73 - Durability of additively manufactured austenitic stainless steels under fatigue-hydrogen coupling

Abstract

Selective Laser Melting (SLM) process has the potential to fabricate complex parts which can find applications in hydrogen-based systems, due to their improved strength-ductility trade-off and inherent austenite phase traditionally resistant to Hydrogen Embrittlement (HE). Knowledge on the HE resistance of these new class of SLM materials is crucial, especially under fatigue loading conditions. In this context, the goal of this work is to experimentally assess and understand the susceptibility of additive manufactured stainless steels to hydrogen embrittlement under coupled mechanical testing conditions. For this purpose, three grades of SLM fabricated stainless steels are chosen : 316L and 304L austenitic stainless steels and 17-4PH martensitic stainless steel. Heat treatments (HT) are performed to modify the dislocation sub-structure and the consequent impact on hydrogen diffusion is studied under both electrochemical and high-pressure hydrogen gas charging conditions. Insitu tensile and fatigue tests under these two charging conditions are performed to understand and reveal the impact of hydrogen on the tensile and fatigue properties, with a focus on the impact of hydrogen on crack initiation and propagation. The results of this work illustrate the degree of sensitivity of the microstructure of SLM and heat-treated materials to HE under various hydrogen-charging conditions. Due to a slower diffusion of hydrogen in the dislocation-rich microstructure and a localisation of hydrogen within 100 µm from the charging surface, electrochemical pre-charging of SLM-316L samples does not show any impact of hydrogen on the mechanical properties. The dominant role of defects impedes the evaluation of the impact of hydrogen on both ductility and fatigue crack initiation in SLM-316L. Insitu electrochemical mechanical tests affect the strength and ductility of the chosen materials more than the high-pressure hydrogen gas condition. Softening due to hydrogen under tension is evident on the dislocation-rich microstructures of SLM-316L and SLM-304L samples, while it is absent in the case of dislocation-free microstructures after HT. H-dislocation interaction varies depending on the severity of H-charging and the nature of the dislocations. The fatigue-hydrogen coupling with electrochemical charging results in a loss of fatigue properties in SLM-304L. Complex phenomena are evidenced during insitu electrochemical fatigue tests on SLM-316L, and the observed loss of fatigue properties are explained through supplementary evidence. Proof of increase in fatigue crack growth rates in the presence of hydrogen is demonstrated through observations of fatigue striation spacing, indicating that SLM-fabricated austenitic stainless steels are susceptible to HE under adverse H-charging conditions.

Speaker

Rahul Subramanian Girija

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Institut de Mécanique et d'Ingénierie (I2M) UMR 5295, Bordeaux, France

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Conference

73 - Durability of additively manufactured austenitic stainless steels under fatigue-hydrogen coupling

Date/Time

21/03/2024

5:25 pm -5:45 pm

Location

Room 8