Austenitizing Temperature Effects on the Martensitic Transformation, Microstructural Characteristics, and Mechanical Per
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INTRODUCTION
COMPARED with austenitic stainless steels, high chromium ferritic/martensitic (FM) steels have been considered as candidate materials for high-temperature in core applications, such as cladding, wrappers, and ducts in fast fission reactors due to their superior swelling resistance and excellent thermal properties.[1] Since the 1960s, three generations of FM steels have been developed, and the fourth generation is currently under development with the goal of increasing the maximum use temperature to 650 C. In the fourth generation of FM steels, the prevailing approach is to increase the W content and add a certain amount of Co.[2] Ferritic/martensitic steels have been typically produced in normalized and tempered conditions with a microstructure characterized by a tempered
XIAOSHENG ZHOU, YONGCHANG LIU, CHENXI LIU, LIMING YU, and HUIJUN LI are with the State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 25, 2017. Article published online June 5, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
martensitic lath structure containing large-sized M23C6 (M = Cr, Fe) carbides and fine MX (M = V, Nb, and X = C, N) carbonitrides.[3,4] The normalizing temperature has a significant effect on the microstructural stability and mechanical performance of FM steels. Pandey et al.[5] found that the sizes of precipitates at prior austenite grain boundaries (PAGBs) and within grains increased as the normalizing temperature increased up to 1100 C. Increasing the normalizing temperature would result in the enhanced tensile strength and the reduced toughness. Yan et al.[6] investigated the microstructure and tensile strength of tempered 9Cr martensitic steels that were normalized at 900 to 1200 C, and found that higher normalizing temperatures resulted in the redissolution of the coarse particles formed during manufacturing and that fine particles precipitated when the steels were tempered. As the normalizing temperature increased from 900 to 1000 C, the tensile strength of the martensitic steel increased. From 1000 to 1100 C, the strength changed only slightly, but continued to increase as the normalizing temperature rose to 1200 C. Moreover, according to Barbadikar et al.,[7] the increase in the normalizing temperature does not significantly affect the carbide particle size, although the size increases as the tempering temperature increases. In spite of the above reports, it is VOLUME 49A, AUGUST 2018—3525
desirable to further clarify effects of the austenitizing temperature on the microstructural characteristics and mechanical performance of FM steels. During the manufacturing process, it is often necessary to join ferritic heat-resistant steels when fabricating structural materials.[8] When these are joined by welding, both the welded joint and the heat-affected zone (HAZ) may exhibit microstructures and mechanical performances different from those of the parent
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