Through-thickness microstructural effects on time-dependent hydrogen diffusivity in X70 pipeline steel

Through-thickness microstructural heterogeneity and gradient deformation can yield varying hydrogen-microstructure interactions within a single plate. Therefore, we extracted samples from three thickness-wise regions from a X70 pipeline steel, i.e., outer, middle, and inner, and these were subjected to detailed microstructural characterisation and electrochemical hydrogen permeation testing to examine hydrogen diffusion and trapping across the wall thickness. All regions have a ferritic–bainitic microstructure with ∼6 vol% of degenerated pearlite but varying grain characteristics. Effective diffusivity is the fastest, ∼7.5 × 10−7 cm2/s, in the middle region with larger equiaxed grains and the lowest apparent trap density, whereas the inner and outer regions have similar reduced diffusivities of ∼4.5 × 10−7 cm2/s due to higher dislocation densities and either smaller grain size (inner) or elongated grains (outer). Variations in subsurface hydrogen concentration were used to track time-dependent changes in diffusivity, and the largest variation was 0.3 wt.ppm for outer region with elongated grains.