چکیده
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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.
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