Researchers are constantly developing processes aiming to improve the properties of metal surfaces, especially related to the wear resistance of components, as in the case of the nitrided layer obtained by die-sinking electrical discharge machining (EDM). Following this line of research, this work investigated the effects of sodium octaborate (Na2B8O13.4H2O), mixed into deionized water as a dielectric fluid on AISI 4140 steel surfaces machined by die-sinking EDM. An adapted EDM machine was employed in the process using electrolytic copper as tool. The effects on AISI 4140 steel-machined surfaces were evaluated by optical microscopy, Vickers microhardness, X-ray diffraction, and energy dispersion X-ray spectroscopy (EDS) analyses. The results showed a hardness gain of approximately 146.8% in the modified layer when compared to the AISI 4140 steel (base material). This suggests the formation of a borided layer, such as the Fe2B phases identified on sample surfaces, which can be explained by the boron element decomposed from the dielectric solution.