Jinhai Gu

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

Abstract 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.

Investigation of the Continuous Cooling Transformations in Rotor Steel 30CrNi4MoV

A diagram of continuous-cooling transformations in rotor steel 30Cr2Ni4Mo (30Kh2N4MF) in the range of cooling rates from 0.01 to 10 K/sec is plotted with the help of a dilatometric method. The changes in the morphology of the products of transformation of austenite and in the Vickers hardness, which occur upon growth in the cooling rate, are studied and analyzed.