高Crフェライト系耐熱鋼溶接継手のクリープ劣化

概要

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Microstructures of Gr.122 and Gr.92 weld joints were investigated mainly by electron backscatter diffraction pattern (EBSP) in order to clarify the difference of creep void initiation site between 9Cr and 12Cr steels. Creep tests of welded specimens were conducted at 898 and 923K for 90MPa. Most of the initial grain boundaries of Gr.122 had straight configurations and their rotation angles were around 60° in both the base metal and HAZ. In the creep ruptured specimens, many fine grains and sub-boundaries were observed in addition to the block boundaries and consequently the decrease of the boundaries with 60° misorientation angle and the increase of boundaries with low misorientation angle were detected. This tendency was relatively remarkable in fine grained HAZ. It seemed that the recovery of martensite structure in fine grained HAZ was faster than other region. In addition, creep voids formed on the prior austenite boundaries in fine grained HAZ of Gr.122 weld. On the other hand, the ratio of random grain boundaries in fine grained HAZ of the as-heat treated conditions was higher in the Gr.92 than in the Gr.122. Many creep voids formed on the random grain boundaries in the Gr.92 weld. It seemed that the formation of creep voids in Gr.122 weld was suppressed because the martensite structure in fine grain HAZ was more stable for the Gr.122 weld than for Gr.92 weld.