Heterogeneous recovery and precipitation of Z phase during long term creep deformation of modified 9Cr-1Mo steel

Degradation of 9Cr-1Mo-V-Nb steel during long term creep deformation is caused by a preferential recovery at the vicinity of prior austenite grain boundary (PAGB). Changes in microstructure, especially on precipitates have been investigated in order to understand a mechanism of such heterogeneous recovery. Heterogeneous recovery has been quantitatively demonstrated by die distribution of subgrain size. Three types of the precipitates, M(23)C(6), V(C, N) and Nb(C, N), have been found in the as tempered condition. Precipitation of Laves phase and Z phase have been found in the creep ruptured specimen. Laves phase is found in all the creep ruptured specimen. On the other hand, Z phase is found in the specimens creep ruptured after more than 10000 h. Complex nitride phase of Z phase is rich in Cr, Nb and V of the form Cr(Nb, V)N. Precipitation of Z phase takes place in the long term region and coarsening rate of it is much faster than that of MX, such as V(C,N) and Sb(C,N). It has been concluded that a precipitation of Z phase in the long term region is one of the reason of preferential recovery at the vicinity of PAGE in 9Cr-1Mo-V-Nb steel.