Effect of Y/Zr Ratio on Inclusions and Mechanical Properties of 9Cr-RAFM Steel Fabricated by Vacuum Melting
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Effect of Y/Zr Ratio on Inclusions and Mechanical Properties of 9Cr-RAFM Steel Fabricated by Vacuum Melting Guoxing Qiu, Dongping Zhan, Changsheng Li, Min Qi, Zhouhua Jiang, and Huishu Zhang (Submitted August 30, 2018; in revised form November 25, 2018; published online January 22, 2019) This study investigates the inclusions, microstructures, tensile properties, and impact toughness of reducedactivation ferritic/martensitic (RAFM) steels with different Y/Zr ratio contents. An optical microscope and a scanning electron microscope (SEM) were employed to determine the size and number of inclusions, and SEM and transmission electron microscope were employed to examine the inclusion types. The increase in Y/Zr ratio influenced the prior austenite grain size owing to the pinning effect of the inclusions. The result of this examination showed that the mechanical properties of the steel are closely related to the refinement of prior austenite grain size and as well as the fine submicron-sized inclusions. The average prior austenite grain sizes of the alloys were 14.5, 13.5, and 13.4 lm for the steels with different Y/Zr ratio contents (i.e., 1/ 2, 5/3, and 11/2, respectively). Moreover, the submicron-sized inclusions in the alloys achieved densities of 4.71 3 1016, 4.49 3 1016, and 3.84 3 1016/m3. The ductile-to-brittle transition temperature (DBTT) decreases when the Y/Zr ratio increases to 5/3. However, the DBTT increases when the Y/Zr ratio reaches 11/2 owing to inclusion coarsening. The RAFM steel with Y content of approximately 0.005 wt.% and Zr content of 0.002 wt.% was verified to exhibit an optimized combination of microstructures, tensile properties, and impact toughness. Keywords
reduced-activation ferritic/martensitic steel, Y/Zr ratio, inclusion, tensile strength, ductile-to-brittle transition temperature
1. Introduction Reduced-activation ferritic/martensitic (RAFM) steels are the primary candidates for first wall and blanket structures of D–T fusion reactors owing to their good swelling resistance and thermophysical and thermomechanical properties (Ref 1-3). The popular solid solution and precipitation strengthening elements (i.e., niobium (Nb) and molybdenum (Mo)) are replaced with tantalum (Ta) and tungsten (W) to ensure low activation of the RAFM steel (Ref 4). To further improve the mechanical properties of RAFM steels, many studies were conducted by scientists worldwide. Their research mainly focused on two aspects: optimizing the previously obtained alloy content and adding new alloying elements. The former aspect mainly aims to optimize the content of W and Ta (Ref 5, 6). The chemical composition optimization has been extenGuoxing Qiu, State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China; and School of Metallurgy, Northeastern University, Shenyang 110819, China; Dongping Zhan and Zhouhua Jiang, School of Metallurgy, Northeastern University, Shenyang 110819, China; Changsheng Li and Min Qi, State Key Laboratory of Rolling and Au
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