Abstract
This study investigates the hydrogen embrittlement (HE) susceptibility of two martensitic ultra-high-strength steel (M-UHSS) grades, focusing on their impact toughness and microhardness behavior following different durations of hydrogen cathodic charging (1, 2, and 4 h). While some mechanisms, such as the interaction between microstructural defects and hydrogen, are well established, the effects of hydrogen on the absorbed energy during impact tests or at high strain rates have been less studied. This study correlates the microstructural characteristics, Charpy-V absorbed energy, and microhardness with fractographic analysis to assess the HE susceptibility. The results show a decrease in both microhardness and toughness after one hour of charging, with the reductions ranging from 32% to 40%. However, as the charging time increased, both properties exhibited an increase, attributed to the interaction of hydrogen and its saturation on the steel's surface. Fractographic analysis reveals a morphological change from brittle fracture to brittle fracture with localized plastic zones, driven by the interaction of hydrogen with the trapping sites within the steel. Permeability tests are conducted to quantify the hydrogen concentration, diffusion coefficients, and trapping sites. The results indicate significant hydrogen embrittlement in both steels, driven by hydrogen diffusion and accumulation in the entrapment zones, leading to increased brittleness over time. This study provides insights into the micromechanisms influencing mechanical properties and fracture behavior under hydrogen exposure.