A low alloy, medium carbon steel was heat treated to form different microstructures and cathodically hydrogen charged at room temperature in the absence of external stress. The hydrogen-induced microstructural damage was studied using high voltage electron microscopy (HVEM). Direct HVEM studies of three very different microstructures showed that the introduction of hydrogen results in the formation of dislocations, dislocation cells, small-angle subgrain boundaries and microbubbles in heavily charged fully spheroidized air-cooled-and-annealed (AA) and partially spheroidized (LS) samples. The hydrogen-induced dislocations are frequently tangled or piled up at grain boundaries and carbide particles. Hydrogen-induced subgrain boundaries are believed to be formed by a dynamic recovery process. Unlike AA or LS samples, the microstructure of quenched-and-annealed samples is not significantly affected by the charging treatment except for the formation of hydrogen-induced microbubbles. These hydrogen-induced microbubbles are observed to nucleate preferentially at internal interfaces such as grain boundaries and carbideferrite interphase boundaries. Locally high dislocation densities are usually observed to be generated around the microbubbles, indicating large localized strains accompanying bubble nucleation and growth. In the highly hydrogen-charged LS microstructure, the microbubbles are observed to nucleate predominantly at the already spheroidized carbide particles and at the tips of the partially spheroidized carbide lamellae. The more coherent carbide lamellar broad faces are apparently ineffective bubble nucleation sites.
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