Growth kinetics of the FeB/Fe2B boride layer on the surface of 4Cr5MoSiV1 steel: experiments and modelling

The boriding of 4Cr5MoSiV1 steel was performed in the temperature range of 860°C–980 °C to examine the influence of boriding conditions on the boride layers. The experimental results show the formation of FeB and Fe2B layers with the predominant saw-tooth morphology. The boride layer depth increases with increase the boriding temperature and time. Thick and compact boride layers could be obtained at temperatures higher than 900 °C. The growth kinetics of boride layers is characterized by a parabolic curve. FeB and Fe2B exhibit a hardness of 1600HV0.1 and 1300HV0.1, respectively, which is 4 and 3 times that of the substrate. A diffusion model, based on the boron concentration profiles of the surface layers and the parabolic growth law, was established to predict the growth kinetics of the boride layers including both the FeB and the Fe2B layer. A satisfactory agreement between the model and experimental results was obtained. The work thus provides an approach to investigate and to predict the boride layer growth of 4Cr5MoSiV1 steel in the boriding process.