H isao Matsunaga

Quenched and tempered low-alloy steels with a martensitic microstructure show promise as materials, particularly in the realm of pressure vessels intended for storing hydrogen gas at hydrogen refueling stations. According to NASA’s database, low-alloy steels fall into either the “severely embrittled” or “extremely embrittled” categories when exposed to hydrogen. Therefore, when these steels are intended for use in a hydrogen gas environment, it is essential to thoroughly consider the hydrogen-induced degradation of their mechanical performance in the strength design. The hydrogen sensitivity of low-alloy steels varies based on their strength levels and microstructural morphologies. This susceptibility is especially noticeable in the fatigue crack growth (FCG) properties. The presenter’s research group conducted FCG tests on low-alloy steels JIS-SCM435 and JIS-SNCM439, with tensile strengths (TSs) ranging from 820 to 1130 MPa and test frequencies from 0.001 to 1 Hz. The key findings are summarized as follows:

• When TS is below 900 MPa, FCG is accelerated by a factor of 20 to 30 in a hydrogen gas environment compared to an in-air condition. However, the acceleration rate is unaffected by the test frequency. Specifically, the acceleration is “cycle-dependent”, and the fatigue life (i.e., the number of cycles to failure) can be determined through crack-growth analysis, assuming the presence of pre-cracks.
• For TS greater than 900 MPa, the rate of hydrogen-induced crack acceleration progressively intensifies with a decrease in test frequency. This acceleration is “time-dependent”, making the fatigue life unpredictable in terms of the number of pressurization cycles to failure. Consequently, fatigue life design is not applicable.

Despite significant progress in phenomenological understanding, the rationale behind the strength-dependence of hydrogen sensitivity remains poorly established. This knowledge gap is crucial not only for ensuring the reliable use of components but also for developing novel materials with both superior strength and adequate hydrogen compatibility. The presentation will showcase a series of experimental results on the fatigue crack-growth properties of various steels in high-pressure hydrogen gas, along with microscopic processes of hydrogen-induced degradation. Based on these findings, the hydrogen compatibility of steels will be discussed, and future research challenges in the field of hydrogen gas embrittlement will be proposed.

Session

Room	Date	Hour	Subject
Room 9	Thursday 30th November	15:00-15:30	Hisao Matsunaga S10-2 Fatigue under severe environmental conditions 149 - Fatigue crack acceleration in martensitic steels under high-pressure hydrogen