Surface modification of SCM420 steel by plasma electrolytic treatment

Recently, much research has been carried out regarding techniques to improve the properties of steel surfaces, such as via ion-nitriding and gas nitriding. However, this needs a high vacuum rate or very expensive equipment and a long treatment time. In this study, a nitriding process is proposed which has a much shorter treatment time with very simple equipment by using plasma electrolytic treatment. To produce the plasma in the electrolyte, a JIS-SCM420 specimen was used as the cathode and a rolled punched stainless steel plate (SUS304) was used as the anode. Voltage was applied between the electrodes using a pulsed DC power supply. The pulsed power supply was used to prevent the discharge from concentrating on only one point. Urea (H2N)2C = O, which was added into distilled water as an electrolyte, was used as the supply source of nitrogen or carbon. In addition, potassium hydroxide (KOH) was added for adjusting the electrical conductivity. The treatment voltage was 600 V, and experiments were conducted by varying the conditions, such as the treatment time and the concentration of the electrolyte. After the plasma electrolytic treatment, many fine discharge pores could be seen on the treated steel surface. In addition, a modified layer which has 1.9–2.7 μm deep was formed on the steel surface within 5 min of the start of the plasma electrolytic treatment. From the results of the XRD analysis, iron nitrides and chromium iron carbides were detected with α-Fe peaks, and the hardness of the modified-layer was found to be about 800 HV.