Adding small amounts of boron in low carbon steels has a positive effect on hardenability. This is commonly explained by boron segregation at prior austenite grain boundaries delaying the austeniteto ferrite phase transformation during cooling, or during isothermal bainitic transformation. We investigated boron segregation at austenite grain boundaries after soaking in a high-strength low carbon steel using high resolution secondary ion mass spectrometry (nano-SIMS) and atom probe tomography (APT). We found that boron segregation at grain boundaries increases with the soaking temperature. This is due to boride precipitate dissolution, which increases the amount of solute boron in the grains. Using a finite difference modeling of the Onsager equation for diffusion, together with the hypothesis of local equilibrium at the grain boundaries, it was possible to fit the concentration profiles of boron in the vicinity of the grain boundaries. The diffusion coefficient and this gregation enthalpy of boron were identified. These results have important practical consequences for controlling the levels of segregated boron in steels.
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