KEY ENGINEERING MATERIALS

The present study reports on boride layer growth kinetics of borided AISI 4140 steel. Steels were boronized in molten borax, boric acid and ferro-silicon bath at 1123degreesK 1173degreesK and 1223degreesK for 2, 4, 6 and 8 hours. Boride layer thickness ranged from 38.4 to 225 mum. Layer growth kinetics were analysed by measuring the extent of penetration of FeB and Fe2B sublayers as a function of boronizing time and temperature in the range of 1123-1223degreesK. The depth of the tips of the most deeply penetrated FeB and Fe2B needles are taken as measures for diffusion in the fast directions. The kinetics of the reaction, K= K-o exp (-Q/RT) have also been determined by varying the boriding temperature and time. The results showed that K increase with boronizing temperature. Activation energy (Q) for present study was determined as 215 kj.mol(-1). The diffusion coefficient (K) ranged from 3x10(-9) cm(2)s(-1) to 2x10(-8) cm(2)S(-1). Also temperature-dependent constant (K-o) at temperatures 1123degreesK, 1173degreesK and 1223degreesK was 179.4 cm(2)s(-1)