Computational modelling of hydrogen embrittlement in welded structures

This paper deals with the modelling of the combined hydrogen embrittlement phenomena: hydrogen-enhanced local plasticity (HELP) and hydrogen-induced decohesion (HID) in dissimilar welds through a cohesive zone modelling approach (CZM). Fractured samples of dissimilar weld interfaces in AISI8630/IN625 show that cracks propagate in a region called the featureless region located in the Nickel side of the weld. This region is characterized by the presence of a distribution of fine carbides. We model the effect of hydrogen on the material toughness as the result of a synergistic effect of HELP and HID mechanisms where (i) hydrogen enhanced dislocation mobility promotes the development of dislocation structures at the carbides which increases the stress on the particles; while the presence of hydrogen also results in (ii) a reduction in the (a) cohesive strength of the carbide/matrix interface and (b) in the local flow stress of the matrix. The decohesion mechanism at the carbide/matrix interface is modelled through a two-dimensional user-defined cohesive element implemented in a FORTRAN subroutine (UEL) in the commercial finite element code ABAQUS and the effect of the hydrogen on the plasticity properties of the matrix is coded in a UMAT routine. Preliminary analysis on a unit cell representing the matrix/carbide system under plane strain shows that HELP and HID are competitive mechanisms. When the combined mechanism HELP+HID occurs microcracks form at the matrix/carbide interface due to decohesion process followed by localization of plastic flow responsible for the link-up of the microcracks.