Study of the isothermal bainitic transformation and austenite stability in an advanced Al-rich medium-Mn steel

Isothermal bainitic transformation of a lean medium-Mn steel containing (in mass%) 0.18C-3.6Mn-1.7Al-0.23Si-0.2Mo-0.04-Nb after full austenitization at 1100 oC was studied by means of high resolution dilatometry. The effects of isothermal holding temperatures ranging from 450 to 350 degrees C on the bainitic transformation kinetics was studied experimentally characterizing the microstructure present after a holding time ranging from 15 min to 2 h. The obtained results showed that the bainitic transformation is uncompleted at temperatures above 425 degrees C. The carbon enrichment of the austenite during isothermal treatment at 450 degrees C and 425 degrees C is not enough to avoid martensitic transformation of the austenite during cooling to room temperature. Thus, it is obtained a mixed structure including bainitic ferrite and martensite. Decreasing the austempering temperature resulted in a more pronounced bainite formation. The kinetics of the transformation during austempering at 350oC is quite similar to that observed at 400 degrees C, so that 60 min are needed in both cases to complete the reaction. However, local variations in chemical composition associated to segregation of Mn and Al during casting solidification results in differences in the transformation rate of bainitic reaction between different areas in the material. A balance difference between the dendritic and interdendritic areas is responsible for the differences observed among test samples in the kinetics of the isothermal bainitic transformation and the final microstructure.