A low alloy medium carbon steel was cathodically hydrogen charged in the absence of external stress, and hydrogen-induced blistering and stepwise cracking examined. Three types of microstructure (two spheroidized and one pearlitic) were produced through different heat treatment schedules. The main cracks involved in the blistering process were observed to nucleate preferentially at interphase boundaries between the manganese sulfide inclusions and the ferrite matrix. These cracks are usually parallel to the free surface and do not break the free surface for relaxation of the stress exerted by the gaseous hydrogen. Instead, release of the precipitated hydrogen gas occurs by a mechanism involving the formation of stepwise secondary cracks between the main inclusion-matrix interface cracks and the external surface. The stepwise cracks were observed to nucleate at microstructural discontinuities such as grain boundaries, carbide-matrix interphase boundaries, pearlite colony boundaries and non-metallic inclusion-matrix interphase boundaries. Configuration of the stepwise cracks observed in this study was generally more consistent with Iino's blistering mechanism than that proposed by Akhurst and Pumphrey. In addition to the stress state as discussed in the Iino model, the results of this study suggest that microstructural discontinuities and anisotropy of crystalline orientation, which always exist in polycrystalline commerical ferrous alloys, must be taken into account to describe the hydrogen-induced blistering and stepwise cracking process in more detail.
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