Effects of Nickel Content, Inclusion Type and Heat Treatment on the Microstructures and Strength and Toughness of New, Cobalt Free, Ultra-high Strength Steels of High Toughness

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Currently available ultra-high strength steels of high fracture toughness contain large amounts of cobalt and nickel. Because of these large amounts of cobalt and nickel these steels are relatively expensive. The ultimate objective of this research is the further development of new ultra-high strength steels of high fracture toughness, which contain no cobalt and much less nickel than currently available ultra-high strength steels of high fracture toughness. Explicitly the work will provide a basic understanding as to the effects of composition and heat treatment on the strength and toughness of the new steels. This basic understanding will facilitate the eventual commercial development of a new class of ultra-high strength steels of high fracture toughness, which are much less expensive than currently available ultra-high strength steels of high fracture toughness. Based on the initial results obtained in this work there has been interest in the new steels from steel producers and users of such steels and thus it is felt that the new steels have the potential to be of lasting technological and engineering significance. As the new steel contains no cobalt, which must be imported, the new steels also have benefits in terms of sustainability and national security. This work will contribute to the scientific/engineering training of a woman graduate student who will be supported by the project, and several undergraduates during the school year and summers through the NSF Research Experience for Undergraduates supplemental grants. The new steels are medium carbon secondary hardening steels in which nickel additions, rather than cobalt, are used to enhance strengthening by secondary hardening, which is the strengthening by the precipitation of fine alloy carbides. Nickel is used instead of cobalt because nickel is less expensive than cobalt and because nickel lowers the ductile-to-brittle transition temperature while cobalt raises the ductile-to-brittle transition temperature. Both the strength and toughness of the new steels are strongly influenced by the amount of nickel added to the steel. Thus a major objective of the proposed work is to understand how nickel additions influence the microstructure and hence the strength and toughness of the new steels. The second objective of the proposed work is to examine the effects of inclusion type on the toughness of the new steels. Inclusions in steels are second phase particles used to remove from solid solution certain impurities such as sulfur. The inclusion type can influence the toughness when the fracture mode is ductile and can also influence the ductile-to-brittle transition temperature. Our specific interest in this work is to assess the effects of inclusion type on the ductile-to-brittle transition temperature of the new steels.