| An investigation was performed to examine the formation process and morphology of the precipitates on the prior-austenite grain boundary of boron bearing steel during continuous cooling. During the faster cooling rate of water quenching from 1350℃ or 20℃/sec from 1350℃ to 750℃ and 850℃, elements of boron and carbon were segregated to the prior-austenite grain boundary, and transformed to the unstable amorphous phase or a meta-stable and a partially crystallized one. The stable phases were Fe_(23)(B,C)_6 at step quenching temperature of 850℃ and Fe₃(C,B) at 750℃, respectively. But during the slower cooling rate of 1℃/sec from 1350℃ to 750℃ and 850℃, segregated boron and carbon were not transformed to a unstable amorphous phase but directly transformed to the stable Fe_(23)(B,C)_6 or Fe₃(C,B). Line fraction and volume fraction of the stable phase on the prior-austenite grain boundary were not depended upon the step quenching temperature and the holding time, but mainly upon the cooling rate from 1350℃ to 750℃ and 850℃. Morphology of precipitates on the prior-austenite grain boundary was changed from small and thin type at the faster cooling rate of water quenching to large and thick one at the slower cooling rate of 1℃ /sec. |
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