Long-Term Exposure in Liquid Hydrogen: Mechanical Properties and Microstructural Investigation of 304 Austenitic Steel After 30 Years of Service

Although austenitic steels have been implemented in direct liquid hydrogen (LH₂) contact for decades, detailed microstructural and mechanical studies are still rare at a temperature of 20 K and inexistent for long-term exposure in LH₂. Therefore, austenitic stainless-steel parts, which were in direct contact with LH₂, from a container for LH₂ transport from the company Linde GmbH that has been in service for over 30 years, was chosen as a material model system for this investigation. In the present work, the possible influence of cryogenic gaseous and liquid H₂ (GH₂ and LH₂) on the micro- and macroscopic as well as mechanical properties of the container was investigated. Monitoring the properties after long-term GH₂ and LH₂-exposed material assesses the durability and the failure characteristics of these austenitic steels. A mean content of 2.5 ppm H was detected in the container walls after the long-term exposure. The microhardness of the long-term GH₂ and LH₂ are similar to an H₂ non-exposed sample. Based on the SEM investigations, no microstructural change could be detected in the material after long-term H₂ exposure and the residual tensile properties are still similar to those of ‘fresh’ non-exposed material. The hydrogen embrittlement (HE) occurred in the container material only after additional thermal H-charging, where the ductility reduced to about 50% at 200 K.