Formation of Bainite in Advanced High-Strength Steels During Isothermal Transformation

Abstract

The isothermal transformation of bainite in an advanced high-strength steel (AHSS) in the temperature range of 422°C to 482°C for transformation times of 10–6000 s has been examined using light optical microscopy (LOM), and orientation imaging microscopy (OIM). Thermodynamic calculations were done using equilibrium thermodynamic simulation software to produce a theoretical time-temperature-transformation (TTT) diagram, to which the experimental results of this study were compared. Advanced microstructural techniques, such as orientation-imaging-microscopy–electron-backscatter-diffraction (OIM-EBSD), image quality (IQ) phase identification, and polarized light microscopy, were used to observe and quantify the progress of the bainitic transformation during isothermal transformation. Orientation imaging microscopy using electron backscatter diffraction image quality (EBSD-IQ) was used with each sample to quantify the amount of bainite transformed. Results showed that the isothermal transformation at 422°C had 80.1 % bainite, whereas the isothermal transformation at 482°C had 85.5 % bainite transformed after 1200 s; while, after 6000 s, the amount of bainite increased to 90.1 % and 93.4 % respectively. As a result the chemical composition of the steel investigated and the microstructural characteristics of the bainite observed, it was not possible to discern the type of bainite formed at higher and lower temperatures because of the absence of carbides. The kinetics of transformation from the experimental results clearly indicated that the isothermal transformation is slower than that predicted by the theoretical thermodynamic program. The results of this study are presented and discussed.