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In order to select the most appropriate steel to deal with pressurized hydrogen over long periods of time, the fatigue crack propagation rate of quenched and tempered CrMo and CrMoV steel grades was assessed by means of tests performed on thermally pre-charged specimens in a hydrogen reactor at 195 bar and 450ºC during 21 hours. Cylindrical samples were used to measure the hydrogen content and their desorption kinetics at room temperature and compact tensile specimens to determine the fatigue crack growth rate. Under the aforementioned pre-charging conditions, significant amounts of hydrogen were introduced, being much larger in the CrMoV steel grades, which also have much lower apparent diffusion coefficients, as precipitation of fine vanadium carbides during tempering provides strong hydrogen traps. Moreover, the fatigue crack growth rate increased significantly due to the presence of internal hydrogen in the CrMo grades for test frequencies lower than 10 Hz in comparison with tests performed in air. Furthermore, the presence of vanadium carbides in the CrMoV steel significantly improved fatigue crack growth performance, the effective hydrogen diffusion distance per cycle and the hydrogen concentration in the process zone ahead of the advancing crack being considerably reduced.

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The influence of hydrogen on the mechanical behaviour of a 42CrMo4 tempered martensitic steel was investigated by means of tensile tests on both smooth and circumferentially-notched round-bar specimens pre-charged with gaseous hydrogen in a pressurized reactor. Hydrogen solubility was seen to decrease with increasing tempering temperature. Moreover, hydrogen embrittlement measured in notched specimens was much greater in the grades with higher hardness, tempered at the lowest temperatures, where a change in the fracture micromechanism from ductile in the absence of hydrogen to intermediate and brittle in the presence of hydrogen was clearly observed. Results were discussed through FEM simulations of local stresses acting on the process zone.