Dependence between the boron surface concentration and the growth kinetics of boride layers in AISI 4140 steels

The present work estimated the growth kinetics of Fe₂B layers formed at the surface of AISI 4140 steels. The thermochemical treatment was applied in order to produce the Fe₂B phase, considering temperatures of 1123, 1173, 1223 and 1273 K with five exposure times (2, 4, 5, 6, and 8 h), using a 4 mm thick layer of boron carbide paste over the material surface. The growth of boride layers was described by the mass balance equation between phases in thermodynamic equilibrium, assuming that the growth of boride layers obeys the parabolic growth equation x(t)= u = k(t^1/2 -t ₀ ^1/2(T)) and the boron concentration at the interfaces remains constant. Also, the boron diffusion coefficient at the Fe₂B (D _Fe2_B) was established as a function of boriding temperature. Likewise, the parabolic growth constant (k), the instantaneous velocity (v) of the Fe2B/substrate interface and the weight-gain of borided steels were established as a function of the parameters τ (t) and α(C_S), which are related to the boride incubation time ( t ₀ (T) ) and boron surface concentration (C_S ), respectively.