Dual role of nanosized NbC precipitates in hydrogen embrittlement susceptibility of lath martensitic steel

The effect of dispersed nanosized spherical NbC precipitates (∼10 nm) on the hydrogen embrittlement (HE) of martensitic steel was investigated by slow strain rate tests, hydrogen permeation tests, atom probe tomography and microstructural examination. HE susceptibility decreases as the amount of NbC increases. NbC plays dual roles in HE. Primarily, NbC alleviates hydrogen accumulation in other areas via hydrogen traps which are induced by NbC directly and increased by the additional grain boundaries resulting from NbC, inhibiting the hydrogen-enhanced decohesion. Moreover, NbC impedes the hydrogen-enhanced localized plasticity by pinning hydrogen-dislocation atmospheres and reducing Σ3 boundary fraction. Secondarily, NbC weakens the beneficial {111}//ND texture, reduces the crack-propagation resistance, and plays a harmful role.