Hydrogen generally results in a degradation of fatigue crack growth resistance in metals depending on the nature of the hydrogen interaction with crystalline defects developed during a cyclic loading. In this paper we present the first results regarding the characterization of the microstructure and the mechanical properties of Armco Iron with a ferritic microstructure, and the investigation of embrittlement under gaseous hydrogen environment. For this purpose, fatigue crack propagation tests were performed on CT40 specimens under high pressure gaseous hydrogen using Hycomat test bench, at the Pprime Institute in Poitiers.

The fatigue crack growth rate data obtained so far at the 35 MPa of hydrogen pressure and at loading frequency of 20 Hz indicate a sharp increase in crack growth rates in a narrow range of stress intensity factor amplitudes. Also, it was observed that by decreasing the loading frequency to 2 or 0.2 Hz at the above mentioned hydrogen pressure no significant change in this transition regime happens. Scanning electron microscope observations of the fracture surfaces are used to support the explanations proposed for the hydrogen inducted intergranular failure in this material.

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