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

• Strain-controlled LCF tests in 4.6 MPa H 2 at RT and 120 °C revealed hydrogen embrittlement at both temperatures. • Fracture surfaces in H 2 exhibited cleavage, striations, and secondary cracking. • These secondary cracks were associated with delta-ferrite. • At 120 °C, no martensite was observed and cECCI revealed strain localization at delta-ferrite boundaries. • Highlights the role of delta-ferrite in fatigue failure in H 2 without martensite. 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 CO 2 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.