Effects of Nb and Ta on the Formation of "A" Segregation and Carbo-nitride in 12%Cr Steel

Abstract

Experiments are carried out on 12% Cr steel, which is widely used for such applications as generator components, to study the effect of Nb and Ta on the formation of "A" segregation and eutectic-type carbo-nitrides in the segregates.
(1)Nb and Ta have an effect of making dendrite sturcture fine and reducing "A" segregation. Critical secondary arm spacing whether "A" segregation forms or not is around 600 μm.
(2)In "A" segregation streaks, number of eutectic-type carbo-nitrides increases with increasing solute enrichment. The formation of the carbo-nitrides is promoted more strongly by the addition of Nb than that of Ta.
(3)Carbo-nitrides form when the solidus temperature of "A"segregate is decreased down to the precipitation temperature of the carbo-nitrides which is around 1400°C.