Degradation of Structure and Properties of Metal of Rails at the Long-Term Operation

Abstract

The regularities of changes in structure-phase states and defect substructure of rails surface layers up to 10 mm along the central axis and fillet following the long-term operation (passed tonnage 500 and 1000 millions of tons brutto) are determined by methods of optical, scanning, transmission electron diffraction microscopy and by measuring microhardness and tribological parameters. It is shown that wear rate along the central axis increased by 3 and 3.4 times at passed tonnage 500 and 1000 millions of tons, respectively, and friction factor decreased by 1.4 and 1.1 times. Three layers are isolated and analyzed: surface, transition and main volume of metal by the fracture behaviour and level of imperfection. The cementite plates destruct completely and cementite particles of round shape and 10–50 nm in size form after 500 millions of tons of passed tonnage. The starting stage of dynamic recrystallization is noticed to proceed after 1000 millions of tons. The possible reasons of the observed regularities are discussed. It is noticed that two competitive processes may proceed at rails operation. The first (1) one is process of cementite particle cutting followed by their carrying to volume of ferrite grains or plates (in pearlite structure). The second (2) one is process of cutting, subsequent dissolution of cementite particles, transition of C atoms on dislocations (Cottrell atmospheres), C atoms transfer by dislocations into volume of grains (or plates) of ferrite followed by the formation of cementite nanosized particles. Steel deformation transformation results in increase in scalar and excess dislocation density, curvature-torsion value of crystal lattice and amplitude of internal stress fields. The elements of structure capable of being stress concentrators are detected. Qualitative analysis of rails hardening mechanisms at different distance from tread surface along the central axis and along fillet after long-term operation is done. It is shown that hardening had a multi-factor character and was caused by substructure hardening and brought about by the formation of nanosized fragments; dispersion hardening of carbide phase particles; hardening caused by formation of Cottrell and Suzuki atmospheres on dislocations; internal stress fields being formed by inner- and inter-phase boundaries.

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