4340, 4140, 1045, 3215 and 1020 steel were boronized by plasma electrolysis by using borax aqueous solutions. Fifteen minute treatments were sufficient for notoriously increasing the hardness of these steel samples. Unlike traditional boronizing treatments, a gradual hardness transition from the surface to substrate was obtained. It was found that Cr promotes boron diffusion, and C and Ni complicate it. The greater values of surface hardness and boride layer thickness were obtained for the 1020 steel: about 1600 HV and over 6 mm, respectively. © 2008 Elsevier Ltd. All rights reserved.