Crack Growth in Carbon and C-0.5Mo Steels in High Temperature Hydrogen: Laboratory Data and Fitness for Service Modelling

High temperature hydrogen attack (HTHA) is a known degradation mechanism in the refining industry for carbon and low alloy steels operating at temperatures above 400°F in hydrogen service. Historically the integrity of operating equipment subject to these conditions has been ensured by using the empirically derived Nelson Curves to identify safe operating regions. This approach was largely successful, but failures still occurred and, in some cases, required overly conservative operational limits. Additionally, this approach did not allow for a defect tolerance approach to fitness for service (FFS) assessments. An on-going joint-industry project (JIP) has been addressing these issues by generating laboratory crack growth data and developing models to apply the acquired knowledge in FFS assessments. A testing program was conducted on three (3) C-0.5 Mo steels to generate crack growth data in hydrogen at a range of temperatures (316 to 399°C [600 to 750°F]), 5.52 MPa (800 psig H2) hydrogen pressure, and stress intensity values between (10.5 to 35.4 MPa√m [9 to 32 ksi√in]). These results were used to validate and refine a crack growth model based on the creep crack growth fracture mechanics approach, C*. The results of the test program and modeling efforts are described in detail.