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Hydrogen embrittlement (HE) threatens the safety of hydrogen transportation pipelines. This study investigates HE susceptibility in X52M and X52MH steels, intended for hydrogen transport. Slow strain rate tensile (SSRT) tests are conducted in N₂ and H₂ at 6.3 MPa, with fracture surfaces analyzed using SEM and 3D X-ray micro-CT. Results reveal that X52MH steel is more susceptible to HE than X52M, attributed to its ferrite/bainite (F/B) duplex-phase microstructure, unlike the acicular ferrite (AF) structure of X52M. Segregation of Cu, Cr, and Ni in X52MH also promotes crack initiation and propagation, as hydrogen-induced cracks on the specimen surface and localised concentrations at the F/B interface from uncoordinated deformation increase HE susceptibility in X52MH. [Display omitted] • X52MH has higher hydrogen embrittlement due to its ferrite/bainite structure. • Cu, Cr, Ni segregation in X52MH speeds up hydrogen-induced crack growth. • Uncoordinated ferrite/bainite deformation in X52MH accelerates crack initiation. • Surface cracks in X52MH raise hydrogen embrittlement susceptibility vs X52M. [ABSTRACT FROM AUTHOR]