A Comparative Study of Kinetic Models Regarding Bainitic Transformation Behavior in Carburized Case Hardening Steel 20MnCr5

Isothermal bainite transformation kinetics have been carried out and evaluated for the case hardening steel 20MnCr5 with the carbon content ranging between 0.2 and 0.8 wt pct in a wide span of isothermal temperatures. Multiple austenitization temperatures were chosen to investigate the effect of austenitic grain coarsening on the effect of transformation kinetics. Three well-known transformation kinetic equations were evaluated on more than 60 transformation curves, the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, the Austin–Rickett (AR) equation, as well as an autocatalytic (AC) model proposed by van Bohemen. In terms of fit quality, the AR kinetic is superior, although all models show a good accordance with experimental data. The results demonstrate that the interpretability of overall activation energies can be highly ambiguous. Also, an increase for the growth exponent of JMAK/AR equations was found with a scatter of values between 1.5–4.5 and 2.0–6.0, respectively. This finding demonstrates that the value of the growth exponent cannot be interpreted in terms of growth dimension of the bainitic subunits. Furthermore, the kinetic model on a diffusionless approach was evaluated, and the activation energies were compared to work from van Bohemen. Due to the consideration of the grain size on the delay of transformation a surprisingly constant OAE was evaluated for every alloy irrespective of the austenitization temperature. Also, found OAE values are close to findings by van Bohemen with approx. 190 to 200 kJ/mol. Finally, an exponential dependency of the autocatalytic factor with respect to carbon content was proposed.