Influence of residual stress and strain generated by cold drawing on hydrogen diffusion profiles of steel wires

High strength steel wires are susceptible to hydrogen induced fracture. It is generally considered that fracture will occur when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress assisted hydrogen diffusion process. Residual stresses generated by cold drawing play an important role in hydrogen accumulation. However, plastic strain also has significant effect on the hydrogen diffusion process. In this paper, a numerical model was developed for calculating the accumulated hydrogen concentration in cold drawn steel wires, taking into account the driving effect of both the residual stress and strain generated by cold drawing on hydrogen transport. First, a finite element model, using the code ABAQUS, was developed to reproduce the drawing process, and to determine the residual stress and strain profiles. The results showed that the drawing process generated a residual stress state in the wire with significant tensile stresses at the surface in the axial and hoop directions. Finite difference method was used to solve the stress-strain assisted and stress-only assisted hydrogen diffusion equations. The hydrogen concentration accumulated in stress-strain assisted case is lower than that in stress-only assisted case in shorter time, that was slowed down by plastic strain due to diffusion. However, after long exposure time, the hydrogen concentration was much higher than that in stress-only affected case. The results in this paper prove the relevant role of residual plastic strain in hydrogen diffusion in cold drawn wires, as well as the residual stress.