Changes in type, size and number density of precipitates during creep exposure have been investigated in Mod.9Cr-1Mo steel, and precipitation behaviour of Z-phase and its influence on another precipitates and creep strength has been discussed. Three type precipitates of M23C6 carbide, NbX and VX carbonitrides were detected in the as tempered condition. Precipitation of Laves phase was observed after short-term creep exposure at 600°C and that of Z-phase was found in the long-term region after several thousands hours of creep exposure at both 600 and 650°C. Mean diameter of MX carbonitride was about 1/3 to 1/7 of that of M23C6 carbide, and those phases were roughly separated by size. With precipitation and rapid coarsening of Z-phase, decrease in number density of fine MX carbonitride and disappearance of NbX was observed. Penetration of nitrogen from atmosphere was not observed during creep deformation. A lot of fine MX carbonitride at the vicinity of prior austenite grain boundary caused by coarse primary MX should promote precipitation of Z-phase. Composition of metallic elements in NbX, VX and Z-phase was limited in small range and unchangeable independent of temperature and exposure time. Nucleation and rapid coarsening of Z-phase after long-term creep exposure results in decrease in creep strength as a result of decrease in precipitation strengthening effect of fine MX carbonitride.
