原子炉用60キロ級高張力鋼溶接部の長時間加熱による材質の変化

Abstract

The effects of exposure at 550°C for times up to 5, 000hr on the notch ductility and the microstructure change of welded joints of Ni-Cr-Mo type HT60 tempered steel were investigated. Test steel, 100 mm thick, was produced experimentally for nuclear pressure vessels and welded joints were made by the electroslag welding and the CO2 arc welding.The results of the study indicated that the base metal, the weld metal and the heat-affected zone (HAZ) of base metal exposed at 550°C for times up to 5, 000 hr tended to embrittle owing to the grain growth of ferrite and the cohering of carbide toward grain boundaries. This process of change in microstructure may be divided into the following three kinds of stage. There are in different combination of the three kind of stage due to the structure before exposure.(I) stage ; Tempering of martensite and spheroidizing of carbide(II) stage ; Formation of fine ferrite(III) stage ; Grain growth of ferrite and, cohering and coasening of carbide toward grain boundariesThe chang in microstructure for the base metal occur with (III) stage only ; the HAZ with electroslag welding (II) → (III) stages ; the HAZ with CO2 are welding (I) → (II) → (III) stages, respecitvely.