On the martensite transformation kinetics of AISI 4340 steel

By means of thermal dilatation a AISI 4340 steel was austenitized at some different temperatures in a range from 900 to 1150°C to obtain different austenite grain sizes (AGS), then rapidly cooled to room temperature to obtain martensitic structures. The transmission electron microscopy (TEM) including electron diffraction and microanalysis was used to describe the microstructures. It was found that both AGS and A1N exert a great effect on the martensite transformation kinetics. The larger the AGS, the higher the starting temperature of martensite transformation (t_ms). Within different austenitizing temperature ranges, there are different kinds of compound precipitates found, affecting the nucleation of martensites. At and below 1000°C, since the particles of AlN play an important role during the martensite transformation, t_ms increased rapidly. However at and above 1050°C, t_ms still increases with AGS. This fact may be due to the precipitation of relatively higher dissolution temperature, such as TiC, on the boundary of austenite.