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Acta Metall Sin  2023, Vol. 59 Issue (12): 1590-1602    DOI: 10.11900/0412.1961.2021.00534
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Microstructure and Mechanical Properties of a Novel Designed 9Cr-ODS Steel Synergically Strengthened by Nano Precipitates
RUI Xiang1,2, LI Yanfen1,2,3(), ZHANG Jiarong2,3, WANG Qitao1,2, YAN Wei1,2,3, SHAN Yiyin1,2,3
1School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
2Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
3CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article: 

RUI Xiang, LI Yanfen, ZHANG Jiarong, WANG Qitao, YAN Wei, SHAN Yiyin. Microstructure and Mechanical Properties of a Novel Designed 9Cr-ODS Steel Synergically Strengthened by Nano Precipitates. Acta Metall Sin, 2023, 59(12): 1590-1602.

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Abstract  

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 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 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 980oC to 1200oC. In addition, microhardness decreased significantly after tempering but initially increased and then decreased with the increase of tempering temperature from 700oC to 800oC, with a peak microhardness at approximately 750oC. Excellent strength and ductility were obtained after normalizing at 1100oC for 1 h and then tempering at 750oC 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 700oC, respectively.

Key words:  ODS steel      MX precipitates      nano-oxide      microstructure      mechanical property     
Received:  06 December 2021     
ZTFLH:  TL341  
Fund: National Natural Science Foundation of China(51971217);Excellent Scholar Funding of Institute of Metal Research, Chinese Academy of Sciences(JY7A7A111A1)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2021.00534     OR     https://www.ams.org.cn/EN/Y2023/V59/I12/1590

Fig.1  Dependence of carbon content on the amount of M23C6 and MX precipitates calculated by Jmat Pro software at 800oC
Fig.2  Thermodynamic calculation according to measured chemical compositions (Fe-8.82Cr-0.99W-0.96Mn-0.39V-0.097Ta-0.12N)
(a) relationship between equilibrium phase and temperature (Inset is the local magnified curve)
(b) dependence of carbon content on M23C6 amount
Fig.3  EBSD images and grain statistics of the novel MX-ODS steel (Heat treatment: normalizing at 1100oC for 1 h and tempering at 750oC for 1 h; in Figs.3a and b, blue lines represent large angle grain boundary (≥ 15°), yellow lines represent small angle grain boundary (< 15°))
(a) inverse pole figure (b) band contrast
(c, d) histograms of misorientation distribution (c) and grain size distribution (d)
Fig.4  SEM images of precipitates (a, c, e) and EDS results corresponding to the precipitates indicated by the arrows (b, d, f) for the novel MX-ODS steel at different conditions
(a, b) as rolled (c, d) normalizing at 1100oC for 1 h
(e, f) normalizing at 1100oC for 1 h and then tempering at 750oC for 1 h
Fig.5  TEM images of microstructure and size distribution of nano-scale precipitates for the novel MX-ODS steel (Heat treatment: normalizing at 1100oC for 1 h and tempering at 750oC for 1 h)
(a) low magnification image (b) high magnification image
(c) distribution of nano-scale precipitates (d) size distribution of nano-scale precipitates
Fig.6  Scanning transmission electron microscopy equipped with high angular annular dark field (STEM-HAADF) image and EDS element maps of V, N, Y, O, and Ta in the novel MX-ODS steel (The rectangular boxes show VN precipitates, and the cycles show Y-Ta-O precipitates; heat treatment: normalizing at 1100oC for 1 h and tempering at 750oC for 1 h)
Fig.7  STEM-HAADF and bright-field (BF) images for precipitates and EDS element maps of V, Y, Ta, and O in the novel MX-ODS steel (Heat treatment: normalizing at 1100oC for 1 h and tempering at 750oC for 1 h)
Fig.8  Dependence of microhardness on heat treatments for the novel MX-ODS steel
(a) normalizing temperature
(b) tempering temperature
Fig.9  Tensile stress-strain curves of the novel MX-ODS steel at room temperature (a) and 700oC (b) after different heat treatments (Heat treatments: normalizing at 1100oC for 1 h or at 1150oC for 1 h, and then tempering at 750oC for 1 h)
Phaseλ / nmr / nmσp / MPa
MX precipitate40075226
Nano-oxide40.311.5612
Table 1  Contributions of MX precipitates and nano-oxides to yield strength, respectively
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