Effects of Heat Treatment on Microstructure and Mechanical Properties of a Bimodal Grain Ultra-Low Carbon 9Cr-ODS Steel

doi:10.11900/0412.1961.2020.00507

Acta Metall Sin

2022, Vol. 58

Issue (5): 623-636 DOI: 10.11900/0412.1961.2020.00507

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Effects of Heat Treatment on Microstructure and Mechanical Properties of a Bimodal Grain Ultra-Low Carbon 9Cr-ODS Steel

ZHANG Jiarong¹^,², LI Yanfen²^,³(

), WANG Guangquan²^,⁴, BAO Feiyang²^,⁴, RUI Xiang²^,⁴, SHI Quanqiang²^,³, YAN Wei²^,³, SHAN Yiyin²^,³(

), YANG Ke²

^1.School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
^2.Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
^3.CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
^4.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

Cite this article:

ZHANG Jiarong, LI Yanfen, WANG Guangquan, BAO Feiyang, RUI Xiang, SHI Quanqiang, YAN Wei, SHAN Yiyin, YANG Ke. Effects of Heat Treatment on Microstructure and Mechanical Properties of a Bimodal Grain Ultra-Low Carbon 9Cr-ODS Steel. Acta Metall Sin, 2022, 58(5): 623-636.

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Abstract

Oxide dispersion strengthened (ODS) steel is a promising structural material for advanced nuclear power systems. In this study, an ultra-low carbon 9Cr-ODS steel with a bimodal grain structure was prepared using powder metallurgy, and a superior matching of strength and plasticity was expected by adjusting the soft-hard matching of the coarse-grained and fine-grained regions. The effects of heat treatment on microstructure and mechanical properties of the ultra-low carbon 9Cr-ODS steel were evaluated through OM, SEM, TEM, microhardness, and tensile tests. The results demonstrated that the ultra-low carbon 9Cr-ODS steel exhibited a tempered martensite structure after normalizing at 1050-1200^oC, and then tempering at 700 and 750^oC. Moreover, it presented the microstructure characteristics of coarse-grained and fine-grained regions, in which the average grain size of fine-grained regions was 1.6 μm and that of coarse-grained regions was 4.3 μm. The dislocation density in the ultra-low carbon 9Cr-ODS steel was very high and the number density of nano-scale oxide particles was up to about 10²² m^-3. The microhardness in fine-grained regions was higher than that in coarse-grained regions. As the normalizing temperature increased, the microhardness of the ultra-low carbon 9Cr-ODS steel first increased and then decreased. The microhardness reached the highest after normalizing at 1100^oC. When the normalizing temperature increased to 1200^oC, the microhardness decreased due to the growth of austenitic grains. Regarding the tempering temperature, the microhardness first decreased and then increased as the tempering temperature increased from 700^oC to 800^oC. Furthermore, the decrease in microhardness when tempering at 700 and 750^oC was because the microstructure was recovered and softened. The higher the tempering temperature, the lower the microhardness. However, when tempering at 800^oC, the microhardness increased significantly, mainly due to the partial austenite transformation of martensite. The tensile test results at 25^oC showed that the strength of the ultra-low carbon 9Cr-ODS steel first decreased and then increased by increasing the tempering temperature, which was consistent with the microhardness change while the opposite was observed for elongation. The tensile test results at 700^oC showed that the strength of the ultra-low carbon 9Cr-ODS steel slightly decreased by increasing the tempering temperature. Moreover, the fracture morphology was dominated by fine dimples and secondary tearing, indicating that the ultra-low carbon 9Cr-ODS steel underwent ductile fracture. Combined with the mechanical property and fracture analysis results, the ultra-low carbon 9Cr-ODS steel exhibited superior matching of strength and plasticity after normalizing at 1150^oC for 1 h and tempering at 750^oC for 1 h.

