PSI - Issue 68

Eduard Navalles et al. / Procedia Structural Integrity 68 (2025) 1105–1114 Eduard Navalles et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The urgent need for a sustainable society is driving the adoption of hydrogen technologies as an alternative to carbon based technologies (Corbett et al., 2004). Understanding the mechanical behaviour of materials in hydrogen environment is crucial for their use in applications such as hydrogen gas transportation and its storage (Hardie et al., 2006). Hydrogen embrittlement (HE) is a phenomenon that engineering metals undergo when atomic hydrogen is present within the materials structure (Lee, 2016). Hydrogen inevitably causes a detrimental effect on important mechanical properties of metals, such as toughness and ductility, through sudden failure (Ohaeri et al., 2018). Different terms and types of HE has been distinguished in the literature, depending on how the atomic hydrogen enters the material; Internal Hydrogen Embrittlement (IHE) when the hydrogen comes from the manufacturing process, Hydrogen Reaction Embrittlement (HRE), due to the reaction of hydrogen with the material to form hydrates and, finally, Hydrogen Environmental Embrittlement (HEE), when the metal is exposed in-service to a hydrogen containing environment (Lee, 2016). High strength carbon steels, commonly used in pipelines and pressure vessels, are chosen to be used for transport and storage of hydrogen in a big scale, for its low cost and robust performance (Wu et al., 2022). However, to ensure their long-term durability and prevent failures, it is crucial to investigate their mechanical behaviour in high pressure hydrogen gas conditions (Han et al., 2019). When performing in-situ slow strain rate tensile test (SSRT) under hydrogen environment it is of importance to use a strain rate slower than 10 -5 s -1 (ASTM G142-98, 2022) and ensure that there is time enough to hydrogen to diffuse into the material (Wang et al., 2023). Thus, in this work, strain rate of 10 -6 s -1 is used for the hollow specimen in-situ tests. Fatigue is particularly relevant for in-service applications for pipelines and pressure vessels, as they experience constant pressure fluctuations during their service life, making them susceptible to fatigue related issues (Mohtadi Bonab, 2022; Slifka et al., 2014, 2018; Yamabe et al., 2016; Zhang, 2010). Low cycle fatigue (LCF) is used in many industries where cyclic plasticity is present (Aleksić et al., 2022). Nowadays, the influence of low cycle fatigue has been studied for stainless steels using hydrogen environment in an autoclave (Oliveira et al., 2022). Pipeline and pressure vessels steels has been studied using autoclave method for solid specimens (Cheng & Chen, 2017; Lee, 2016; Slifka et al., 2018; Zhang, 2010), as well as cathodic charging (Tsuchida et al., 2010). From the authors knowledge there is no data available regarding the use of hollow specimen method (HSM) to determine the effect of hydrogen gas on the mechanical properties of pipeline steels under fatigue testing. There have been articles of HSM using pipeline steels under SSRT (Konert, Campari, et al., 2024; Konert, Wieder, et al., 2024) as well as some authors using stainless steel in tensile and fatigue loading (Dey et al., 2018; Ueno & Benjamin, 2019). The aim of this work was to evaluate the effect of hydrogen gas (H 2 ) on the tensile and low cycle fatigue properties as well as evaluating the risk to hydrogen embrittlement of high strength carbon steels using the hollow specimen method. 2. Materials and Methods This study investigated two hot rolled high strength steels, more specifically in the category of low-alloy manganese and silicon steels commonly used in pressure vessel application: a ferritic-pearlitic steel and a bainitic steel. Table 1 shows the nominal chemical composition. Figure 1 illustrates their typical microstructures in the mid-thickness of the plate. The ferritic-pearlitic steel exhibits a band-like microstructure of ferrite and pearlite. The bainitic steel has a more complex microstructure consisting of degenerated bainite (DB), quasi polygonal ferrite (QPF), and polygonal ferrite (PF). QPF is more present in the lower quarter thickness of the plate and PF that forms due to transformation of the parent austenite at high temperatures is more likely to appear in the middle of the plate. PF is a softer constituent than QPF. The microconstituents are embedded in globular bainite (GB) matrix that consist of two phases, bainitic-ferritic and a carbon-rich phase, normally maretensite-astenite (MA) islands. The average grain size of the ferritic-pearlitic was 5 µm, and 1.8 µm for the bainitic steel. Hollow specimens (HS) were used to perform slow strain rate tensile (SSRT) and low cycle fatigue (LCF) testing. Specimens were manufactured according to the geometry shown in Figure 2, transversal to the plate rolling direction. A hole with 2 mm diameter was drilled in the centre of the specimen. The roughness of the inner hole surface for both steels was measured and was as follows Ra = 0.112 ± 0.026 µm and Rz ~ 0.841± 0.16 µm. The exterior surface of the gauge section was ground up to P4000 silicon carbide sandpaper.