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Austenite decomposition in an X80 linepipe steel Tafteh, Reza


The final microstructure and resulting mechanical properties in the heat-affected zone (HAZ) of welded linepipes are predominantly determined by austenite decomposition during cooling after welding processes. Thus, a full understanding of continuous cooling transformation of austenite is a key step toward improving the overall performance of linepipes. The main objective of the current study is to investigate the influence of cooling rate, prior austenite grain size and niobium content of austenite on austenite decomposition kinetics and the resulting microstructures for an X80 linepipe steel. To consider the significant effect of the niobium solid solution level on the transformation of austenite, two thermal histories were developed. For the first case, Nb was dissolved in solid solution prior to austenite decomposition. In contrast, the second scenario involved the formation of Nb(C,N) precipitates prior to austenite decomposition, i.e. leaving a low level of Nb in solid solution. Austenite grain growth studies were conducted to obtain grain sizes similar to those observed in the HAZ of the girth-welded steel, i.e. 5-80μm. Furthermore, employing appropriate thermal cycles, continuous cooling transformation (CCT) tests were conducted to examine the effect of niobium condition, austenite grain size and cooling rate on austenite decomposition behavior of the steel. Cooling rates varied in the range of 3−100ºC/s and dilation measurements were utilized to capture the transformation kinetics of austenite upon cooling. The resulting microstructures, which usually consist of ferrite, bainite and martensite-austenite (MA) constituents, were examined using optical microscopy. They were revealed using appropriate etchants and the corresponding phase volume fractions were subsequently measured in accordance with ASTM standards. Hardness measurements were also conducted on CCT samples.

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