Physical nature of rail strengthening in long term operation

Abstract

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 multifactor character and was caused by substructure hardening brought about by the formation of nanofragments, dispersion hardening by carbide phase particles, hardening caused by the formation of the Cottrell and Suzuki atmospheres on dislocations, and internal stress fields being formed by inner- and interphase boundaries.