PSI - Issue 68

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

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3.3. Effect of hydrogen on tensile and fatigue properties The impact of hydrogen on the tensile and low cycle fatigue properties of the studied steels can be evaluated from this work. However, this is only applicable to the tested conditions, such as a hydrogen pressure of 200 bar and room temperature. From the in-situ SSRT tests using a uniaxial tensile loading at constant strain rate of 10 -6 s -1 , the effect of hydrogen appears after reaching the UTS i.e. when necking begins to form. For ferritic-pearlitic leakage happens rather soon as the UTS/necking was reached, indicating that in a hydrogen may assists faster crack propagation. In contrast, in the case of bainitic steel, crack propagation seems to be slower, as the leakage is delayed after reaching UTS/necking of the specimen. However, even though both steels suffer a loss of ductility and toughness, the tensile strength remains unaffected, as shown in Table 2. On the other hand, when dynamic loading is applied, hydrogen damage seems to be dependent on the total strain amplitude. The steels experienced more severe degradation on the fatigue life to leakage when tested at higher total strain amplitudes. It has been observed that for 1.2 % and 0.6 % total strain amplitudes, where plastic deformation is expected to be more pronounced, premature leakage occurs in a hydrogen environment. In contrast, at a total strain amplitude of 0.3 %, where a negligible plastic deformation is expected, no significant effect of hydrogen on fatigue life was observed. However, these results are just preliminary, since only one specimen was tested at 0.3% total strain amplitude. In general, regardless of the mechanical loading type imposed on the specimens both tested steels, uniaxial tensile and low cycle fatigue, it was observed that the bainitic carbon steel exhibits a slightly higher resistance to hydrogen embrittlement than the ferritic-pearlitic steel. This result is of great importance because these steels have different strength properties and microstructures. 4. Conclusions In this work the effect of hydrogen on the tensile and low cycle fatigue properties was evaluated by means of hollow specimen method. The main conclusions are as follows: - The hollow specimen method is a suitable tool for evaluating the effect of hydrogen on the tensile and low cycle fatigue properties of high strength carbon steels. - For both steels it was found that the tensile properties were not affected when specimens were tested at 200 bar hydrogen environment at room temperature. However, a considerable degradation in elongation and relative reduction of area was measured when comparing the hydrogen to argon-tested specimens. - Hydrogen analysis revealed that higher hydrogen uptake occurred in the ferritic-pearlitic steel than in the bainitic steel and this agreed with the observed effect of hydrogen on the ductility and area of reduction. - The LCF life was dependent on applied total strain amplitude. Highest total strain amplitude resulted in shortest LCF life for both steels. - Fractographic analysis revealed brittle fracture for the specimens tested in hydrogen compared to argon environment. For the hydrogen SSRT specimen quasi-brittle cleavage type of fracture was observed, while for the LCF tested specimens, intergranular cleavage decohesion was the main fracture feature accompanied with much less number of striations within the grains as compared to the argon specimens. - In general, the ferritic-pearlitic steel was more sensitive to hydrogen than the bainitic steel in the testing conditions as ductility and LCF life were more deteriorated. Acknowledgements This work was carried out within the FINAST project (Research and Innovation in Norrbotten for Advanced Green Steel Production and Manufacturing) funded by the EU Just Transition Fund and the Swedish Agency for Economic and Regional Growth under grant number 20358499.