Influence of non-homogeneous microstructure on hydrogen diffusion and trapping simulations near a crack tip in a welded joint

Abstract

Hydrogen assisted fracture near welds is the result of a combination of microstructural changes and the accumulation of hydrogen. With the aim of predicting local hydrogen concentrations, hydrogen redistribution near a crack tip is simulated using a Boundary Layer approach and diffusion modelling is modified by trapping phenomena. The simulated non-homogeneous geometry includes layers that reproduce weld metal, heat affected zones and base metal of a 2.25Cr-1Mo steel; mechanical and diffusion properties have been extracted from references. The hydrogen transport model here considered involves a stress dependency that affects local concentrations; thus, the possible interaction between constraint effects associated to a graded material with hydrogen entry and transport is studied. Results show that the constraint effect is not significative for the loading and for the widths assigned to the weld and the heat affected zones (2.5 to 5 mm); however, for the HAZ-centred crack, a higher hydrostatic peak and the corresponding increase in lattice hydrogen are found. A two-type trapping process is also simulated to reproduce simultaneously the effect of dislocation trapping and microstructure delayed diffusion. Hydrogen is weakly trapped in dislocations and it is added to the lattice concentration to obtain a measure of diffusible hydrogen near a crack tip while effective diffusivity is strongly reduced by deeply trapped hydrogen. Differences between environmental or internal hydrogen sources are expected to be more accurately captured because stress-dependent boundary conditions have been implemented for hydrogen uptake.