Abstract: The synergistic effects of Nb and Mo on hydrogen-induced cracking (HIC) of pipeline steels were studied experimentally and numerically. The results showed that Mo was primarily segregated at grain-boundaries (GBs) or solid-dissolved in the matrix, while most Nb and a small amount of Mo formed dispersed (Nb,Mo)C nano-precipitates and refined the microstructure. Compared with Nb alloying, the multiple additions of Nb-Mo played dual roles in affecting H diffusion: primarily, the H-traps densities such as GBs, precipitates, and solute Mo atoms increased, providing an advantage; however, Mo slightly reduced the H-trapping capacity of precipitates, playing an adverse role. Nonetheless, the beneficial effects far outweighed the adverse effects, thereby reducing H diffusivity and inhibiting crack initiation. Additionally, Nb and Mo hindered crack propagation synergistically as follows: (i) Mo enhanced GB cohesion by repelling H, impeding intergranular cracking and hydrogen-enhanced decohesion (HEDE); (ii) Nb reduced the proportion of Σ3/high-angle grain boundaries, increasing cracking resistance; (iii) (Nb,Mo)C precipitates impeded H-dislocation interactions, reducing the hydrogen-enhanced localized plasticity (HELP).

Key words: Hydrogen-induced cracking, Nb-Mo microalloying, Hydrogen permeation, Precipitate, Pipeline steel