DEVELOPMENT OF LOW-CHROMIUM, CHROMIUM-TUNGSTEN STEELS FOR FUSION

High-chromium (9-12% Cr) Cr-Mo and Cr-W ferritic steels are favored as candidates for fusion applications. In early work to develop reduced- activation steels, an Fe-2.25Cr-2W-0.25V-0.1C steel (designated 2.25Cr -2WV) had better strength than an Fe-9Cr-2W-0.25V-0.07Ta-0.1C (9Cr-2WV Ta) steel (compositions are in weight percent). However, the 2.25Cr-2W V had poor impact properties, as determined by the ductile-brittle tra nsition temperature and upper-shelf energy of subsize Charpy impact sp ecimens. Because low-chromium steels have some advantages over high-ch romium steels, a program to develop low-chromium steels is in progress . Microstructural analysis indicated that the reason for the inferior impact toughness of the 2.25Cr-2WV was the granular bainite obtained w hen the steel was normalized. Properties can be improved by developing an acicular bainite microstructure by increasing the cooling rate aft er austenitization. Alternatively, acicular bainite can be promoted by increasing the hardenability. Hardenability was changed by adding sma ll amounts of boron and additional chromium to the 2.25Cr-2WV composit ion. A combination of B, Cr, and Ta additions resulted in low-chromium reduced-activation steels with mechanical properties comparable to th ose of 9Cr-2WVTa.