A New Systematic Approach Based on Dilatometric Analysis to Track Bainite Transformation Kinetics and the Influence of the Prior Austenite Grain Size

This investigation explores the influence of the austenitisation heat treatment and thus, of the prior austenite grain size (PAGS), on the kinetics of the bainitic transformation, using as A case study two high-carbon, high-silicon, bainitic steels isothermally transformed (T-Iso = 250, 300, 350 degrees C), after being austenised at different temperatures (T-gamma = 925-1125 degrees C). A methodology, based on the three defining dilatometric parameters extracted from the derivative of the relative change in length, was proposed to analyse the transformation kinetics. These parameters are related to the time to start bainitic transformation, the time lapse for most of the transformation to take place and the transformation rate at the end of the transformation. The results show that increasing the PAGS up to 70 mu m leads to an increase in the bainite nucleation rate, this effect being more pronounced for the lowest T-Iso. However, the overall transformation kinetics seems to be weakly affected by the applied heat treatment (T-gamma and T-Iso). In one of the steels, PAGS > 70 mu m (T-gamma > 1050 degrees C), which weakly affects the progress of the transformation, except for T-Iso = 250 degrees C, for which the enhancement of the autocatalytic effect could be the reason behind an acceleration of the overall transformation.

Automated summary

Increasing prior austenite grain size up to 70 μm leads to an increase in bainite nucleation rate, however, overall transformation kinetics seem to be weakly affected by the applied heat treatment.