Characterization of Hydrogen-Assisted Degradation of a Vintage and a Modern Pipeline Steel

Hydrogen gas has gathered much interest as an energy carrier for present and future clean energy needs. It will play an important part in the storage and transport of energy produced by renewable sources. One economically justifiable method for the transport of hydrogen gas is the use of existing pipeline networks. However, the uptake of hydrogen in steel structures is known to cause a degradation of mechanical properties, potentially decreasing toughness and inducing cracks. This phenomenon is called hydrogen embrittlement (HE). Since the existing pipeline systems were not designed with high-pressure hydrogen gas transport in mind, its suitability needs to be investigated. In this study, the hydrogen properties of two grades pipeline steel, a ‘vintage’ API 5L X56 and a ‘modern’ API 5L X70 are investigated. The influence of hydrogen on their mechanical behavior is evaluated by tensile tests on uncharged and hydrogen charged specimens. These tests are performed on notched round bar specimens with different notch radii, allowing for a range of positive stress triaxialities to be examined. Hydrogen charging is done electrochemically and two different hydrogen levels are used: both steels are tested at the same hydrogen concentration and for the X70 steel, a higher hydrogen concentration is considered as well. Hydrogen signatures in the form of fisheyes and quasi-cleavage are detected on the fracture surface. The influence of different stress triaxialities on fisheye formation is investigated, as well as the role of inclusions or other microstructural features as initiation points for fisheyes. Based on the notched specimens’ reduction in area, the vintage X56 steel appears slightly less or similarly sensitive to HE compared to the higher strength, modern X70 steel for similar hydrogen levels. In the X70 steel, a higher hydrogen concentration results in a more severe embrittlement as well as more fisheyes on the fracture surface.