Hydrogen resistant high strength steel microstructures for automotive applications- insitu diffusion and hydrogen embrittlement analysis

Hydrogen embrittlement in high-strength steels creates a significant challenge for their applications in hydrogen storage and transportation systems. Understanding the interaction between hydrogen and these steels is critical to address the embrittlement issues. This study focused on designing hydrogen-resistant microstructures through a unique combination of ausforming and austempering treatment in low-alloy carbon steels intended for automotive applications. A novel microstructure was engineered by applying plastic deformation prior to the bainitic transformation in a thermomechanical simulator (Gleeble 3800). This innovative microstructure was characterised by the presence of nano-scale austenite films between carbide-free bainitic laths. An extensive investigation on the influence of austempering temperatures on the morphology of the retained austenite with three temperatures (475 degrees C, 450 degrees C, 430 degrees C) was undertaken. The retained austenite morphology and volume fraction were analysed by optical, scanning, transmission electron microscopy, and XRD analysis. The hydrogen trapping characteristics as a function of the size and volume fraction of retained austenite were studied using the Oxygen Nitrogen and Hydrogen (ONH) analyzer. Both diffusible and total hydrogen measurements were conducted to elucidate the nature of reversible and irreversible hydrogen traps within the microstructures. Insitu Cryo Atom Probe Tomography (APT) analysis provided the precise localisation of hydrogen atoms in the engineered microstructures and used to explain the hydrogen interactions at austenite-bainite interphase boundaries. The findings from hydrogen trapping and Cryo APT analyses revealed that the dislocation tangles at the bainite-retained austenite interphase boundaries served as diffusible hydrogen traps, releasing hydrogen when heated to 900 degrees C. The retained austenite, present as nanofilms between bainite laths, acted as bulk hydrogen traps, releasing hydrogen upon heating to the melting temperature. Mechanical properties studies on the engineered steels showed comparable ultimate tensile strength and yield strength with and without hydrogen charging.