Key words: ODS steel heat treatment bimodal grain microstructure mechanical property

Received: 18 December 2020

ZTFLH:

TG156.1

Fund: National Natural Science Foundation of China(51971217);Excellent Scholar Funding of Institute of Metal Research, Chinese Academy of Sciences(JY7A7A111A1)

About author: LI Yanfen, professor, Tel: (024)23978990, E-mail: yfli@imr.ac.cnSHAN Yiyin, professor, Tel: (024)23971517, E-mail: yyshan@imr.ac.cn

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	Jiarong ZHANG
	Yanfen LI
	Guangquan WANG
	Feiyang BAO
	Xiang RUI
	Quanqiang SHI
	Wei YAN
	Yiyin SHAN
	Ke YANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00507 OR https://www.ams.org.cn/EN/Y2022/V58/I5/623

Table 1 Heat treatments design for ultra-low carbon 9Cr-ODS steel

Fig.1 SEM images of powders before (a) and after (b) ball milling for 90 h

Fig.2 Distribution diagram of particle size before and after ball milling

Fig.3 OM images of ultra-low carbon 9Cr-ODS steel normalized at 1150^oC, 1 h and tempered at different temperatures
(a) 1150^oC, 1 h, AC (b) 1150^oC, 1 h, AC + 700^oC, 1 h, AC
(c) 1150^oC, 1 h, AC + 750^oC, 1 h, AC (d) 1150^oC, 1 h, AC + 800^oC, 1 h, AC

Fig.4 OM images of ultra-low carbon 9Cr-ODS steel under different heat treatmens
(a) 1050^oC, 1 h, AC + 750^oC, 1 h, AC (b) 1050^oC, 1 h, AC + 800^oC, 1 h, AC
(c) 1100^oC, 1 h, AC + 750^oC, 1 h, AC (d) 1100^oC, 1 h, AC + 800^oC, 1 h, AC
(e) 1200^oC, 1 h, AC + 750^oC, 1 h, AC (f) 1200^oC, 1 h, AC + 800^oC, 1 h, AC

Fig.5 Low (a, b) and high (c, d) magnified bright field TEM images (a, c) and corresponding high angle annular dark field (HAADF) images (b, d) of ultra-low carbon 9Cr-ODS steel normalized at 1150^oC, 1 h and tempered at 750^oC, 1 h

Fig.6 Microhardness of ultra-low carbon 9Cr-ODS steel under different heat treatments
(a) influence of normalizing temperature on microhardness of coarse-grained and fine-grained regions
(b) influence of tempering temperature on microhardness of coarse-grained (b1) and fine-grained (b2) regions after different temperature normalizations

Fig.7 Tensile properties of ultra-low carbon 9Cr-ODS steel under different heat treatments tested at room temperatur (25^oC)
(a) dependence of strength on tempering temperature (σ_b—ultimate tensile strength, σ_0.2—yield strength)
(b) dependence of total elongation on tempering temperature

Fig.8 Tensile properties of ultra-low carbon 9Cr-ODS steel under different heat treatments tested at 700^oC
(a) dependence of strength on tempering temperature
(b) dependence of total elongation on tempering temperature

Fig.9 Low (a, c, e) and locally high (b, d, f) magnified tensile fracture SEM images of ultra-low carbon 9Cr-ODS steel tested at room temperature after normalization at 1150^oC, 1 h + tempering at 700^oC (a, b), 750^oC (c, d), and 800^oC (e, f) (CG represents coarse-grained region and FG represents fine-grained region)

Fig.10 Low (a, c, e) and locally high (b, d, f) magnified tensile fracture SEM images of ultra-low carbon 9Cr-ODS steel tested at 700^oC after normalization at 1150^oC, 1 h + tempering at 700^oC (a, b), 750^oC (c, d), and 800^oC (e, f)

Fig.11 Distributions of grain size of fine-grained regions (a) and coarse-grained regions (b) after normalizing at 1150^oC for 1 h and tempering at 750^oC for 1 h

Fig.12 Schematics of fracture of ultra-low carbon 9Cr-ODS steel with bimodal grain structure
(a) a bimodal microstructure after rolling
(b) cavitation, crack initiation, and crack tip blunting
(c) crack propagation and fracture failure

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