2023 Volume 109 Issue 3 Pages 179-188
In order to understand microstructural changes in 9Cr-1Mo-V-Nb weld metal after long term use, microstructure and precipitates distribution before and after aging at 1013 K were investigated. In the weld metal, regions with coarse or fine prior austenite grains were observed due to thermal cycle during welding. In the coarse grain region, precipitate particles inferred to M23C6 were densely located on grain boundaries, however, in the fine grain regions, they were sparsely observed not only on grain boundaries but also inside grains. Post weld heat treatment (1013 K/7.7 h) followed by aging (1013 K/100 h) led to ferrite grains formation in the fine grain region. EBSD analysis implied that dislocation density in ferrite grains was low. After the aging, mean diameter of particles became coarser and interparticle spacing became sparser in the fine grain region than in the coarse grain region. On the other hand, dislocation density calculated by hardness in martensite structure was almost no deference between these regions before and after the aging. Therefore, it was suggested that ferrite grains were formed because pinning energy by precipitate particles locally reduced in the fine grain region.
