Transformation of Structural-Phase States in the Rail Head after Extremely Long-Term Operation

At the macro-, micro- and nanoscale levels, the optical, scanning and transmission electron diffraction microscopy methods revealed quantitative transformations of the structure at a depth of 0, 2, 5, 10 mm along the central axis and the symmetry axis of the head fillet of long-length differentially hardened rails after extremely long-term operation (passed tonnage 1770 million gross tons). At the macroscale level, numerous shallow parallel contact fatigue cracks are observed on the surface of the working fillet, and small spalls are observed on the surface of the non-working fillet. The side wear of the rail was 2.5 mm, and the vertical wear was 2 mm. The metal microstructure of the rail head complies with the requirements of the standard and technical specifications of Russian Railways. At the microscale level, the transformation of cementite plates was established by cutting them with moving dislocations and dissolving with the escape of carbon to the dislocation lines, low- and high-angle boundaries. There is a decrease in the dispersion of the microstructure with distance from the tread surface. At the nanoscale level, the subgrain structure formed in the surface layers (subgrain size 110–200 μm) contains nanosized cementite particles (25–60 nm) localized at the joints and along the subgrain boundaries. It is suggested that this type of structure is formed as a result of dynamic recrystallization during megaplastic deformation, which occurs during extremely long-term operation of rails. The content of the subgrain structure in the fillet layer is five times higher than the content in the surface layer of the tread surface. It has been established that during operation, the transformation of lamellar perlite along the central axis of the head proceeds more slowly than along the symmetry axis of the fillet.