Christophe Mesplont

Quantitative analysis of the dilatation during an isothermal decomposition of austenites

The dilatation of hypoeutectoid steels during an isothermal transformation has been analysed. A model accounting for the effect of carbon redistribution has been developed which can be applied not only to the calculation of the dilatation during an isothermal transformation but also to the extraction of the phase change kinetics from a dilatation curve which may correspond to a multistep phase transformation. The model is validated by comparing it with the experimental results for an interstitial free steel, The dilatation of Fe-C steels during an isothermal transformation has been calculated and discussed. It is indicated that the isothermal formation of ferrite causes a different linear dependence of the relative length change on the volume fraction of the transformed austenite compared with the pearlite type decomposition. Isothermal transformation experiments have been carried out with an experimental bainitic grade steel. The model was successfully applied to extract the phase transformation kinetics from the dilatation curves of this steel

Effect of austenite deformation on crystallographic texture during transformations in microalloyed bainitic steel

Abstract The evolution of the texture of ferrite as a function of the coiling temperature has been studied in a hot rolled Nb alloyed CMnMoCrB complex phase steel by means of electron backscatter diffraction. Coiling that steel at 720 ° C led to ferrite and pearlite, and coiling at 550 ° C produced a bainite-martensite microstructure. The presence of residual austenite in the steels coiled at 680 and 550 ° C allowed for texture measurements in γ. Analyses of texture gave fundamental information on the decomposition of γ in both the recrystallised state and the deformed state. It was found that austenite, initially deformed below the non-recrystallisation temperature Tnr, recrystallised statically d partially during the γ α and the γ d α b transformations. In the specimen coiled at 680 ° C, primary ferrite and bainite could be distinguished based on the confidence indexof the diffraction pattern. A clear variant selection was observed for the γ d α b transformation, as arotation of ϖ 1 = 30 ° occurred inthe austenite between the ferrite and the bainite formations. The bainite was found to result mainly from the decomposition of the brass {110} 〈 112 〉 and Goss {110} 〈 001 〉 orientations of deformed austenite. The residual austenite was found to be recrystallised γ γ austenite with the cube{001} 〈 100 〉 orientation. Coiling simulations were performed in a dilatometer starting from different austenite grains sizes and deformation states. In the most deformed specimens, the deformation state of the austenite and the combined effects between the different alloying elements presentin the steel were responsible for a solute drag like effect.

Dilatometric study of the effect of soluble boron on the continuous and isothermal austenite decomposition in 0.15C-1.6Mn steel

Abstract The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac! temperature during reheating. Isothermal transformations in the 450–660°C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partition...

Model for extracting phase transformation kinetics from dilatometry measurements for multistep transformations

Abstract A detailed analysis has been made of the length change of a hypoeutectoid steel during a continuous cooling or an isothermal transformation. A model has been developed in which the transient dilatation is calculated based on the fraction of the phases present. The model accounts for the carbon partitioning effects and is applicable to the extraction of the phase change kinetics from a dilatation curve for a multistep phase transformation. It has been validated by comparing the model results with the experimental results of an interstitial free steel. Isothermal transformations and continuous cooling experiments have also been carried out with a bainitic steel. The model was used to extract the phase transformation kinetics from the dilatation curves of this steel. Excellent agreement between the calculated results and the experimental ones has been obtained.