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The orientation dependence of the fracture toughness in hydrogen of X65 D pipeline steel with a banded ferrite-pearlite microstructure

Abstract:

We studied the orientation dependence of the fracture toughness of X65 D pipeline steel with in-situ cathodic hydrogen charging. The fracture toughness values in hydrogen for fracture propagation in the longitudinal and transverse pipe directions were ∼60 % lower than for fracture propagation in the radial direction. This orientation dependence was attributed to the banded ferrite pearlite microstructure elongated in the longitudinal pipe direction. The harder pearlite phase in the banded microstructure caused more crack branching and deflection for fracture propagation in the radial direction. The fractography in hydrogen typically showed brittle fracture characteristics of flat facets and microcracks.