On the relationship between the multiphase microstructure and the mechanical properties of a 0.2C quenched and partitioned steel

TLDR: In this article, a quench dilatometer was used to estimate the fraction of martensite formed at a given temperature below the start temperature of the martensitic transformation profile, and the measured austenite fractions after quench treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions.

Abstract: In the present work, Quenching and Partitioning (Q&P) heat treatments were carried out in a quench dilatometer on a 0.2 wt% carbon steel. The microstructure evolution of the Q&P steels was characterized using dilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate the fraction of martensite formed at a given temperature below the martensite start temperature Ms. Q&P was shown to be an effective way to stabilize retained austenite at room temperature. However, the measured austenite fractions after Q&P treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitive to the quench temperature and were never larger than the ideal predicted maximum fraction. Competitive reactions such as austenite decomposition into bainite and carbide precipitation were found to occur in the present work. Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperatures and partitioning times. The direct contributions between Q&P microstructural constituents -such as retained austenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This was critically discussed and compared to quenching and austempering (QAT) which is a more conventional processing route of stabilizing retained austenite at room temperature. Finally, Q&P steels were shown to exhibit an interesting balance between strength and ductility. The achievement of this interesting combination of mechanical properties was reached for much shorter processing times compared to QAT steels.