Transformation of Carbides in Prolonged Rail Operation

Abstract

The evolution of the carbide phase in the surface layers of bulk-quenched rails (after the passage of 500 and 1000 million t of traffic) and differentially quenched rails (after the passage of 691.8 million t) to a depth of 10 mm at the central axis of the rail cross section and at the nearby rounded section is studied by transmission electron-diffraction microscopy. The grains of plate pearlite, ferrite–carbide mixture, and structure-free ferrite are analyzed. The carbide phase in the surface layers of the steel changes in two mutually complementary processes during rail operation: (1) cleavage of cementite particles with subsequent entrainment in ferrite grains or plates (in the pearlite structure); (2) cleavage and dissolution of cementite particles, with transfer of carbon atoms to dislocations (in Cottrell atmospheres and in dislocational cores), which transport them to the ferrite grains (or plates), where cementite nanoparticles are formed again. In the previous location of the plates, fragmented dislocational substructure appears. The boundaries of the fragments are found at the positions previously occupied by cementite α-phase boundaries. The solution of cementite is mainly due to the energy of carbon atoms at dislocation cores and subboundaries in comparison with the cementite lattice. The binding energy of the carbon atom and the dislocations is 0.6 eV and the binding energy of the carbon atom and the subboundary is 0.8 eV, as against 0.4 eV for the carbon atom in cementite. Elastoplastic stress fields are formed; their stress concentrators are intra- and interphase boundaries of ferrite and pearlite grains, cementite plates and ferrite of the pearlite colonies, and globular cementite and ferrite particles. Those are also the basic sources of curvature and torsion in the crystal lattice of the rail steel. On approaching the contact surface, the number of stress concentrators increases, and the internal long-range stress fields are of greater amplitude.