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Colorado School of Mines

1. Mitchell, Emily B. Toughness and separation improvement of thick plate X70 pipeline steels.

Degree: MS(M.S.), Metallurgical and Materials Engineering, 2019, Colorado School of Mines

The effects of microstructure, texture, and material constraint for four American Petroleum Institute (API) X70 grade steel plates of varying thickness (13.5, 15.5, 22, and 32 mm) on separations and the subsequent effects of separations on toughness and toughness anisotropy were evaluated. The effects of plate thickness and separations on impact behavior were also investigated. Tensile, Charpy V-notch (CVN), and drop-weight tear testing (DWTT) were used to evaluate the plates’ mechanical properties in three orientations: 0°, 45°, and 90° from the plate’s rolling direction. Microstructural characterization determined that the 13.5 and 15.5 mm plate microstructures are primarily fine polygonal ferrite, with the 13.5 mm plate containing some small colonies of pearlite, while the 22 and 32 mm plate microstructures are banded with non-uniformly distributed polygonal ferrite and a bainitic secondary constituent that increases in volume fraction along the plates’ centerline. Energy dispersive X-ray spectroscopy (EDS) results did not correlate the microstructural banding observed in the thicker plates to manganese segregation or chemical banding. The increased volume fraction of the bainitic secondary constituent along the thicker plates’ centerline was correlated to a reduction in strength along the centerline, but may improve toughness measured by CVN testing. Coarser ferrite grain sizes may significantly lower a plate’s toughness measured by both CVN and DWTT. All four plates exhibited mechanical property anisotropy, deleterious in the 45° orientation, which was primarily attributed to texture rather than microstructure. High volume fractions and intensity of the {113}<110> texture component apeared to promote anisotropy in strength. Toughness performance degredation was observed in the 90° orientation for some of the CVN absorbed energy results and was correlated to the increased presence of separations in this orientation. Separation index (SI) measurements were performed in attempt to quantify separation severity, but the SI measurement is dependent on plate thickness and could only be compared between different orientations or through-thickness locations for an individual plate. Separations formed in impact specimens tested mainly at transition temperatures. Microstructure could not be directly correlated to separation formation in all plates, but was found to affect the morphology of the separation crack. Separations in polygonal ferrite microstructures generally were spread across the plate’s through-thickness and had a well-defined crack tip, while separations in a microstructure with a high volume fraction of the bainitic secondary constituent at the plate centerline formed mainly along the centerline and then split into many thin cracks extending from the main separation crack tip. Texture components with low angles between the closest {001} cleavage plane and the normal plane of a texture component were found to promote separation formation in plates that had high intensities and volume fractions of these… Advisors/Committee Members: Clarke, Kester (advisor), Clarke, Amy (committee member), Speer, J. G. (committee member).

Subjects/Keywords: Plate; Steel; X70; Separation; Pipeline; Toughness

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APA (6th Edition):

Mitchell, E. B. (2019). Toughness and separation improvement of thick plate X70 pipeline steels. (Masters Thesis). Colorado School of Mines. Retrieved from http://hdl.handle.net/11124/173297

Chicago Manual of Style (16th Edition):

Mitchell, Emily B. “Toughness and separation improvement of thick plate X70 pipeline steels.” 2019. Masters Thesis, Colorado School of Mines. Accessed December 08, 2019. http://hdl.handle.net/11124/173297.

MLA Handbook (7th Edition):

Mitchell, Emily B. “Toughness and separation improvement of thick plate X70 pipeline steels.” 2019. Web. 08 Dec 2019.

Vancouver:

Mitchell EB. Toughness and separation improvement of thick plate X70 pipeline steels. [Internet] [Masters thesis]. Colorado School of Mines; 2019. [cited 2019 Dec 08]. Available from: http://hdl.handle.net/11124/173297.

Council of Science Editors:

Mitchell EB. Toughness and separation improvement of thick plate X70 pipeline steels. [Masters Thesis]. Colorado School of Mines; 2019. Available from: http://hdl.handle.net/11124/173297

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