Structural phase states and properties of rails after long-term operation

•Long-term operation of rails is accompanied by surface layer strengthening.•Mechanical and tribological properties depend on the distance from tread surface.•Strengthening mechanisms are discussed. The investigations into structural phase states and properties of differentiatedly quenched 100-m rails of DT 350 category after extremely long-term operation (passed tonnage of 1411 mln. t brutto) were performed by the methods of modern physical material science. It has been established that rail operation results in the essential strengthening of surface layer up to 80–100 μm in thickness independent of the studied area of rail head. The microhardness of rail surface is 1.5–2-fold higher in comparison with volume and it decreases with the distance from the surface. A 7-fold increase in wear resistance of tread surface and 1.3-fold increase in friction coefficient with respect to volume have been revealed. The values of α-Fe crystal lattice parameter of tread surface (a = 0.28699 nm), the distortions of cementite crystal lattice (Δd/d = 2.37·10-3) are higher but the relative content of cementite (3.31 mas. %) and the size of coherent-scattering region (D = 20.6 nm) are lower than the corresponding values in volume. The long-term operation of rails is accompanied by the failure of cementite plates and carbon being liberated goes into solid solution and precipitates on the defects of crystal structure (dislocations, interphase and intraphase interfaces).