Fatigue Crack Growth Rates of API X70 Pipeline Steels in Pressurized Hydrogen Gas Compared with an X52 Pipeline in Hydrogen Service

The current ASME B31.12 code used to guide the design of hydrogen pipelines favors the use of API 5L X52, but is being modified to include higher strength steels, such as X70 to enable cost reductions without affecting safety. To provide a scientific basis for code modification, fatigue crack growth (FCG) tests were conducted on an X52 pipeline steel that is currently in service transporting hydrogen gas, as well as two X70 pipeline steels designed for natural gas. Compact tension specimens were tested in hydrogen gas pressurized to 5.5 MPa or 34 MPa. A comparison of these tests, conducted at a cyclic loading frequency of 1 Hz, shows that there is very little difference between the FCG rates of the base metal among the three steels at a given hydrogen pressure. All three metals exhibited some increase in FCG rate at a hydrogen pressure of 34 MPa compared with 5.5 MPa. Analysis of the data provide a rationale for allowing higher strength steels to be used for hydrogen gas transport. A recommendation was made to the ASME B31.12 Committee on Hydrogen Piping and Pipelines to allow higher-strength steels that is based on this and other data acquired at hydrogen pressures ≤ 21 MPa.