Internal stresses and their sources in steels with BCC lattice

Abstract

The paper studies fine structure of industrial steels with BCC lattice (pearlite, ferrite-pearlite and martensite) using transmission diffraction electron microscopy. The internal structure of the grains was analyzed; the scalar density of dislocations in various parts of the material, the sources of internal stresses and their amplitude were determined. The use of a method based on the analysis of bending extinction contours allowed us to study internal stresses. We analyzed the internal stresses and their sources using the example of 0.76С–Cr–V–Fe rail steel with a lamellar pearlite structure after ultra long-term operation with the tonnage of 1770 million gross tons. The metal of the rails was examined along the central axis of symmetry (rolling surface) and the rounding radius (working fillet) of the railhead at distances of 0, 2 and 10 mm from the surface. As one approaches the head surface, regardless of the research direction (along the fillet rounding radius or along the axis of symmetry), the lamellar pearlite is gradually replaced by destroyed pearlite with formation of a ferrite-carbide mixture and formation of a fragmented structure. These processes occur more intensively in the working fillet. Along the entire central axis of symmetry of the rail head (rolling surface), there is a plastic bending-torsion of the crystal lattice, along the rounding radius of the rail head (working fillet) at a distance of 10 mm from the surface – also plastic, and at a distance from 0 to 2 mm – elastic-plastic. The main source of internal torque (long-acting) stresses in rail steel is the excessive density of dislocations. Using the example of 34CrNi3MoVN steel of the martensitic class, the type of bending extinction contour was determined using mathematical equations. At low degrees of plastic deformation, extinction contours are contours of bending or torsion, at high degrees they are of a mixed type.