An alternative technology of surface hardening by the electrolytic-plasma modification (EPM) of a low-carbon alloy steel has been developed. The processing of 20KH (207) alloy steel samples to achieve surface hardening was conducted using electrolytic-plasma surface modification in a Na2CO3 solution electrolyte. The optimal processing modes of the electrolyte-plasma modification treatment was determined experimentally using voltage and applied current measurements. The quenching of the steel was performed in the electrolyte stream. The plasma was ionized after excitation. A mathematical model using thermal conductivity equations and regression analysis relating the key parameters of the hardening process was also carried out. Our experimental and mathematical model results both confirmed that the EPM processing clearly leads to a significant reduction in the time for hardening relative to other traditional hardening methods for steels. We observed that electrolyte-plasma modification (EPM) processing promotes the transformation of coarse-grained pearlite-ferrite microstructure into quenched martensite in 20KH (207) alloy steel samples using inverted light optical microscopy. Our Vickers micro-hardness indentation measurements showed an increase in hardness relative to the initial state of the steel samples after electrolyte-plasma surface treatment. The major advantage of the method of electrolytic-plasma treatment includes a low energy consumption at high quenching rates and the possibility of a local surface treatment. They also provide an overall cost reduction in the surface treatment of steels for various industrial applications.