Optimisation de l'état métallurgique en vue d'une meilleure résistance à la fragilisation par l'hydrogène : Comparaison des aciers martensitiques et bainitiques et impact de l'état de précipitation

Hydrogen embrittlement (HE) is a critical issue affecting various industries, including the automotive sector, where there is a demand for high-strength steels with high mechanical properties. However, these properties can be severely compromised by hydrogen, particularly in martensitic microstructures, which are highly susceptible to hydrogen embrittlement. In contrast, bainitic steels offer a promising combination of high strength and enhanced ductility, but their susceptibility to hydrogen embrittlement is not yet fully understood. Furthermore, the involvement of micro-alloying elements on the resistance to the HE in both types of microstructures also remains to be clarified. This thesis investigates different strategies to improve resistance of carbon steel to hydrogen embrittlement, focusing on the effects of microstructure and the influence of micro-alloying elements (Nb, Ti, B), with particular attention to the role of their precipitates. This research study an industrial alloy and four variations, each with different combinations of micro-alloying elements Nb, Ti and B. Two types of heat treatments were applied: quenching and tempering to produce martensite, and bainitic quenching to produce bainite, with different treatment durations to investigate the evolution of the precipitation state. The first part of the research involved microstructural characterization using various techniques such as Electron Backscatter Diffraction (EBSD), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) to identify differences between the two microstructures and alloys, with a particular focus on alloying carbides (composition, size, coherency). Simultaneously, hydrogen solubility and trapping behaviors were investigated using electrochemical charging and Thermal Desorption Spectroscopy (TDS), complemented by electrochemical permeation tests to determine diffusion coefficients and hydrogen states. The obtained results provided insights into the relationship between microstructural defects, the state of trapping, and their impact on hydrogen diffusion. Finally, mechanical characterization was performed through tensile tests at strain rates range 10⁻³ to 10⁻⁶ s⁻¹ on both uncharged and hydrogen-charged specimens and two notch radius, to assess the impact of hydrogen embrittlement on mechanical properties. The thesis discusses the correlation between microstructural observations, hydrogen behavior, and mechanical performance, with a particular emphasis on the precipitation state and the comparison of the two types of microstructures.