INFLUENCE OF PLASTIC DEFORMATION ON THE STRUCTURE OF LOW-CARBON STEEL AFTER SURFACE HARDENING AND HEAT TREATMENT

The structural and phase state, durometry, and metallography of samples cut from flat rolled steel of grade 20 subjected to plastic tensile deformation are considered. Some of the samples were subjected to surface hardening by chemical heat treatment (CTO) before stretching, the second part was subjected to heat treatment (TO). Both treatment methods were carried out at the same temperature regime (850 C with 180 min exposure). At the same time, the task of changing the mechanical properties during heat treatment was not set for the material in question. The microstructure was studied in the directions along and along the rivers of the rolling direction. The effect of thermal and chemical-thermal treatments on the static strength and impact strength of the samples after the corresponding processes was investigated. During plastic stretching deformation, the thickness of the diffusion layer does not change, except for the area located in the fracture zone: the thickness of the diffusion layer increases slightly, but the layer itself is already a conglomerate of fragmented fragments of boride needles, nevertheless, quite firmly connected to the matrix material. It is shown that plastic deformation leads to an increase in the anisotropy of the grain in the direction of the forces, and during plastic deformation, it is crushed by crushing excessively elongated inclusions into smaller fragments in the direction perpendicular to the action of the deforming force. The measurements of grain anisotropy show that in both cases it is close to unity (0.99 for CTO and 1.02 for TO), which suggests that in both cases equi-axial grains are observed, which are close to globular in shape. A comparison of the microstructure of the core of both samples (borated and non-borated), which underwent identical thermal treatment, showed that the microstructure of the core of the samples under consideration is absolutely identical: both the phase composition and the structural state coincide.