Formation and kinetics of FeB/Fe₂B layers and diffusion zone at the surface of AISI 316 borided steels

The kinetics of the FeB/Fe₂B layers and diffusion zone at the surface of AISI 316 steels exposed to the powder-pack boriding process were studied in this work. FeB/Fe₂B layers and diffusion zone measurements were taken at different temperatures and exposure times to validate diffusion-controlled growth during the boriding process. In order to obtain the boron diffusion coefficients at the FeB/Fe₂B layers and diffusion zone, a mathematical model based on the mass balance at the growing interfaces was proposed. The activation energy values estimated for the FeB and Fe₂B layers were 204 and 198kJmol^-1 respectively. In addition, the activation energy value obtained for the diffusion zone was 116kJmol^-1. The diffusion model was extended to estimate the FeB/Fe₂B layer thicknesses, and the depth of the diffusion zone at the temperature of 1243K with 3 and 5h of exposure, based on the experimental parameters ascribed to the boriding process. Finally, the effects of the FeB/Fe₂B growth and diffusion zone, on the weight gain of borided steels and on the instantaneous velocity of the interfaces were incorporated in the model.