Correlation of microstructure and fracture properties of API X70 pipeline steels |
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Authors: | Byoungchul Hwang Young Min Kim Sunghak Lee Nack J Kim Seong Soo Ahn |
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Affiliation: | (1) the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, 790-784 Pohang, Korea;(2) the Materials Science and Engineering Department, Pohang University of Science and Technology, Korea;(3) Technical Research Laboratories, POSCO, the Plate Research Group, 790-785 Pohang, Korea |
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Abstract: | Effects of microstructure on fracture toughness and transition temperature of high-toughness X70 pipeline steels were investigated
in this study. Three types of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures
were varied by rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN)
impact tests and pressed notch drop-weight tear tests (DWTT) were conducted on the rolled steel specimens. The Charpy impact
test results indicated that the specimens rolled in the single-phase region of the steel containing a reduced amount of C
and Mo had the highest upper shelf energy (USE) and the lowest energy transition temperature (ETT) because of the appropriate
formation of acicular, quasipolygonal, or polygonal ferrite and the decreased fraction of martensite-austenite constituents.
Most of the specimens rolled in the single-phase region also showed excellent DWTT properties as the percent shear area (pct
SA) well exceeded 85 pct, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens
rolled in the two-phase region. Thus, overall fracture properties of the specimens rolled in the single-phase region were
better than those of the specimens rolled in the two-phase region, considering both USE and pct SA. |
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