Bainite Growth Behavior at Early Stage of Transformation in a High Carbon Silicon-Containing Steel

The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy, X-ray diffraction analysis and transmission electronic microscopy. The bainite was obtained by isothermal transformation at 200 °C only for a short time in a high carbon silicon-containing steel after austenitization at 200 °C only for 20 min. Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm, and that the interface of the bainite leading tip is wedge shaped. X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase, with absence of carbides. The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip. Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary. The austenite grain boundary impedes the longitudinal growth of the bainite plate, i. e., the growth of bainite plate stops at the austenite grain boundary. The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.