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 treatment time of 2 to 8 h. The mixture of powders containing 20% B₄C, 10% KBF₄and 70% SiC has been used for producing a single boride 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 sample treated at 1173 K during 2 h. Kinetically, different approaches have been used to estimate the boron diffusion coefficients in the Fe₂B layers and to predict the value of Fe₂B layer thickness obtained at 1253 K for a treatment time of 2.5 h. The estimated values of activation energies for boron diffusion in AISI 4150 steel have been in the range of 193.45 to 199.74 kJ mol^-1. These values of activation energies have been depended on the diffusion models used. In addition, a good agreement has been observed between the experimental value of Fe₂B layer thickness obtained at 1253 K for 2.5 h with the predicted values from these different diffusion models.