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 1123°K 1173°K and 1223°K for 2, 4, 6 and 8 hours. Boride layer thickness ranged from 38.4 to 225 μm. Layer growth kinetics were analysed by measuring the extent of penetration of FeB and Fe_2B sublayers as a function of boronizing time and temperature in the range of 1123-1223°K. The depth of the tips of the most deeply penetrated FeB and Fe_2B 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~2s~(-1) to 2x 10~(-8) cm~2s~(-1). Also temperature-dependent constant (K_o) at temperatures 1123°K, 1173°K and 1223°K was 179.4 cm~2s~(-1).
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