An approach to kinetic study of borided steels

In present study, kinetic studies on borided AISI 5140, AISI 4340 and AISI D2 steels are reported. Steels were borided in a salt bath consisting of borax, boric acid and ferro-silicon between 1073 and 1273 K for 2, 4, 6 and 8 h. The morphology and types of borides formed on the surface of steel substrates were confirmed by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Boride layer thickness formed on the borided steels ranged from 21 to 238 μm depending on process temperature, treatment time and alloying elements of the substrates. The hardness of borides formed on the samples changes between 1077 and 2140 HV0.1 according to treatment time and temperature. Layer growth kinetics were analyzed by measuring the extent of penetration of FeB and Fe2B sublayers as a function of boronizing time and temperature in the range of 1123-1273 K. The depth of the tips of the most deeply penetrated FeB and Fe2B needles are taken as measures for diffusion in the growth directions. The kinetics of the reaction, K=Ko exp (−Q/RT), have also been determined by varying the boriding temperature and time. The results showed that K increase with boriding temperature. Activation energies (Q) of borided AISI 5140, AISI 4340 and AISI D2 steels at present study were determined as 223, 234 and 170 kJ/mol, respectively. Moreover, an attempt was made to investigate the possibility of predicting the contour diagrams of boride layers variation and to establish some empirical relationships between process parameters and boride layer thicknesses.