In the last years, many hydrogen production projects from renewable energies (electrolysis, power to gas…) have been submitted to store and transport clean energy. Therefore, to be a part of a big contribution to this energetic transition, GRTgaz is studying the possibility of using its existing Natural Gas (NG) pipeline grid for the transport of NG-H~2~ blend. In order to control the risks linked to hydrogen transport, it is important to study the impact of hydrogen injection on the mechanical behavior of pipeline steels, particularly in presence of surface defects , depending on the hydrogen content in the blend.

The purpose of the present work is to study the effect of the hydrogen content in an x%H~2~-NG blend on the mechanical behavior of a C-Mn low alloy steel L485-MB (NF EN ISO 3183) under monotonic loading. Different testing conditions were explored for the blend: 2%H~2~-NG, 25%H~2~-NG, 100%H~2 ~and 100%NG at a total pressure of 85 bars using two slow strain rates: 10^-4^ and 10^-6^ s^-1^. Tests in the blend were compared to those under pure hydrogen at the same partial pressures. Likewise, two surface roughness have been explored, one corresponding to a mirror polish, the other being higher. The embrittlement has been assessed by necking measurements and fracture analysis for the whole testing conditions. In addition, a tensile test interrupted before the ultimate tensile stress was carried out under pure hydrogen to provide more understanding on hydrogen damage, particularly in the first stages of crack initiation and propagation.

The results of tensile tests under the different gas mixtures at 85 bars show a drop of elongation to fracture with the increase of hydrogen content for the two strain rates. Compared to those under pure hydrogen at equivalent partial pressures, we observed a slight positive effect of the blend on the behavior of the material. On the contrary, increase in roughness seems detrimental for the material resistance to hydrogen embrittlement. Likewise, the lower strain rate increases the embrittlement. Under hydrogen, the general appearance of the fracture shows a remarkable 45° fracture to the tensile axis, which corresponds to the maximum shear stress plane in the specimen. At a finer scale observation of the specimen surfaces, crack initiation appears to be localized at the intersection of shearing bands when a substantial plastic deformation of the sample has been applied, and crack propagation follows these directions. Unlike the tests under hydrogen, this localization of the deformation does not appear for the tests in air and NG, which present a much more diffuse necking along the barrel of the specimens. The analysis of fracture surfaces confirmed the consistence between hydrogen content on NG and partial pressure of hydrogen. Two main regions were distinguished on fracture surfaces, a zone characterized by large dimples which corresponds to a ductile fracture, and a zone named “hydrogen affected zone” (HAZ) which corresponds to some quasi-cleavage zones mixed with small dimple areas. The HAZ becomes dominant with the increase of hydrogen partial pressure in NG. 


Effect of Hydrogen Content in Natural Gas Blend on the Mechanical Properties of a L485-Mb Low Alloy Steel

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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