| The influence of solidification rates on the morphology and distribution of δ-ferrite and eutectic niobium carbides was studied using Bridgman-type unidirectional solidification apparatus in order to simulate the solidification pattern of a large-scale ingot of 12% chromium rotor steel. The volume fractions of the δ ferrite contained in the specimens solidified with the withdrawal speeds of 225 ㎜/hr, 45 ㎜/hr, and 4.5 ㎜/hr were measured to be 7%, 5%, and 30% respectively. δ-ferrite was finely distributed in the case of the withdrawal speeds of 225 ㎜/hr and 45 ㎜/hr, but the distribution was coarse network type in the case of the withdrawal speed of 4.5 ㎜/hr. When hot compressive working was applied in temperatures from 950℃ to 1250℃, 3-ferrite was transformed into carbides and austenite at the interface of δ-ferrite with the matrix. Subsequent heat treat melts result in a complete annihilation of δ-ferrite in the case of fast withdrawal speeds, but result in only a partial annihilation in the ca -e of the slow withdrawal speed, 4.5 ㎜/hr. |
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