Tensile Testing of Carbon Steel in High Pressure Hydrogen

An infrastructure of new and existing pipelines and systems will be required to carry and to deliver hydrogen as an alternative energy source under the hydrogen economy. Carbon and low alloy steels of moderate strength are currently used in hydrogen delivery systems as well as in the existing natural gas systems. It is critical to understand the material response of these standard pipeline materials when they are subjected to pressurized hydrogen environments. The methods and results from a testing program to quantify hydrogen effects on mechanical properties of carbon steel pipeline and pipeline weld materials are provided. Tensile properties of one type of steel (A106 Grade B) in base metal, welded and heat affected zone conditions were tested at room temperature in air and high pressure (10.34 MPa or 1500 psig) hydrogen. A general reduction in the materials ability to plastically deform was noted in this material when specimens were tested in hydrogen. Furthermore, the primary mode of fracture was changed from ductile rupture in air to cleavage with secondary tearing in hydrogen. The mechanical test results will be applied in future analyses to evaluate service life of the pipelines. The results are also envisioned to be part of the bases for construction codes and structural integrity demonstrations for hydrogen service pipeline and vessels. . INTRODUCTION The effect of gaseous hydrogen on materials is not a new phenomenon. Science and industry have noted a propensity for reduced ductility and resistance to fracture in ferritic steels as a result of exposure to hydrogen environments. This reduction is most prevalent at ambient or just below ambient temperatures and is also enhanced at slow strain rates. The tensile properties found in the literature typically include one or more of the following: yield stress, ultimate tensile strength, elongation, and reduction of area. They were reported mainly to demonstrate the hydrogen effects at various levels of pressure or concentration. The data may be useful for codified analyses which require strength information of the steels. However, for a realistic structural analysis or fracture performance analysis with the finite element method, in general, a full stress-strain curve beyond linear elasticity (up to failure) would be required. Furthermore, a mechanical properties database should be developed for each material selected for this type of application, in order to provide a consensus of properties for design of hydrogen systems. This requires the use of a statistically valid number of specimens for each testing condition. Much of the experimental in this area has yielded data which is more applicable to academic study of the subject. A report on the hydrogen embrittlement effects on various structural alloys including (but not limited to) carbon steels can be found in the literature, which is a summary of a research project sponsored by National Aeronautics and Space Administration (NASA) prior to 1973. In the experimental programs for the tensile properties, the researchers used unnotched and notched specimens. The notched specimens provided stress concentration in the gage section so the hydrogen concentration is enhanced locally resulting in a more pronounced effect. This technique is a good screening criterion for determining hydrogen effects on specific metals and alloys. However, the test data generated this type of specimen may be