改良9Cr-1Mo鋼のクリープ変形に伴うZ相の析出と析出物変化

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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 M_<23>C_6 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℃ and that of Z-phase was found in the long-term region after several thousands hours of creep exposure at both 600 and 650℃. Mean diameter of MX carbonitride was about 1/3 to 1/7 of that of M_<23>C_6 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.
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