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UBC Theses and Dissertations

Microstructure evolution in the coarse grain heat affected zone of line pipe steels Romualdi, Nicolas


Line pipe steels are manufactured by thermomechanically controlled processing (TMCP), which combines selected alloying additions and processing conditions, to produce engineered microstructures that confer steels their properties. During pipeline construction, segments of pipe are sourced from numerous suppliers, each of whom may have different alloying strategies, and welded by varied technologies and procedures, leading to numerous different resulting microstructures in the weld heat affected zone (HAZ). Consequently, it is critical to develop a chemistry sensitive model that describes austenite grain growth and decomposition in the coarse grain heat affected zone (CGHAZ). In this work, austenite grain growth in the CGHAZ is investigated by using Laser Ultrasonics for Metallurgy (LUMet) in 37 steels of chemistries in a range relevant to X80 line pipe steels. A model which considers the effect of solutes on the mobility of the austenite grain boundary and the pinning pressure exerted by Ti-rich precipitates was developed. The amount of C and Nb in solution were found to reduce the mobility of the austenite grain boundary. The size distribution of Ti-rich precipitates was analyzed and verified by scanning transmission electron microscopy (STEM). The model was validated for laboratory thermal cycles and industrial welding trials. Austenite decomposition kinetics were measured for 16 steels with systematic variations of C, Mo, Nb, and Cr, at continuous cooling rates of 3, 5 and 10 °C/s, and their final microstructures were characterized using electron backscatter diffraction (EBSD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness. Microstructures at 30 and 50°C/s were also produced and characterized with the same techniques. The effect of solutes in decomposition kinetics was analyzed and C, Nb, and Mo were found to decrease the transformation start temperature in continuous cooling. A novel finding was the role of Ti-rich particles. It was observed that a decrease in the interparticle spacing of these precipitates leads to an increase in the transformation start temperature, which was rationalized by their ability to pin moving bainite grain boundaries. A microstructure-properties model is proposed where the hardness in the CGHAZ is a function of the high angle grain boundary density.

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