Mechanical properties and fracture behavior of hydrogen charged AHSS/UHSS grades at high- and low strain rate tests

The present study focuses on the impact of hydrogen on mechanical properties of four typical advanced and ultra-high strength steel (AHSS/UHSS) grades (complex-phase-, dual-phase- and tempered martensitic steel grades with tensile strength between 1200MPa and 1400MPa) at two significantly different loading rates (10^-5s^-1 and 20s^-1). At very low strain rates a strong reduction of tensile strength and elongation at fracture is observed for all grades charged with hydrogen and the fracture appearance is typical for hydrogen embrittlement, HE. At the high strain rate an impact of hydrogen on the mechanical properties is not detected and the fracture type is ductile. From a comparison between HE-mechanisms in literature and the fracture surfaces of low strain rate samples it is assumed, that hydrogen induced failure is a combination of enhanced dislocation mobility by hydrogen (hydrogen enhanced localized plasticity, HELP) and decohesion promoted by hydrogen transport through dislocations. An estimation of the critical dislocation velocity for hydrogen to move along with dislocations according to Tien et al. [1] reveals, that even at the high strain rate applied hydrogen transport by dislocations is still possible. Therefrom the non-existing effect of hydrogen on mechanical properties at high strain rates is assumed to be due to the absence of hydrogen accumulation at highly stressed microstructure regions and an insufficient increase of hydrogen concentration through hydrogen transport by dislocations to initiate decohesion.