Influence of Hardness on Fatigue Life of Vintage X42 Pipeline Steel in Gaseous Hydrogen

Abstract Compatibility of existing natural gas pipelines with conveyance of hydrogen is of interest to the pipeline industry as a potential strategy to enhance energy security. Vintage pipelines (pre-1970) are of particular concern due to the known presence of microstructural hard spots associated with processing and welding. Gaseous hydrogen exposure is known to accelerate fatigue crack growth rates and lower fracture toughness in pipeline steels, and research has correlated higher hardness and strength to lower fracture toughness. Yet, the relationship between hardness and fatigue crack initiation in pipeline steels is not well understood. The current work evaluates the hypothesis that hydrogen promotes crack initiation and shortens the fatigue life of hard spots in transmission pipelines. Simulated hard spots were created from a vintage X42 pipeline steel via austenization and direct water quenching. Fatigue crack initiation and fatigue life were evaluated using instrumented circumferentially notched tension (CNT) specimens in air and in high-pressure gaseous hydrogen for both the base material and the simulated hard spot material. Our results show that the increased strength of the simulated hard spot increases the number of stress cycles to crack initiation and failure in both air and hydrogen for maximum applied stresses relevant to operation of a pipeline.