Effect of Normalizing Temperature on Microstructure and Mechanical Properties of a New-type 12%Cr Ferritic/Martensitic Heat Resistant Steel for Sodium Cooled Reactor
GUAN Songyuan1,2, XING Weiwei2, YANG Yaqian2, CHEN Deli3, ZHAO Xia2, LIANG Tian2, MA Yingche2,*, HAO Xianchao2,*
1 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 2 Shi-changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 3 Fushun Special Steel Co., Ltd. Technical Center, Fushun 113001, Liaoning, China
Abstract: To address the limitation that the service temperature of ferritic/martensitic (F/M) heat-resistant steels for sodium-cooled fast reactor (SFR) cladding is restricted below 620 ℃, this work investigated a nuclear-grade modified HT-9 F/M steel (HT-9G). The evolution of the microstructure under various normalizing temperatures was systematically examined, along with the analysis of tensile and creep properties. The results indicate that HT-9G exhibits a fully martensitic microstructure after normalizing at different temperatures followed by tempering at 720 ℃. The primary precipitates are identified as M23C6 carbides, metal-carbon carbides (MC), and metal-carbon/nitrogen compound (MX) phases. As the normalizing temperature increases from 1 000 ℃ to 1 150 ℃, both room-temperature and elevated-temperature (630 ℃) tensile strengths show a monotonic increase. This enhancement is attributed mainly to the refinement of MX and M23C6 precipitates along grain boundaries and the ove-rall rise in dislocation density at higher normalizing temperatures. Strengthening effects due to precipitates and dislocations are thus considered the dominant contributors to the improved tensile strength. However, the coarsening of prior austenite grains reduces their ability to impede dislocation motion, resulting in slightly diminished plasticity at both room temperature and 630 ℃ due to decreased deformation compatibility. In terms of high-temperature performance, the creep resistance of the alloy is significantly improved with increasing normalizing temperature. Under the condition of 700 ℃ and 100 MPa, the creep rupture life is extended from 62.51 h to 658.13 h, while the steady-state creep rate decreases markedly from 0.084 0%/h to 0.006 5%/h. This improvement is primarily ascribed to the growth of prior austenite grains and the corresponding reduction in grain boundary area, which effectively suppress grain boundary sliding and the nucleation and growth of creep cavities, leading to a substantial reduction in the steady-state creep rate.
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2026, Vol. 40 
