Diffusion induced decomposition of cementite layers during gas nitriding of 4140 steel: thermodynamical and kinetical aspects

Abstract In this paper, some new aspects of the decomposition of cementite layers by diffusion of nitrogen during gas nitriding are investigated. Therefore, AISI4140 steel samples were heat-treated to harden by heating up to 1113 K for 120 min, followed by quenching in agitated mineral oil. Then samples were tempered at 863 K for 60 min. Nitriding was carried out for 1400 up to 21000 min. Samples were characterized by optical microscope, scanning electron microscope, x-ray diffractometer, and HSC thermodynamical software. Results showed that the rate of diffusion was higher in the early stages of nitriding. Due to nitrogen of diffusion, some nitrides are formed in grain boundaries and act as diffusion barriers. In this case, the diffusion mechanism from grain boundary diffusion changes to the volume diffusion or a combination of both mechanisms. Meanwhile, diffusion of nitrogen, in addition to the ferrite layers and grain boundaries, is also performed into the cementite layers. Although these layers are not the preferred areas of diffusion, volume diffusion of nitrogen through cementite layers leads to decomposition of them and turning them into iron nitrides and carbo-nitrides. Results show that almost 42 percent of cementite layers decomposed by nitrogen diffusion.