| Abstract | Hydrogen embrittlement is a well-documented yet complex phenomenon that remains only partially explained, despite extensive research across multiple scales, from quantum mechanics and molecular dynamics to macroscopic investigations. Its multifaceted nature poses significant challenges to achieving a comprehensive understanding. In this work, various pipeline steels were subjected to ex-situ hydrogen charging by exposing them to pure gaseous hydrogen at a pressure of 500 psi for four weeks to induce embrittlement. Following this charging process, tensile test was performed to assess the impact of hydrogen on the steels' mechanical properties. In addition, in-situ tensile tests were conducted using a specially designed hollow dogbone specimen exposed directly to hydrogen and helium gases during testing, allowing real-time observation of mechanical behavior under gaseous environments. Electron Backscatter Diffraction (EBSD) was employed as the primary technique for microstructural characterization. Analyses were conducted on specimens in their initial state, post-fracture without hydrogen, post-fracture with hydrogen charging, and following in-situ gas testing. The EBSD results revealed that, in most of the pipeline steels studied, both grain orientation spread (GOS) and the maximum geometrically necessary dislocations (GNDs) density were significantly lower under hydrogen charging compared to uncharged conditions. Simultaneously, hydrogen exposure was associated with grain sizes exhibiting less deformation, characterized by aspect ratios closer to that of a circle, and with an increase in microstructural texture intensity (expressed as multiples of uniform distribution). These microstructural changes are consistent with, and likely contribute to, the observed alterations in fracture surface morphology and the reductions in tensile properties. Specifically, lower GOS and GND density may indicate reduced intragranular plasticity accommodation, while increased texture intensity may promote strain localization, both mechanisms that can accelerate hydrogen-assisted degradation. These findings demonstrate the utility of EBSD in identifying and tracking microstructural changes associated with hydrogen embrittlement. To the authors' knowledge, no comprehensive study systematically examines various pipeline steels in service or operation, tested using both in-situ and ex-situ methodologies, and exclusively correlates primary microstructural changes using EBSD metrics. Our research pursues to address this gap, offering novel insights into the metallurgical intricacies of hydrogen embrittlement in pipeline steels. |
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