In order to provide the technical basis for the judgment of the needs of surveillance specimens of heat-affected zone (HAZ) in reactor pressure vessel (RPV) steels, we performed a research on the characterization of metallurgical and mechanical properties of the HAZ in RPV steels. The distributions of grain size and phases were drawn as a map based on temperature histories around HAZ obtained from welding simulation. Referring to the HAZ map, typical HAZ materials were made by simulating temperature histories including postweld heat treatment (PWHT) within the HAZ. Metallurgical and mechanical characteristics for those HAZ materials were compared with those of base metal. From the results, it is clear that mechanical properties of HAZ materials depend not on the prior austenitic grain size but mostly on the phases. Concerning on the fracture toughness in HAZ, HAZ materials close to weld metal indicated higher toughness than that of base metal due to mixed structure of martensite and lower-bainite, while HAZ materials close to base metal were equivalent or slightly lower toughness than that of base metal due to the same phase as base metal of upper-bainite.

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