Screening the hydrogen embrittlement of novel high strength intermetallic strengthened steels

Maraging steels are a class of ultrahigh strength steels with a martensitic microstructure.  The main characteristic of them is that they are not hardened by carbon and carbide precipitation, but by the precipitation of intermetallic phases during the aging.

Large number of intermetallic phases have been reported in the literature  as hardening phases for maraging steels such as: Laves phases, Fe₃Si, μ-phase, θ-phase, Ni₃(Ti,Mo,Al), G-phases, R-phases, etc ^[1-3]. Each type has different formation kinetics and shows different effect on properties such as strength and toughness. In the RFCS project iNiTiAl, the individual and combined influence of Ni₃Ti and β-NiAl on properties, including strength, toughness and hydrogen embrittlement, are thoroughly studied.

In this study, 6 generic alloys have been synthetized via vacuum induction melting, which are expected to form different type and volume fractions of the precipitates mentioned above after the aging treatment. After the homogenization treatment, they were hot rolled to 8mm and further austenitized and quenched to  fully martensitic microstructure.  For each composition, 4 aging treatments were performed.  The tensile properties of the alloys were characterized  in air and in hydrogen atmosphere through in-situ charging in aqueous solution. To get further understanding in the trapping capacity of the different microstructural features, the samples were charged with gaseous deuterium and measured via thermal desorption spectroscopy. 

To link the observed properties with the microstructure, especially the precipitates, advanced high-resolution techniques are key. The morphology, type and chemical composition of the intermetallic precipitates are extensively characterized via atom probe tomography (APT) and transmission electron microscopy (TEM).  Fractions of retained or reverted austenite (if any) are quantified via X-ray diffraction (XRD). The influence of precipitates as well as the martensitic microstructure on hydrogen embrittlement will be discussed in detail.