In situ measurement and modelling of austenite grain growth in a Ti/Nb microalloyed steel

Using a novel laser ultrasonics technique in situ measurements of austenite grain growth were conducted during continuous heating (10 °C s⁻¹) and subsequent isothermal holding at various temperatures in the range 950-1250 °C in a microalloyed linepipe steel. Based on the experimental results, a grain growth model was developed, which includes the pinning effect of precipitates present in the steel. Analyzing the grain growth behaviour and using the advanced thermo-kinetic software MatCalc, an approach was developed to estimate the initial distribution of precipitates in the as-received material and their dissolution kinetics. The evolution of the volume fractions and mean particle sizes of NbC and TiN leads to a time-dependent pinning pressure that is coupled with the proposed grain growth model to successfully describe the observed kinetics of austenite grain growth. The predictive capabilities of the model are illustrated by its application to independent grain growth data for rapid heat treatment cycles that are typical of the weld heat affected zone.