## 2 Materials and Methods  
### Experimental Approach  
The experiments were conducted at the Engineering Center of the Shakarim University in Semey. An experimental setup for electrolytic plasma treatment was constructed, and its schematic representation is provided in Figure 1a. This setup comprised a 50 kW DC source, an electrolyte bath, an electric pump, and a cone-shaped stainless steel anode through which the electrolyte was supplied. A 20% aqueous solution of sodium carbonate (Na\({}_{2}\)CO\({}_{3}\)) served as the electrolyte. The samples, made of 20 GL steel, were prepared as parallelepipeds and manually polished using sandpaper with progressively finer grit sizes, ranging from P100 to P2500. The samples had the following geometric dimensions: a length of 25 mm, a width of 20 mm, and a height of 15 mm. The electrolyte flow rate was 10 L/min. The electrolyte flow rate was measured using an electronic meter, the Piusi K24 Meter (Piusi, Suzzara, Italy). To evaluate the overall structure, a metallographic microscope HL-102 AW (Shanghai Hualong Test Instruments Corporation, Shanghai, China) was utilized. For metallographic microanalysis, sections of steel samples were polished using chromium dioxide paste and subsequently etched with a 4% alcohol solution of nitric acid. The microhardness of the steel samples was then measured using an HV-1 DT device (Shanghai Hualong Test Instruments Corporation, Shanghai, China), with an indenter load of \(p=1\) N and a holding time of 10 s (in accordance with the requirements of GOST 9450-76). The electrolyte temperature following each hardening cycle was measured using an LT-300 (Thermex, Tosno, Russia) laboratory electronic thermometer, which has a measurement range of \(-\)50 \({}^{\circ}\)C to +300 \({}^{\circ}\)C and a division value of 0.01 \({}^{\circ}\)C. Measurements were taken by immersing the thermometer probe into the electrolyte solution after each hardening stage. During electrolytic plasma hardening (EPH), the current and voltage values in the electrolytic cell were recorded using an analog signal measurement circuit. Voltage was read via a PC817 optocupler (Sharp Microelectronics, Sakai, Osaka, Japan), and current strength was read using an SCT-013-000 current transformer (Shenzhen Zhuang Yi Electronics Technology, Shenzhen, China). Both signals were fed to an ATmega328P microcontroller (Microchip Technology Inc., Chandler, AZ, USA) for primary processing. The sample temperature was monitored using a MEGEON26003 K-type thermocouple (MEGON, Omsk, Russia) with a measurement range of (\(-\)50...+1200) \({}^{\circ}\)C and a MAX6675 digital module (Shenzhen Qinyuansheng Electronics, Shenzhen, China). The data from the thermocouple and module were also transmitted in digital form to the microcontroller for primary processing. The thermocouple was installed in the sample at a distance of 2 mm from the surface through a hole in the back of the sample (Figure 1b). All obtainedparameters--current, voltage, and temperature--were transmitted to a personal computer, where they were saved for subsequent analysis and processing. The electrical schematic diagram of the measuring system used to register the parameters of the EPH process is illustrated in Figure 2. This diagram was divided into three functional sections. Section A measures the voltage in the electrolyte circuit, using a PC817 optocoupler for galvanic isolation and safe transmission of the analog signal to the ATmega328P microcontroller. Section B measures current with an SCT-013-000 current sensor, which converts the current value into an analog signal that is then fed to the ADC input of the microcontroller. Lastly, Section C regulates temperature using the MEGEON26003 thermocouple connected to the MAX6675 digital module, which converts the thermocouple signal into digital format before transmitting it to the microcontroller.