Physical Metallurgy for Superior Toughness of Pipeline Welds - Intercritical phase transformations in microalloyed high-strength steels

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***The long-term objective of this program is to develop the welding metallurgy for next generation steel linepipe for safe transportation of fossil-fuel and renewable energy. The short-term research objectives are to develop a theory for non-equilibrium transformations in X70 pipeline steel thermally cycled between lower and upper critical temperatures, and to optimize the low-temperature toughness by controlling the multipass weld microstructure. The educational objective of this program is to train highly qualified personnel in physical metallurgy of ferrous alloys, advanced mechanical testing methods, and analytical and numerical modeling of heterogeneous material performance.******Possessing exceptional strength and ductility, advanced steels alloyed with small amounts of elements, 0.01 to 0.10 wt% of Nb, V, Cr, and Mo each, find widespread use in critical structures from automotive to gas and petroleum pipelines. As an indispensable fabrication process, welding usually damages the optimized microstructure and properties of the original pipeline steels. How to properly weld these pipeline steels for superior performance has been the “holy grail” not only technologically, but also economically. The applicant proposes the following research tasks to quantitatively investigate:******1. Conduct welding tests on X70 pipeline steel, and characterize the thermal cycles typical for the intercritical heat-affected zone for gas-metal arc welding and submerged arc welding. Measure the impact toughness of the weld bondline and the heat-affected zone for the temperature range of -20 to -60C. ******2. Numerical simulations will be conducted to understand the temperature fields as influenced by the weld geometry and heat input. Reproduce the thermal cycles on the test alloys using the Gleeble and a quench dilatometer. ******3. Develop a microstructural evolution theory for optimizing the impact toughness by quantifying the peak temperatures on austenite formation and carbonitride dissolution (on-heating), and ferritic-type transformations (on-cooling). This theory will be applied to multipass deposits of 3D printed tool steel parts.******The economic impact of the proposed program promises to be very large. The technical findings will directly enhance the performance and safety of welded steel structures in petrochemical and power generating industries. There will be direct impact on Canadian metals and steels, manufacturing, petrochemical, and energy sectors. The scientific findings will contribute to materials engineering and physical metallurgy by providing new knowledge on microstructure-processing-properties relationship for an important class of engineering alloys. For training of highly qualified personnel, this research program serves as an ideal platform to educate graduate and undergraduate students in physical metallurgy.*****