Abstract
A countermeasure for hydrogen embrittlement is to limit the diffusible hydrogen content in the microstructure by deliberately introducing irreversible hydrogen traps, which act as sinks for diffusible hydrogen. Finely dispersed nano-carbides have been put forward as prime candidates for this purpose. Although the solubility of hydrogen in most carbides is low, the interplay between hydrogen and carbon vacancies can lead to significantly increased hydrogen solubilities. The local atomic environment at the precipitate-matrix interface as well as interface geometry are also vital in this regard. In this study, we consolidate the effect of aforementioned parameters by studying the hydrogen trapping effect in various carbides of interest. Our investigations are focused on single phase Extra Processing Formability (XPF) steels which contain finely dispersed nano carbides. Different carbide characteristics, such as coherency and composition, were obtained by altering the steel chemistry and heat treatment. Density Functional Theory (DFT) has been used for quantitative analysis of hydrogen solution enthalpies in the carbides and at the carbide-matrix interfaces. Input parameters for these calculations were informed by microstructural characterization of the steel samples. Samples were cathodically charged with hydrogen, and the corresponding hydrogen content was measured via Thermal Desorption Spectroscopy (TDS) and hydrogen permeation tests. Our combined ab-initio – experimental approach not only provides validation for the beneficial role of carbides but also illuminates the effect of fundamental properties like magnetic ordering and charge density on hydrogen trapping.