The aim of the project is to understand the role of retained austenite (RA) and its connection to a transforming microstructure in hydrogen-related crack initiation and propagation in advanced highstrength steels under realistic conditions and thereby assist the future development and the acceptance of these new materials in a wide variety of industrial applications.
- The focus on RA will be achieved by performing the investigations mainly on a single composition (Melt I), for which varying concentrations of RA will be attained through different heat treatments. To modify the thermodynamic stability of the austenite, the Mn content is adjusted for a secondmodel alloy (Melt II). To get close to applications with RA, a third melt (Melt III) is also taken into account.
- In order to investigate hydrogen-related crack initiation and propagation approaches such as fractography, tensile testing of notched specimens and advanced characterisation techniques such as Scanning Kelvin Probe Force Microscopy (SKPFM), Electron Channeling Contrast Imaging (ECCI) and in-situ SEM will be performed.
- In order to study the transforming microstructure a series of different mechanical tests, including constant load test (CLT), slow strain rate tensile tests (SSRT) and cyclic loading tests (CYT), will be performed. While a CLT will keep the microstructure largely unchanged, a slow increase of straincan eventually trigger a martensitic transformation. Hence, hydrogen needs to redistribute either in the RA or the surrounding martensite. The dynamics of this process and the evolving hydrogen concentration profile can be triggered by applying cyclic loadings conditions.
- At the same time, the application of cyclic loading follows our goal to come close to realistic conditions. While fluctuating stress states definitely occur in applications, such an approach has not been performed in previous RFCS projects and literature to date has not explored this testing withautomotive grade steel. Another step towards realistic conditions is the comparison of different hydrogen absorption modes, including high temperature absorption during annealing under H2 rich atmospheres. The latter conditions are probably the main source of trapped hydrogen in steels andtherefore particularly relevant in the context of retained austenite. A third strategy towards reality, isthe consideration of wired material that is as close as possible to real application (stud anchor) in construction.
Alongside the experimental investigations, atomistic calculations will be performed on the microscopic and nanoscale features of the various materials in order to understand the influence that RA, the neighboring microstructure (e.g. carbides) and point defects (e.g. vacancies) have in the hydrogenembrittlement process. Furthermore, these ab initio calculations will serve as inputs for scale-bridging simulations at the meso- and macro-scale in an effort to understand hydrogen-related phenomena at the component-scale. This two fold concept is the key to connect the microstructural characterization of the different steels with the fractography and the results obtained from cyclic loading tests.