A low-alloy medium carbon steel with a variety of microstructures was cathodically hydrogen charged and the preferred microstructural sites for hydrogen-induced damage examined. Microcracks or microvoids initiated by the charging treatments were found to nucleate at both the grain and the interphase boundaries. Severe surface blistering was always associated with molecular hydrogen recombination and gas precipitation at manganese sulfide inclusions. The combination of spheroidized cementite particles on the grain boundaries provided more favorable microvoid and/or microcrack nucleation sites than for either individual grain boundaries or carbide particles within the grain interiors. Subgrain boundaries in quenched-and-annealed samples were found to be much less favorable sites than the large-angle grain boundaries for microvoid and/or microcrack nucleation under the same charging conditions. A mixed spheroidized and lamellar cementite (pearlite) microstructure was produced to examine the role of carbide-ferrite interfacial coherency in susceptibility to hydrogen damage. The nucleation of microcracks and/or microvoids along the broad faces of the cementite plates was observed to occur much less frequently than at the cementite plate edges or the spheroidized cementite-ferrite interphase boundaries. The results of this research indicate that the tendency for hydrogen to interact with and cause irreversible damage at a grain or interphase boundary increases with decreasing interfacial coherency.
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