## 3 Results and Discussion
The average cooling rates at a depth of 2 mm following the EPD, as determined numerically, were 16.7 \({}^{\circ}\)C/s (150 V), 34.6 \({}^{\circ}\)C/s (200 V), and 59.8 \({}^{\circ}\)C/s (250 V). Experimentally, slightly lower values were observed at 15.0 \({}^{\circ}\)C/s, 30.4 \({}^{\circ}\)C/s, and 47.8 \({}^{\circ}\)C/s, respectively. This difference can be attributed to the thermocouple's inertia and potential degradation of thermal contact at elevated temperatures. Notably, during the initial seconds after voltage termination, instantaneous cooling rates can reach hundreds of degrees per second. Such rapid thermal unloading fosters conditions conducive to martensite and bainite formation, even in low-carbon steel 20 GL, particularly within the surface layers. The elevated values obtained from the numerical calculations can be primarily attributed to inherent simplifications within the model. These simplifications pertain to the boundary conditions, specifically the assumption of a heat-insulated surface, given the substantial size of the holder. Additionally, the model does not account for the oscillatory nature of heating as a boundary condition with heat flow. For preliminary engineering assessments, such assumptions are deemed reasonable to avoid unnecessary complexity. The primary objective of this study was to investigate the feasibility of hardening low-carbon steel 20 GL through electrolytic plasma treatment and to evaluate the depth of thermal influence on the samples. Future research endeavors will aim to incorporate a more comprehensive analysis of all conditions involved in the electrolytic plasma hardening of components. The distribution of microhardness by depth (Figure 10) clearly shows how the hardening of 20 GL steel depends on the electrolytic plasma treatment mode. Initially, the samples had a microhardness of approximately 150-160 HV. The cross-section for measurement was made at a distance of approximately 5 mm from the edge of the sample. At 150 V, hardening is minimal, with a maximum hardness of about 200 HV. This aligns with the calculated temperature of less than 400 \({}^{\circ}\)C at a depth of 2 mm, indicating an inability to reach the critical temperature required for phase transformations. At 200 V, a notable increase in microhardness to 500 HV occurs in the surface zone, followed by a sharp decline after 1 mm of depth. Although the numerical temperature distribution predicts heating to 700-800 \({}^{\circ}\)C at a depth of 1-2 mm, the reduction in hardness suggests an inadequate cooling rate in these layers, which restricted the formation of bainite or martensite structures. The most substantial strengthening is observed at 250 V, where high microhardness (750-850 HV) is sustained in the zone up to approximately 2.2-2.4 mm. This aligns with calculations showing that the temperature at this depth exceeded 750 \({}^{\circ}\)C, and the cooling rate (>50 \({}^{\circ}\)C/s) was sufficient to develop a martensitic structure. Therefore, for 20 GL steel, which has limited hardenability, the effective strengthening depth depends not only on the heating temperature but also on achieving critical cooling rates, especially during high-voltage EPH. The sharp drop in hardness observed at 200 V can be explained as follows. At the end of the heating stage (\(t\) = 10 s), the surface and subsurface layers (up to \(\sim\)1.5 mm) had reached temperatures in the range of 723-875 \({}^{\circ}\)C, sufficient for austenitization. During the subsequent 5 s cooling phase in a vigorously circulated electrolyte (at \(\sim\)20 \({}^{\circ}\)C), a rapid temperature drop occurred. The computed cooling rate at the sample surface reached approximately 77 \({}^{\circ}\)C/s, gradually decreasing with depth. At depths of 1.25 mm and 1.5 mm, the cooling rate fell to around 52 \({}^{\circ}\)C/s and 48 \({}^{\circ}\)C/s, respectively--close to the critical threshold for martensitic transformation in low-carbon steels.