PHASE INSTABILITY AND TOUGHNESS CHANGE DURING HIGH-TEMPERATURE EXPOSURE OF VARIOUS STEELS FOR THE FIRST WALL STRUCTURAL-MATERIALS OF A FUSION-REACTOR

The objective of the present research is to clarify the phase instabil ity, particularly, the precipitation behavior of carbide and nitride d uring the long term aging in the non-irradiation state of the material s proposed for the first wall structural component of fusion reactors, such as a type 316 austenitic steel, its modified steels, ferritic he at resisting steels and reduced radio-activation materials. The effect of the precipitation behavior on the toughness is also investigated. It is noticed that the toughness was much deteriorated by the formatio n of large amounts of coarse carbides within grains and on grain bound aries during 2.88 x 10(4) ks (8000 h) aging at 873 K and that intergra nular fracture occurred by the impact test at room temperature even in the type 316 steel.