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

Degradation of 9Cr-1Mo-V-Nb steel during long term creep deformation is cau sed by a preferential recovery at the vicinity of prior austenite grain bou ndary (PAGB). Changes in microstructure, especially on precipitates have be en investigated in order to understand a mechanism of such heterogeneous re covery. Heterogeneous recovery has been quantitatively demonstrated by die distribution of subgrain size. Three types of the precipitates, M23C6, V(C, N) and Nb(C, N), have been found in the as tempered condition. Precipitati on of Laves phase and Z phase have been found in the creep ruptured specime n. Laves phase is found in all the creep ruptured specimen. On the other ha nd, 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 c oarsening 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 ter m region is one of the reason of preferential recovery at the vicinity of P AGE in 9Cr-1Mo-V-Nb steel.