Surface Modification of AISI 4140 Steel Using Electrolytic Plasma Thermocyclic Treatment

Surface modification of AISI 4140 steel has been carried out by using electrolytic plasma thermocyclic treatment (EPTT). Cyclic voltage between 320 and 250 for stipulated period of 1, 2, 3, 4 s and 2, 3, 4, 5 s, respectively, has been performed. At the same time, the effect of average voltage and duration on process has been studied. Microhardness hardness profile measurements have been used before and after thermal treatment to analyze the hardened layer. Scanning electron microscopy (SEM), stereo, optical microscopy, and X-ray diffraction (XRD) have been employed to characterize microstructure and phase composition of the treated layer. The hardening layers are composed of martensite, fine pearlite, oxides with iron, such as FeCr2O4, Fe3O4, and FeO. According to the treatment parameters, max 770 HV hardness and fine martensite, transition zone, and fine pearlite phases have been examined from the surface to the bulk body. When the substrate has been processed at 300-320 V for 42 s, the highest thickness is about 10 mm, and the corresponding microhardness is HV 500-650, which is 4 or 5 times higher than that of the substrate. The depth of hardened layer can be 6-8 mm, depending on the EPTT conditions.