Microstructural evolution and the effect on hardness of Sanicro 25 welded joint base metal after creep at 973 K

Abstract

The microstructural evolution and the hardness of Sanicro 25 welded joint base metal after creep at 973 K were investigated, aiming to determine which precipitate is the most important to affect the change in hardness. The precipitates in as-received specimen consist of primary NbCrN and NbC. Creep at 973 K results in the precipitation of secondary NbCrN and Cu-rich particles which grow slightly and M23C6 which coarsens noticeably. The precipitation of secondary NbCrN, Cu-rich particles and M23C6 greatly increases the hardness in the early stage of creep. However, when the creep rupture time falls in the range from 582 to 4265 h, the hardness is reduced mainly owing to the growth of Cu-rich particles. With further creep, the growth of both secondary NbCrN and Cu-rich particles decreases the hardness furthermore. It is thus concluded that both secondary NbCrN and Cu-rich particles are the key precipitates to affect the hardness change in Sanicro 25 welded joint base metal after creep at 973 K.