Austenite Grain Growth Kinetics during Continuous Heating of a Microalloyed X-80 Linepipe Steel

Abstract

Non-isothermal austenite grain growth kinetics has been studied in a microalloyed linepipe steel with complex precipitates containing Ti, Nb and/or Mo. The goal of these experimental studies is to provide the basis for the development of a grain growth model to predict the austenite grain size evolution in the weld heat affected zone (HAZ). Detailed electron microscopic investigations of the as received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. The steel was subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350 °C at heating rates of 10, 100 and 1000 °C/s, respectively. Thermal cycles have been found to have a strong effect on the austenite grain size. Austenite grain sizes increase with peak temperature and decreasing heating rate. However, the increase in heating rate from 100 to 1000 °C/s has a negligible effect on the austenite grain size. The observed austenite grain growth kinetics can be explained taking into account the potential dissolution of Nb-rich precipitates.