Analyses of Transformation Kinetics of Carbide-Free Bainite Above and Below the Athermal Martensite-Start Temperature

The isothermal transformation kinetics of austenite decomposition in Fe-0.4C-2.78Mn-1.81Si was analyzed by an electrical resistivity technique in the temperature interval 723 K to 418 K (450 °C to 145 °C). The analysis of transformation kinetics of the bainite transformation was performed using the Johnson–Mehl–Avrami–Kolgomorov (JMAK) and Austin–Rickett (AR) approaches. The kinetic parameters, the reaction constant n , rate constant k = k ( T ), and apparent activation energy Q were evaluated for isothermal transformations below and above the martensite-start temperature M S = 548 K (275 °C), which was determined experimentally. The formation of strain-induced martensite, which starts to accompany the bainite transformation at just above M S , increases the rate of transformation and decreases the apparent activation energy of austenite decomposition.