Influence of hydrogen and orientation on the fracture toughness and strength of X70 pipeline steel

This study investigated the influence of hydrogen and orientation on the strength and fracture toughness of an X70 pipeline steel, measured following ASTM E1820 using single edge-notched bend specimens in (a) air and (b) electrochemically hydrogen charged during the test at 9 or 25 mA/cm 2 in 0.10 M NaOH and for 72 h prior to the test to ensure an equilibrium hydrogen concentration in the specimen, with the electrochemical hydrogen charging continuing during the mechanical testing. The fracture toughness was measured using two specimen orientations: circumferential-radial (C-R) and circumferential-longitudinal (C-L). In the C-R orientation, the fracture toughness with hydrogen was 114-348 kJ/m 2 (162-283 MPa √ m ). Fracture was predominantly ductile with a minor influence of hydrogen. The fracture toughness with hydrogen in the C-R orientation was comparable to the values in air of 184-214 kJ/m 2 (206-222 MPa √ m ). In contrast, hydrogen caused a marked reduction of fracture toughness in the C-L orientation. The fracture toughness with hydrogen of the C-L specimens measured at the slower testing speed was 27 ± 11 kJ/m 2 (77 ± 17 MPa √ m ) , which was nevertheless comparable with literature values for good quality pipeline steels. The lower values in the C-L orientation were attributed to the facilitation of hydrogen assisted fracture in this orientation by the banded X70 microstructure. Complementary Linearly Increasing Stress Tests indicated that hydrogen reduced ductility without significant strength loss. The tensile strength is need for a structural integrity evaluation along with the fracture toughness values for the pipeline should the pipeline be considered for hydrogen service. • The fracture of an X70 pipeline steel was studied in air and with hydrogen. • Radial fracture was more difficult because of the microstructure elongated in the longitudinal direction. • The fracture toughness with hydrogen (114–348 kJ/m 2 ) in the C-R orientation was comparable to that in air (184–214 kJ/m 2 ). • The fracture toughness with hydrogen in the C-L orientation was 16–92 kJ/m 2 . • There was no hydrogen induced subcritical cracking.