Microstructure and Mechanical Property Performance of Commercial Grade API Pipeline Steels in High Pressure Gaseous Hydrogen

The continued growth of the world’s developing countries has placed an ever increasing demand on traditional fossil fuels. This increased demand for fossil fuels has lead to increasing research and development of alternative energy sources. Hydrogen gas is one of the potential alternatives under development. It is anticipated that the least expensive method of transporting large quantities of hydrogen gas is through steel pipelines. It is well known that hydrogen embrittlement has the potential to degrade steel’s mechanical properties. Consequently, the current pipeline infrastructure used in hydrogen transport is typically operated in a conservative fashion, in particular lower operating pressures, lower strength steels, and heavier pipe wall thicknesses. This operational practice is not conducive to economical movement of significant volumes of hydrogen gas as an alternative to fossil fuels. The degradation of the mechanical properties of steels in hydrogen service depends on the microstructure of the steel. An understanding of the relationship of mechanical property degradation of a given microstructure on exposure to hydrogen gas under pressure can be used to evaluate the suitability of the existing pipeline infrastructure for hydrogen service and guide alloy and microstructure design for new hydrogen pipeline infrastructure. To this end, the microstructures of relevant steels and their mechanical properties in relevant gaseous hydrogen environments must be fully characterized to establish suitablity for transporting hydrogen. A project to evaluate four commercially available pipeline steels alloy/microstructure performance in the presences of gaseous hydrogen has been funded by the US Department of Energy along with the private sector. The microstructures of four pipeline steels were characterized and tensile testing was conducted in gaseous hydrogen and helium at pressures of 5.5 MPa (800 psi), 11 MPa (1600 psi) and 20.7 MPa (3000 psi). Based on reduction of area, two of the four steels that performed the best across the pressure range were selected for evaluation of fracture and fatigue performance in gaseous hydrogen at 5.5 MPa (800 psi) and 20.7 MPa (3000 psi). This paper describes the work performed on four commercially available pipeline steels in the presence of gaseous hydrogen at pressures relevant for transport of hydrogen in pipelines. Microstructures and mechanical property performances are compared. In addition, recommendations for future work related to gaining a better understanding of steel pipeline performance in hydrogen service are discussed.