The Effect of Graphite Size on Hydrogen Absorption and Tensile Properties of Ferritic Ductile Cast Iron

Ductile cast iron (DCI) is one of prospective materials used for the hydrogen equipment because of low-cost, good workability and formability. The wide range of mechanical properties of DCI is obtained by controlling microstructural factors such as graphite size, volume fraction of graphite, matrix structure and so on. Therefore, it is important to find out an optimal microstructural condition that is less susceptible to hydrogen embrittlement. In this study, the effects of graphite size on the hydrogen absorption capability and the hydrogen-induced ductility loss of ferritic DCI were investigated.
Several kinds of ferritic DCIs with a different graphite diameter of about 10 µm - 30 µm were used for the tensile test and the hydrogen content measurement. Hydrogen charging was performed prior to the tensile test by exposing a specimen to high-pressure hydrogen gas. Then, the tensile test was performed in air at room temperature. The hydrogen content of a specimen was measured by a thermal desorption analyzer.
It was found that the amount of hydrogen stored in DCI was dependent on the graphite size. As the graphite diameter increased, the hydrogen content sharply increased at a certain graphite diameter, and then it became nearly constant irrespective of increase in graphite diameter. In other words, there was the critical graphite diameter that significantly changed the hydrogen absorption capability. The ductility was decreased by hydrogen, and the hydrogen-induced ductility loss was dependent on the hydrogen content. Therefore, the hydrogen embrittlement of DCI became remarkable when the graphite size was larger than the critical value.