Abstract
Plastic deformation is key to understanding hydrogen embrittlement in steels. Although macroscopic to microscopic observations and analyses have reported softening and hardening behavior in the presence of hydrogen, the mechanisms causing these effects and the overall mechanisms remain unclear. Therefore, this study applies nanoindentation tests to the ferrite phase of bcc-structured polycrystalline carbon steel S25C and single-crystalline iron with 3 wt.% silicon (Fe-3wt.%Si). The change in indentation work is evaluated by varying the exposure time in air between the uncharged and hydrogen-charged materials, focusing on the hydrogen concentration. The change in work per unit indentation volume caused by hydrogen (hydrogen-induced work) is investigated, revealing that the specimens harden immediately upon hydrogen charging, and then gradually soften with increasing exposure time before returning to their original state. This variation is attributed to the change in hydrogen concentration. The softening and hardening behavior with and without hydrogen, as confirmed by nanoindentation tests, is suggested to quantitatively affect the macroscopic mechanical response, which is determined by the mobility of screw dislocations in these materials.
