The effect of tungsten on creep
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I.
INTRODUCTION
T E M P E R E D martensitic steels, such as modified 9Cr-lMo (9Cr-IMo-0.2V-0.08Nb-0.1C) and HT-9 (12Cr1Mo-0.5W-0.3V-0.5Ni-0.2C), are being considered alternate candidate structural materials to austenitic type 316 stainless steel (SS) for application to first-wall and blanket-structure of demonstration and commercial fusion reactors. [l'/m A primary reason for the use of tempered martensitic steels is a higher resistance to swelling compared to type 316 SS. tL2,4] Recently, tungsten stabilized, tempered martensitic Cr-W steels have become of interest as:a replacement for conventional Cr-Mo steels, such as modified 9Cr-lMo and HT-9, from a viewpoint of reduced radioactivation of fusion components. 12,5J In order to simplify special waste storage of highly radioactive structures of a fusion reactor after service, fast decay characteristics of induced radioactivity are also imposed on the candidate steels. In particular, the concentrations of Mo and Nb in the candidate steels must be severely restricted, because they transmute to longlived radioactive nuclides in high-energy fusion neutrons. The alloying elements that can be used in a reduced radioactivation steel include C, Cr, W, V, Ta, Ti, Mn, and Si. At present, Cr-W steels offer the best possibility for the base composition of reduced radioactivation bainitic or martensitic steels. T M u Of the Cr-W steels, 9Cr-W steels are the most promising as structural materials because of their excellent high-temperature creep strength and toughness, t12,~3]We already showed that the creep-rupture strength of experimental steels of Cr-2W0.1C (wt pct) with various Cr concentrations from 2 to 15 wt pct had its maximum at about 9 to 10 wt pct Cr at 873 K. In previous reports, t14'151 we examined the microstructural evolution in the tempered martensitic 9Cr-W FUJIO ABE, Senior Researcher, Second Research Group (Nuclear Materials), and SHIZUO NAKAZAWA, Senior Researcher, Materials Design Division, are with the National Research Institute for Metals, Sengen 1-2-1, Tsukuba 305, Japan. Manuscript submitted April 13, 1992. METALLURGICAL TRANSACTIONS A
steels with various W concentrations from 0 to 4 wt pct during tempering and subsequent aging and the effect of microstructural evolution on creep behavior of the tempered martensitic 9Cr-2W steel. The microstructural evolution, such as precipitation behavior, in the tempered martensite during aging depended on the concentration of W and aging temperature. During creep of the 9Cr-2W steel at 873 K, the microstrucaual evolution (e.g., the recovery of excess dislocations, the agglomeration of M23C 6 carbides, and the growth of martensite lath subgrains) occurred with the aid of stress, while it was negligibly small during aging in the unstressed condition. The minimum creep rate was shown to be influenced by the microstructural evolution. Therefore, because the rate of microstructural evolution in the 9Cr-W steels during aging depended on the concentration of W, the creep behavior would also be influenced by the W conce
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