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 conven-tional M23C6-type carbides and by increasing the content of nitrogen and vanadium to form MX-type pre-cipitates.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 forg-ing 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 μm.Moreover,the preferential orientation of coarse-grained and fine-grained mixed grains was not detected.By diminishing carbon content,M23C6-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,nano-scale 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 1.9×1023 m-3.Furthermore,"core-shell"structure precipi-tates were found,which were spherical in shape with a diameter ranging from 10 to 20 nm.Such precipi-tates 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℃ to 1200℃.In addition,microhardness decreased significantly after tempering but initially increased and then decreased with the increase of tempering temperature from 700℃ to 800℃,with a peak microhard-ness at approximately 750℃.Excellent strength and ductility were obtained after normalizing at 1100℃ for 1 h and then tempering at 750℃ 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℃,respectively.
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