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This study evaluated the growth kinetics of Fe2B hard coatings obtained by the paste boriding process at the surface of AISI 1018 steels. The treatment was carried out at temperatures of 1123, 1173, 1223 and 1273 K with different exposure times for each temperature using a 4 mm thick layer of boron carbide paste over the material surface. The boron diffusion coefficient was determined by the mass balance equation at the Fe2B/substrate interface, assuming that the growth of Fe2B layers obey the parabolic growth law. Likewise, the growth constant was related as a function of the boride incubation time, the boron surface content and the boron diffusion coefficient, in order to estimate the weight gain in the borided samples and the instantaneous velocity of the Fe2B/substrate interface.