A physically based approach to model the incomplete bainitic transformation in high-Si steels

Abstract

This paper describes an approach to simulate the incomplete bainitic transformation observed in high-Si steels. The model can capture the incomplete transformation when a simple procedure accounting for carbon enrichment in the remaining austenite is applied to calculate the change in the zero transformation temperature, which is a key parameter controlling the transformation kinetics. The carbon concentration in the remaining austenite is determined by the volume fraction of bainitic ferrite and the effective carbon concentration of the bainitic ferrite. Comparison with experimental data for Fe-0.3C-2.4Mn-1.8Si is made, which demonstrates that the isothermal kinetics of bainite formation in the range 370–480°C can be satisfactorily described with the model by using a single set of model parameters and only adjusting the carbon concentration of the bainitic ferrite. A good agreement is found between predicted final carbon contents of retained austenite and those measured using X-ray diffraction, which indicates that a plausible carbon dependency of the zero transformation temperature has been assumed in the calculations.

Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel

“…In order to quantify the fresh martensite fractions, [24,25] the net dilatations at 298 K (25°C) of all isothermal treatments are compared with the maximum net dilatation with respect to the austenitic phase obtained at the same temperature in the direct-quench treatment, which is 0.84 pct (see Figure 9). Net dilatations (indicated by double-ended arrows) are obtained calculating the difference of change in length between the experimental curves and the dashed lines.…”

Modeling retained austenite in Q&P steels accounting for the bainitic transformation and correction of its mismatch on optimal conditions

“…However, the predictions of the CCET model, regardless of the alloy system examined, seem to be more consistent with the experimental observations because the bainitic transformation occurs in both alloys. Although the formation of bainite consumed austenite, carbon diffused from the carbonsupersaturated subunit to the surrounding remaining austenite [40], further enriching the remaining austenite, which eventually increases the stability of this phase and prevents its transformation to secondary martensite during the final cooling to room properties. Moreover, it is worth noting that the predictions of steel B showed relatively greater deviation from the experimental observations, which will be discussed in Section 5.4.…”

An Alternative to Avrami Equation

“…The incomplete Fe-0.29wt%C-2.39wt%Mn-1.76wt%Si bainite transformation curves in ref. 14 also could be fitted with Eq. (3) with high fitting correlations -see Fig.…”

“…Since anti-thermal φ K refers to mechanical-autocatalysis, such counterpoint suggests that the feedback from auto-accommodation of the transformation strains did not compensate the drag effect imposed by Si influence on the carbon concentration in the austenite, thence the incomplete transformation as reasoned in ref. 14 .…”