کلیدواژهها
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back-scattered electron, energy-dispersive spectroscopy, fatigue crack, pipeline steel, scanning electron microscopy
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چکیده
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In the current research, we investigate fatigue crack growth in an API X65 pipeline steel by using an
Instron fatigue testing machine. To this, first the microstructure of steel was accurately investigated using
scanning electron microscope. Since nonmetallic inclusions play a key role during crack propagation, the
type and distribution of such inclusions were studied through the thickness of as-received X65 steel using
energy-dispersive spectroscopy technique. It was found that the accumulation of such defects at the center
of thickness of the pipe body was higher than in other regions. Our results showed that there were very fine
oxide inclusions (1-2 lm in length) appeared throughout the cross section of X65 steel. Such inclusions were
observed not at the fatigue crack path nor on both sides of the fatigue crack. However, we found that large
manganese sulfide inclusions (around 20 lm in length) were associated with fatigue crack propagation.
Fatigue experiments on CT specimens showed that the crack nucleated when the number of fatigue cycles
was higher than 340 3 103
. On fracture surfaces, crack propagation also occurred by joining the microcracks at tip of the main crack.
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