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

目前可用的高断裂韧性的超高强度钢含有大量的钴和镍。由于这些大量的钴和镍，这些钢相对昂贵。 本研究的最终目标是进一步开发具有高断裂韧性的新的超高强度钢，其不含钴，并且比目前可用的具有高断裂韧性的超高强度钢含有少得多的镍。解释这项工作将提供一个基本的了解，成分和热处理对强度和韧性的新钢的影响。这种基本的理解将促进高断裂韧性的新型超高强度钢的最终商业开发，其比目前可用的高断裂韧性的超高强度钢便宜得多。根据这项工作中获得的初步结果，钢生产商和此类钢的用户对新钢产生了兴趣，因此认为新钢具有持久的技术和工程意义。由于新钢不含钴，钴必须进口，因此新钢在可持续性和国家安全方面也有好处。这项工作将有助于在学年和夏季通过国家科学基金会本科生研究经验补充赠款对一名将得到该项目支助的女研究生和几名本科生进行科学/工程培训。新的钢是中碳二次硬化钢，其中镍添加物而不是钴用于通过二次硬化来增强强化，二次硬化是通过细合金碳化物的沉淀来强化。 使用镍代替钴是因为镍比钴便宜，并且因为镍降低了韧脆转变温度，而钴提高了韧脆转变温度。新钢的强度和韧性都受到加入钢中的镍量的强烈影响。 因此，所提出的工作的一个主要目标是了解镍添加剂如何影响微观结构，从而影响新钢的强度和韧性。所提出的工作的第二个目标是检查夹杂物类型对新钢的韧性的影响。 钢中的夹杂物是用于从固溶体中去除某些杂质（如硫）的第二相颗粒。当断裂模式为韧性时，夹杂物类型会影响韧性，也会影响韧脆转变温度。 在这项工作中，我们的具体兴趣是评估夹杂物类型对新钢的韧脆转变温度的影响。

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.