The microstructure of chromium-tungsten steels
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I. I N T R O D U C T I O N
THE steels
that have been proposed for the first wall and blanket structure of a magnetic fusion reactor will become highly radioactive during service. The complexity of the waste disposal procedure for these radioactive components after service depends on the time required for the induced radioactivity to decay to safe levels. The more rapid the decay, the simpler is the disposal task. Alloying elements that result in radioactive isotopes that decay over a long period of time are nickel, molybdenum, nitrogen, copper, and niobium. An alloy-development program was proposed for fast induced-radioactivity decay (FIRD) versions of present fast-wall and blanket structural candidate alloys. I~} It was proposed that alloys be developed by replacing the nonFIRD alloying elements in the steels presently of interest for fusion reactor components. The ferritic steels now being considered are the following commercial Cr-Mo steels: 2 . 2 5 C r - l M o (2.25 pet Cr-1 pct Mo-0.1 pet C),* *Throughout this report, concentrations are given in wt pct.
9Cr-IMoVNb (9 pet Cr-1 pet Mo-0.2 pet V-0.06 pet Nb0.1 pct C), and 12Cr-IMoVW (12 pct Cr-1 pct Mo-0.25 pct V-0.5 pct W-0.5 pct Ni-0.2 pct C) steels. Because tungsten is found to behave like molybdenum in simple steels, 12j it was proposed as a replacement for molybdenum.l~l It was suggested l~l that the strengthening function of niobium could be replaced by a greater use of vanadium, by titanium, and especially by tantalum, which has characteristics in c o m m o n with niobium. A series of experimental steels was proposed (Table I), tll and in this paper the results of some of the initial studies on those alloys will be presented. The designations to be used for each of the steels also are given in Table I (i.e., 2.25CRV is used to designate the steel with 2.25 pct Cr and 0.25 pct V, etc.).
R.L. KLUEH and P.J. MAZIASZ are Research Metallurgists with the Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37831-6376. Manuscript submitted February 1, 1988. METALLURGICAL TRANSACTIONS A
The compositions for these new experimental steels were based on variations of the compositions of the three commercial ferritic steels currently of interest in the fusion-reactor materials program. A range of chromium compositions from --2.25 to 12 pct was proposed, tll An atom-for-atom replacement of molybdenum by tungsten was chosen; this required - 2 wt pet W, since the atomic weight of tungsten is approximately twice that of molybdenum. A 0.25 pet V content was used, which is similar to the amount used in the Cr-Mo steels. Alloys with 0, 1, and 2 pct W were proposed to determine the effect of tungsten, and an alloy with tungsten and no vanadium and one with vanadium and no tungsten were proposed to determine the effect of these elements, tll Tantalum was added to a 9Cr-2WV steel to get a steel analogous to the 9 C r - I M o V N b steel. A carbon level of 0.1 pct was proposed for all steels to help ensure weldability, t~l Of the Cr-Mo ste
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