Steel grades with levels of strength beyond 1000MPa and containing between 10 and 20% retained austenite are increasingly developed notably for lightweight automotive applications requiring high ductility. This enhanced ductility is achieved by Transformation-Induced Plasticity (TRIP) effect induced by the retained austenite during forming. As a consequence of the steel manufacturing process, hydrogen absorption and desorption into and from the microstructure will occur. As the interactions between hydrogen and ferritic microstructures have been widely studied, this is not the case for these new steel grades combining ferritic and austenitic phases.
This work aims at investigating the role and impact of diffusible hydrogen on the mechanical behavior of advanced high strength steels with retained austenite.
To correctly assess the influence of hydrogen on such multiphase microstructures, model microstructures were processed from steel grades differing by their global carbon content. Single phase martensitic and bainitic microstructures with or without retained austenite were processed and characterized. The samples were then cathodically charged with hydrogen up to saturation. Thermal desorption analysis (TDA) was subsequently used to quantify the hydrogen content, derive the activation energy associated to the desorption peaks and study the degassing behavior of the different microstructures.
An influence of the constituting phases on the hydrogen content and diffusivity is demonstrated. Hydrogen microprint technique used to visualize the hydrogen diffusion paths through the different microstructures confirms the nature of the dominant trapping sites. Finally, the hydrogen influence on the mechanical properties of the investigated microstructures is also scrutinized.