Non-isothermal prior austenite grain growth of a high-Nb X100 pipeline steel during a simulated welding heat cycle process

Highlights: • The austenite grain rapidly grows at high heating and cooling temperature. • The Nb-rich carbonitrides quickly dissolve as heating temperature is over 1200 °C. • The kinetics of austenite coarsening and dissolution of the precipitates are relevant. • Adding Nb decreases the Ti-rich carbonitride thermostability. The behavior of non-isothermal prior austenite grain growth of heat-affected zone (HAZ) for a high-Nb X100 pipeline steel was investigated on a Gleeble 3500 thermal simulator by simulating weld thermal cycle. The chemical composition, size, and volume fraction of carbonitride precipitate particles were determined using transmission electron microscopy (TEM) and energy dispersive X-ray (EDX). The effects of adding Nb on carbonitride were also analyzed using Thermo-Calc software based on TCFE7 database. Prior austenite grain rapidly grew not only at > 1200 °C but also at high temperature during cooling process. Result of non-isothermal austenite grain growth method based on pinning by carbonitride precipitate showed that austenite grain growth was strongly related to carbonitride precipitates. Adding Nb decreased the Ti-rich carbonitride thermostability and the full dissolution temperature. However, some remaining Ti-rich carbonitride particles and dissolved Nb restrained austenite grain growth during the welding thermal cycle in rapid heating and cooling processes. Therefore, the prior austenite grain size was still maintained at a small level under high welding heat input condition.