In this research, we investigated microstructural aspects of intergranular and transgranular fatigue crack propagation in an API X65 steel pipelines. For this purpose, the fatigue testing, based on ASTM E647 Standard, was carried out on CT specimens. The micro-texture measurement results showed that fatigue micro-cracks propagates dominantly in transgranular manner through differently oriented grains. It was also observed that most of the grains involved in crack propagation are broken during the crack growth. Coincidence site lattice (CSL) boundaries are considered as crack resistant paths; however, there was an accumulation of Σ3 boundaries around the fatigue micro-cracks showing that such boundaries acts as high energy boundaries during crack propagation. Elongated manganese sulphide inclusion was determined as the most detrimental inclusion for crack propagation due to the highly disordered boundaries between metal matrix and this inclusion. In other words, this type of inclusion may initiate the fatigue crack due to a high stress concentration factor that can provide in some sharp edges. Moreover, it can facilitate fatigue crack growth by reducing the local fracture toughness. Many manganese sulphide inclusions were found on fatigue fracture surfaces.
