Effect of Loading History on Hydrogen Content in Pipeline Steels

The application of high strength pipeline steels for oil and gas transmission is believed to provide greater gas flow capacity due to increased design pressure, and reduced line pipe cost due to material tonnage savings. However, the use of high strength pipeline steels is concerned with high risk of brittle failures such as hydrogen induced cracking, fractures due to low ductility. In this study, three grades of modern pipeline steel (X65, X80, X100) were examined to determine their susceptibility to hydrogen permeation and hydrogen trapping under the influence of various mechanical loading conditions. The steel samples were placed in a solution of sulfuric acid poisoned with arsenic trioxide to create an environment where hydrogen can enter the steel. Initially, round bar samples were charged for various times at a low current density to establish that 24 hours was a sufficient charging time for the three steels. Tensile samples were loaded and held at stress levels corresponding to the respective yield strength and the amount of hydrogen entering the steel was then measured. The stress, normalized to the yield strength, and hydrogen contents, normalized to as received contents, were used to rank the three steel grades and to find the steel that was the most susceptible to hydrogen entry. For the samples charged prior to loading, two times as much diffusible hydrogen was found in the X100 as compared to the other steels, but the trapped hydrogen content was equivalent. Four loading conditions were used for each grade of steel: 1) 2% strain; 2) 2% strain and hold at load for 24 hours; 3) 2% strain then 100 cycles at R = 0.1; and 4) 2% strain, 100 cycles at R = 0.1 then hold at load for 24 hours. For the loaded samples, the amount of hydrogen, both diffusible and trapped increased with load severity, with the highest amounts found in the highest grades of steel. The most pronounced increase was not found in the X100, but in the X-80 steel. Micro structural features, such as banded structure, seemed to have a more prominent role on the hydrogen content of the X100 than in the other steels as it seemed less affected by the loading condition than by charging time.