austenite grain boundary in quenched specimens increases with increasing qued- c hing temperature and is related to and dominated by a process other than eq- uilibrium adsorption.In the present work,it has been found that at any posit- ion of the Jominy bar,the grain boundary segregation develops continuously during cooling and the final extent of segregation varies with Jomiay 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 bouudary segregation increases with increasing quenching tempe- rature.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 A,where Ai is the cooling rate or X= H(Ti)/vA.H (Ti)increase with increasing quenching temperature and is a ref- lection of the rale 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 boun- dary and the incubation of the austenite decomposition,and is at its best when the correlation of both rates is optimum. 硼钢淬透性与硼偏聚图版 (a)800C 7mm 10.5mm 图6 163立 立 级 五 直 ， ， 一 一 ， ， 一 亿万 ， 侧 · ， 。 硼钢淬透性与硼 偏聚图版 。 图