Thermal processing of ferritic 5Mn steel for toughness at cryogenic temperatures
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THE steels commonly specified for structural applications at L N G and lower temperatures, 9 pct Ni steel, austenitic stainless steels, and Invar alloys, all have a relatively high nickel content. While the nickel addition contributes significantly to the good low-temperature properties of these alloys, it also adds substantially to the cost. Consequently, there is an incentive to develop alloys which retain good cryogenic properties with reduced nickel content. Steels containing 5 to 6 pct nickel were recently introduced in the United States 1 and in Japan 2 in response to this need. Further decreases in the nickel content would be desirable. Of the common alloying elements in steel, manganese is the most obviously attractive as a substitute for nickel in cryogenic alloys. Manganese is readily available, relatively inexpensive, and has an intriguing metallurgical similarity to nickel in its effect on the microstructures and phase relationships of iron-base alloys. Research on the cryogenic properties of Fe-Mn alloys has hence been undertaken in a number of laboratories.3 8 While this research has yielded several alloys which retain excellent toughness at cryogenic temperatures, the alloys are austenitic grades which are relatively low in structural strength and very high in manganese content (18 to 25 pct by weight). In contrast to the Fe-Ni cryogenic steels, whose metallurgy and cryogenic mechanical properties have been extensively investigated, until very recently there had been relatively little research on the low temperature mechanical properties of ferritic Fe-Mn steels. The published work s 10was largely discouraging. The ferritic Fe-Mn grades were found to have high transition temperatures and to be particularly liable to intergranular fracture, either because of a sensitivity to temper embrittlement, in the low M n grades, or because M. NIIKURA, formerly Visiting Scientist, Lawrence Berkeley Laboratory, is Assistant Manager, Technical Research Center, Nippon Kokan K.K., Kawasaki, Japan. J. W. MORRIS, JR. is Professor of Metallurgy, Department of Materials Science, University of California, Berkeley, and Senior Faculty Scientist, Materials and Molecular Research Division, Lawrence Berkeley Laboratory. Manuscript submitted November 5, 1979.
of an apparently inherent bias toward intergranular failure at higher Mn contents. Despite the discouraging results of earlier research, the metallurgical similarity of Mn and Ni in ferritic steels seemed sufficient to warrant a program of research into the cryogenic potential of ferritic Fe-Mn steels, which was begun in this laboratory several years ago. This program took two rather different technical paths, which addressed the two distinct types of ferritic alloys which may be based on the Fe-Mn bianary, 3,11 illustrated in Figs. 1 and 2. When the manganese concentration is relatively low (4 to 10 wt pct), an Fe-Mn binary alloy will transform on quenching to ~' (bcc) martensite with a dislocated lath substructure (Fig. 2(a)) strongly resembling that of Fe-Ni alloys o
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