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UBC Theses and Dissertations

Continuous cooling transformation from coarse-grained austenite in X80 line pipe steels Yamada, Kaiya


Oil and natural gas are important contributors to Canada’s energy sector, and pipelines are the most prevalent means of transporting these products. Modern Canadian pipelines are produced from advanced line pipe steels that have suitable properties for their applications as they are manufactured with the thermomechanical-controlled process (TMCP) and contain an optimized steel chemistry, including a low carbon content usually below 0.1wt% and specific additions of microalloying elements. When the line pipe segments are welded, the region next to the fusion zone experiences rapid heating and subsequent rapid cooling, creating a heat-affected zone (HAZ). The microstructure and, thus, the mechanical properties of the HAZ may be different from that of the as-rolled steel and may have a detrimental effect on pipeline integrity. The area of the HAZ closest to the fusion boundary, referred to as the coarse-grain heat-affected zone (CGHAZ), is subjected to temperatures high enough to dissolve precipitates, such as niobium carbonitrides. The dissolution of these precipitates may result in significant coarsening of the austenite grains. The resulting microstructural changes can lead to inadequate properties, such as insufficient toughness and strength. The purpose of this study was to investigate austenite decomposition kinetics during continuous cooling from coarse-grained austenite in three different line pipe steels with variations in chromium and molybdenum levels. Thermal conditions were simulated with a Gleeble 3500 thermomechanical simulator. Samples were continuously cooled at rates of 3, 5, 10, 30 and 50℃/s. Resulting microstructures were examined with optical microscopy and electron backscatter diffraction (EBSD) analysis. Hardness was measured with a Vickers hardness tester. An increase in molybdenum content from 0.11wt% to 0.26wt% resulted in a decrease in transformation start temperature by approximately 15℃ for all cooling rates. Lower transformation temperatures generated an increase in the high-angle grain boundary density (HAGB) of bainite, which was found to correlate with hardness. The analysis of the role of chromium was inconclusive and will require further studies. The continuous cooling transformation kinetics were modeled for each steel using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model with the additivity principle.

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