A European 10Cr cast steel, GX12CrMoWVNbN10-1-1, which has been recently used in the steam turbine components of USC power plant, was subjected to thermal aging at 610, 630, and 650°C and creep test. The small punch (SP) test was carried out at the temperature range from -196 to 80°C using those aged and creep damaged steels to examine an applicability of the SP testing technique to the assessment of aging degradation. The experimental results revealed that the maximum load and the displacement to fracture changed depending on the aging and creep conditions, respectively. The temperature corresponding to the SP fracture energy of 1 J, T1J, was employed for investigating the change in ductile-brittle transition property for convenience. The T1J, which changed to the higher temperature side due to aging and creep at the beginning, decreased gradually with increasing Larson-Miller parameter (LMP), resulting in the recovery of embrittlement. Laves phase was clearly observed at the initiation site of quasi-cleavage cracking and the inside of dimple on the fracture surfaces. The T1J was well correlated with the number density of Laves phase, and it increased with increasing density. These results indicated that the precipitation and coarsening of Laves phase was closely associated with the change in SP ductile-brittle transition behavior. Additionally, it was also found that the tensile strength predicted based on the SP test result was relatively in good agreement with the measured one, and it had a tendency to decrease with increasing LMP.
