In situ observation of bainite to austenite transformation of microalloyed high strength steel during simulated welding cycle

A confocal scanning laser microscope, equipped with a high temperature stage, was used for in situ observation of the bainite to austenite transformation in a microalloyed high strength steel. Austenite grain growth was observed and measured directly from 1040°C to the peak temperature of 1340°C during the heating process, as well as the subsequent cooling down to 1040°C. The grain growth rate versus temperature was analysed. It was demonstrated that the austenite grains grew up simultaneously by means of grain boundary migration and ‘grain swallowing’ phenomena. The variation in grain growth rate was attributed to the presence of impeding precipitates, e.g. carbonitrides Nb,Ti(C,N). The work also showed that coarsening and/or dissolution of carbonitride precipitates, above a certain temperature, led to a fast grain growth. The in situ observation by confocal scanning laser microscope can provide valuable information on the austenite reformation and final microstructure of weldments.