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Grain growth and austenite decomposition in two niobium containing line pipe steels Robinson, Isaac Dalton Gordon


Niobium is a common microalloying element for line pipe steels for the promotion of fine ferrite microstructures following hot rolling. During construction via welding, niobium carbonitrides formed during coiling may dissolve depending on peak temperatures reached. This has a strong influence on the microstructure and mechanical properties of the heat affected zone. The primary objective of the current work is to study the effect of niobium microalloying on heat affected zone microstructures. Two steel compositions were selected for this study, possessing a high (0.091 wt%) niobium content and differing (0.028 and 0.058 wt%) carbon contents. To study the effect of niobium on grain growth, thermal histories were designed in the context of continuous heating and isothermal holding for a range of heating rates and holding temperatures. Laser ultrasonics were used to observe in-situ grain growth during these histories and experimental results were confirmed by ex-situ metallography using appropriate etchant. Niobium is shown to be effective in restricting grain growth behavior at temperatures when precipitates are stable. An increase in heating rate was found to reduce overall grain growth (from 48 to 25 μm between 10 and 1000°C/s), eventually reaching a limiting behavior at the maximum heating rate (i.e. 1000°C/s). To study the effect of niobium on austenite decomposition, thermal histories were designed to produce two grain sizes (5 and 35 μm) and two states of niobium (in and out of solution). The latter was investigated through precipitation experiments. Cooling rates were varied between 3 and 30°C/s and mechanical dilatometry was used to measure the phase transformation. As expected, an increase in cooling rate, carbon content, and prior austenite grain size results in lower transformation temperatures. The effect of niobium in solution was strongly dependent on prior austenite grain size. Microstructures formed by austenite decomposition were characterized via metallography and microhardness. The microstructural constituents include ferrite, bainite, and M/A. Samples representative of each microstructure were selected to quantify M/A fraction through etching and point counting. Niobium was shown to have a strong effect on hardness when the prior austenite grain size was large (up to 70 HV).

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