Hydrogen-enhanced deformation in pearlite

Hydrogen-induced degradation of pipeline steels is a serious safety challenge for the transport of hydrogen. Steel pipes contain a large proportion of the pearlite phase, which consists of lamellar cementite in ferrite. How hydrogen interacts with pearlite and degrades the mechanical properties remains unclear. Here we have studied the deformation behavior of pearlite in the presence of hydrogen. In-situ micromechanical results revealed that H softens the pearlite. Uncharged samples deform by slip at the ferrite–cementite interface, while deformation in H-charged pearlitic samples occurs predominantly via slip within the ferrite phase, with some shearing of the cementite. Cryo atom probe tomography confirms that hydrogen is associated with defects in the ferrite, away from the interface, and is trapped within the cementite, which is thought to be the result of a high concentration of C vacancies. Hydrogen is not observed at the ferrite–cementite interface. First principal simulations show that the expected influence of lattice strain and cementite vacancies on hydrogen trapping is consistent with the experimental results. The softening is attributed to a reduction in the energy required for dislocation motion in both the ferrite and the cementite.