PSI - Issue 54

Robin Depraetere et al. / Procedia Structural Integrity 54 (2024) 172–179 R. Depraetere et al. / Structural Integrity Procedia 00 (2023) 000–000

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infrastructure poses challenges with respect to the structural integrity, as many mechanical properties of metals are degraded due to hydrogen. This phenomenon is also known as ‘hydrogen embrittlement’ [Laureys et al. (2022)]. De spite numerous research e ff orts, it is still not comprehensively understood, which hampers the repurposing of pipelines towards hydrogen gas transport and storage. Accordingly, increased insight into the fundamental mechanisms respon sible for hydrogen embrittlement is required. One of the key areas of interest within this context is the e ff ect of hydrogen on the fracture behavior of pipeline steels. Fracture of those steels in the absence of hydrogen typically occurs in a ductile way, driven by the action of void nucleation, growth and coalescence [Pineau et al. (2016)]. The presence of hydrogen has been shown to accelerate these microvoid processes, as indicated by various studies [Cialone and Asaro (1979); Matsuo et al. (2008); Depraetere et al. (2023)]. However, a definitive consensus on the exact process(es) being influenced has not yet been reached. Some studies highlight the dominance of accelerated void nucleation [Depraetere et al. (2023); Nagumo and Takai (2019)], while other studies suggest that mainly accelerated void growth and coalescence are responsible for hydrogen embrittlement [Garber et al. (1976); Yu et al. (2018)]. One possible explanation for this could be linked to the observation that diverse materials, with varying microstructures, exhibit di ff ering susceptibilities to hydrogen embrittlement [Laureys et al. (2022)]. To better understand the fracture behavior of metals, high resolution X-ray computed tomography (X-ray micro CT) can be employed [Azman et al. (2022)], since it provides three-dimensional information regarding the distribution of ductile damage. Multiple studies have used X-ray micro-CT to investigate the e ff ect of hydrogen on the fracture behavior of various materials [Maire et al. (2019); Depraetere et al. (2023); Lee et al. (2022); Wang et al. (2022)]. It is worth noting that only the study by Depraetere et al. focused on a pipeline steel, in particular a grade API 5L X70 [American Petroleum Institute (2018)] which was produced by a thermomechanically controlled rolling process (TMCP). We observed for this particular material that hydrogen changed the microvoid processes through a combina tion of void nucleation at lower plastic strains, and accelerated void growth perpendicular to the loading direction. The current study analyses the fracture behavior of a di ff erent pipeline steel, grade API 5L X56, with X-ray micro CT. The goal is to evaluate the potential di ff erences between the involved micromechanisms, both in the absence and presence of hydrogen charging.

2. Materials & Methods

2.1. Materials characterization

The investigated pipeline steel is a grade API 5L X56 (ISO equivalent L390) and was produced in 1965 by a normalizing rolling process. The pipe has an outside diameter of 914 . 4 mm (36”) and a measured wall thickness of around 15 . 5 mm. The chemical composition, as measured by spark source optical emission spectroscopy is given in Table 1. Light optical microscopy (LOM) provides a basic microstructural characterization in Figure 1a, showing a heterogeneous microstructure in the form of alternating ferrite-pearlite layers, which is common for pipeline steels [Ronevich et al. (2016)]. A more detailed microstructural characterization of the tested material is reported in Cauwels et al. (2022).

Table 1: Average chemical composition of the investigated X56 pipeline steel in weight percentage.

C Mn Si

Cr

Ni

Nb V Mo

Cu S

P

Fe

0.230 1.392 0.364 0.067 0.062 0.002 0.002 0.022 0.122 0.036 0.024 Balance

2.2. Ex-situ tensile testing after hydrogen charging

Smooth and double-notched axisymmetric tensile specimens, as displayed in Figure 1b, were extracted from the pipe at mid-thickness. The loading axis was oriented in the pipe longitudinal-to-rolling (L) direction. By using four