Abstract
A non-isothermal transformation model was proposed to determine the austenite formation kinetics in a steel alloyed with 2.6% wt. Si by dilatometric analysis, considering that the nucleation mechanism does not change with the heating rate. From the dilatometric analysis, it was observed that the austenite formation occurs in two stages; critical temperatures, degree and austenite formation rate were determined. The activation energies associated with each of the stages were obtained employing the Kissinger method (226.67 and 198.37 kJ·mol−1 for the first and second stage) which was used in concert with the austenite formation rate in the non-isothermal model as a first approximation, with acceptable results in the second stage, but not in the first due to the activation energies magnitude. Then, the activation energies were adjusted by minimizing the minimal squares error between estimated and experimental austenite formation degree, obtaining values of 158.50 kJ·mol−1 for the first and 165.50 kJ·mol−1 for the second stage. These values are consistent with those reported for the diffusion of carbon in austenite-FCC in silicon steels. With these activation energies it was possible to predict the austenite formation degree with a better level of convergence when implementing the non-isothermal model.
Highlights
The austenite formation is an important part as a previous stage in the heat treatment of steels to obtain microstructures with specific mechanical properties
It was shown that the austenitic transformation kinetics is highly sensitive to the content of the alloying elements by promoting or delaying the austenite formation owing to the elements partition that occurs during the reaction as a function of the phases and microconstituents present
The degree and the austenite formation rate were determined by dilatometric analysis
Summary
The austenite formation is an important part as a previous stage (austenitization) in the heat treatment of steels to obtain microstructures with specific mechanical properties. There are studies on other types of steels such as: medium and high carbon [4,5,6], microalloyed steels [7,8], cast irons [9,10,11] and alloys with certain quantity of alloying elements such as silicon, manganese and chromium [12,13,14] On this basis, it was shown that the austenitic transformation kinetics is highly sensitive to the content of the alloying elements by promoting or delaying the austenite formation owing to the elements partition that occurs during the reaction as a function of the phases and microconstituents present. This type of transformation is carried out in two stages, contrary to what happens in plain medium-carbon steels where a similar microstructure is presented [14]
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