Effect of hydrogen pressure and test frequency on fatigue crack growth of a ferrite-pearlite steel in hydrogen gas

Fatigue crack growth behaviour of a ferrite-pearlite steel SM490B has been investigated in hydrogen gas and air. In order to investigate the effects of hydrogen pressure and test frequency on fatigue crack growth, fatigue crack growth tests were carried out under the conditions of hydrogen pressures p=0.1, 0.4, 0.7 MPa (absolute pressure) and test frequencies f=0.1, 1, 5 Hz. Fatigue crack growth was accelerated in hydrogen gas under all the conditions with the combination of the test frequency and hydrogen pressure. Fatigue crack growth rates in hydrogen gas were about 30 times higher than those in air. There existed the upper limit of the acceleration of fatigue crack growth at 30 times at frequency below 1 Hz in hydrogen gas. The striations formed in hydrogen gas were flat and unclear, compared to the striations in air. Hydrogen that enters the material in hydrogen gas is external hydrogen. On the other hand, hydrogen that exists in the hydrogen-charged material is internal hydrogen. The difference in the fatigue crack growth acceleration between external hydrogen and internal hydrogen for the ferrite-pearlite steel was discussed in comparison between the results of SM490B steel in hydrogen gas and the results of hydrogen-charged specimens of ferrite-pearlite-steel SGP in the previous study(4). The highest test frequency to obtain the upper limit of crack growth rate acceleration for SM490B in hydrogen gas was 1 Hz. On the other hand, the highest frequency to obtain the upper limit for the hydrogen charged steel SGP was 0.01 Hz. Accordingly, there was the difference in the hydrogen supply mechanism to the crack tip between external hydrogen and internal hydrogen.