EXPERIMENTAL AND MACHINE LEARNING-BASED INVESTIGATION OF HYDROGEN EMBRITTLEMENT AND ITS IMPACT ON THE FRACTURE TOUGHNESS OF PIPELINE STEELS

Abstract

Pipelines are expected to play a crucial role in the large-scale transportation of hydrogen. However, the mechanical properties of pipeline steels can degrade due to hydrogen embrittlement (HE), even at low hydrogen concentrations (partial pressures). This embrittlement significantly reduces the fracture toughness of steels commonly used for containment and distribution. Conservative operating conditions with prominent safety factors must be employed to limit these effects.Operating conditions, such as temperature and hydrogen concentration, are critical factors in ensuring the safe transportation of hydrogen. Temperature variations across different regions necessitate investigation, as areas like Alaska, with low temperatures, can exacerbate HE. In contrast, high temperatures in regions like Texas could alter the diffusion and interaction of hydrogen with steel. These geographical variations make it essential to tailor operating conditions and mitigation strategies to ensure safe and effective hydrogen transportation across various climates. In addition to optimizing operating conditions, it is necessary to evaluate the sensitivity of pipeline steels to degradation by gaseous hydrogen. Methods like adding inhibitor gases, like oxygen, should be explored to mitigate HE. A comprehensive understanding of the effects on pipeline steels' fracture toughness is critical for ensuring structural integrity in hydrogen environments. This study experimentally examines fracture toughness in X60, X70, and X52 steels, focusing on temperature, gas composition, and impurities, to develop a robust ML model for accurate toughness prediction. The reduction of the fracture toughness of commonly used pipelines steels (X52, X60, and X70) is evaluated according to relevant industrial standard (ASTM E1820 and G142), using compact tension specimens. The steels are tested in natural gas/hydrogen mixtures, with hydrogen pressures ranging from 0 to 6.9 MPa, oxygen concentrations between 0 and 1000 ppm, and temperatures from 50 to 122°F. Results indicate that steels tested in this investigation are susceptible to HE, with vulnerability increasing with hydrogen content. When tested at 6.9 MPa hydrogen pressure, X52 steel exhibits the highest reduction in fracture toughness, decreasing by 15.7%, followed by X70 with 12.9% and X60 with a 10.8% reduction. Additionally, oxygen concentrations up to 750 ppm significantly mitigate embrittlement in X70 and X60, nearly eliminating it, while X52 shows a reduction from 15.6 to 9.9%. A critical temperature is identified for all materials, at which the material exposed to hydrogen embrittlement has the maximum fracture toughness. For X52 and X70 steels, the critical temperature is 77°F, while for X60, it is 95°F. This threshold highlights the sensitivity of each material to temperature variations in hydrogen-rich environments. The ML model accurately predicted the effect of hydrogen embrittlement over a wide range of partial pressures, indicating a saturation point at 7 MPa, beyond which HE of fracture toughness plateaus and stabilizes. The model also identified hydrogen partial pressure, yield strength, and manganese content as the most significant parameters influencing fracture toughness.