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 (LH$_2$) contact for decades, detailed microstructural and mechanical studies are still rare at a temperature of 20 K and inexistent for long-term exposure in LH$_2$. Therefore, austenitic stainless-steel parts, which were in direct contact with LH$_2$, from a container for LH$_2$ 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 H$_2$ (GH$_2$ and LH$_2$) on the micro- and macroscopic as well as mechanical properties of the container was investigated. Monitoring the properties after long-term GH$_2$ and LH$_2$-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 GH$_2$ and LH$_2$ are similar to an H$_2$ non-exposed sample. Based on the SEM investigations, no microstructural change could be detected in the material after long-term H$_2$ exposure and the residual tensile properties are still similar to those of ‘fresh’ non-exposed material. ... mehrThe hydrogen embrittlement (HE) occurred in the container material only after additional thermal H-charging, where the ductility reduced to about 50% at 200 K.