PSI - Issue 54

Margo Cauwels et al. / Procedia Structural Integrity 54 (2024) 233–240 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Given the current trend towards clean energy and net zero carbon emissions, natural gas pipelines are considered for conversion to hydrogen gas transport and storage, as this provides an economical and efficient way of hydrogen gas distribution. The problem of hydrogen embrittlement is well known, however, and the ingress of hydrogen into the pipeline wall can cause a reduction in the mechanical performance of the pipeline steels. Fracture toughness is an important factor in the structural integrity assessment of pipelines, and the effect of hydrogen on the fracture toughness should be reliably measured and investigated. Many studies have investigated hydrogen embrittlement using a variety of test procedures and specimen geometries, such as Chatzidouros et al. (2019), (2011); Fassina et al. (2012) and Nguyen et al. (2020). For pipeline steels, fracture toughness is often determined by single edge notched tension (SENT) testing, as SENT testing provides a similar constraint to the actual pipeline application (Cravero and Ruggieri (2007)). Yang et al. (2009) tested pipeline base metal and welds by SENT specimens in-situ in a sour environment, with additional pre-charging and preloading, and found both a fracture toughness reduction of 44% compared to air, as well as a change in fracture surface appearance, with fisheyes appearing at the crack tip of hydrogen charged specimens. Furthermore, an X70 pipeline steel was tested by Wang (2009), finding no significant reduction in fracture toughness when the hydrogen concentration was below a critical concentration. Above this threshold, they observed a significant effect of in-situ charging, combined with a shift to quasi-cleavage fracture. Given the different levels of complexity for electrochemical or gaseous hydrogen charging, and in-situ or ex-situ testing, the applicability of these test methods is also relevant. Álvarez et al. (2020) compared gaseous and electrochemical hydrogen (pre-)charging for a CrMoV steel and found higher embrittlement in the electrochemical case, ascribed to the higher diffusible hydrogen content in the sample in that case. Lower strain values were needed to trigger crack growth in the cathodically pre-charged specimen, which was not the case for the gaseous pre-charging. They also observed a detrimental influence of the displacement rate on the fracture toughness in hydrogen charged condition, which also points to the importance of the test parameters in interpreting and assessing hydrogen-assisted degradation of the fracture toughness. This work studies the effect of electrochemical hydrogen charging on the fracture toughness and crack growth mechanism of an X70 pipeline steel, considering both in-situ and ex-situ test methodologies. 2. Materials and Methods The investigated material was an API 5L X70 grade pipeline (American Petroleum Institute, 2018), produced in 1991. The pipeline had a measured wall thickness of 15.8 mm and a nominal diameter of 1016 mm and had been in service for natural gas transmission. The microstructure is given in Fig. 1 and mostly consists of banded ferrite and pearlite, as well as some segregation bands in centerline region, containing bainitic and martensitic constituents. Inclusion content includes both oxides, mostly containing Al, Ca and/or Si, and sulfides, containing Ca, Mn or both. Elongated MnS as well as large Nb-rich precipitates were found in and near the hard segregation bands. Single edge notched tension tests were performed both in air and in hydrogen charged condition. The specimen was extracted from the pipe mid-thickness along the L direction, with the notch oriented so that the crack grows in the S direction ( denoted as ‘LS’ by BS 8571 (British Standard Institution, 2018)). The specimen had a square section with a side of 10 mm. The notch had an initial crack length of 3.3 mm ± 0.2 mm, consisting of a 1.5 mm deep machined notch followed by fatigue pre-cracking until the initial crack length was reached. The sample geometry is schematically represented in Fig. 2. Crack growth was monitored using the direct current potential drop (DCPD) method (Van Minnebruggen et al. (2017)). Crack tip opening displacement (CTOD) was monitored using the double clip gauge method. No side-grooves were used. The samples were manually ground with a #320 SiC grinding paper to obtain a consistent surface finish before charging. Specimens were pre-charged for 48h in a 0.1M NaOH electrolyte poisoned with 1 g/l NH 4 SCN at a constant current density of 0.8 mA/cm 2 . Afterwards, these pre-charged specimens were tested either ex-situ or in-situ. For the ex-situ case, there was a gap of 35 min between the end of the pre-charging and the start of the tensile test, including mounting time and stabilization of the DCPD current. For the in-situ test, a setup was developed to allow for concurrent tensile testing and hydrogen charging. The time between the end of pre-charging and the beginning of the application of force in the SENT test included 45 min in air and 45 min of additional pre charging. This was needed for mounting the specimen in the testbench and also allowed sufficient time for stabilization