Fatigue crack growth rates of API X70 pipeline steel in a pressurized hydrogen gas environment

Hydrogen is known to have a deleterious effect on most engineering alloys. It has been shown repeatedly that the strength of steels is inversely related to the ductility of the material in hydrogen gas. However, the fatigue properties with respect to strength are not as well documented or understood. Here, we present the results of tests of the fatigue crack growth rate (FCGR) on API X70 from two sources. The two materials were tested in air, 5.5 and 34 MPa pressurized hydrogen gas, and at both 1 and 0.1 Hz. At these hydrogen pressures, the FCGR increases above that of air for all values of the stress intensity factor range (Delta K) greater than ~7 MPa center dot m(1/2). The effect of hydrogen is particularly sensitive at values of Delta K below ~15 MPa center dot m(1/2). That is, for values of Delta K between 7 and 15 MPa center dot m(1/2), the FCGR rapidly increases from approximately that found in air to as much as two orders of magnitude above that in air. Above 15 MPa center dot m(1/2), the FCGR remains approximately one to two orders of magnitude higher than that of air.