Hydrogen embrittlement at elevated temperature during low cycle fatigue of AISI 321 stainless steel

Journal article, 2026

Adapting industrial gas turbines, which are traditionally fuelled by natural gas, to run on pure hydrogen or hydrogen-natural gas blends is critical for eliminating or lowering CO2 emissions in power generation. However, hydrogen embrittlement challenges the mechanical integrity of stainless steel 321, commonly used in fuel supply pipes. While deformation-induced martensite formation is widely recognized as a key factor contributing to hydrogen embrittlement in metastable austenitic stainless steels. The influence of delta-ferrite, temperature, and the type of mechanical loading have received less attention. In this study, AISI 321 stainless steel was thermally precharged in gaseous hydrogen with a pressure of 4.6 MPa at 350 °C for 672 h. Strain-controlled low cycle fatigue tests were performed at room temperature and at 120 °C in a hydrogen atmosphere at the same pressure. Emphasis was placed on fractographic analysis and phase evolution during deformation. A reduction in fatigue life was observed in hydrogen atmosphere at both temperatures. When compared to ambient temperature, fatigue life is enhanced at 120 °C in air due to the absence of martensite transformation. Nonetheless in hydrogen, this improvement is compromised by delta ferrite. Its phase boundary acts as a place for faster crack initiation and propagation, resulting in embrittlement. The underlying hydrogen embrittlement mechanisms is discussed. These findings provide crucial insight for material selection in hydrogen-fuelled gas turbines, underscoring the need to minimize delta-ferrite content to enhance the resistance to HE up to 120 °C.