Isothermal Phase Transformations and Stability of Retained Austenite during Quenching and Partitioning Process for 0.15C Steel

Abstract

The microstructure and dilatation for 0.15C steels were investigated to define the phase transformation during the quenching and partitioning (Q&P) process. For the one step Q&P dilatation, the isothermal martensite/bainite transformation occurred because the holding temperature was between Ms and Mf. The isothermally transformed martensite/bainite and the athermally transformed martensite were produced by a loss of retained austenite. As the holding time increased, new martensite-start (Ms) temperature produced from the final quenching process decreased due to the carbon partitioning from the martensite to the retained austenite. This was the direct evidence of increment for the retained austenite stability. For the two step Q&P dilatation, the isothermal bainitic transformation occurred because the partitioning temperature was larger than the Ms and new Ms. The partitioning at 400°C indicated the short incubation period for the bainite transformation than the 350°C partitioning because the partitioning at 400°C should acquire the larger thermal driving force for carbon partitioning than the 350°C partitioning. A quick drop of Ms and short period of bainite incubation for the 400°C partitioning steel were also the direct evidence of significant effects of carbon partitioning on the stability of retained austenite.