Shigeru Asano1, Kazuhisa Hara2, Yoichi Nakai3 and Namio Ohtani4
1Department of Metallurgical Engineering, Nagoya Institute of Technology, Nagoya
Diffusion coefficients of hydrogen in a mild steel were measured by the method of electrochemical permeation at room temperature and above. In order to evaluate the trapping effect of dislocations, the influence of cold work and quenching treatments on hydrogen diffusion was examined in detail, together with the observation of recovery processes in annealing. The results obtained are summarized as follows: (1) The diffusivity of hydrogen was significantly decreased by cold work. Its variation ranged over three orders of magnitude at 20°C. (2) It was shown from recovery that the trapping effect due to cold work was attributed to dislocations, but not to void-type defects as often suggested. (3) The dislocations produced by martensitic transformation gave rise to just the same trapping effect as the dislocations produced by cold work. (4) It was shown from temperature dependence of hydrogen diffusion that the binding energy of hydrogen with dislocations was 6300 cal/mol, which agreed well with the estimate from internal friction. (5) The method of estimation of dislocation density from the apparent diffusivity of hydrogen was established, which was confirmed to provide reasonable values of dislocation density.
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