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Swan, Joshua

University of British Columbia

The oil and natural gas industry, as well as newly developing technologies such as carbon sequestration and hydrogen transport rely on pipelines for transportation of their products. As the demand for safety and efficiency of pipelines increase, the properties of the line pipe steel from which they are made must improve. To do this, line pipe steel is manufactured using a thermomechanically controlled process (TMCP) which combines specific alloying elements with processing conditions to produce an engineered microstructure. Within the TMCP, the austenite grain size and structure are important factors in determining the final microstructure and thus mechanical properties. The purpose of this work is to investigate the austenite grain growth evolution during the thermal treatment in the reheat furnace and compare austenite grain growth characteristics in as-cast slab and hot-rolled material. Two different line pipe steels with varying amounts of manganese and niobium were studied. Thermal conditions were simulated with a Gleeble 3500 thermomechanical simulator while the austenite grain size was measured in-situ with a laser ultrasonics for metallurgy (LUMet) system. The reheat furnace treatment was simulated with a slow heating rate of 5°C/min to 1200°C followed by a 2hr hold. Grain growth comparison experiments consisted of a high heating rate of 50°C/s to one of three holding temperatures of 1100°C, 1200°C and 1300°C. Results of the LUMet measurements were validated by reconstructions of prior-austenite microstructures using electron backscatter diffraction (EBSD) data. LUMet measurements and reconstructions of austenite grain structures for the as-cast slab material undergoing the reheat furnace simulation show a bimodal grain size distribution with a few extremely large grains and many small grains clustered together. A similar but less extreme version of this microstructure was found in slab material undergoing grain growth comparison experiments. This bimodal microstructure was not observed in the hot rolled material. Current hypothesis is that solute segregation of Niobium and Titanium into the inter-dendritic zones is occurring during solidification. These alloying elements promote the formation of (Ti,Nb)(C,N) precipitates that inhibit grain growth and when concentrated into specific areas, allow for a bimodal microstructure to occur.

Text

2024-07-08

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/88593

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/