Bainite: Overall Transformation Kinetics

H. K. D. H. Bhadeshia

Abstract

New experimental results on the overall transformation kinetics of the bainite reaction (in three different steels) are interpreted in terms of recent work on the mechanism of the bainite reaction. The analysis is shown to be consistent with (a) a linear dependence of the activation energy for isothermal nucleation, on the transformation free energy change, (b) the sub-unit mechanism of bainitic transformation, (c) the variation of nucleation rate with the degree of transformation, (d) hard and soft impingement effects and (e) alloying element effects. It is also suggested that the more conventional methods of analysing the overall transformation kinetics are inconsistent with the experimental results, and with the need to understand alloying element effects.

This research investigates the overall transformation kinetics of the bainite reaction in silicon-rich steels. By utilising isothermal dilatometric analysis on three different alloy compositions, the author demonstrates that conventional mathematical models fail to accurately represent the experimental data. Instead, the study proposes a mechanism-based theory which suggests that bainite forms through a diffusionless, martensitic-like process involving discrete sub-units. This new framework accounts for factors such as isothermal nucleation activation energy, autocatalysis, and the incomplete-reaction phenomenon. The resulting calculations provide a more consistent fit for the progress of the transformation across various temperatures compared to traditional methods. Although some minor discrepancies remain regarding specific alloying effects, the work establishes a stronger link between the physical mechanism of growth and observed kinetic behavior.

Journal de Physique, Colloque C4, No. 12, Vol. 43, 1982, pp. C4-443-448.

Download PDF file of paper from HAL open access

Download PDF file of paper

Download dilatometric data

Audio podcast Podcast about the work