Effect of cooling conditions on plasma-carbonitrided iron surfaces

Plasma carbonitriding (nitrocarburizing) has been used to improve the tribological and mechanical properties of materials, especially iron-based alloys. In this work, the pulsed glow discharge technique was used for nitrocarburizing pure iron in order to investigate the effect of the cooling conditions on the characteristics of the formed layers. Three sample sets were nitrocarburizing in a gas mixture of 2 vol % $CH4,$ 78 vol % $H2,$ and 20 vol % $N2,$ under a pressure of 400 Pa, discharge frequency of 9 kHz, temperature of 580 °C, during 90 min. One set was cooled under a vacuum, the second set was cooled in air, and the third set was rapidly removed from the furnace at the treatment temperature and quenched in oil. All samples were characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, and Vickers microhardness. Differences in surface composition and morphology were observed, indicating the importance of the cooling rate on the formed layer in these three processes. For all of the samples, three layers were observed: A compound layer, a transformed zone, and a diffusion zone.