Hardening of surface layers of long rail head during long-term operation

Starting in 2018, JSC EVRAZ United West Siberian Metallurgical Plant (EVRAZ ZSMK) produced rails of the DT400IK category with increased wear resistance and cyclic crack resistance for heavy traffic and difficult sections of track with steep curves with a radius of less than 650 m. The method of transmission diffraction electron microscopy was used to study the structural and phase states and defect substructure at different distances (0, 2, 10 mm) from the “wheel – rail” contact surface along the central axis of symmetry of the rail head (tread surface) and along the radius of rounding of the rail head (fillet) of differentially hardened long rails of the DT400IK category made of hypereutectoid steel after long-term operation on the experimental ring of Russian Railways (passed tonnage of 187 million tons). Based on the obtained structure parameters, the quantitative estimates were made of the dislocation substructure and main strengthening mechanisms (strengthening of the pearlite component, incoherent cementite particles, grain boundaries and subboundaries, dislocation substructure and internal stress fields) in various morphological components and in the material as a whole, forming the additive yield strength in the studied steel. A comparison of the quantitative parameters of the fine structure and contributions to strengthening on the tread surface and fillet was carried out. It was established that near the “wheel – rail” contact on the tread surface the prevailing morphological component is the subgrain structure, in the fillet – a ferrite-carbide mixture (completely destroyed pearlite). Strength of the rail head metal depends on the distance to the “wheel – rail” contact surface. It is shown that the main streng­thening mechanisms on the tread surface are strengthening by internal stress fields, in the fillet – strengthening by incoherent particles.