, dynamic recrystallization develops in the material. Two competitive mechanisms are suggested for such evolution: (1) decomposition of cementite particles with their transfer to the volume of ferrite grains or plates in pearlite and (2) decomposition and dissolution of cementite particles, transition of carbon atoms to dislocations (to Cottre...

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.