“Modeling boride layer formation under ultrafast boriding in low- and high-alloy steels”

Ultrafast boriding (UFB) has emerged as an electrically assisted thermochemical surface treatment capable of producing boride layers within remarkably short processing times. In this study, a comprehensive microstructural, kinetic, and mechanistic analysis of boride layer formation on AISI 1045 and AISI 316L steels treated by UFB is presented. Boriding treatments were conducted at different temperatures and exposure times using molten borax as a fully liquid electrolyte under a direct-current electric field. Layer growth was quantitatively described using a mass balance model, enabling the estimation of B diffusion coefficients and activation energies for both FeB and Fe2B layers. The results demonstrate that boride layer growth under UFB is governed by a combined electrochemical-diffusional mechanism, in which electrolysis-driven boron dissociation and electrolysis within a fully molten medium enhance B availability and directed transport toward the substrate. A comparative epistemological analysis reveals that the physical state of the boriding medium plays a decisive role in defining the activation energies and operational efficiency of electrically assisted boriding processes. These findings provide a mechanistic framework for understanding fast boriding and support the rational design of energy-efficient surface treatments.