Transformation of fine structure of lamellar pearlite under deformation of rail steel

Abstract

The defective substructure of polycrystalline bodies causes substructural hardening and mechanical properties. Perlite, which is the main structural component of rails, undergoes a significant transformation during deformation, which is accompanied by a number of processes. In this paper, the methods of modern physical materials science were used to study and analyze the defective substructure of perlite of lamellar morphology and the properties of rail steel subjected to destruction under deformation conditions by single-axis stretching of flat samples. It was established that the ultimate strength varies from 1247 to 1335 MPa, and the relative deformation to failure – from 0.69 to 0.75. The formation of three zones of the fracture surface was observed: fibrous, radial and shear zones. Their shape and sizes were analyzed. Deformation of rail steel is accompanied by destruction of cementite plates of perlite colonies and repeated release of nanoscale particles of tertiary cementite approximately 8.3 nm in size in the volume of ferrite plates. The main mechanisms of destruction of cementite plates are cutting and dissolution. The dislocation substructure is represented by a chaotic distribution of dislocations and their clusters. The scalar density of dislocations in ferrite increases from 3.2·1010 cm–2 in the initial state to 7.9·1010 cm–2 when it is destroyed. Deformation is accompanied by formation of internal stress fields, manifested in the form of bending extinction contours. The sources of stress fields are the interface of cementite and ferrite plates, as well as grain boundaries. Fragmentation of ferrite and cementite plates was revealed. The average size of cementite fragments is 9.3 nm. In fracture zone of the rail steel sample, rotation of perlite grains was noted, indicating the presence of a rotational mode of deformation. Electron microscopic images of cementite plates show a change in the contrast, which may be associated with the formation of Cottrell’s atmospheres.