PSI - Issue 68

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

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the ferritic-pearlitic steel showed unexpected behaviour at this total strain amplitude, as indicated by the red circle in Figure 7a. The specimen tested with argon failed earlier than the one tested with hydrogen. Note that only a single specimen was tested, so further testing is necessary to clarify this unexpected result. Figure 8 shows overview of SEM micrographs from the LCF specimens tested at a total strain amplitude of 1.2 % for the ferritic-pearlitic steel. Figure 8a shows the fracture overview of the specimen tested in argon, while Figure 8b shows the fracture surface for the hydrogen-tested specimen. In Figure 8b the crack initiation site as well as fatigue striations that evidence the crack propagation are clearly visible. The crack propagated in the direction indicated by black arrows, also, it is possible to observe traces of the last stages of crack propagation during the final moments of the test.

Figure 8. Low magnification 40X SEM images of ferritic-pearlitic steel after LCF test of 1.2% strain range amplitude a) Ar test b) H 2 test.

Figure 9 shows high magnification SEM images of the fracture surface revealing distinct differences on the fracture mode between argon and hydrogen tested specimens. For the argon-tested specimen, the features observed are indicating ductile fracture appearance with large number of closely spaced striations, typical for material that resists fracture and promotes slow crack propagation. In contrast, the hydrogen-tested specimen at high magnification, appears less deformed and brittle. Secondary cracks are also seen. The fracture surface evidence intergranular cleavage decohesion with a presence of a couple of striations within the grain (see inserted SEM image in Figure 9b). The impact of hydrogen gas on the fracture type is evident and it could be concluded that hydrogen is responsible for accelerated crack propagation, leading to a reduced LCF life of the steel. Similar fracture behaviour has been observed on the bainitic steel LCF measurements, further analysis needs to be done to determine the full extent of the damage caused due to hydrogen embrittlement in the fatigue test.

Figure 9. Low magnification 5000X SEM images of ferritic-pearlitic steel after LCF test of 1.2% strain amplitude a) Ar test b) H 2 test.