Two Kinds of Boron Segregation at Austenite Grain Boundaries

It has been considered that the boron segregation at austenite grain boundaries is responsible for the effect of boron addition on the properties of the steel, such as hardenability, creep fracture, etc.By means of particle tracking autoradiograph (PTA) the grain boundary segregation of boron has been investigated in Fe-30% Ni alloys, quenched from 550-1200℃ at different cooling rates.It is shown that two kinds of grain boundary segregation, equilibrium and aonequilibrium segregation, caused by different mechanisms take place.The nonequilibrium grain boundary segregation during cooling with boron depleted zone adjacent to the grain boundary is very sensitive to e cooling rate, and can be inhibited by rapid quenching.
The temperature effects on these two kinds of grain boundary segregation are different.As the quenching temperature increases, the grain boundary segregation of boron due to equilibrium adsorption decreases, while the non-equilibrium segregation brought to the grain boundaries kinetically increases. The nonequilibrium segregation therefore dominates in the specimens quenched from high temperatures, and the equilibrium segregation dominates in those cooled from low temperatures. For the cooling used in the work described above, the transition temperature for the dominance of cither segregation in the alloys lies in the range of 650-750℃9 increasing with increasing cooling rate.