EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Ultra-high-strength steel (UHSS) fasteners, like screws, bolts or clamps, are characterized by a good combination of strength and toughness. The utilization of UHSS opens up new possibilities for reducing the weight of cars and, therefore, for reducing greenhouse gas emissions. However, the distinctive susceptibility of UHSS to hydrogen embrittlement has been limiting the use of fasteners with a higher strength. Due to the high strength small hydrogen concentrations, which are absorbed during processing and service, are already critical and may cause brittle fracture of components. Hydrogen-induced cracking (HIC) occurs either instantaneously during mechanical loading or delayed after a period of time. For that purpose, the present work is dedicated to understand the role of the microstructure on the hydrogen embrittlement resistivity of bainitic and tempered martensitic UHSS. Two different heat treatments, namely isothermal bainite (IB) treatment and quenching and tempering (Q&T) treatment, were applied to CrMoV steel. Microstructural characterization revealed intensive precipitation of carbides in both the bainitic as well as the tempered martensitic steels. However, the morphologies, nucleation sites and number densities of the carbides were different in both steels. The dislocation substructures were characterized by transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS). Experimentally recorded TDS spectra were evaluated by numerical bulk diffusion simulations. The results revealed a structure-property relationship between hydrogen uptake and diffusion, number density and carbide size distributions. Finally, incremental step load testing (ISLT) was applied to measure the fracture strength under hydrogen donating conditions.