Microstructure and Phase Transformations in High-Strength Bainitic Forging Steel

In this work, the microstructural evolution and the phase transformations on a 0.22 wt.% carbon bainitic forging steel are investigated using a deformation dilatometer. The microstructure evolution was characterized using light microscope (LM), scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and electron backscatter diffraction (EBSD). It was tried to predict the phase transformations via a continuous cooling transformation (CCT) diagram calculated by JMatPro using the results of prior austenite grain size measurement. In addition, critical temperatures were calculated using Thermo-Calc, JMatPro, and empirical formulations. Based on the dilatometric tests, hardness, and microstructural investigation, a deformation-CCT diagram was plotted. It was found that a bainitic microstructure can be obtained at cooling rates from 0.15 to 8 K/s. The microstructures varied from a mixture of bainite and blocky martensite/austenite islands at lower cooling rates to a bainitic ferrite with martensite at higher cooling rates. Finally, an industrially hot-forged sample cooled at 1 K/s was investigated. The microstructural evolution showed a microstructure of carbide-free bainitic ferrite along with film and island of retained austenite.