Mechanical and Microstructural Characterization of Heat-Affected Zone Materials of Reactor Pressure Vessel

In order to monitor the neutron irradiation embrittlement of the reactor pressure vessel (RPV) steels for the safe operation of light-water reactors, surveillance specimens of representative materials, i.e. base metal, weld metal and heat affected zone (HAZ), are installed in the RPV during reactor operation according to the regulation. Among these materials, HAZ specimens exhibit a relatively large scatter in Charpy impact properties because of the microstructural inhomogeneity due to multi-pass welding. ASTM E185 and JSME S NC1 stipulate the exception of HAZ specimens from surveillance test. However, the technical basis on the exception has not been established. Therefore, we have started a research on the irradiation embrittlement in HAZ material of RPV steels. Typical RPV steel plates with different impurity levels and their weldments were fabricated to characterize the microstructures and subsequent mechanical properties of typical HAZ materials. Simulated HAZ materials were also made by applying several heat treatments representative of HAZ. Finite element analysis was conducted to draw maps of distributions of grain size and phase-fraction. Using simulated HAZ materials with different grain size and phase before irradiation, mechanical properties such as hardness, Charpy impact property and fracture toughness were measured in comparison with those of base metals and actual HAZ materials. Through the comparison, it was indicated that mechanical properties such as fracture toughness in some simulated HAZ materials were different from base metal and dependent significantly on the metallurgical structure, particularly phase but prior austenitic grain size. Higher fracture toughness in CGHAZ (Coarse-Grain HAZ) materials compared to base metal is due to mixed structure of martensite and lower-bainite phases. Upper-bainite phase in FGHAZ (Fine-Grain HAZ) and base materials causes lower fracture toughness than CGHAZ materials.