Kinetics of Martensite/Austenite Decomposition during Tempering of Ultrafine Nano-Bainitic Steels

The decomposition of martensite/austenite (M/A) microconstituent in bainitic steels was analyzed using differential scanning calorimetry (DSC) data in conjunction with Kissinger’s and Johnson-Mehl-Avrami-Kolmogorov (JMAK)'s formulas. The results show that: in bainitic steel subjected to austempering heat treatment, the presence of M/A microstructure adversely affects the mechanical properties. According to the kinetic equations derived, it is observed that after tempering at 600 ℃ for 4000 s, the generation of each phase in the sample reaches its maximum. The SEM images taken before and after tempering reveal extensive decomposition of the M/A constituent in the microstructure. The proportion of the M/A microstructure decreases significantly from about 10% before tempering to less than 1% after tempering. Additionally, the content of residual austenite also reduces nearly to zero. These observations are consistent with the predictions of the kinetic equations.