Using transmission electron microscopy methods at various distances from the rolling surface along the central axis, changes in structure, phase composition, and defective substructure of the head of differentially hardened rails were studied after passed tonnage of 691.8 million tons of gross weight. It is confirmed that prolonged operation of rails is accompanied by two simultaneous processes of transformation of structure and phase composition of plate-pearlite colonies: cutting of cementite plates and dissolution of cementite plates. The first process is carried out by mechanism of cutting carbide particles and removing their fragments, accompanied only by change in their linear dimensions and morphology. The second process of dest ruction of the cementite plates of perlite colonies is carried out by leaving carbon atoms from crystalline lattice of cementite on dislocation, as a result of which phase transformation of rails metal is possible. This is due to a noticeable relaxation of mean energy of carbon atom s binding to dislocations (0.6 eV) and to iron atoms in cementite lattice (0.4 eV). The stages of transformation of cementite plates are considered: enveloping the plates with sliding dislocations and then splitting them into weakly oriented fragments; penetration of sliding dislocations from ferrite lattice into lattice of cementite; dissolution of cementite and formation of nanoscale particles. The presence of nanosized cementite particles in ferrite matrix is noted due to their removal during dislocation slide. Using expressions of modern physical materials science and X-ray diffraction analysis, influence of content of carbon atoms on structural elements of rail steel was estimated. It is shown that prolonged operation of rails is accompanied by a significant redistribution of carbon atoms in surface layer. In the initial state, the main quantity of carbon atoms is concentrated in cementite particles, and after a long operation of rails, along with cementite particles, carbon is located in defects of crystal structure of steel (dislocation, grain boundaries and subgrains), and in the surface layer of steel atoms carbon is also found in crystal lattice based on α-iron.
RAIL STEEL OPERATION: DEGRADATION OF STRUCTURE AND PROPERTIES OF SURFACE LAYER
