PSI - Issue 42

Aleksander Omholt Myhre et al. / Procedia Structural Integrity 42 (2022) 935–942 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Climate change is already affecting several regions of the world, and actions are being taken to accelerate the transition toward carbon neutrality. CO 2 emissions must be reduced through the energy value chain from generation to end-use by switching out fossil fuel power generation for renewable energy sources. A challenge with renewable energy sources, such as wind and solar, is balancing the supply and demand. One way to achieve this is to use hydrogen as an intermediate storage medium for excess energy. The increasing attention and use of hydrogen as an energy carrier, introduces a transport demand in large volumes in the future (Gerwen, Eijgelaar, and Bosma 2019). As the third largest exporter of natural gas in the world, Norway plays a central role in Europe as an energy provider and operates one of the world's most extensive and integrated subsea transport systems for natural gas (Gassco 2022). The available infrastructure, consisting of 8800 km of natural gas pipelines, is highly relevant as a large-scale transport system of H-gas to Europe (Gassco 2022). However, most of the subsea pipelines on the Norwegian continental shelf are designed according to the code DNV-OS-F101, which does not address hydrogen transport. A key challenge is the reliable assessment of the negative impact of hydrogen uptake into metals: it is well known that the ingress of hydrogen may cause a detrimental effect on the mechanical properties, often referred to as hydrogen embrittlement (HE) (Johnson 1875). The Norwegian pipeline infrastructure mainly consists of ferritic steels (carbon steel, carbon manganese steel and low alloy steel) of Grade X52 to X70 (i.e., specified minimum yield strength (SMYS) between 360 and 480 MPa) (API Specification 5L 2000). Ferritic steels have a body-centred cubic (BCC) lattice structure implying a low solubility and high diffusivity of hydrogen. The high mobility of hydrogen in the lattice leads to the easy access of hydrogen to microstructural features and local stress concentrations that may initiate premature and unexpected failures (Nowick 2012). Hydrogen may cause a detrimental effect on mechanical properties involving a significant loss of ductility (San Marchi and Somerday 2012; J. Cialone and H. Holbrook 1988) reduced fracture toughness (Olden, Alvaro, and Akselsen 2012; Alvaro et al. 2014) and degradation of fatigue properties, resulting in the accelerated crack growth rate (Ronevich, Somerday, and San Marchi 2016; Nanninga et al. 2012), whereas the yield and ultimate tensile strength are barely affected by pressurized hydrogen gas (Laureys et al. 2022) A measure of hydrogen embrittlement is often given by the embrittlement index, EI, defined as the difference between the reduction of area (RA) in a slow strain rate tensile (SSRT) test in a reference condition and in hydrogen environment normalized by the reduction of area in the reference condition, i.e., air (Michler, Yukhimchuk, and Naumann 2008; Takasawa et al. 2012; Moro et al. 2010). Commercial pipeline steels commonly possess a banded microstructure consisting of alternating layers of ferrite and pearlite in the rolling direction (Krauss 2015). As a result, mechanical properties can differ in different directions. Specifically, the lower fatigue crack growth rates in hydrogen gas are observed when the crack is oriented perpendicularly to the banded pearlite structure (Ronevich, Somerday, and San Marchi 2016). X65 steel with a homogenous microstructure showed a larger reduction in the fracture toughness in a hydrogen environment compared to a conventional pipe with a banded ferritic-pearlitic structure (Chatzidouros, Papazoglou, and Pantelis 2014). In the on-going project, HyLINE - Safe Pipelines for Hydrogen Transport (RCN project no. 294739), material challenges related to pipeline transport of pure pressurized hydrogen gas are being addressed. During the first phase of the project, a Materials screening program has been conducted. The main purpose was the selection of pipeline steels for further investigations of hydrogen influence on fracture toughness and fatigue properties. This brief communication presents the finding of the screening program including the steel microstructure and the results of SSRT in air and under electrochemical hydrogen charging conditions. Finally, the SSRT results are discussed in relation to the microstructure and the influence of hydrogen. 2. Materials and Methods 2.1. Materials The steels samples were cut from four different pipelines of the strength classes X65 and X70: three vintage pipelines (denoted as Material A, Material B and Material D) and a new pipeline (Material C). Material C is retrieved from a seamless QT pipe, whereas the others from hot rolled and longitudinally welded ones. Chemical compositions of the steels are presented in Table 1. Due to microstructural differences throughout the wall thickness, three areas were investigated both in the longitudinal and transversal direction: the outer, mid, and inner area.