Effect of integrated austempering and Q&P treatment on the transformation kinetics, microstructure and mechanical properties of a medium-carbon steel

It is found that integrated bainite austempering with quenching & partitioning process is effective to refine microstructure and improve ductility of bainitic steels. However, the effect of this new processing route on the microstructure and mechanical properties as well as transformation kinetics of Q&P steels is not clarified. The present work investigates the effects of austempering and quenching & partitioning (AQ&P) process on the transformation kinetics, microstructure and mechanical properties of a medium-carbon steel by metallography, dilatometry, X-rays diffraction, high-temperature laser scanning confocal microscopy (HT-LSCM) and tensile tests. The bainitic transformation during partitioning was firstly dynamically observed with HT-LSCM. Results demonstrate that prior bainite formation before quenching was is effective to refine the microstructure, especially the blocky M/A islands emerged in mere austempering treatment and large-size tempered martensite induced by conventional Q&P process. In addition, compared to individual austempering or Q&P process, more retained austenite with higher carbon content, especially filmy retained austenite, was obtained by AQ&P process. Moreover, compared to individual austempering or Q&P process, mechanical properties were enhanced through pre-formed bainite in AQ&P process.