The paper deals with the modelling of the combined hydrogen embrittlement phenomena: hydrogen enhanced local plasticity (HELP) and hydrogen induced decohesion (HID) in subsea dissimilar welds. Fractography analysis of dissimilar welds of 8630 steel / IN625 nickel alloy demonstrate that a cleavage-like fracture path is located through an area called the “featureless zone” characterized by the presence M7C3 carbides. This fracture morphology, which is noticed only in the presence of hydrogen, seems to be consistent with a hydrogen-induced decohesion (HID) mechanism along the M7C3 -matrix interface succeeded by a ductile-type fracture. In light of this the purpose here is to develop constitutive models of the “featureless zone”. We present two types of models: the former (i) is based on a classical continuum mechanics formalism; the latter (ii) is a mescoscopic type of model to represent more accurately the dislocation structure that develops in the vicinity of the carbide-particles. The model (i) describes some of the aspects involved in the failure process of a dissimilar AISI8630/IN625 weld, however continuum models are not appropriate for the scale of the carbide particles in the featureless region. The scale of the particles is such that the interaction of the particles with discrete dislocations and the dislocation structures generated around the particles play an important role in determining the constitutive response.

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