CHANGES IN STRUCTURE AND PHASE COMPOSITION OF THE SURFACE OF DIFFERENTIALLY HARDENED 100-METER RAILS IN OPERATION

Abstract

By the methods of optical and transmission electron diffraction microscopy the evolution of structural-phase states of the surface layers in the head of differentially hardened rails of category DT350 was studied to the depth up to 10 mm in the fillet after the passed tonnage of 691.8 million gross tones on the experimental ring of JSC “VNIIZhT”. It is shown that in the initial state the following structural constituents present in the rail head: grains of lamellar perlite (relative content is 0.7), grains of ferrite-carbide mixture (0.25) and grains of structurally free ferrite. After the passed tonnage of 691.8 million gross tones this state is preserved only at a depth of more than 10 mm. A distinctive feature of the structure at this distance is a large number of bending extinction contours, which reveals elastoplastic distortions of the crystal lattice in the material. Stress concentrators of the steel under study are intra- and interphase interfaces of grains of ferrite and perlite, cementite plates and ferrite of perlite colonies, globular cementite particles and ferrite. The transformation of the structure is manifested at the macro level in the formation of microcracks at an acute angle to the surface to a depth up to 140 μm, and in the formation of the decarburized layer. At the micro level, the formation of elastoplastic stress fields and the destruction of cementite plates of perlite colonies have been revealed. It is shown that stress concentrators are intra- and interphase interfaces of ferrite and perlite grains, cementite and ferrite plates of ferrite colonies, globular cementite particles and ferrite. In the grains of structurally free ferrite, the formation of nano-sized particles of cementite was observed. The comparison is made with the results of the evolution of structural-phase states in the fillet of volume-hardened rails after the passed tonnage of 500 million gross tones: the most significant transformations of structural-phase states are observed in the surface layers. The evolution of the structural-phase state of perlite of lamellar morphology consists in the dissolution of cementite plates, which leads to the formation of a chain of particles of the carbide phase with globular shape in place of the cementite plate. This is possible due to the transfer of carbon atoms from the crystalline lattice of cementite to dislocations.