Microstructure characteristics of 12Cr ferritic/martensitic steels with various yttrium additions

12Cr ferritic/martensitic steels with 0, 0.1 wt%, 0.2 wt% and 0.3 wt% theoretical yttrium (Y) additions were fabricated by vacuum inducting melting and casting method. Solubilities of Y in the 12Cr steels are 0.027, 0.078 and 0.17 for 12Cr-0.1Y, 12Cr-0.2Y and 12Cr-0.3Y, respectively. Phase transformations and microstructure characteristics under different heat-treatment schedules were investigated. The starting temperature of ferrite-to-austenite transformation A(c1) are maintained about 850 degrees C, but the finishing temperature of ferrite-to-austenite transformation A(c3) are about 950, 970, 980 and 1000 degrees C for 12Cr-0Y, 12Cr-0.1Y, 12Cr-0.2Y and 12Cr-0.3Y, respectively, which indicates that Ac3 increases gradually with the addition of Y. Martensite accompanied with a few delta-ferrite is the dominant structure in all the steels. The amount of delta-ferrite shows a strong dependence with the Y content and austenitizing temperature. Area fraction of delta-ferrite increases with the content of Y, which is the ferrite favouring element. The minimum amount of delta-ferrite are achieved at 950 degrees C for 12Cr-0Y, 12Cr-0.1Y, 12Cr-0.2Y and 1000 degrees C for 12Cr-0.3Y. Besides, more carbides precipitate along the martensite laths and grain boundaries in the Y-bearing steel due to the redistribution of carbon between austenite and ferrite resulting from the ferrite favouring element of Y. (C) 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.