Hydrogen embrittlement in stainless steel 321: Influence of temperature, loading mode and cyclic pre-deformation

Licentiate thesis, 2025

Driven by the shift towards clean energy, industrial gas turbines engineered for operation with natural gas are increasingly being adapted to operate on hydrogen-natural gas blends or even pure hydrogen. However, the use of hydrogen introduces concerns related to its absorption into structural components, such as fuel supply pipes. Austenitic stainless steel AISI 321, widely used for this purpose, is susceptible to hydrogen embrittlement, a degradation phenomenon due to hydrogen uptake that may result in premature failure. This thesis investigates the influence of temperature, loading mode, and cyclic pre-deformation on hydrogen embrittlement in AISI 321 stainless steel.

In the first part, the influence of temperature and loading mode on hydrogen embrittlement susceptibility is examined. Slow strain rate tensile tests (1 × 10^-5 1/s) at 150°C in gaseous hydrogen showed no signs of embrittlement, whereas strain-controlled low-cycle fatigue tests exhibited significant embrittlement at 120°C. Although deformation-induced martensite was absent at elevated temperatures in both loading modes, fracture analysis of fatigued specimens revealed cracking along δ-ferrite phase boundaries. Further insight from controlled electron channelling contrast imaging (cECCI) of fatigue specimens tested at 120°C revealed strain localization near these boundaries. This suggests that localized deformation, when combined with the presence of hydrogen, could play more critical role in fatigue by promoting crack initiation and propagation. In contrast, tensile failure requires widespread embrittlement (e.g., through martensite formation) to embrittle a larger cross-sectional area. This was observed in tensile specimens tested in hydrogen at room temperature, where cleavage fracture and δ-ferrite cracking occurred. The second part explores the influence of deformation history on hydrogen embrittlement through cyclic pre-deformation. Specimens were subjected to strain-controlled cyclic pre-deformation (100 cycles) across different strain amplitudes. At all strain amplitudes tested, dislocation cells and slip/shear bands were observed. Minimal martensite was present at the highest strain amplitude of 0.6%. Regardless of pre-cycling, hydrogen pre-charging lowered the tolerable plastic strain range at all strain amplitudes. A re-hardening was observed in pre-cycled test bars due to thermal exposure from hydrogen pre-charging or reference exposure in air. Despite this, low-cycle fatigue (LCF) life of these pre-cycled test bars remained comparable to that of non-pre-cycled test bars, with and without hydrogen pre-charging. This finding supports the use of simplified testing procedures where hydrogen charging before LCF loading is performed as a practical approximation of service conditions involving continuous hydrogen exposure during cyclic loading with prolonged hold times, like that of a gas turbine.