Effect of hydrogen on the fatigue crack growth rate of quenched and tempered CrMo and CrMoV steels

Abstract

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 450 °C during 21 h. 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.