The effect of process parameters on microstructure and corrosion behavior of AISI 4140 steel modified by pulse plasma treatment

The pulse plasma system is a surface modification method applied to steel samples. In the present study, the effect of the nozzle distance and pulse number parameters were investigated on the modification process. AISI 4140 steel was preferred as the substrate for modification. In this system, the battery capacity and voltage were constant, and, were selected as 800 mF and 3000 V, respectively. The AISI 4140 steel samples were modified by applying 5, 10, and 15 pulses with 40, 50 and 60 mm nozzle distance. The molybdenum consumable electrode was used during the process. The modified surfaces were examined by optical and scanning electron microscope (SEM) and analyzed by X-ray diffractometer (XRD). Vickers microhardness on the cross-section surface of the samples was measured under a load of 50 g for 15 s. Finally, the specimens were exposed to corrosion in 0.5 M NaCl solution. Corrosion tests were realized using the potentiodynamic polarization method. A modified layer on the steel was determined to consist of two layers, the compound layer and the diffusion layer. It was observed that the structure and thickness of the modified layer affect by pulse number and nozzle distance. New phases such as Fe2N, FeN, MoN, and γ-Fe in the modified layer have occurred. The hardness value of the treated sample has risen about 4-5 times than the untreated, depending on applied process parameters. In general, the pulse plasma treatment has improved the corrosion resistance of treated samples. It was observed that while intergranular corrosion has formed on the unmodified surface, pitting corrosion has appeared on the unmodified surfaces.