Fatigue crack growth properties of steels in high-pressure hydrogen gas environment

Quenched and tempered low-alloy steels having martensitic microstructure are promising materials, primarily in the context of pressure vessels destined for the storage of hydrogen gas in hydrogen refueling stations. According to the database established by NASA, low-alloy steels are classified as either “severely-” or “extremely-embrittled” materials with the occlusion of hydrogen. Thus, when low-alloy steels are destined for use in hydrogen gas environment, the hydrogen-induced degradation of their mechanical performance should be thoroughly taken into account in the strength-design.

The hydrogen sensitivity of low-alloy steels varies depending on their strength levels and microstructural morphologies. Such a variation in susceptibility is particularly prominent for FCG properties as elaborated by Matsuoka et al., who carried out FCG tests of low-alloy steels JIS-SCM435 and JIS-SNCM439 with tensile strengths (TSs) of 820 ~ 1130 MPa at various test frequencies ranging from 0.001 to 1 Hz. Their key discoveries are summarized as follows:

1. When TS is less than 900 MPa, FCG is accelerated by a factor of 20 ~ 30 in a hydrogen gas environment, as compared to an in-air condition. However, the acceleration rate is not affected by test frequency. Specifically, the acceleration is “cycle-dependent”, for which the fatigue life (i.e., number of cycles to failure) is determinable by crack-growth analysis, under the assumption of the presence of pre-cracks.
2. In the case of TS greater than 900 MPa, the rate of hydrogen-induced crack acceleration monotonically intensifies with a reduction in test frequency. That is, the acceleration is “time-dependent”, for which the fatigue life is unpredictable in terms of the number of pressurization cycles to failure. Thus, fatigue life design is not applicable.

However, even after the remarkable progress of phenomenological understanding, the rationale behind the strength-dependence of hydrogen sensitivity has not been well-established, despite its crucial importance not only for the reliable use of components, but also for the development of novel materials with both superior strength and adequate hydrogen compatibility.

In the presentation, a series of experimental results on fatigue crack-growth properties of various steels in high-pressure hydrogen gas will be exhibited together with the microscopic processes of the hydrogen-induced degradation. Based on them, hydrogen compatibility of steels will be discussed and future challenge for research in the field of hydrogen gas embrittlement will be proposed.