The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels
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I.
INTRODUCTION
T E M P E R E D martensitic steels, such as modified 9Cr1Mo (9Cr-IMo-0.2V-0.08Nb-0.1C) and HT-9 (12Cr1Mo-0.5W-0.3V-0.5Ni-0.2C), are being considered as candidate structural materials for fusion reactors because of their excellent swelling resistance compared with austenitic stainless steels. [~j Recently, however, in order to simplify special waste storage of the highly radioactive blanket and first-waU structures from fusion reactors after service, the development of low activation steels has received attention. [2] In low activation steels, the concentration of Mo, Nb, and Ni, which are principal alloying elements in conventional Cr-Mo steels, such as modified 9Cr-lMo and HT-9, must be severely restricted. These elements transmute into long-lived radioactive nuclides when they are irradiated with the high-energy neutrons (10 to 14 MeV) found in the first-wall and blanket structure. [31 Alloying elements that can be used in low activation steels include C, W, V, Ta, Ti, Mn, and Si. At present, Cr-W steels offer the best possibility for the base composition of low activation ferritic/martensitic steels that can replace conventional Cr-Mo steels. Although the irradiation response, including irradiation hardening, irradiation embrittlement, and irradiationinduced microstructural evolution, of these newly developed Cr-W steels is now being extensively studied, [4,5,61 research on the physical metallurgy of unirradiated material is still insufficient, t7,sm Tempered martensitic steels usually consist of lath subgrains which contain a high density of dislocations or dislocation net-
F. ABE, Senior Researcher, H. ARAKI, Researcher, and T. NODA, Head of the Second Laboratory, are with the Second Research Group (Nuclear Materials), National Research Institute for Metals, Tsukuba Laboratories, Tsukuba 305, Japan. Manuscript submitted October 22, 1990. METALLURGICAL TRANSACTIONS A
work and fine carbides, depending on tempering temperature. However, 8-ferrite can also form during austenitizing, and some steels can form intermetallic phases during tempering as well. These microstructural constituents significantly affect the mechanical properties of tempered martensitic steels. Therefore, it is important to understand the effect of W on microstructural evolution in martensitic Cr-W steels. The purpose of the present research is to investigate systematically the effect of W on dislocation recovery and precipitation behavior of carbides and intermetallic compound in martensitic 9Cr steels containing from 0 to 4 wt pet W. Microstructural observations using transmission electron microscopy (TEM) were made after quenching, after tempering, and after prolonged aging. Hardness measurements were also made to study the secondary hardening and softening that occur during tempering of quenched material. The effect of W on microstructural evolution is comprehensively discussed. II.
EXPERIMENTAL PROCEDURE
The chemical composition of the steels examined is given in Table I. Only the concentration of W was varied from 0
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