THE MECHANISM OF NON-EQUILIBRIUM SEGREGATION OF BORON TO AUSTENITE GRAIN BOUNDARIES

It has been demonstrated that the boron segregation at austenite grain boundaries in quenched steel is a non-equilibrium grain boundary segregation phenomena occurred during cooling. In this paper, by way of resolving nonisothermal diffusion equation, a theoretical formula for non-equilibrium grain boundary segregation has been derived, and the relationship among width of boron depleted zone, quenching temperature, cooling rate, activation energy and diffusion constant of non-equilibrium grain boundary segregation has been established. The theoretical prospects agreed with experimental results satisfactorily.From experimental results and theoretical analysis, an atomic model for nonequilibrium grain boundary segregation has been suggested, based on the moving of boron atoms along with supersaturationies vacancies or divacancies to grain boundaries (sink of vacancy) during cooling. In light of this new idea of nonequilibrium grain boundary segregation, the complicated effects of boron on hardenability of steel can be explained unambiguously.