PSI - Issue 68

Jan Kec et al. / Procedia Structural Integrity 68 (2025) 1091–1097 Jan Kec / Structural Integrity Procedia 00 (2025) 000–000

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properties is shown in the Fig. 2. (a), and can be attributed to the scatter of the mechanical properties of the steel. No fundamental changes are apparent in the tensile elongation and reduction of area (Z), see Fig. 2. (b).

Fig. 2. (a) tensile strength without hydrogen and after pre-charging; (b) tensile elongation and reduction of area without hydrogen and after pre charging.

Fig. 3. (a) show the decrease in CVN energy after pre-charging in a hydrogen/natural gas blend. The decrease is probably most noticeable after 3000 hours of pre-charging, it was also observed at TT. At subsequent pre-charge time, the same values or slightly increasing values can be observed, due to the scatter of the measured data. The micromorphology of all samples is predominantly composed of fine dimples, Fig. 4. (a). No significant fractographic differences were observed between the reference (without hydrogen) and pre-charged samples. All samples tested at 0 °C are characterized by microviods coalescence fracture mechanism. All samples also exhibit short central delamination, usually associated with the occurrence of HE, but it was found to be a material characteristic in this case. Areas of cleavage facets are seen near the delamination, Fig. 4. (b). For FT, a pop-in of about COD 2 mm was observed in most samples. However, prior and after pop-in, there was a steady extension in the crack tip; therefore, both size-independent and fracture-dependent fracture mechanical parameters are evaluated. The size-independent ones include the J-integral initiation value (J i ) and at 0,2 mm crack extension (J 0,2 ). The fracture-dependent ones include the fracture resistance value at pop-in (J u(20) ). Thus, it is evident from the FT results indicating a decrease in FT parameters. Similar to the other tests, the most noticeable decrease occurred after the pre-charge of 3000 hours, see Fig. 3. (b). The pre-charged 6000 hours samples show very similar values of the J-integral parameters. The micromorphology of the crack extension of the specimens after FT testing is composed of fine dimples (see Fig. 5. (a)) and thus, even at higher pre-charge time, the change in mechanism from microvoid coalescence to quasi-cleavage fracture was not confirmed. During the pop-in, a central delamination was formed on the fracture surface and, similar to the CVN test, cleavage facets were observed around this delamination, see Fig. 5 (b). It should be emphasized that delaminations were present in both the samples tested after pre-charging in hydrogen and in the reference samples (without hydrogen), therefore this is a material feature of this steel.