Microstructural Effects on the Fracture Resistance of the Base Metal in Line Pipe Steels in High Pressure H2 Gas

Abstract Cost-effective transportation of gaseous hydrogen requires the use of long-distance low-carbon steel pipeline networks. Employing higher strength (X65-X70) grades can allow for economic viability by enabling operation at higher pressures. While higher strength steel grades are expected to exhibit decreased fracture resistance due to exposure to hydrogen, recent data shows that fracture resistance can vary considerably for different pipeline steels within a similar strength range. The present work aims to understand microstructural effects in determining the fracture response of modern pipeline grades in hydrogen gas. ASTM E1820 fracture toughness tests were performed in 210 bar of high purity hydrogen gas on CT and SENB samples extracted from modern thermomechanically processed (TMP) line pipe including an X70 (banded ferrite-pearlite), an X65 (fully ferritic with some dispersed pearlite colonies), an X65 (fully ferritic with a higher volume fraction of dispersed pearlite colonies), and an X70 (fully ferritic with some dispersed pearlite colonies). While the fracture resistance in hydrogen (KJQ-H) did not correlate strongly with strength properties, some correlation with the amount and morphology of secondary microconstituents was observed, primarily with the low KJQ-H of the banded ferrite-pearlite microstructure. A stronger correlation was observed between increasing KJQ-H values and increasing bainitic nature of the microstructures, as measured by histograms of grain boundary misorientation angle collected from electron backscatter diffraction (EBSD) scans. Spacing measurements between grain boundaries also revealed a correlation between smaller effective grain sizes and higher KJQ-H.