Abstract
As the global energy landscape transitions toward hydrogen, understanding the impact of hydrogen on the fracture behavior of pipeline steels is essential for the safe deployment of hydrogen infrastructure. Hydrogen embrittlement (HE) presents a significant challenge to the integrity of line-pipe steels, particularly in applications involving the transport and storage of hydrogen gas. For this purpose, fracture toughness tests of industrial grade line-pipe steels were performed using the reversing direct current electric potential difference (DCEPD) method to enable a real time crack initiation and crack growth measurement. On one side, a pipe (γ-Pipe) was tested under two different conditions to study the effect of pressure: 100 bar pure H2 and 80% N2/20% H2 gas mixture (100 bar). Two other pipes (Pipe X & Pipe Y) of similar strength levels but different “thickness-to-outer diameter” ratios were tested in 100 bar H2. The influence of H2 partial pressure on fracture toughness appears to be present, and a negative trend between the “thickness-to-outer diameter” ratio and fracture toughness also seems possible. To advance further understanding, the fracture toughness tested samples were studied using fracture surface mapping and advanced characterization techniques. The objective is to bridge the knowledge gaps for evolving gaseous testing methodologies by focusing on the interplay between local stress-state and hydrogen concentration at the crack tip leveraging finite element analysis (FEA), while developing fundamental understanding on the HE mechanisms. A detailed fracture surface mapping of the pipes reveal that their morphology is affected by HE when tested in 100 bar H2 compared to the ductile dimple morphology of their air tested counterparts. Several out-of-plane “secondary cracks”, perpendicular to the primary crack propagation direction was observed both in the stretch zone (or boundary) and crack extension zone in the fracture surface. A good correlation was observed between the fracture toughness results and the stretch zone morphology and secondary crack formation. An advance postmortem analysis into the mechanism(s) of the secondary crack formation reveal that these cracks are associated with hydrogen enhanced dislocation activity just underneath the fracture surface, leading towards hydrogen enhanced localized plasticity (HELP) mechanism. A quantitative comparison of the total secondary crack area using a machine learning approach rooted in computer vision suggests more secondary crack formation results in lower crack growth resistance.