Defect substructure change in 100-m differentially hardened rails in long-term operation

•The dependences of change in scalar and excess dislocation density are established.•The passed tonnage is 691mln.tbrutto.•The mechanisms of cementite plates failure are considered. By methods of optical and transmission electron diffraction microscopy the evolution in structural phase states of surface layers head of differentially hardened rails of DT 350 category to the depth up to 10mm along the central axis after the passed tonnage of 691.8mln.tbrutto was studied. The formation of gradient substructure being expressed in regular change in scalar and excess density of dislocations, curvature-torsion amplitude of steel crystal lattice, degree of deformation transformation of lamellar pearlite structure was defected. The failure mechanisms of cementite plates were considered.