Improved nitrogen transport in surface nanocrystallized low-carbon steels during gaseous nitridation

In the thermochemical process of low-carbon steel nitridation, improvements of mechanical properties are governed by the nature (hardness, thickness, etc.) of the compound layer since the compound layer will be kept under working conditions. Because of low nitrogen diffusivity in steel and low mass transfer between gaseous reaction and steel, long duration and high temperature are classically used for thermal nitridation. The present study shows that improved nitrogen transport can be obtained by gaseous nitridation after the samples have been surface nanocrystallized by ultrasonic shot peening (USSP). Comparing to the untreated samples, the nitridation efficiency of treated samples has been improved dramatically. The growth of γ′ nitride layer conforms to a parabolic relationship with nitridation duration during the whole nitridation process in treated samples, while in untreated samples, the growth of nitride layer could be traditionally divided into two stages: the first linear growth stage and the second parabolic growth stage. This difference clearly indicated that the whole nitridation process is diffusion-controlled in treated samples instead of interface reaction-controlled stage and then diffusion-controlled stage in untreated samples. The high nitridation efficiency obtained may be explained by an increased volume fraction of grain boundary and an enhanced reactivity of surface atoms of the steel samples due to the nanostructure layer generated by surface nanocrystallization of USSP. © 2002 Elsevier Science B.V. All rights reserved.