PRECIPITATION OF Z-PHASE AND DEGRADATION BEHAVIOUR OF Mod.9Cr-1Mo STEEL

摘要

Changes in precipitates and microstructure during creep deformation at 873 and 923K have been investigated on a modified 9Cr-lMo steel, in conjunction with degradation behaviour. Three types precipitates of M{sub}23C{sub}6, Nb(C,N) and V(C,N) have been observed in the as tempered condition and in the creep ruptured specimens. Precipitation of Laves phase (Fe{sub}2Mo) has been detected in the specimens creep ruptured at 873K. Moreover, precipitation of Z-phase which is a complex nitride in a form of Cr(Nb,V)N, has been found in the specimens creep ruptured after long-term creep deformation for more than about 10,000h and 5,000h at 873K and 923K, respectively. Precipitation of Z-phase preferentially takes place at the vicinity of prior austenite grain boundary (PAGB), as a result of presence of coarse primary MX type carbonitride particles on PAGB. The coarsening rate of Z-phase is larger than those of M{sub}23C{sub}6 and MX, and it is much fester at 923K than that at 873K. Tendency of decrease in the number density of MX particles has been observed with progress in precipitation and coarsening of Z-phase. It has been concluded that precipitation of Z-phase is one of the reasons which promote a preferential recovery at the vicinity of PAGB during long-term creep exposure.