Fig. 1 Geometry of the tubular specimen

Fig. 2 Metallography images of X52 pipeline steel in the a TD-RD, b TD-ND, c RD-ND planes, d image position diagram

Fig. 3 EBSD patterns of the base metal of X52 pipeline steel: a IPF map, b IQ + GB map, c KAM map

Fig. 4 Engineering stress-strain curves of the tubular specimens under different gaseous hydrogen charging time

Fig. 5 a Tensile data and b HEI of the tubular specimens as functions of the hydrogen charging time

Fig. 6 Overall fracture morphologies of the specimen under different experimental conditions: a 4 MPa N2 charging for 0 h, b 4 MPa H2 charging for 0 h, c 4 MPa H2 charging for 24 h, d 4 MPa H2 charging for 48 h, e 4 MPa H2 charging for 96 h

Fig. 7 Fracture morphology of X52 pipeline steel specimen charged with 4 MPa nitrogen for 0 h: a fracture surfaces with a high proportion of ductile shear fracture region, b fracture surfaces with a high proportion of dimpled region, c dimpled region, d dimpled region with secondary phase particles

Fig. 8 Fracture morphologies of X52 pipeline steel specimen charged with 4 MPa hydrogen for 0 h: a overview of the fracture surface, and magnified images of b brittle fracture region, c region c, d dimple region

Fig. 9 Fracture morphologies of X52 pipeline steel specimens charged with 4 MPa hydrogen for different time: a 24 h, b enlarged view of area A in a, c 48 h, d enlarged view of area B in c, e 96 h, f enlarged view of area C in e

Fig. 10 Inner surface, and secondary cracks in the necking region of the specimens: a, b charged with 4 MPa N2 for 0 h, c, d charged with 4 MPa H2 for 48 h

Fig. 11 EBSD patterns and corresponding SEM image of the secondary crack on the fracture of the specimen charged with 4 MPa H2 for 48 h: a SEM image, b IPF + GB map, c KAM map

Fig. 12 Engineering stress-strain curves of the tubular specimens under different gaseous hydrogen charging pressures

Fig. 13 a Tensile data and b HEI of the tubular specimens as functions of hydrogen charging pressure

Fig. 14 Overall fracture morphologies of the specimen charging hydrogen 48 h for different pressures: a 0.3 MPa, b 0.8 MPa, c 2 MPa, d 4 MPa

Fig. 15 Fracture morphologies of X52 pipeline steel specimens charged hydrogen 48 h for different pressures: a1-a3 0.3 MPa, b1-b3 0.8 MPa, c1-c3 2 MPa