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This study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071 wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071 wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034 wt-%.

Process of adding yttrium into the melt is not a common method for ODS steel production. In this paper, 9Cr-ODS alloy is prepared by adding Y or Y and Ti into 9Cr steel through vacuum induction melting. Optical microscopy is employed to determine the size and number of particles, scanning electron microscopy is used to examine particle type, and transmission electron microscopy (TEM) is used to determine the compositions of smaller particles. Micron-size inclusions are observed in the ingots of the #2 and #3 steels; over 30% of the inclusions are 0.5-1 mm in size, and more than 85% are 0.5-3 mm. A large number of Y-Ti-O particles smaller than 0.5 mm are found in the #3 steel, and the density of inclusions reaches 2.69 Â 10 19 m À3 . After heat treatment, the particles size decreases, and Ti effectively promotes carbide precipitation. Some nano-sized Y-Ti-O is detected by TEM. The tensile strength of Y (Y-Ti) alloyed steel reaches 640 (700) MPa at 298 K, and fracturing mainly occurrs via micro-hole aggregation. At 923 K, the tensile strength of the Y (Y-Ti) alloyed steel reaches 254 (270) MPa. These properties are superior to those of 9Cr steel prepared using the same process.

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