Combined Ab-initio and Experimental Study of Hydrogen Sorption in Dual-Phase Steels

Abstract

The formability of Advanced High Strength Steels is critical for their usability in automotive applications. It has been observed that the presence of hydrogen, even in concentrations of the order of 1 ppm, leads to a considerable drop in formability. Hydrogen atoms may get absorbed during steel-making and are known to get trapped at various sites in the lattice. When sufficient activation energy is made available, hydrogen atoms that are weakly trapped can diffuse towards critical regions in the microstructure, such as crack tips and voids, where one or more embrittlement mechanisms might be activated. On the other hand, a strongly trapped hydrogen atom remains immobile and plays no part in the embrittlement process. Precipitates of transition metals are known to be strong traps for hydrogen. It is speculated that by promoting the formation of strong traps in the microstructure, the amount of freely diffusible hydrogen can be limited, which would lead to an improvement in mechanical performance.

In this work, a combined ab-initio - experimental approach was used to study the absorption of hydrogen in dual-phase steel. Density Functional Theory (DFT) calculations were employed to study and compare the trapping of hydrogen by carbide and nitride of titanium and vanadium. A carbon or nitrogen vacancy in the bulk of the precipitate was found to be the most efficient trap site. When coupled with the vacancy formation energy, trapping was found to be more efficient in off-stoichiometric vanadium carbide and nitride than that in titanium carbide and nitride. To validate the theoretical findings, cyclic voltammetry experiments were conducted on two grades of DP800 steel with different concentrations of vanadium and titanium. The amount of diffusible hydrogen in the vanadium grade was found to be approximately 25 \% higher than that in the titanium grade. This was in contradiction to the theoretical results. Characterisation of the specimen post testing revealed that an oxide film had formed on the sample surface and while the film on vanadium grade was uniform and dense, that on titanium grade was sparse and irregular. It was evident that the oxide layer contributed to trapping of hydrogen, however the amount of hydrogen trapped by the oxide could not be specified. Overall, designing steels resistant to hydrogen embrittlement by promoting the formation of precipitates of a particular element is theoretically attainable, however, it was not possible to obtain experimental validation with the method employed.