Growth kinetics of Fe2B layers formed on the AISI 4150 steel by different approaches

Abstract

In the present work, the AISI 4150 steel has been pack-borided in the temperature range of 1123-1273 K for a treatmenttime of 2 to 8 h. The mixture of powders containing 20% B4C, 10% KBF4 and 70% SiC has been used for producing a singleboride layer (Fe2B) at the surface of AISI 4150 steel. The presence of Fe2B phase has been confirmed by XRD analysis.The SEM observations have been done to investigate the morphology of boride layers and measure their thicknesses.The cohesion of boride layers has been evaluated by using the Daimler-Benz Rockwell-C indentation technique.The borided sample at 1173 K for 8 h has shown a best cohesion of boride layer to the substrate in comparison to the sampletreated at 1173 K during 2 h. Kinetically, different approaches have been used to estimate the boron diffusion coefficients inthe Fe2B layers and to predict the value of Fe2B layer thickness obtained at 1253 K for a treatment time of 2.5 h. Theestimated values of activation energies for boron diffusion in AISI 4150 steel have been in the range of 193.45 to 199.74 kJmol-1. These values of activation energies have been depended on the diffusion models used. In addition, a good agreementhas been observed between the experimental value of Fe2B layer thickness obtained at 1253 K for 2.5 h with the predictedvalues from these different diffusion models.