Tribological and Corrosion Behavior of Oxidized and Plasma Nitrided AISI 4140 Steel

Medium alloy steels are used in several components in the industry where wear and corrosion resistance are required. Different treatments can be used to enhance the surface properties. In this work, the tribological and corrosion behavior of the nitrided and post-oxidized AISI 4140 steel was studied. The samples were plasma nitrided in a 25 %/75 % (N-2/H-2) gas mixture at 500 degrees C for 15 h. They were oxidized in the same chamber for 1 h in a water steam atmosphere at 400 degrees C. The microstructure of the nitrided and oxidized layer was analyzed using X-ray diffraction (XRD) and scanning electron microscopy. The pin-on-disk tests were performed according to ASTM G99-17, Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus, with a 6-mm diameter alumina ball as counterpart, a sliding distance of 500 m and 1,100 MPa of applied Hertzian pressure. Moreover, reciprocating sliding tests with a stroke length of 5 mm were performed using am AISI 52100 steel ball as a counterpart under a Hertzian pressure of 354 MPa in both dry and lubricated conditions. Salt spray tests following ASTM B117-19, Standard Practice for Operating Salt Spray (Fog) Apparatus, and electrochemical tests in sodium chloride were carried out to evaluate the corrosion behavior. The thickness was 0.5 mu m and 3 mu m for the oxide layer and the nitrided layer, respectively. Magnetite was detected by XRD. In the pin-on-disk tests conducted under high pressure, the nitrided + oxidized and only nitrided samples presented similar wear resistance. However, the oxide layer allowed a reduction in the friction coefficient in the first cycles with respect to the only nitrided sample in the sliding reciprocating test under low pressure. This behavior was observed in both the dry and lubricated conditions, but it was more noticeable in lubricated ones. Regarding the corrosion behavior, the nitrided + oxidized samples showed better corrosion resistance than the other samples in a chloride solution.