Abstract
New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.
Highlights
THE continuous cooling transformation (CCT) diagram is a common tool in use for designing the heat treatment process of the steel
The current study introduces the new optimized CCT-equations and shows their validation with the applied experimental CCT measurements
This experimental CCT database is extended with CCT diagram compilations made in the USA[19] and Slovenia,[20] besides the CCT diagrams obtained from several literature studies.[20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]
Summary
THE continuous cooling transformation (CCT) diagram is a common tool in use for designing the heat treatment process of the steel. Due to solute microsegregation and formation of precipitates, tying up certain solutes from the grain boundary solid solution phase, the grain boundary compositions, become very different from the nominal ones, and so do the corresponding CCT-equations optimized from the experimental CCT diagrams. In our earlier study,[8] thirteen CCT equations were optimized to be able to calculate the phase transformation temperatures and the corresponding critical cooling rates applied in the austenite decomposition simulations using the IDS software. The optimization was made using the CCT measurements conducted in Germany[16,17] and Britain[18] on low-alloyed steels, applying the IDS-calculated grain boundary soluble compositions for these steels.
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