Papers by Keyword: Transformation Kinetics

Abstract: The effect of deformation temperature (DT) and thickness reduction on the bainitic structure was investigated under various test conditions by using hot compression on a Gleeble-1500 thermo-mechanical simulation machine, and electron back scattering diffraction (EBSD) technique. In the case of the bainitic structure consisting of granular bainite (GB), lath bainite (LB) and a little ferrite (AF) under the given deformation conditions, DT and thickness reduction have remarkable effect on the transformation kinetics, starting temperature (B) of bainite fast transformation, and the type of bainitic structure. With the decreasing of DT from 810 °C to 730 °C, the starting temperature of transformation B increase from 585 °C to 595 °C. When the thickness reduction was 0 % and 20 %, the microstructure consists of GB, LB and a little AF, whereas as the thickness reduction increase to 40 %, large grain size of LB and GB disappear, and only AF and M/A remained. With the thickness reduction increases from 0 % to 40 %, the effective grain size decreases from 4 μm to 2 μm, and the fraction of HGB increases from 48 % to 57 %.

Abstract: Three ductile irons with different aluminum-and manganese-content were subjected to two thermo-mechanical schedules. In the first schedule, a total deformation of φ_t = 0.3 is applied on the ductile irons in the austenitic region before the austempering process. In the second schedule, the materials are subjected to deformation of 0.2 in the austenitic region and deformation of 0.1 during austempering (ausforming). Mechanical deformation of austenite prior to the transformation “stage I” pronouncedly accelerated the transformation due to increasing the nucleation sites of ausferrite. This increase has its impact on enhancing the microstructural uniformity and refining the ausferrite platelets. On the other hand, the retained austenite content was not significantly affected by the applied ausforming. Remarkable increase in hardness, strength and ductility of the ausformed ductile iron due to the latter effects is observed.

Abstract: The effects of the stress state and temperature on the martensitic phase transformation behavior in TRIP 780 steel were investigated using multi-axial experimental techniques. Various mechanical experiments are performed to differentiate the stress state and temperature effects. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equibiaxial tension, plane strain tension and simple shear. A temperatures both 25 and 60 °C for each stress state condition except the simple shear test were investigated. In-situ magnetic measurements were performed to mesure the evolution of the martensite content throughout each experiment. Finally, a new martensitic transformation kinetics model for the TRIP 780 steel is proposed to take the effect of stress state and temperature into account.

Abstract: A modified model describing the austenite reaction was developed that took into account the effect of heating rate. The model considered the variation of activation energy during non-isothermal heating and one set of model parameter was adequate to predict the formation of austenite. To verify the theoretical model, the process of austenite formation during continuous heating in Cr5 roller steel with pearlite and ferrite mixed initial microstructure was analyzed by dilatation experiment. The results show that a strong logarithmic relationship between apparent activation energy and heating rate. Experimental kinetic transformations as well as critical temperatures of austenite reaction are in good agreement with the calculations. The model can be used to describe the transformation kinetics at an intermediate heating rate.

Abstract: The microstructure of metastable alloys varies with the thermomechanical history of the material. During a thermomechanical loading, different phases can be present in the material simultaneously. They can be at the origin of macroscopic stress and strain. Consequently, it is important to determine the proportion of each phase to understand the transformation kinetic. However, the techniques usually used to carry out these measurements such as magnetic properties measurements, neutron or X-ray diffraction, are heavy and require a lot of resources. This study, thanks to experimental tests based on electric resistance measurements, permits to determine the volume fraction of the different states present in the material. The study has been realized on a 301 stainless steel in order to take into account the effects of elasticity, transformation and plasticity. Then the volume fraction of the different phases of the material has been determined during transformation with the plasticity presence.

Abstract: Mechanical stability of retained austenite and its effect on mechanical properties of high strength TRIP steel were studied by means of OM, SEM, TEM, XRD, and mechanical testing after various heat treatments. Results revealed that the film-type retained austenite located between bainite laths with high carbon content showed gradual martensitic transformation with strain, demonstrating a good TRIP effect. Samples annealed at 800°C and held at 420°C showed an optimum value of strength and ductility product up to 18381.2MPa%. Transformation kinetics of the retained austenite were evaluated through tensile tests and fitted by the function y=0.86-0.86×exp(-Ax). The fitting results were good.

Abstract: The effects of aging temperature and aging time on properties of Cu-2.1Ni-0.5Si -0.2Zr-0.05Cr (wt.%) alloy were studied. The transformation ratio of new phase in Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr alloy was calculated when aging at 400°C, 450°C and 500°C by measuring electrical conductivity, the relation between the electrical conductivity and the quantity of new phase. The Avrami-equation of phase transformation kinetics and the Avrami-equation of electrical conductivity during aging were established for Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr alloy, on the basis of linear relationship between the electric conductivity and the volume fraction of precipitates. The calculated values of electrical conductivity well consistent with those of experiment can provide reference on the alloy of production process. The characteristics of precipitates in the alloy after solution treatment and cold rolling were established, and the results show the precipitate was δ-Ni2Si phase.

Abstract: A mathematical model was developed predicting the effects of alloying and thermomechanical processing on the final microstructure of steel. Various factors influencing transformation kinetics, including microalloying with Nb and plastic deformation of austenite, are considered. Subsequent stages of the model development and calibration are described.

Abstract: The thermo-mechanical behaviour of the hot rolled dual-phase steel 10MnSi7 has been determined as a function of temperature and time within the two-phase field α+γ as well as at lower deformation temperatures down to Ms-temperature. The corresponding microstructures, phase hardness, state of recrystallisation have been determined using quantitative metallography. Using this information, a finite element model of a representative volume element (RVE) has been set up. Each element represents a subdomain of a grain that transforms according to the kinetics found in the measurements. The RVE has been subjected to the same loading history as in the accompanying experiments and the overall stress-strain response is monitored during ongoing transformation. The effect of a preferred orientation of the two phase structure has been studied taking into account the effects of a significant plastic deformation. The model is validated by comparison with the experimental evidence.

Abstract: This paper presents a transformation kinetics model of NiTi shape memory alloy (SMA) wires based on electrical resistivity (ER) derivative study under the assumption that the derivative of electrical resistivity with respect to temperature is in linear relationship with the derivative of free energy change with respect to temperature. Free energy change and electrical resistivity properties of SMA are analyzed based on differential scanning calorimetry (DSC) experiments during phase transformation. The simulated evolution of electrical resistivity during thermomechanical transformation is presented using the proposed model.