Evaluation of ductile–brittle transition temperature before and after neutron irradiation for RPV steels using small punch tests

Small punch (SP) tests were performed to evaluate the ductile–brittle transition temperature before and after a neutron irradiation of reactor pressure vessel (RPV) steels produced by different manufacturing (refining) processes. The results were compared to the standard transition temperature shifts from the conventional Charpy tests and the Master Curve fracture toughness tests in accordance with the American Society for Testing and Materials (ASTM) standard E1921. Small punch specimens were taken from a 1/4t location of the vessel thickness and machined into a 10 mm × 10 mm × 0.5 mm dimension. The specimens were irradiated in the research reactors at Korea Atomic Energy Research Institute Nuclear Research Institute in the Czech Republic at the different fluence levels of about 290 °C. Small punch tests were performed in the temperature range of RT to −196 °C using a 2.4 mm diameter ball. For the materials before and after irradiation, the small punch transition temperatures (TSP), which are determined at the middle of the upper small punch energies, showed a linear correlation with the Charpy index temperature, T41 J. TSP from the irradiated samples was increased with the fluence levels and was well within the deviation range of the unirradiated data. However, the transition temperature shift from the Charpy test (ΔT41 J) shows a better correlation with the transition temperature shift (ΔTSP(E)) when a specific small punch energy level rather than the middle energy level of the small punch curve is used to determine the transition temperature. TSP also had a correlation with the reference temperature (T0) from the Master Curve method using a pre-cracked Charpy V-notched (PCVN) specimen.