Microstructure and Mechanical Properties of a Novel Designed 9Cr-ODS Steel Synergically Strengthened by Nano Precipitates

Oxide dispersion-strengthened (ODS) steels with nano-scale Y2O3 or Y-Ti-O oxides have been considered as potential structural materials used in advanced nuclear systems. In this work, a novel 9Cr-ODS steel, namely, MX-ODS steel, was designed by decreasing carbon content to eliminate conventional M23C6-type carbides and by increasing the content of nitrogen and vanadium to form MX-type precipitates. In addition, the MX-ODS steel was synergistically strengthened by nano-scale MX precipitates and oxides. After fabrication by powder metallurgy, microstructural observation, and mechanical property tests were conducted after different heat treatments. The density of the prepared materials using hot forging instead of hot isostatic pressing was about 98%. Results of the microstructure observation of the MX-ODS steel indicated that after normalizing and tempering, the tempered martensitic structure dominated, and the mean effective grain size was approximately 1 mu m. Moreover, the preferential orientation of coarse-grained and fine-grained mixed grains was not detected. By diminishing carbon content, M C-23(6)-type carbides precipitated at the grain and sub-grain boundaries were eliminated. By contrast, MX-type precipitates with a diameter of approximately 30-200 nm were formed in the matrix. Furthermore, nanoscale Y-rich oxides with an average size of approximately 3.0 nm were dispersed in the matrix, and a number density can reach to approximately 1.9 x 10(23) m(-3). Furthermore,"core-shell"structure precipitates were found, which were spherical in shape with a diameter ranging from 10 to 20 nm. Such precipitates also contained Y, Ta, and O as the core and V as the shell. The mechanical properties indicate that microhardness decreased from 372 to 320 HV with the increase of normalizing temperature from 980 degrees C to 1200 degrees C. In addition, microhardness decreased significantly after tempering but initially increased and then decreased with the increase of tempering temperature from 700 degrees C to 800 degrees C, with a peak microhardness at approximately 750 degrees C. Excellent strength and ductility were obtained after normalizing at 1100 degrees C for 1 h and then tempering at 750 degrees C for 1 h. Yield strength, ultimate tensile strength, and total elongation were 1039 MPa, 1103 MPa, and 20.5% when tested at room temperature and 291 MPa, 333 MPa, and 16% at 700 degrees C, respectively.