Atomic-scale investigation of H-trapping by fine NbC precipitates in a low C ferritic steel

The origin of hydrogen trapping by the Nb-carbide precipitates in a ferritic steel was studied at the atomic scale with the help of HRTEM (high resolution transmission electron microscopy) and 3D-APT (atom probe tomography). A Nb-added ferritic steel was prepared in the laboratory and subjected to aging after solutionizing treatment to have fine Nb-carbide precipitates in ferrite matrix. The precipitation kinetics were simulated beforehand using TC-Prisma in Thermocalc software. A large number of nanometer-sized Nb-carbide precipitates were observed in the ferrite matrix using TEM. 3D-APT studies on the steel sample electrolytically charged with D (deuterium) showed that the D atoms are trapped more within the precipitates than at the precipitate-matrix interfaces. It has been argued that H atoms occupy the vacant lattice sites on carbon sub-lattice sites in non-stoichiometric NbC0.83.