Abstract: The process of boron segregation to austenite grain boundary has been inv-erstigated by particle-tracking autoradiography of Jominy bar of a medium carbon boron steel (0.54%C, 0.75%Mn and 0.0027% acid soluble boron) quenched from different temperatures (800-1150℃) after heating to a same high austen-itizing temperature (1150℃). It has been shown that the boron segregation to austenite grain boundary in quenched specimens increases with increasing quenching temperature and is related to and dominated by a process other than equilibrium adsorption. In the present work, it has been found that at any position of the Jominy bar, the grain boundary segregation develops continuously during cooling and the final extent of segregation varies with Jominy distance. The grain boundary segregation intensifies as the cooling rate decreases and changes from continuous boron-enriched networks to discontinuous aggregation and even to discrete precipitation of boron constitutents. The developing rate of grain boundary segregation increases with increasing quenching temperature. The boron concentration profiles and the widths of boron-depleted zone have been experimentally measured, and the developing rate of grain boundary segregation estimated and related to the hardenability of boron steel.
Under a given quenching temperature, the width of the boron-depleted zone X is inversely proportional to $\sqrt {\rm{A}} $, where Ai is the cooling rate or X=H(Ti)/$\sqrt {\rm{A}} $.H (Ti) increase with increasing quenching temperature and is a reflection of the rate of grain boundary segregation and effects of composition and other structural factors. It is suggested that the effect of boron on the ha-rdenability depends on the relative rate of segregation to austenite grain boundary and the incubation of the austenite decomposition, and is at its best when the correlation of both rates is optimum.