Mechanical and microstructural effects of high pressure hydrogen up to 1000 bar and 300°C on high strength austenitic steels with varying austenite stability

• Applicant: Professor Dr. Peter Felfer, Ph.D.
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537479235

In the coming decades, hydrogen is going to play a key role in energy systems. Compressed hydrogen at 70 MPa (700 bar) offers an attractive energy density of 4752 kJ/l, while keeping the hydrogen at purity levels required e.g. for fuel cell operation. However, due to currently lacking understanding of the deformation and failure mechanisms, the expensive cold worked austenitic steels used to face high pressure hydrogen in tanks, valves and piping components are not used to their full potential. Here, the amount of expensive austenite stabilizing elements needed is a critical factor. Therefore, it is highly interesting to understand the interaction of hydrogen with the microstructure in fairly hydrogen resistant structural materials that have potential uses in hydrogen applications. This has been a real challenge, since it requires creating these high-pressure conditions at temperatures that allow for diffusion of the hydrogen into the material in practical timescales for fcc materials with low hydrogen diffusion constants compared to hydrogen embrittlement susceptible bcc materials. In preliminary work, the group of the applicant has already found that the influence of hydrogen on the austenite stability drives the loss in ductility due to hydrogen in metastable austenitic steels with strength levels of 1400 MPa. In this project, we will use several innovative approaches, developed and built up in the applicant’s group to reveal the effects of hydrogen on the deformation behavior of austenitic stainless steels of varying austenite stability from the macroscopic, down to the atomic scale. We have built up a high-pressure hydrogen charging facility, that can reach 100 MPa (1000 bar) and 300 °C to create a thermodynamically defined level of hydrogen charging in the specimen. We will use this to charge specimens of cold worked fcc stainless steels of varying austenite stability levels with hydrogen and then deform them in tensile loading. We will then combine the mechanical testing with scale-bridging characterization using EBSD to investigate microstructure evolution, TEM for the imaging of the resulting crystal defects and a special titanium atom probe to measure hydrogen concentrations and interactions with crystal defects. This atom probe is currently the only instrument worldwide that can do so without the use of tracers, making it practical for high pressure experiments. Additionally, the relative amounts of hydrogen at specific crystal defects will be assessed using a newly developed thermal desorption spectroscopy system, which has the same titanium vacuum system as the titanium atom probe. The results of this project will provide essential insights into the behavior of high-strength metastable austenitic steels under hydrogen conditions. This understanding will contribute to the safe and efficient use of these materials in hydrogen systems, supporting the broader adoption of renewable energy and greenhouse gas reduction.

• DFG Programme: Research Grants