Hot crack susceptibility in multi-pass welds of reduced activation ferritic/martensitic steel F82H

The research and development of nuclear fusion reactors are going on conducting to acquire a next-generation energy source. It is supposed to be that the reduced activation ferritic/martensitic (RAFM) steel F82H is adopted as a structural material of the blanket module, which is the device set on the inner wall of fusion reactors. As welding for thicker plates of F82H, multi-pass welding will be adopted to the joints. In the case of multi-pass welding, hot cracking is one of serious defects in welds and is concerned in welds reheated by following weld passes. Therefore, the objectives of this study are to evaluate hot crack susceptibility in multi-pass welds of F82H and to clarify the cause of hot cracking in multi-pass welds of F82H. The hot crack susceptibility in multi-pass welds is evaluated by the longitudinal Varestraint test with double-bead and triple-bead types. From the observation of fractured surface occurred in welds of F82H after the Varestraitnt test, the crack is identified as ductility-dip crack. The cause of hot crack in multi-pass welds of F82H is clarified by Vickers hardness test, SEM microstructure observations and X-ray diffraction pattern analyses. Based on these tests, when multi-pass welding is conducted for the F82H steels with high Ta contents, it should be considered to control welding conditions for prevention of hot cracking in weld metal as well as high strength martensitic steels. These results suggest that the cause of the crack is intragranular hardening by the precipitation of TaN during welding thermal cycles.