The effect of an Al-induced ferritic microfilm on the hydrogen embrittlement mechanism in martensitic steels

Abstract

The addition of 2 wt% aluminium to a martensitic Fe-0.4C steel leads to a ferritic microfilm along the prior austenitic grain boundaries, which was demonstrated to significantly increase the ductility without affecting the bulk hardness. This study investigates its effect on the hydrogen embrittlement behavior. For Al-free martensitic steel without grain boundary ferrite, the presence of hydrogen causes a brittle cleavage type of fracture due to the embrittlement of the martensitic packets/blocks according to the hydrogen enhanced decohesion mechanism. Al-containing steels, with a ferritic grain boundary microfilm, on the other hand, experience a delay in hydrogen-induced fracture, and the crack propagation is redirected along the ferrite/martensite interface resulting in an intergranular fracture surface. This can be linked to an interplay between the hydrogen enhanced decohesion and hydrogen enhanced localized plasticity mechanism. However, by applying slow deformation rates in a hydrogen rich environment, the ferritic microfilm itself embrittles and loses its capability to influence the crack propagation, thus resulting in a similar fracture surface and ductility as the samples without the ferritic microfilm.