Study on microstructural changes in thermally-aged stainless steel weld-overlay cladding of nuclear reactor pressure vessels by atom probe tomography

Takeuchi, Tomoaki   ; Kameda, Jun*; Nagai, Yasuyoshi*; Toyama, Takeshi*; Nishiyama, Yutaka ; Onizawa, Kunio 

Microstructural changes by thermal aging in stainless steel weld overlay cladding of nuclear reactor pressure vessels were investigated using atom probe tomography. The cladding material was composed of about 90% austenite phase and 10% -ferrite phase and thermally aged at 400C for 10,000 h. In the ferrite phase, the thermal aging increased a fluctuation of Cr concentration due to spinodal decomposition and caused the precipitation of G phase with chemical composition of Ni:Si:Mn = 16:7:6. Moreover, significant hardening of the ferrite phase was induced by the thermal aging. On the other hand, the thermal aging did not affect on the microstructures and the hardness in the austenite phase, which indicates the microstructural changes were responsible for the hardening in the ferrite phase. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.