The effect of TiC and VC nano-precipitate size on the hydrogen embrittlement of ferritic steels was studied in this work. Steels containing two size distributions (10 nm or less and 10 - 100 nm) of TiC and VC carbides are subjected to tensile tests in-situ in an electrochemical hydrogen charging environment. Hydrogen is found to be trapped in interstitial matrix sites on the precipitate/matrix interface with activation energies of 14 - 20 kJ/mol and inside misfit dislocation cores with energies of 27 - 37 kJ/mol. All steels are embrittled by 15 to 20%, except the TiC steel with semi-coherent carbides up to 100 nm, which is embrittled by 37%. This is caused by accelerated intergranular fracture as a result of hydrogen trapped in dislocation pile-ups around grain boundary precipitates. The steel with coherent VC nano-carbides retained the highest strength and ductility during in-situ testing. This is therefore the optimal carbide configuration for use in hydrogen environments.@en

Abstract: This work studies the effect of TiC and VC precipitate sizes on hydrogen trapping and embrittlement. Two experimental ferritic HSLA steels containing either TiC or VC carbides for precipitation strengthening are annealed in nitrogen and hydrogen gas. This results in a hydrogen uptake of up to 0.91 and 0.44 wppm in the TiC and VC steels, respectively. TEM and TDS analysis indicate that semi-coherent TiC particles trap hydrogen in misfit dislocations with an activation energy of 43 kJ/mol. Coherent VC particles are suggested to trap hydrogen in interface carbon vacancies, with an energy between 53 and 72 kJ/mol. Carbon vacancies are the likely trapping site in incoherent precipitates, where SIMS imaging confirms that incoherent TiC precipitates trap preferentially near the interface, whereas incoherent VC precipitates trap throughout their bulk. Neither alloy is embrittled in SSRT tests after hydrogen absorption, which shows that these precipitates can be used as both a hydrogen sink and a strengthening mechanism in steels. Graphical abstract: (Figure presented.)

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Controlling the detrimental effect of hydrogen on the mechanical behaviour of advanced high strength steels is decisive for their application. Precipitates in steels can be useful in irreversibly trapping the hydrogen atoms, thereby preventing their diffusion to critical regions in the microstructure where they can be most detrimental. In this work, the capability of precipitates of transition metals in limiting the amount of diffusible hydrogen has been examined. A combined ab initio–experimental approach was used to study the hydrogen sorption in two DP800 steel grades with different concentrations of titanium and vanadium using cyclic voltammetry. Under the same charging conditions, diffusible hydrogen concentration was found to be higher in the vanadium grade as compared to the titanium grade. Scanning electron microscope characterisation revealed a more compact layer of oxide on the vanadium grade which contributed to more hydrogen absorption on the surface. Density Functional Theory calculations were performed to determine the trapping strength of precipitates of titanium and vanadium. C vacancy in titanium carbide was found to be the strongest hydrogen trap, but the C vacancy formation energy was much lower in vanadium carbide. At finite temperatures, however, both precipitates are experimentally known to be off-stoichiometric. Our DFT-based finding of the titanium grade being irreversible hydrogen trap is thus in agreement with the experimental results.

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