The growth kinetics of borides formed on boronized AISI 4140 steel

The growth kinetics of boride layer on boronized AISI 4140 steel is reported. Steel samples were boronized in molten borax, boric acid and ferro-silicon bath at 1123, 1173 and 1223 K for 2, 4, 6 and 8 h, respectively. The morphology and types of borides formed on the surface of AISI 4140 steel substrate were analyzed by means of optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (XRD). The boride layer thickness ranged from 38.4 to 225 μm. Iso-thickness diagrams for pre-determined thickness according to treatment time and temperature, were graphed by MATLAB 6.0 software. The hardness of borides formed on the samples changed between 1446 and 1739 HV0.1, according to treatment time and temperature. Layer growth kinetics way analyzed by measuring the extent of penetration of FeB and Fe2B sublayers as a function of treatment time and temperature in the range of 1123-1223 K. For practical use, an iso hardness diagram was established as a function of treatment time, temperature and boride layer thickness. The depth of the tips of the most deeply penetrated FeB and Fe2B needles were taken as measures for diffusion in the growth directions. The kinetics of the reaction, K=Koexp(-Q/RT) were also determined by varying the treatment temperature and time. The results show that K increased with boronizing temperature. The activation energy (Q) was formed to be 215 kJ mol−1. The growth rate constant (K) ranged from 3×10−9 to 2×10−8 cm2s−1.