Abstract
The low cycle fatigue crack initiation in a low-alloy steel in hydrogen/natural gas (H2/NG) mixtures has been investigated and compared to behaviour in air. Stress controlled fatigue tests and plastic strain controlled fatigue tests were both performed. Mechanisms of crack initiation and the propagation path in the ferritic pearlitic microstructure were analysed using fine SEM and EBSD surface and sub-surface analysis. The results revealed that at low applied plastic strains, fatigue cracks initiate inside ferrite grains at intrusions-extrusions locations. Fatigue performance of the steel in H2/NG mixtures under different hydrogen partial pressures has been assessed using load-controlled tests on smooth and notched specimens. The results show that a threshold quantity of hydrogen is necessary to observe an effect of hydrogen on the initiation stage. Under high hydrogen partial pressure, mechanisms of embrittlement induced by hydrogen dominate and decrease the number of cycles of initiation. Fatigue crack growth tests confirm that hydrogen has a strong impact on crack propagation.