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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embrittlement mechanism from hydrogen-enhanced localized plasticity (HELP) in the ex-situ case to the hydrogen enhanced decohesion (HEDE) mechanism in the in-situ case. Li et al. (2018) also ascribed the embrittlement in their case to hydrogen decreasing the cohesive interface energy, as they mentioned only limited dislocation density in hydrogen-charged specimens. Still, this limited ductility on the fracture surface does not appear to be fully reflected in the mechanical fracture toughness data. Given the divided crack front as a result of the crack branching, indicated on Fig. 5 (c) by the dash- dot line, the meaning of ‘crack tip’ become s ambiguous. While the DCPD data will still reflect the smallest remaining cross-sectional area (i.e. the furthest grown crack front), the crack tip opening cannot be accurately captured by the clip gauges.

Fig. 7. Stable crack growth region (a) dimples in an air-tested specimen (b) region of quasi-cleavage (fisheye) in the ex-situ tested specimen, surrounded by dimples, yellow arrow marks an MnS inclusion (c) quasi-cleavage in the in-situ tested specimen, blue arrow indicates secondary crack in L direction 4. Conclusion The hydrogen-assisted toughness degradation of an API 5L X70 pipeline steel was investigated using SENT testing. Ex-situ testing and in-situ testing were compared after the same electrochemical hydrogen pre-charging procedure. Hydrogen charging influences fracture toughness of an X70 pipeline steel only minimally when tested ex-situ, but significantly when tested with in-situ hydrogen charging. For one, hydrogen appears to promote splitting along the banded microstructure: in the ex-situ case, this manifests as splitting at the start of crack growth. For the in-situ case, a split can be found in the crack growth region. Secondly, hydrogen causes a (partial) shift to quasi-cleavage. For the ex-situ case, this manifests as fisheyes initiating from the hard segregation bands. For the in-situ case, the active hydrogen uptake during the test results in an almost fully quasi-cleavage crack growth, as well as crack branching. The interpretation of mechanical data is complicated by both microstructural effects, in the case of crack-arrester splits along the banded microstructure, and hydrogen-related effects, in the case of crack-branching due to active hydrogen uptake. Both aspects affect the ability to accurately capture the crack tip opening displacement by clip gauges. Acknowledgements The authors acknowledge the support from Research Foundation - Flanders (FWO) via grant G056519N and grant

12ZO420N. References

Álvarez, G., Zafra, A., Belzunce, F.J., Rodríguez, C., 2020. Hydrogen embrittlement analysis in a CrMoV steel by means of SENT specimens. Theor. Appl. Fract. Mech. 106, 102450. https://doi.org/10.1016/j.tafmec.2019.102450 American Petroleum Institute, 2018. API Specification 5L - Line Pipe, n° 46. British Standard Institution, 2018. BS 8571:2018, Method of test for determination of fracture toughness in metallic materials using single edge notched tension (SENT) specimens.