Microstructual change of welded joints in 2.25Cr-1Mo steel during a creep deformation was observed with focusing attention on the crystal orientation and the growth of voids. Orientation Imaging Microscopy was used to analyse the crystal orientation. From results, it was revealed that the plastic deformation intensively occurred in the heat affected zone (HAZ) at early stage of accelerated creep region. Generations of void were observed in HAZ of a creep interrupted sample whose test time was 70% of a rupture time. Therefore, it is suggested that the concentration of voids bought about the plastic deformation in HAZ. Voids were generated at prior austenite boundaries in the coarse grained HAZ. In the fine grained HAZ, they were generated at the triple junction of grain boundary. Therefore, the growth and coupling of voids were comparatively easy in the coarse grained HAZ.
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