Characterization of dual phase boride coatings on Sverker 3 steel and simulation of boron diffusion

The Sverker 3 steel was treated by solid boriding in the range 1173-1273 K for holding times ranged from 1 to 7 h. The processes resulted in two-phase (FeB + Fe${}_2$B) boride layers except the treatment at 1173 K for 1 h where the Fe${}_2$B was formed only. The growth of borides obeys a typical parabolic law, with the maximum thickness of 120 ± 4.5 µm. Considerable redistribution of carbon and alloying elements took place during boronizing; carbon and silicon were pushed out from borides while chromium and tungsten were rather gathered in these compounds. The microhardness of Fe${}_2$B ranged between 1600 and 1700 HV 0.1, and that of FeB reached 2100-2200 HV 0.1. The average diffusion coefficient (ADC) approach was applied by assuming the linearity of boron concentration profiles across the iron boride layers. This approach allowed us to obtain the boron diffusivities in both the FeB and Fe${}_2$B. Afterwards, the boron activation energies in both layers were obtained by fitting the temperature evolution of calculated boron diffusivities in the two iron boride phases with the Arrhenius relations. The assessed boron activation energies in FeB and Fe${}_2$B were, respectively, 215.18 and 203.6 kJ mol${}^{-1}$. Finally, a comparison of these values of energies was made with literature results.