Abstract
Hydrogen interactions with different microstructural defects are analysed in ultra-low carbon steel, with specific focus on the influence of the carbon distribution. For this purpose, the steel is cold rolled and subjected to various annealing treatments, obtaining microstructures ranging from as cold rolled up to fully recrystallized. Information on the dislocation density is obtained by scanning transmission electron microscopy analysis, while positron annihilation spectroscopy provides information on the presence of all open volume defects. The carbon distribution is characterized by internal friction experiments. The hydrogen-material interactions are evaluated based on thermal desorption spectroscopy and internal friction measurements of samples pre-charged with hydrogen.
Dislocations are identified as the most dominant feature contributing to hydrogen trapping in the cold rolled material. However, their role is strongly reduced after annealing at temperatures in the range between 150°C and 350°C due to dissolution of metastable kink-pairs and small carbon-vacancy clusters. Dissolution of these clusters provides fresh supply of carbon to dislocations, as such reducing the dislocation trapping capacity for hydrogen. The presence of carbon also promotes the formation of vacancy clusters during cold rolling, which can act as strong hydrogen trapping sites. Hydrogen release from vacancy clusters is determined by the energy required for complete cluster dissolution.