Effect of microstructure and composition on hydrogen permeation in X70 pipeline steels

The influence of microstructure and composition on permeation of hydrogen in 1.2 and 0.5 wt.% Mn X70 pipeline steels after different processing was investigated using an electrochemical permeation technique.For 1.2 wt.% Mn (standard Mn) steel, the microstructure of normalised transfer bar was coarse equiaxed ferrite grains. This sample exhibited the highest diffusivity, followed by transfer bar, with a mixed ferrite–bainitic ferrite microstructure; and hot rolled strip, with fine elongated ferrite grains.The 0.5 wt.% Mn (medium Mn) strip displayed lower diffusivity than the 1.2 wt.% Mn strip, due to hydrogen trapping by finer ferrite grains and a higher density of carbonitride precipitates. Moreover, the medium Mn strip exhibited a uniform microstructure and consequently similar diffusion coefficients for the edge and centreline regions, whereas the finer grains of the edge region of the standard Mn strip resulted in a lower diffusivity than the centreline region.

► Permeation of hydrogen in 1.2 and 0.5 wt.% Mn X70 pipeline steel was examined.
► The normalised transfer bar having coarse grains showed the highest diffusivity.
► The hot rolled strips showed intermediate values of diffusivity.
► The 0.5 wt.% Mn strip showed the lowest diffusivity due to precipitates.
► Edge and centreline regions of 0.5 wt.% Mn strip exhibited similar diffusivities.