EFFECTS OF ADDITIONAL ELEMENTS ON THE EVOLUTION OF SECOND PHASES IN 9-12% Cr STEEL AND RESULTING MECHANICAL PROPERTIES

Investigated in this study are precipitate evolution with and without addition of W, Co, and B in two kinds of 9-12% Cr steels (named as A and B) used for power plants after various aging time and temperature using OM, SEM, TEM, etc. Three kinds of precipitates (Cr-rich M23C6, Nb-rich and V-rich MX, W-rich and Mo-rich Laves phase) were observed and investigated in the two alloys. Upon aging, the area fraction of M23C6 increased whereas that of Laves phases decreased despite of increase in size. The area fraction of W-rich Laves phase was much higher than that of Mo-rich Laves phase, indicating that W addition, compared to that of Mo addition, is more powerful in the formation of Laves phase precipitation (specimen A). The martensitic microstructure of specimen B was more stable than that of specimen A due to the addition of cobalt and boron. The tensile test and impact test were measured and studied in relation to the long term aging effect.