The role of microstructural instability on creep behavior of a martensitic 9Cr-2W steel
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INTRODUCTION
FERRITIC and martensitic steels, such as Mod.9Cr-IMo(0.1C-9Cr-IMo-0.2V-0.08Nb) and HT9(0.2C-12Cr-IMo-0.5W-0.3V-0.SNi), are being considered as candidate structural materials for first-wall and blanket-structure components of fusion reactors because of their excellent resistance to swelling compared with austenitic stainless steels. Recently, in order to simplify special waste storage of highly radioactive structures of fusion reactors after service, fast decay characteristics of induced radioactivity were also imposed to the candidate steels. ~q This problem would be alleviated by the use of low activation steels. Molybdenum and niobium, which are principal alloying elements in conventional Cr-Mo steels, transmute to long-lived radioactive nuclides in a fusion environment of high-energy neutrons. Therefore, from a viewpoint of reduced activation, molybdenum and niobium must be substituted with elements whose decay times of induced radioactivity are shorter than that of iron. The alloying elements that can be used in low activation steel include C, Cr, W, V, Ta, Ti, Mn, and Si. Under these circumstances, martensitic Cr-W steels have become of much interest as a replacement for conventional Cr-Mo steels. At present, a detailed understanding on creep behavior of martensitic Cr-W steels is lacking, although rupture data have been reported by some research groups, t2-5] The high-temperature strength and irradiation embrittlement are very important issues for the development of low-activation Cr-W steels. It is well established that creep and fracture behavior of engineering multiphase alloys depends critically on microstructures such as particle size, spacing of particles, grain size, dislocation density, e t c . I6"71 F. ABE, Senior Researcher, Second Research Group (Nuclear Materials), S. NAKAZAWA, Senior Researcher, Materials Design Division, H. ARAKI, Researcher, Second Research Group, and T. NODA, Head of the Third Laboratory, Second Research Group, are with the National Research Institute for Metals, Tsukuba Laboratories, Tsukuba 305, Japan. Manuscript submitted January 14, 1991. METALLURGICAL TRANSACTIONS A
However, the initial microstructure can change markedly during creep at high temperature. This means inevitably that resistance to creep can alter with time. Previously, we investigated the effect of aging treatment at temperatures between 823 and 973 K on the microstructure of martensitic 9Cr steels with several levels of tungsten. [8,9,1~ Fine distributions of M23C6-rich carbides of 0.1 /xm or less in size, high density of dislocations, and fine distributions of martensite lath subgrains of about 0.5 ~ m in width were observed in quenched and tempered condition. The microstructure of the steels changed gradually with time at high temperatures above 873 K. The purpose of the present research is to investigate the microstructural instability during creep and its effect on creep behavior for a martensitic 9Cr-2W steel. The creep rupture test and interrupt test were carried out at 873 K (600 ~
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