Hydrogen-induced ductility loss in cast irons

The effect of hydrogen-charging was investigated with respect to the tensile properties ofthree types of ductile cast irons: JIS FCD400, FCD450 and FCD700. In this study, hydrogen chargingled to a marked ductility loss in all the ductile cast irons. The thermal desorption spectroscopy and thehydrogen microprint technique revealed that, in the hydrogen-charged specimens, most of solutehydrogen was diffusive and mainly segregated at the graphite, the graphite/matrix interface zone andthe pearlite. In the fracture process of the non-charged specimen, the neighboring graphites wereinterconnected with each other mainly by ductile dimple fracture. On the other hand, in the fractureprocess of the hydrogen-charged specimen, the graphites were interconnected by cracks. Thedifference in the fracture morphology between the non-charged and the hydrogen-charged specimensis attributed to the presence of diffusive hydrogen in graphite and graphite/matrix interface. Duringearly stage of fracture process in hydrogen-charged specimen, the interspace between graphite andmatrix is filled with hydrogen gas, which leads to the ductility loss of matrix in the vicinity of graphite.Even after the initiation of crack from graphite, hydrogen is continuously outgassed from graphite andsupplied to the crack tip. Therefore, concerning the effect of hydrogen on the strength of cast irons, arole of subsurface graphite as a "local hydrogen supplier" should be taken into consideration.