Assessing the scale contributing factors of three carbide-free bainitic steels: A complementary theoretical and experimental approach

Adriana Eres-Castellanos,Javier Hidalgo,Muftah Zorgani,Mohammad Jahazi,Isaac Toda-Caraballo,Francisca G Caballero,Carlos Garcia-Mateo

doi:10.1016/j.matdes.2020.109217

Adriana Eres-Castellanos, Javier Hidalgo + Show 5 more

Open Access

https://doi.org/10.1016/j.matdes.2020.109217

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Abstract

The bainitic ferrite plate thickness is the main parameter that controls the strength of this type of microstructures. Such thickness has been proved to mainly depend on the austenite yield strength, the driving force for the transformation and the transformation temperature. However, no research has focused on how these parameters evolve throughout the transformation and how this evolution can affect the outcome. In this study, thermal and thermomechanical treatments have been performed in three selected steels. The treatments have been designed in such a way that all the mentioned parameters are comparable, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness. However, significant differences have been found among the microstructures, with variations in plate thickness larger than 100 nm. These results indicate that there might be other factors that take part in the scale of bainitic microstructures. To explain these differences and based on the kinetics of the transformation and on the carbon content of austenite at the end of the transformation, a possible explanation has been proposed.

Highlights

The YSTiso has been commonly adjusted by changing the transformation temperature [10], more recent works have modified it by work hardening prior to the transformation during the thermomechanical treatment of ausforming [7,11,12]
The process parameters were selected in such a way that YSTiso, Prior Austenite Grain Size (PAGS), ΔGγ→α and Tiso are similar in all cases, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness
For the 04C3Si and 477 L steels subjected to low temperature (LT) ausforming, this behavior was kept for the entire holding

Summary

Introduction

The impingement events influence the size of the bainitic plates [13] Such events are mainly dependent on the Prior Austenite Grain Size (PAGS) [14] and on the driving force for the transformation of austenite into ferrite, ΔGγ→α [6]. The above parameters (YSTiso, ΔGγ→α and Tiso) have been linked to the final scale of the bainitic microstructures, no research has been focused on how they evolve with respect to time throughout the transformation and how such evolution could alter the overall bainitic ferrite plate thickness. The process parameters were selected in such a way that YSTiso, PAGS, ΔGγ→α and Tiso are similar in all cases, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness. In order to analyze whether the mentioned aim had been achieved, a thorough microstructural characterization was carried out, including a quantification of the plate thickness on Scanning Electron Microscopy (SEM) micrographs [19]

Experimental

Methodology for the selection of pure isothermal and ausforming parameters

Experimental results

Results on pure isothermal and ausforming treatments

Dynamic changes of the system as the transformation progresses

Conclusion