An Extensive Study on the Hydrogen Permeation Characteristics of API 5L X70 Steel Affected by Microstructure and Texture Evoluted by Severe Cold Rolling

In this study, effect of severe cold rolling of API 5L X70 pipeline steel on the microstructure and texture evolution and hydrogen permeability is deeply investigated. Optic microscopy, X-ray diffraction technique, and Devanathan-Stachurski hydrogen permeation test are conducted on two different cross sections of the sample: the normal surface and 45 deg twice-tilted surface. Results indicate that the microstructures of the initial and cold-rolled steel include granular ferrite and conventional upper bainite. However, the equi-axed grains of the as-received steel is changed to elongated ones with low-angle grain boundaries due to cold rolling. The X-ray diffraction macro-texture analysis revealed a rotation in crystallographic texture from the predominant {110}||ND to the {110}||ND on the normal surface and in the 45 deg tilted cross section indicate that the predominant {110}||ND texture is replaced by an intense texture of {100}||ND due to cold rolling. Investigation of hydrogen permeability in cold-rolled samples with a textured microstructure shows a higher diffusion coefficient into {100} planes compared to {110}. Also, it was observed that the increase in the defects due to applying cold work results in a decrement of the hydrogen diffusivity into the same planes and an increase in its solubility.