Effect of Microstructural Parameters on Fatigue Crack Propagation in an API X65 Pipeline Steel
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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 µm 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 µm 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 × 103. On fracture surfaces, crack propagation also occurred by joining the microcracks at tip of the main crack.
Keywordsback-scattered electron energy-dispersive spectroscopy fatigue crack pipeline steel scanning electron microscopy
We would like to thank Natural Sciences and Engineering Research Council of Canada for the financial support of this project. We also sincerely thank for the support received from Centre for Engineering Research in Pipelines (CERP) located in Windsor, ON, Canada.
- 11.J.W. Sowards, T. Gnäupel-Herold, J.D. McColskey, V.F. Pereira, and A.J. Ramirez, Characterization of Mechanical Properties, Fatigue-Crack Propagation, and Residual Stresses in a Microalloyed Pipeline-Steel Friction-Stir Weld, Mater. Des., 2015, 88, p 632–642Google Scholar