Development of manufacturing method of excess oxygen controlled ODS martensitic steel

ODS martensitic steel is a prospective candidate cladding material for the advanced Fast Breeder Reactor (FBR) core components. A previous result showed that the high temperature strengths were deteriorated by the oxide particle coarsening and the reduction of the residual-alpha grains as excess oxygen (Ex.O) increased. In this study, the manufacturing conditions for reducing excess oxygen were examined in order to obtain the target strength. High excess oxygen steels with higher Y2O3 addition were also manufactured with the intention of manufacturing the steel which has both fully martensitic microstructure and enough high temperature strength. The derived results can be summarized as follows. (1) It was shown that 0.04wt% excess oxygen was brought from the pre-alloyed raw powders, and 0.04-0.1 wt% excess oxygen was mixed in the powders during the Mechanical Allowing (MA). (2) It was shown that excess oxygen could be reduced less than 0.1wt% by applying an ultra high purity Ar gas (99.9999wt%Ar) to MA atmosphere as well as reducing the agitating energy by using pin agitator with shorter length. (3) Residual alpha-grains and transformed alpha-grains were mixed in the furnace-cooled low excess oxygen steels (< 0.1wt%Ex.O). These steels are expected to have the equal high temperature strength to Mm11 because their Vickers hardness is approximately the same as Mm11. (4) Full martensitic microstructure was obtained in the higher Y2O3 and excess oxygen-added steels (E1, E2). However enough high temperature strength are not expected in these steels because the Vickers hardness of these steels after the furnace-cooling (FC) or normalizing and tempering (NT) are apparently low. (author)