Influence of temperature and duration on the nitriding behavior of 40Cr low alloy steel in mixture of NH3 and N2

40Cr low alloy steel samples was nitrided by plasma nitriding with hollow cathode discharge assistance in a gas mixture of N-2 and NH3 with different ratios at temperatures ranging from 520 degrees C to 560 degrees C for processing time from 2 to 6 h. The influences of processing temperature and time on the thickness and phase composition of nitrided layer were researched. Nitrided layers were evaluated using scanning electron microscope (SEM), X-ray diffraction and microhardness tester. The thickness of white layer increased with the processing temperature and time increasing. For samples nitrided at 540 for 4 h, the white layer is 5.6 mu m in ammonia and 10.2 mu m in mixture gas respectively. The maximum thickness of white layer about 15 mu m thick is obtained after nitrided at 560 degrees C for 6 h in this study. The phases of nitrided layer are mainly composed of epsilon-Fe2-3N with a small amount of Fe4N. Such nitrided layer was beneficial to reducing the friction coefficient. The corrosion resistance of the nitrided sample was improved with the increasing thickness of the white layer.