Success in Development of 1800 MPa Class Ultra-High Strength Bolt
Realizing Excellent Fracture Resistance in an Ultra-High Strength Product
National Institute for Materials Science
Dr. Yuuji Kimura, Senior Researcher of Structural Metals Center and Dr. Tadanobu Inoue, Senior Researcher of the Exploratory Materials Research Laboratory for Reliability and Safety of National Institute for Materials Science, in joint research with Kyowakogyosyo Co., Ltd. and Fusokiko Co., Ltd., succeeded in the development of an ultra-high strength bolt which is both strong and resistant to breakage.
- Dr. Yuuji Kimura, Senior Researcher of Structural Metals Center (Managing Director: Kaneaki Tsuzaki) and Dr. Tadanobu Inoue, Senior Researcher of the Exploratory Materials Research Laboratory for Reliability and Safety (Managing Director: Kohmei Halada) of National Institute for Materials Science (NIMS; President: Sukekatsu Ushioda), in joint research with Kyowakogyosyo Co., Ltd. and Fusokiko Co., Ltd., succeeded in the development of an ultra-high strength bolt which is both strong and resistant to breakage.
- The basic performance requirements for structural materials are strength, that is, the ability to support large loads, and toughness, meaning tenacity and fracture resistance. In previous work, NIMS developed an ultra-high strength steel with excellent fracture resistance at low temperatures by controlling the metal microstructure of a steel containing small amounts of added alloying elements (Science, 320 (2008), p. 1057, press release dated May 23, 2008).
- In the work reported here, the NIMS researchers realized an ultra-high strength bolt based on a new concept using the previously-developed ultra-high strength steel with low temperature fracture resistance by establishing a so-called warm-forming technology, in which the steel material is formed into bolts in a temperature region (500-700°C) higher than the cold forming region (around room temperature) used with conventional bolts. At the ultra-high strength level of 1800 megapascals (MPa; 1MPa=1N (approximately 0.102kgf)/mm2 of cross-sectional area), bolts manufactured by the conventional method fracture easily in the tensile test. In contrast, the developed bolt resists fracture, showing a fracture mode like that when wood is broken, as illustrated in Fig. 1. This means that the safety of the 1800 MPa class ultra-high strength bolt is greatly improved.
- Until now, the strength characteristics of bolts were heterogeneous from the head through the thread. In contrast to this, the developed bolt has a strength (hardness) distribution which shows a decreasing gradient from the underhead fillet across the head. Japanese swords, which represent the essence of Japan’s unique, outstanding steelmaking technology, are also strong and resistant to breaking. These weapons have a composite microstructure in which low carbon steel, which is relatively soft but tough, is encased in a layer of hard high carbon steel, and thus are also a type of functionally graded material (FGM). At a glance, a Japanese sword appears to have a graded function which is similar to that of the developed bolt. However, while the tip of a Japanese sword is hard and brittle, the thread part of the developed bolt using developed steel as material shows high resistance to fracture in spite of its high hardness. In other words, this new bolt was created based on a design concept which is completely different from the conventional thinking.
- In recent years, increasingly high strength has been demanded in iron and steel materials, with the aim of realizing next-generation steel structures and further weight reductions in transportation equipment for purposes of resource saving, energy saving, and CO2 reduction. As part of this trend, simultaneously with high strength in steel sheets/plates and shape steel, higher strength has also been desired in the bolts which are used to join those high strength steel materials. The realization of this new bolt, which combines high strength and excellent fracture resistance, is expected to make an important contribution to expanding the range of use of high strength steel products.