OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 of the rod after CRF to a 95 % area reduction was taken as the initial state. Then, the cold-forged rod was subjected to heat treatment at temperatures of 600 °C (95 % CRF + HT 600 °C) and 700 °C (95 % CRF + HT 700 °C) followed by air cooling. The holding time was 2 hours. For microstructure characterization, steel disks with a diameter of 3 mm, mechanically thinned to 100– 150 μm and electrolytically perforated, were examined using a JEOL JEM-2100 transmission electron microscope (TEM) with an accelerating voltage of 200 kV. To confirm structural heterogeneity, blanks for TEM studies were cut in the cross-section from the center and edge of the studied steel rod. Perforation was performed on a TenuPol-5 setup using an electrolyte consisting of 10 % perchloric acid and 90 % acetic acid. Uniaxial tensile tests of steel specimens were carried out at room temperature using an Instron 5882 testing machine at a strain rate of 1.15 × 10⁻³ s⁻¹. For a more accurate determination of the deformation degree, the VIC-3D visual inspection system was used. Subsequent processing of the obtained data was carried out using VIC-2D software. To determine the mechanical properties of different regions, specimens were cut from the center (sample C – center) and edge (sample E – edge) of the rod. To determine the mechanical properties of a specimen with a heterogeneous structure, flat specimens were cut along the entire diameter of the rod (sample H – with a heterogeneous structure), with a width corresponding to the diameter of the rod. The cutting diagram is shown in Fig. 1. The gauge length of the steel tensile specimens was calculated according to GOST 1497–23: 0 0 5.65 l F , (1) where l0 is the gauge length of the specimen; F0 is the cross-sectional area. Fig. 1. Uniaxial tensile specimen cutting scheme and dimensions (mm) for: central rod portion (C – center), subsurface layer (E – edge), and the entire structural zone (H – heterogeneous) For identification of samples in the case of subsequent heat treatment, a number denoting the annealing temperature is added to the corresponding letter, defining the area of samples cutting (e.g., E600 is a sample cut from the rod edge subjected to heat treatment at 600 °C). For a deformed sample, the number 95 is added, denoting the area reduction (e.g., E95 is a sample cut from the edge of a rod subjected to CRF). To determine the magnitude of back stresses in each region, load-unload tests were carried out on the corresponding samples under conditions of uniform plastic deformation at ε = 2–5 %. Calculations were performed according to the methodology presented in Refs. [17, 23]. The samples were tested in the state after CRF to 95 % followed by heat treatment at 700 °C for 2 hours. During the tests, stress-strain curves with hysteresis loops were obtained (Fig. 2). The level of back stresses was determined by the following equation [17, 23]:
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