CORROSION RESISTANCE OF PLASMA NITRIDED STRUCTURAL STEELS

Plasma nitriding technology, based on plasma assisted nitrogen diffusion into the steel surface, mostly used for improvement of wear resistance and fatigue strength is well known. Additional benefit of plasma nitriding is heat and corrosion resistance improvement. This study is focused on evaluation of corrosion resistance of plasma nitrided structural steels under several nitriding conditions. Experiments were performed on structural steels CSN 41 2050 (C45E), CSN 41 4220 (16MnCr5) and CSN 41 5230 (DIN 1.7361). Selected structural steels were manufactured for experimental specimens (size of 80x50x4 mm), heat-treated and plasma nitrided under following conditions: T = 500 degrees C, process duration t = 10 and 20 h, p = 280 Pa, U = 510 V and variable gas mixture ratio of 3H(2):1N(2) [l/h] and 1H(2):3N(2) [l/h] for different compound layer phase composition. Corrosion tests were performed in the Liebisch GmbH & Co (S 400 M-TR) corrosion chamber in the mist of neutral sodium chloride dilution (NSS method), evaluated and documented in accordance to ISO 9227 standard and results compared to different plasma nitriding conditions. Under different plasma nitriding conditions were different nitride layer characteristics obtained. Nitride layers were by surface hardness (HV), metallographic testing and microhardness testing (HV0,05) classified. Using the microhardness testing were different values of microhardness and nitride layer depth detected. Metallographic testing and additional concentration profiles measuring (using GDOES method) displayed different compound layers thickness, which tends to be a significant factor to corrosion resistance of plasma nitrided structural steels.