13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Ductility loss in Hydrogen-charged Ductile Cast Iron Hisao Matsunaga1, 2, 3, 4, *, Teruki Usuda5, Keiji Yanase4, 6, Masahiro Endo4, 6 1 Department of Mechanical Engineering, Kyushu University, Fukuoka 819-0395, Japan 2 International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka 819-0395, Japan 3 Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University 4 Institute of Materials Science and Technology, Fukuoka University, Fukuoka 814-0180, Japan 5 Graduate School of Fukuoka University, Fukuoka 814-0180, Japan 6 Department of Mechanical Engineering, Fukuoka University, Fukuoka 814-0180, Japan * Corresponding author: matsunaga@mech.kyushu-u.ac.jp Abstract Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile by accompanying the interconnecting cracks between the adjacent graphite nodules. Inside of the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss. Keywords Hydrogen, Hydrogen embrittlement, Ductile cast iron, Ductility loss 1. Introduction Ductile cast iron (DCI) is widely used for pipes in city gas pipelines and is a prospective candidate material for hydrogen gas pipes. However, to the authors’ knowledge, there exist few studies about the effect of hydrogen on the strength properties of cast irons. On the other hand, with respect to the hydrogen-induced ductility loss in the ferritic or ferritic-pearlitic steels (e.g. carbon steels), a number of studies have examined the degradation process in the tensile properties [1-8]. These studies clarified that the ductility loss was mainly attributed to the hydrogen-induced acceleration of the fracture process (e.g. nucleation of voids or cracks and their subsequent growth). In addition to the ferritic-pearlitic matrix, DCI contains numerous spheroidal graphites, which can serve not only as stress-concentration sites but also as hydrogen-trap sites. Therefore, the process of hydrogen-induced degradation in cast iron, if any, could be more complex than carbon steels due to the presence of graphites. In this study, as a first attempt to understand the hydrogen effect on the various strength properties of DCI, our focus is primarily on the ductility loss due to internal hydrogen. A unique degradation process associated with solute hydrogen behavior is demonstrated and discussed based on the observations of the fracture process and the microscopic hydrogen segregation behavior. 2. Material and experimental methods 2.1. Material and specimen The test specimens were cut out from a ductile cast iron pipe that was produced by centrifugal casting. The chemical composition in mass% was 3.70 C, 1.78 Si, 0.34 Mn, 0.058 P, 0.007 S and bal.
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