Influence of urea content blended with deionized water in the process of nitriding using electrical discharge machining on AISI 4140 steel

Abstract

The process of nitriding with electrical discharges makes use of electrical discharge machining (EDM) machines. Nitrogen is supplied by a dielectric fluid comprised of distilled water or deionized and urea solution (CH4N2O). The underlying objective of this study was to investigate the influence connected to the amount of urea diluted in deionized water during the nitriding process of AISI 4140 steel. The tests were performed using a sinker EDM machine. As a tool electrode, the authors used an electrolytic copper cylinder, and as workpiece electrodes, they used cylindrical AISI 4140 steel. Samples of deionized water were used as a dielectric fluid and urea was added at different concentrations. The results showed the progressive loss of dielectric strength with the addition of different amounts of urea, mixed to the water. There was also a noticeable change in the kinetics of the plasma channel formation. Urea quantities higher than 10 g/l did not produce correct plasma arc formation. Variations in urea content did not significantly change in the morphology of the machined surface, the thickness of the nitrided layer, the type of nitrides formed, or the final hardness of the enriched surfaces. However, the increase in the amount of urea caused the loss of dielectric strength of the fluid with a consequent decrease in the material removal rate and therefore, the best material removal rate (MRR) was found when machined at a lower urea content level.