Modified 9Cr-1Mo steel is a primary candidate material for reactor pressure vessel of Very High Temperature Gas-Cooled Reactor (VHTR) in Korean Nuclear Hydrogen Development and Demonstration (NHDD) program. In this study, T0 reference temperature, J-R fracture resistance and Charpy impact properties were evaluated for commercial Grade 91 steel as preliminary tests for the selection of the RPV material for VHTR. The fracture toughness of the modified 9Cr-1Mo steel was compared with those of SA508-Gr.3. The objective of this study was to obtain pre-irradiation fracture toughness properties of modified 9Cr-1Mo steel as reference data for the radiation effects investigation. The results are as follows. Charpy impact properties of the modified 9Cr-1Mo steel were similar to those of SA508-Gr.3. T_0 reference temperatures were measured as -67.7°C and -72.4°C from the tests with standard PCVN (pre-cracked Charpy V-notch) and half sized PCVN specimens respectively, which were similar to results for SA508-Gr.3. The K_(Jc) values of modified 9Cr-1Mo with test temperatures are successfully expressed with the Master Curve. The J-R fracture resistance of modified 9Cr-1Mo steel at room temperature was almost the same as that of SA508-Gr.3. On the other hand it was a little bit higher at an elevated temperature.
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